STUDIES ON GROWTH, CHARACTERIZATiuj
PROPERTIES OF LAYER PEROVSKITES
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STUDIES ON GROWTH, CHARACTERIZATION AND MAGNETIC PROPERTIES OF LAYER PEROVSKITES
BIBLIOTHEEK TU Delft P 1185 6279
STUDIES ON GROWTH, CHARACTERIZATION AND MAGNETIC
PROPERTIES OF LAYER PEROVSKITES:
The bis-(alkylammonium) nietal(II) tetrahalides
(CnH2n+lNH3)2MX4
y/cPs ^-^^9
PROEFSCHRIFT
ter verkrijging van de graad van doctor in de
technische wetenschappen aan de Technische
Hogcschool Delft, op gezag van de rector magnificus
prof. ir. L. Huisman, voor een commissie aangewezen
door het college van dekanen, te verdedigen
op woensdag 30 november 1977 tc 16.00 uur
door
Frederik Hendrik Maria Mischgofsky
natuurkundig doctorandus
geboren te Roermond
Dit proefschrift is goedgekeurd door de promotoren
Lector dr. P. Bennema en Prof.dr. P. Wyder
Dit proefschrift draag ik op aan alien die hebben bijgedragen aan het tot stand komen ervan.
CONTENTS
CHAPTER I:
INTRODUCTION 15 1. Growth and characterization 17
2. Monte Carlo simulations 19
3. Magnetism 19
PART ONE: GROWTH AND CHARACTERIZATION
CHAPTER II:
LAYER PEROVSKITES OF THE (C H„ ,NH,)„MX, AND NHo(CH„) NH,MX, n 2n+l 3 2 4 3 2^n 3 4 FAMILIES WITH M = Cd, Cu, Fe, Mn or Pd, AND X = CI or Br:
IMPORTANCE, SOLUBILITIES AND SIMPLE GROWTH TECHNIQUES 25
1. Introduction 25 2. Role in solid state sciences 25
2. 1 Crystallography and chemistry 26
2.2 Substances known 28 2.3 Magnetism 29 2.4 Structural phase transitions 29
2.5 Thermal decomposition 29
2.6 Crystal growth 30
3. Synthesis 30 4. Solvents and solubilities 32
4.1 Solvents 32 4.2 Solubility measurements 32
4.2.1 Aqueous solutions 32
4.2.2 Other solvents 35
5. Crystal growth techniques 35 5.1 Morphology and growth characteristics 36
5.2 Isothermal evaporation of unseeded solutions 37
5.3 Unseeded cooling 38 5.4 Seeded cooling 39 5.5 Handling 40 6. Characterization 41
6.1 Chemical analysis 41 6.2 Crystal perfection 41
CHAPTER III:
OPTICAL AND X-RAY CHARACTERIZATION OF THE LAYER TYPE PEROVSKITES
(C H„ ^,NHO,CuCl, IN RELATION TO THEIR GROWTH 47 ^ n 2n+l 3 2 4
1. Introduction 48 2. Growth techniques 49
2.1 Controlled cooling 50 2.2 Temperature gradient method 50
2.3 Isothermal evaporation 50 2.4 Sample preparation 51
3. X-ray techniques 51 3.1 X-ray absorption 51 3.2 Laue diffraction 51 3.3 Continuous X-ray topography 52
3.4 Lang topography 53 4. Growth history and perfection: an example 54
4.1 Microstructure 54 4.2 Growth history 56 4.3 Discussion 58
5. Characterization of -in situ grown crystals 60
5. 1 Selection of samples 60 5.2 Volume defects 61 5.3 Planar defects 62 5.4 Line defects 64
5.4.1 Density and Burgers vector 64
5.4.2 Influence of step bunching 64
5.4.3 Distribution 65
6. Conclusions 66
CHAPTER IV:
MICROSCOPIC OBSERVATIONS OF VARIATIONS IN THE GROWTH RATE OF THE
LAYER PEROVSKITE (C2H^NH3)2CuCl^ 69 1. Introduction ^ 69
2. Structure and morphology 70
3. 1 Apparatus 3.2 Temperature registration 3.3 Solution 3.4 Experimental procedure 4. Observations 4. 1 {111} faces 4.1.1 Stagnant solution 4.1.2 Stirred solution 4.1.3 Flowing solution 4.2 {100} and {0)0} faces 4.3 {001} faces 4.3.1 Step bunching 4.3.2 Satellites 4.3.3 Growth rate
5. Discussion and conclusion
5.1 Variations in the growth rates of {111} faces
5.1.1 Growth mechanism
5.1.2 Influence of volume diffusion 5.1.3 Role of organic chains
5.2 Variations in the growth rates of {010} faces 5.3 Stability of {111} faces
5.3.1 Concentration profiles 5.3.2 Instabilities
5.4 Step bunching on {001} faces
CHAPTER V:
GROWTH OBSERVATIONS ON THE LAYER PEROVSKITE (C^H^NH^)^MnCl^ WITH A COMPOUND HOLOGRAPHIC INTERFERENCE AND CONVENTIONAL MICROSCOPE
1. Introduction
2. Coherent light microscopy
3. Interference microscopy and crystal growth 4. The compound microscope
4.1 General principles 4.2 Wavelength separation
4.3 The incoherent light microscope
4.4 The holographic interference microscope 4.5 Independence of optimization
4.6 Recording 102 5. Experiment 102
5.1 Ihe model substance (C^H NH2)2MnCl, 102
5.2 The growth cell 103 5.3 Refractive indices 104 5.4 Vertical resolution 104
0. Observations 105 6.1 Comparison with growth from the bulk 105
6.2 High index faces 105 6.3 Step bunching 107
6.2.1 Grooves 107
6.3.2 Etch pzts 110
6.3.3 Shockwaves 110
6.3.4 Step bunch profvlea 114
6.3.5 Intra-layer bond amsotropy 115
7. Conclusion 116
PART TWO. MONTE CARLO SIMULATIONS
CHAPTER VI:
THE STRUCTURE OF A SINGLE STEP ON THE SURFACE OF A KOSSEL
CRYSTAL IN EQUILIBRIUM: A MONTE CARLO SIMULATION 120
1. Introduction 120 2. The computer model 121
2.1 Two-dimensional simulation model 121
2.1.1 Surface model 121
2.1.2 Kznetvas 121
2.1.3 Ledge traczng 121
2.1.4 Data sampltng 121
2.2 Ont-dimensional simulation model 122
3. Results 122 3.1 Jump properties 122
3.1.1 One-dvmens%onal svmulavion model 123
3.1.2 Two-dtmensvonal stmulatton model 123
3.1.3 Overhatig denszty 123
3.2 Clustering 125 4. Conclusions and discussion 125
CHAPTER VII:
APPLICATION OF STATISTICAL TESTS TO MONTE CARLO SIMULATIONS
OF CRYSTAL SURFACE STRUCTURES 127
1. Introduction 127 2. Simulation models 130
2.1 Two-dimensional simulation model 130 2.2 One-dimensional simulation model 131
3. Hypotheses 131 4. Results 133
4.1 The hypotheses of symmetry 133 4.2 Jump frequency distribution 137
4.3 Jump correlation 138
5. Conclusions 139
PART THREE: MAGNETISM
CHAPTER VIII:
A SIMPLE GLASS DEWAR FOR HIGH RESOLUTION MICROSCOPIC OBSERVATIONS
OF MAGNETO-OPTICAL PHENOMENA AT LIQUID-HELIUM TEMPERATURES 142
CHAPTER IX:
OBSERVATION OF MAGNETIC DOMAINS IN A TWO-DIMENSIONAL HEISENBERG
FERROMAGNET 144
CHAPTER X:
MAGNETO-OPTICAL OBSERVATIONS OF THE MAGNETIC DOMAINS IN THE
NEARLY TWO-DIMENSIONAL HEISENBERG FERROMAGNET (C2H^NH3)2CuBr, 146
1. Introduction 146 2. Experiment 146 3. Observations 147 4. Discussion 148 5. Conclusion 150 SUMMARY 153 SAMENVATTING 156 11
CHAPTER I: INTRODUCTION
Crystals are a sine qua non for solid state physics and chemistry, and have
numerous technological applications. This has led to extensive research on crystal growth: between 1918 and 1968 the number of publications on single crystal growth has developed exponentially, doubling every five years, where-as the duplication period for physics where-as a whole wwhere-as 10-15 years [1].
The discovery by Frank in 1949 [2] of a relation between the rate at which crystals grow and the presence of screw dislocations, was an impetus for a rapid development of the theory. In the last decade, particularly Monte Carlo simulations and the investigations on the stability of crystal surfaces against perturbations have contributed to a better understanding of the basic processes. Unfortunately, the verification of the theories by experiments is lagging be-hind. Recent reviews for growth from the vapour phase [3], the melt [4] and solution [5-10] clearly show the gap between theory and experiment.
There are several reasons for this gap. We will try to illustrate this by considering the relationship between growth rate and supersaturation: one of the basic and most investigated -both experimentally and theoretically- topics in the field. All theoretical "growth curves" (linear, parabolic or exponential, according to the supposed basic mechanism (cf. e.g. [11])) have in common that the growth rate is a continuously increasing function of supersaturation. In many cases the experimental data are insufficient to discriminate between them
[7,10], and also in many cases there exists no additional evidence in favour of any of the theoretical curves.
Of course, it is possible to determine the parameters of any theoretical growth curve by fitting. These parameters, however, are composed of a series of system variables. For crystal growth from solution some of these are the diffusion coefficient of the growth units in the solution, at the crystal sur-face and along a step on the sursur-face. Many of these system variables depend on temperature, orientation of the growing surface, and the orientation of the processes with respect to this surface. Some of these can be measured, al-though this is very tedious in many cases, for instance the concentration and temperature dependence of the diffusion coefficient of the growth units in the solution. Others cannot be measured directly. This holds particularly for the
parameters governing the elementary processes at the surface, for instance the diffusion coefficient of the growth units on the surface and along a step. Even their order of magnitude is an open question. Thus even a good fit of the experi-mental data to any theoretical curve, does not warrant any real insight in many of the elementary processes.
There is still another serious problem concerning the experimental verifica-tion of crystal growth theories: almost all parameters menverifica-tioned above strongly depend on the solid and liquid under investigation. This makes it impossible, or at least questionable, to apply the variables found for one crystal-solution system to another one.
Knowing this, and the importance of obtaining crystals for numerous purposes, it can be easily understood, that the majority of experimental crystal growth studies is devoted to the improvement of the product (the crystal) and not to the understanding of the process (the growth of crystals). In practice, often trial and error, skill and experience, intuition and improvisation are a better help than an advanced knowledge of the theory. Oilman emphasized this by giving his well known book the title "The Art and Science of Growing Crystals" [12].
It is obvious that bond anisotropy in crystals raises both the number of in-dependent parameters, and the practical problems in obtaining good specimens. On the other hand anisotropic crystals possess quite interesting physical pro-perties, which stimulate the efforts to grow them.
The bis-(alky1ammonium) metal(II) tetrahalides (C H„ ,NH_)„MX, constitute a n 2n+l 3 2 4
family of such anisotropic crystals, which have played an important role in solid state physics and chemistry (as will be discussed in chapter II), espe-cially since the last decade. For crystal growth research these compounds pro-vide a promising model system too, in particular for comparative studies as will be explained in the next section.
In this thesis we will describe various aspects of crystal growth and charac-terization of (C H NH„) MX, crystals (Part One). Subsequently we shall deal with Monte Carlo simulations of a monatomic step on a Kossel crystal. These were inspired by the step bunch configurations on the crystals studied (Part Two). Finally we will discuss the magnetic ordering of the nearly two-dimen-sional Heisenberg magnet (C H NH ) CuBr. (Part Three).
1. Growth and characterization
The advantage of the layer perovskites with organic chains in comparison to more well known inorganic and organic families, is the greater variety in near-ly isomorphous compounds of different character. The crystals consist of layers of corner sharing octahedra MX,, separated by a double layer of organic chains C H„ ,NH, (see fig. 1). A bivalent metal M = Cd, Cu, Fe, Mn, or Pd is situated
n 2n+1 3
in the centre of the octahedra, whereas the corners are formed by the halogens X = Br or CI. The organic chains are connected to the octahedra by N-H...X hydrogen bonds, and to each other by van der Waals bonds. The chain length n varies from 1 to 18. It is possible to "eliminate" the van der Waals bonds by
taking the nearly isomorphous NH (CH„) NH„MX, (or diammonium) family in which m varies from 1 to 8 (cf. fig. 1). Thus one can study crystal growth in depen-dence on chain type and length, metal ion and halide.
bis-(alkylammonium) metaUII) tetrahalides la,Wl- polyethylene diammonium metal (11) tetrahalides ! I 1 I I I I i I I I I I I I
Fig. 1. Schematic repre-sentation of the crystal structure of the
mono-ammonium and the diammonium compounds.
(Cn H2n.l NH3I2 MX^ INHj-ICHzlm" NH3I MX4
n = 1 . 2 . ,18 M=Cd, Cu, Fe, Mn, Pd m = 2 , 3 . ,8
The compounds dissolve in a variety of solvents with different solvation [13-15] and solubilities ranging from large to very low. The crystals grow as platelets perpendicular to the longest or c-axis. The main faces are {001} and {111}, followed by the {100} and {010} faces. Occasionally also other faces may appear [15]. The growth rate as a function of
tion, the bunching of steps and the formation of liquid inclusions differ considerably for the various faces.
The bond anisotropy within a growth layer and between successive growth
layers strongly varies from face to face. Moreover, this anisotropy depends on the choice of metal atom, halogen atom and length (and type) of the orga-nic chains. Also doping with isomorphous metal and/or halide compounds is possible. Mixed crystals with X, = Br C ] , , where 0 < x < 4, are known [16].
1- ^ 4 X 4-x •• •'
The relatively simple structure and morphology of the crystals, together with the possibility to vary composition, bond anisotropy and solvent, make the layer perovskites interesting model substances for comparative growth measurements. Comparison of the conduct of these compounds might reduce some
of the problems caused by the specific character of the critical variables governing the growth process, as discussed above. In Part One of this thesis an account of some pilot studies on the growth and characterization of the compounds will be presented.
In chapter II [17] we give an account of preparation, purity, solvents, solu-bilities, simple growth techniques and characterization of (C H ^ NH )„MX, and NH.,(CH,,) NH.,MX, crystals with n = 1-10; m = 2-8; M = Cd, Cu, Fe, Mn or Pd;
3 z m 3 4
and X = Br or CI.
In chapter III [18,19] we describe the application of various optical and X-ray techniques for a quick selection on lattice perfection of
(C H„ ,NH.,),,CuCl, crystals with n = 1,2,3. The microstructure is refined in n 2n+l 3 2 4
more detail by Lang topography. The relations to the growth history are dis-cussed .
In chapter IV [20] growth rates, face stability and step bunching are in-vestigated for (C.H,.NH,)„CuCl, crystals in aqueous solution. In order to perform the experiments under well defined conditions two complementary growth apparatuses have been developed, suited for microscopic observations of crystal growth in a stagnant, stirred and flowing solution.
In chapter V [21] step bunching is studied in more detail for
(C_H NH ) MnCl, crystals in aqueous solution. In order to carry out quantita-tive observations which are representaquantita-tive for growth from a bulk solution, we have developed a combination of a conventional transmission microscope and a holographic interference microscope. This enables us to record growing crystals continuously and simultaneously on both micrographs and inter-ferograms, which yields an optimal lateral and vertical resolution.
2. Monte Carlo simulations
The configuration of step bunches on the {001} crystal surfaces of the layer perovskites, as seen by conventional and electron microscopy, deviates strongly from the configurations which one should expect on the basis of existing theo-ries. These, in general, consider straight parallel steps only [22]. This assump-tion is certainly justified for step distances which are large compared to the step extent and to the mean free path for surface diffusion. In order to get an impression of what might happen, when step overlap starts to change this picture, we took a closer look at a single step.
Chapter IV [22] describes the Monte Carlo simulation of a monatomic single step on the (001) face of a monocomponent isotropic cubic (Kossel) crystal in equilibrium. We used both a two-dimensional model of 40x20 units and an one-dimensional model up to 960 units.
Chapter VII [23] deals with the statistics and the additional simulations necessary to verify the reliability of the data sampled during the simulation of the step.
3. Magnetism
In 1974 De Jongh and Miedema [24] gave an extensive review of experiments on simple magnetic model systems. Among them, the (C H NH )MX, crystals played in the last decade a very prominant role as model substances for two-dimensional magnetic systems. The metal ions, namely, are situated in MX layers at short intra-layer distances, whereas the inter-layer spacing can be regulated by the length of the carbon chain (n = 1-18). This structure,
to-. to-. to-. to-. to-. 2+
gether with the possibility to occupy the M-sites by both paramagnetic Cu , 2+ 2+ . . 2+ 2+.
Fe or Mn , and diamagnetic Cd or Pd ions, make these substances an ideal object for observations on various types of quasi two-dimensional mag-netic ordering.
The crystals of the Cu compounds form a series of nearly two-dimensional Heisenberg ferromagnets. One of the crucial problems for this magnetic type is the occurrence of a phase transition to the so-called Stanley-Kaplan state
(see below). In 1966 Mermin and Wagner [25] rigourously proved the absence of spontaneous magnetization in two-dimensional Heisenberg magnets at any finite temperature in the absence of an external magnetic field. In 197 1 the validity
of the proof was extended by Fischer and Jasnow [26].
In 1958 Rushbrooke and Wood [27], however, derived from extrapolation of high temperature expansions of the initial susceptibility, the possibility of the existence of a finite temperature at which the susceptibility diverges. Stanley and Kaplan [28; see also 29-30] extended the number of known terms m these series expansions. They, however, also pointed out the possibility of a phase transition at a finite temperature T , to a state with no
spontane-SK
ous magnetization but with an infinite susceptibility m the absence of an external magnetic field, the Stanley-Kaplan state.
Small deviations from the ideal Heisenberg magnetism, however, may also induce a transition to long range order at a finite temperature T . Lines [40] and others 133, 41-43] have studied the dependence of T on the strength of such deviations. The proof of the existence of a Stanley-Kaplan state, typical for the two-dimensional Heisenberg and XY-model [24] can only be given by a substance with T < T .
C bK
In 1969 Miedema and co-workers seemed to have found such a substance. Speci-fic-heat measurements of Bloembergen et al. [44] on (C H NH.)„CuX, with n = 1,2,...,5 and X = CI,Br indicated a finite transition temperature for the
ideal two-dimensional Heisenberg magnet. De Jongh et al. [45] measured the a.c. susceptibility m most of these compounds and found for (C^H NH,) CuBr an in-dication for two transition temperatures. The only direct proof for the exis-tence of the Stanley-Kaplan state could be given by the direct observation of the absence of magnetic domains between the two transition temperatures. It was Miedema who drew our attention to this problem.
In Chapter VIII [46] the Dewar is described which we developed for high reso-lution microscopic observations of magnetic phenomena at liquid-helium tempera-tures .
In chapter IX [47,48] our observations of magnetic domains in (C.H-,NH,,) „CuBr, i / i I 4 are described. We found a structural phase transition and two types of magne-tic domains.
In chapter X [49] this magnetic behaviour is explained with the aid of group theoretical arguments and energy calculations. Also more evidence is given about structural phase transitions.
References
[1] A.A. Chernov, Ann.Rev.Mat.Sci, 3 (1973) 397 [2] F.C. Frank, Disc. Faraday S o c , no 5 (1949) 48,67
[3] P. Bennema and C. van Leeuwen, J.Cryst.Growth 31 (1975) 3 [4] K.A. Jackson, J.Cryst.Growth 24/25 (1974) 130
[5] P. Bennema, J. Boon, C. van Leeuwen, and G.H. Gilmer, Krist.u.Techn. 8 (1973) 559
[6] P. Bennema, J.Cryst.Growth 24/25 (1974) 75
[7] J. Garside, R. Janssen-van Rosmalen, and P. Bennema, J.Cryst.Growth 29 (1975) 353
[8] P. Bennema, in "Industrial Crystallization", edited by J.W. Mullin (Plenum, New York, 1976) p. 91
[9] R. Boistelle, ibid., p. 203
[10] J. Garside, in "1975 Crystal Growth and Materials; A selection of Review Papers from the First European Conference on Crystal Growth ECCG-1 and Materials Symposium, Zurich", edited by E. Kaldis and H.J. Scheel
(North-Holland Publ., Amsterdam, in press)
M. Ohara and R.C. Reid, "Modeling crystal growth rates from solution" (Prentice Hall, Englewood Cliffs, N.J., 1973)
J.J. Oilman, "The Art and Science of Growing Crystals" (John Wiley, New York, 1965)
R.D. Whealy, D.H. Bier, and B.J. McCormick, J.Am.Chem.Soc. 81 (1959) 5900
R. F i l l e r and R. G o r e l i c , J . I n o r g . N u c l . C h e m . 24 ( 1 9 6 2 ) 1297
P . Bennema, H . J . Human, F . H . M i s c h g o f s k y , and C F . W o e n s d r e g t , t o b e p u b l i s h e d A. Daoud, T h e s i s ( U n i v e r s i t y of D i j o n , F r a n c e , 1976) H. A r e n d , W. H u b e r , F . H . M i s c h g o f s k y , and G.K. R i c h t e r - V a n L e e u w e n , t o be p u b l i s h e d F . H . M i s c h g o f s k y and R. D e l h e z , t o b e p u b l i s h e d R. D e l h e z , F . H . M i s c h g o f s k y , and N.M. v a n d e r P e r s , i n " T h i r d E u r o p e a n C r y s t a l l o g r a p h i c M e e t i n g ( Z u r i c h ) , C o l l e c t e d A b s t r a c t s " ( 1 9 7 6 ) p . 412 [ 2 0 ] F.H. M i s c h g o f s k y , t o b e p u b l i s h e d [ 2 1 ] F . H . M i s c h g o f s k y , t o b e p u b l i s h e d
[ 2 2 ] C. van Leeuwen and F . H . M i s c h g o f s k y , J . A p p l . P h y s . 46 ( 1 9 7 5 ) 1055
21 [ 1 1 ] [ 1 2 ] [ 1 3 ] [ 1 4 ] [ 1 5 ] [ 1 5 ] [ 1 7 ] [ 1 8 ] [ 1 9 ]
[23] C. van Leeuwen and F.H. Mischgofsky, J.Phys.A 9 (1976) 1827 [24] L.J. de Jongh and A.R. Miedema, Adv. in Phys. 23 (1974) 1-260 [25] N.D. Mermin and H. Wagner, Phys.Rev.Lett. 17 (1966) 1133 [26] M.E. Fisher and D. Jasnow, Phys.Rev. B 3 (1971) 907 [27] G.S. Rushbrooke and P.J. Wood, Molec.Phys. 1 (1958) 257
[28] H.E. Stanley and T.A. Kaplan, Phys.Rev.Lett. 17 (1966) 913; J.Appl. Phys. 38 (1967) 975
[29] H.E. Stanley, Phys.Rev. 158 (1967) 537, 546; ibid. 164 (1967) 709
[30] H.E. Stanley, Phys.Rev.Lett. 20 (1968) 150, 589 [31] Berezinskii, Zh.eksp.teor.Fiz. 59 (1970) 907
[32] D.D. Betts, C.J. Elliott, and R.V. Ditzian, Can.J.Phys. 49 (1971) 1327 [33] T. Ishikawa and T. Oguchi, J.Phys.Soc.Japan 31 (1971) 1021
[34] R.P. Kenan, Phys.Rev. Bl (1970) 3205
[35] M.E. Lines, J.Appl.Phys. 40 (1969) 1352; Phys.Rev. B3 (1971) 1749 [36] M.A. Moore, Phys.Rev.Lett. 23 (1959) 861
[37] V. Mubayi and R.V. Lange, Phys.Rev. 178 (1959) 882
[38] R.E. Watson, M. Blume, and G.H. Vineyard, Phys.Rev. B2 (1970) 684 [39] D.W. Wood and N.W. Dalton, J.Phys.C 5 (1972) 1575
[40] M.E. Lines, Phys.Rev. 133 (1954) A841; J.Phys.Chem.Solids 31 (1970) 101 [41] N.W. Dalton and D.W. Wood, Proc.phys. Soc. 90 (1967) 459
[42] E.I. Kats, Zh.eksp.teor.Fiz. 56 (1969) 2043; Sov.Phys.J.E.T.P. 29 (1969) 1098
[43] T. Obokata, I. Ono, and T. Oguchi, J.phys.Soc.Japan 23 (1967) 516 [44] P. Bloembergen, K.G. Tan, F.H.J. Lefevre, and A.H.M. Bleyendaal,
J.Phys. C 32 (1971) Cl-878 Suppl.Met.Phys.
[45] L.J. de Jongh, A.C. Botterman, F.R. de Boer, and A.R. Miedema, J.Appl. Phys. 40 (1969) 1363
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[47] H. van Kempen, F.H. Mischgofsky, and P. Wyder, in "Proceedings of the Twelfth International Conference on Low Temperature Physics" edited by E. Kanda (Academic Press of Japan, Tokyo, 1971) p. 811
[48] F.H. Mischgofsky, doctoral thesis (Physics Laboratory, Catholic University, Nijmegen, 1970)
CHAPTER II:
LAYER PEROVSKITES OF THE (C H„ ,NH,)„MX, AND NH,(CH.) NH.MX, FAMILIES WITH n 2n+l 3 2 4 J 2 n 3 4
M = Cd, Cu, Fe, Mn or Pd AND X = CI or Br: IMPORTANCE, SOLUBILITIES AND SIMPLE GROWTH TECHNIQUES.
H. Arend and W. Huber, Laboratory of Solid State Physics, Swiss Federal Institute of Technology, Zilrich, Switzerland, and F.H. Mischgofsky and G.K. Richter-Van Leeuwen, Laboratory of Physical Chemistry, Delft University of Technology, Delft, The Netherlands.
Abstract
Two families of layer-structure halide-perovskites with unbranched organic chains are considered: (C H. ,NH,),MX, and NH-(CH„) NH.MX, with M = Cd, Cu,
n 2 n + l 3 2 4 3 2 m 3 4
Fe, Mn or Pd, X = Br or CI, n = 1,2,..., 18 and m = 2,3,...,8. Their im-portance for solid state research (in particular two-dimensional magnetism and structural phase transitions) is discussed. Synthesis, solubility and crystal growth are investigated. The crystals dissolve in a large variety of solvents, from extremely soluble to nearly insoluble. A few compounds dissolve incongruently. Depending on solvent and growth method, the thick-ness to length ratio of the "platelets" can be varied from 1 to 10 . Under stable conditions large single crystals can be grown. The crystals are characterized by chemical, optical and X-ray methods. The lattice perfection depends on chain type and metal.
1. INTRODUCTION
Progress in solid state physics results mainly from the study of materials exhibiting both simple crystal structures and interesting properties. So the field of ferroelectricity and the related development of knowledge about structural phase transitions was basically shaped by oxide perovskites such as BaTiO,.
The step from oxide to halide perovskites made it possible to introduce bivalent metal ions (particularly those of magnetically ordering transition
metals) into a perovskite structure and lead also to new layer structures, such as K„NiF,. Among them an important role is played by the bis-(alkylam-monium) metal(II) tetrahalide (C H„ ,,NH-)„MX, (or mono-ambis-(alkylam-monium), and the
n zn+1 J z 4
(a)U~) polymethylene diammonium metal(II) tetrahalide NH,(CH„) NH MX, (or diammonium) compounds.
Recently, investigations of these substances have spread over many fields of solid state physics and chemistry. Little attention, however, is focused on growth and characterization of the crystals.
In this paper we shall shortly review their importance and report on pre-paration, solubility data, simple growth techniques for various applications and characterization of both families of perovskite layer structures.
2. ROLE IN SOLID STATE SCIENCES
2.1 Crystallography and chemistry
The first studies on mono-ammonium compounds were carried out by crystal-lographers. Observations on the crystal habit and early crystallographic considerations were published by Topsoe in 1882 [l] and Von Fedorow in
1904 [2]. In 1933 Greenwood [3] discussed the problem of "correct setting" of crystals on the example of these compounds.
Also in 1933, an early study in the field of complex chemistry by Remy
2-and Laves [4] dealt with the stability of CuCl, complexes. Later the com-pounds served as an example for studies on absorption spectra of transition metal ions in different ligand coordination [5] and on luminescence of man-ganese chlorides of different amines [6].
Their interest in a possible square planar CuCl.-coordination, directed Willett and Steadman [7] to the first detailed structure analysis.
The basic structural element of both families are MX.-layers consisting of more or less regular halide octahedra, sharing corners in two dimensions only. NH--groups in the cavities between octahedra are attached by hydrogen
CnHznn NH3 alky! ammonium groups ( 3 X halide ions
O M metal ions
Fig. 1. Schematic representation of a single layer of the mono-ammonium family.
bonding to the halide atoms, and hydrocarbon chains in their turn are attach-ed by covalent bonding to the NH,-groups. In the first family the chains are connected only from one side to the octahedra-network, as shown schemati-cally for one layer in fig. 1. Such layers are bound together by van der Waals forces only, whereas in the second family the chains bearing
NH,-bis-(alkylQmmoniuml metal (11) tetrahalides
iAA
(Q UJl- polyethylene diammonium metal (11) tetrahalidesi.i
ICHjInlA
CnH2n»iI.Li
• A •r ^ r i
T ' T ' T - T
Fig. 2. Schematic representation of mono-ammonium and diammonium families,
ICnHin.i NHjIjMXj I N H j - I C H j l ^ - N H j l MXt . 1 2 19 M = C d C u F e M n P d m . 2 3 B
groups at both sides are connecting adjacent MX, layers. A schematic cc parison of both structures is shown in fig. 2.
2.2 Substances known
Both families offer the following possibilities of crystal engineering, helpful to the understanding and influencing of physical properties:
- the occupation of metal sites - the occupation of halide sites - the length of carbon chains
- the isotopic substitution, either as partial deuteration (i.e. either in the ammonium groups or in the carbon chains) or perdeuteration.
Al known substances fall within the frame work of table I, except for (CH.NH,)-Cdl,, [8], the only known iodide compound. Crystal lattice para-meters are given in ref. [8 - 13]. Some room temperature structures have been determined in detail [7, 14 - 23]. Structure isomers with branched chains, e.g. [24], have been omitted.
Table I
SYNTHESIZED LAYER PEROVSKITES WITH UNBRANCHED HYDROCARBON CHAINS
A. Mono-ammonium family B. Diammonium family
(V2n.l'^«3^
CH, ^^18^37 CH^ CD, ND, ND. Cd Cu Fe Mn Pd 4 Cl^Br^ Br, NH, - (CH,) -NH, M 3 2 m JS»4
^8 "16 Cd Cu Fe Mn CI, Br, 282.3 Magnetism
2+ 2+ The possibility to occupy the M-sites by both paramagnetic Cu , Fe or Mn and diamagnetic Cd or Pd at short intra-layer distances, together with the possibility to regulate the inter-layer spacing by the length of the carbon chain, make these substances an ideal object for observations on various types of quasi two-dimensional magnetic ordering. In 1974 a review of the problems involved was given by De Jongh and Miedema [25]. Recent developments are the studies on field induced magnetic phase transi-tions [10, 26 - 2 8 ] , on the transition from two- to three-dimensional mag-netic ordering [lO, 27, 29] and on thermal conductivity by magnon transport
[30].
2.4 Structural phase transitions
Structural phase transitions in these "two-dimensional" perovskites offer another fascinating problem. The first indications came from the thermochro-mic behaviour of the copper compounds [7, 12]. Later theoretical and experi-mental work [17, 18, 31 - 3 7 ] has shown that the driving forces of the tran-sitions - differing completely from the three-dimensional perovskites - are primarily related to changes in the motion or configuration of the organic chains. In connection to this, the lattice vibrations [38, 39] , dielectric properties [40, 41 ] and critical behaviour [42 - 47] have been studied.
The behaviour of the organic chains raised also interest because of the resemblance of the compounds to organic membrane structures and smectic crystals [32, 48] (cf. also [49]). "Chain melting" has been observed recently for compounds with long chains [50, 5 1 ] .
2.5 Thermal decomposition
The thermal decomposition of the (C H NH ) MCI, compounds offers another interesting aspect to the solid state scientist. A first study was made by Wendlandt and Whealy in 1959 [52]. Later it was shown that different rate-determining steps govern the two-stage decomposition of the
ride family to MnCl [53], and that the first step leading to (C^H ^^jNH )MnCl is probably an example of a topotactic chemical reaction [54] . Recently we found that Cu and Pd compounds can be decomposed to highly poreous metal films which might be of interest for catalytic studies.
2.6 Crystal growth
Finally crystal growth itself is an important reason for studying these substances. Their structure and morphology is relatively simple [55]. The intra-layer bond anisotropy, and the ratio between inter- and intra-layer bond anisotropy vary from face to face and, moreover, depend on the choice of metal atoms, halogen atoms and length or type of the organic chains. The crystals grow from a variety of solvents with different solvation [56, 57] and solubility (cf. next section). In preliminary studies we have found that step bunching patterns, lattice perfection [58 - 60] and habit [55] depend on, besides others, chain type, metal and solvent. So we believe that these nearly isomorphous families of layer perovskites offer interesting possibilities for comparative studies of crystal growth from solution.
3. SYNTHESIS
If necessary, the first step of a synthesis is the preparation of the corresponding alkylammonium or polyethylene diammonium chloride. Therefore either gaseous HCl is introduced into an ether solution of the amine, or hydrochloric acid is added to an aqueous amine solution. The reactions are very exothermal and the solutions have to be cooled. The dry substances may be washed with ether in order to remove traces of unreacted amine.
The next step is the formation of the desired substances. For this purpose stoichiometric amounts of the mono- or diammonium halide and the metal halide are dissolved in a suitable solvent (mostly water) and the compound is pre-cipitated by evaporation. The product is subsequently filtrated and twice recrystallized from the same solvent, sometimes after a repeated purification by means of active carbon.
This procedure is obviously suitable only for congruently soluble compounds. From our experiments it follows that all mono-ammonium compounds with n<_5 30
gas outlet gas inlet glass diaphragm . _ thermostating bath , _ thermostating bath gas niet gas outlet
comply with this condition. For some compounds with longer chains and some diammonium compounds with short chains (see below and [13]), however, we found evidence for in-congruent solubility at room tem-perature. Therefore these substan-ces are prepared either at an ele-vated evaporation temperature or from alcoholic solutions. In excep-tional cases, they were prepared by partial precipitation from off-stoichiometric solutions.
Essentially similar procedures are used for the bromide compounds, for which more details are mentio-ned in refs. [ 5] and [ 13] .
Whealy et.al. [56] have described a completely waterfree preparation of the compounds in methanol.
A special procedure has to be used for Fe compounds. Traces of oxygen easily will oxidize iron into the trivalent state. Therefore they were synthesized under argon
Fig. 3. Apparatus to prepare solutions in the reaction vessel shown in
of the iron-compounds under argon atmos- fig. 3. The starting materials were
phere. dissolved in the double-jacket
thermostated, stirred vessel. The solution was slightly acidified by addition of hydrochloric acid and reduced by adding iron powder. The saturation temperature was adjusted to approxima-tely 60 C. The solution was filtrated at about 70 C into an argon filled am-poule, which was sealed subsequently and crystals could be obtained by slow cooling.
The chemical analysis of starting materials for crystal growth will be discussed in § 6.1.
4 . SOLVENTS AND SOLUBILITIES
4.1 Solvents
The dominant chemical property for the selection of crystal growth techni-ques is the low decomposition temperature of all compounds. This fact limits feasible crystal growth techniques to growth from solution.
A variety of solvents can be used, each of them leading to different solva-tion effects. Whealy [56] found an increasing solubility with increasing die-lectric constant of the solvent. Potential solvents are (cf. also [5, 56, 57]): n-methyl-2-pyrrolidone, water (very good); hydrochloric acid, glycol, methanol, ethanol, aceton, glacial acetic acid (good); and isopropylalcohol, Tf-butyro-lactone (moderate).
Little is known about the structure of the solutions. The differences be-tween spectra of some solutions of (C H NH,)^CuCl, in, among others, metha-nol, acetone and isopropylalcohol indicate a different solvation [56, 57].
2- . . . . . .
The CuCl, ion may partially decompose depending on concentration and acidity [55]. In aqueous solution different types of aquo-complexes should be present. No systematic studies, however, exist of the influence of solvation on crystal growth. Therefore we shall limit ourselves to the solvents we have used: main-ly water (occasionalmain-ly acidified with hydrochloric acid), less frequentmain-ly me-thanol, ethanol and n-methyl-2-pyrrolidone (hereafter abbreviated by nmpd). 4.2 Solubility measurements
Solute and solvent were weighed into glass ampoules. The ampoules were sealed and rotated head-over in a glass thermostat, the temperature of which was increased in steps of 0.5 C every 12 hours. The disappearance of the last
solute yields the saturation temperature. This procedure has an accuracy bet-ter than + 0.5°C.
4.2.1 Aqueous solutions
Characteristic data for congruently soluble substances are listed in table II [61]. The table contains the solubilities C,. at 20 C and the temperature
coefficients of solubility a = -—— as determined from the practically linear
^20"^
Table II: CHARACTERISTIC SOLUBILITY DATA FOR H O AS SOLVENT [61] A. Mono-ammonium family Solute '20 (CH,NH,),CdCl, 3 S I 4 (C2H^NH2)2CdCl^ (C3H^NH^)2CdCl^ (CH,NH,),CuCl, 3 i I 4 (C2H^NH2)2CuCl^ (C3H^NH3)2CuCl^ (C^Hj jNH^)2CuCl^ (CH,NH,),MnCl, i i I 4 (C2H^NH3)2MnCl^ (C^H^NH^)2MnCl^ (C4HgNH3)2MnCl^ 0.0353 0.0220 0.0107 0.0519 0.0386 0.0314 0.0183 0.0899 0.0689 0.0621 0.0547 0.01615 0.0171 0.0283 0.0099 0.0089 0.0121 0.0053 0.0013 0.0028 0.0042 0.0016 B. Diammonium family Solute "20 a NH,(CH,),NH,CdCl, 3 2 / 3 4 NH, (CH,),NH,CdCl. 3 2 3 3 4 NH2(CH2)^NH3CdCl^ NH2(CH2)5NH3CdCl^ NH ( C H , ) , N H , C u C l , 3 2 2 3 4 NH2(CH2)^NH2CuCl^ NH2(CH2)3NH3MnCl^ 0.0059 0.0062 0.0077 0.0080 0.0230 0.0300 0.0717 0.0246 0.0274 0.0291 0.0345 0.0068 0.0087 0.0004
C IS the saturation concentration at 20 C in mol solute/mol solvent.
" = (C^O-So)/20C2o
o 0 -0 1-0 2-0 3-0 4-0 5-0 6-0 7-0 8-0 temperature °C Fig. 4. Solubility curves of with n - 1,2, . . . ,5 in water (full lines) and 96% ethanol (dashed lines).
Table III
CHARACTERISTIC SOLUBILITY DATA FOR 96% ETHANOL AS SOLVENT [61],
Solute (CH3NH3)2MnCl,^ (C2H^NH3)2MnCl^ (C3H^NH3)2MnCl^
(S»ll^"3^2"""4
^^20 0.0062 O.OlOi 0.0144 0.0128 20^40 0.0055 0.0095 0.0188 0.012934
temperature dependence -r— of the solubility in the temperature range between 20° and 40 C. This linearity can be seen in fig. 4 where experimental results for the first four members of the mono-ammonium manganese(II) tetrachloride series are shown.
No data are listed for NH,(CH,) NH,MnCl, with m=2,4. Both substances exhi-3 2 m J 4
bit an incongruent solubility in the temperature range between 20 and 40 C, but crystallize congruently at higher temperatures.
4.2.2 Other solvents
The solubilities in alcohol are usually much lower than in water as can be seen from table III [51] and fig. 4.
Nmpd is a very good solvent with an extremely high boiling temperature, thus facilitating the preparation of water free solutions. Characteristic solubility data for (C3H^NH3)2CuCl^ are [61]: 0^0=0.0310, a=0.0229, i.e. a much higher solubility with a much smaller temperature dependence than in the case of water is experienced (cf. table II).
The solubility of (C,H NH,)„CdCl, in heavy water has been determined for growth experiments of deuterated compounds. The solubility is about 10% less than in normal water [61].
5. CRYSTAL GROWTH TECHNIQUES
Solid state research requires crystals in a large variety of size and quality. For example, for transmission optics and magneto-optics (in par-ticular of the bromides) in many cases large and extremely thin single crystalline films are essential, whereas for reflection optics, nuclear mag-netic resonance, neutron scattering and thermal conductivity large, thick, single crystals are needed. For the observation of magnetic domains some twinning is allowed [28], whereas the thermal conductivity in untwinned samples depends even on the dislocation density [30]. From the mono-ammonium compounds the bromide crystals are brittle and the chlorides are very plastic. Thus cleaving and cutting easily may damage and deform the weak crystals.
Therefore, it is advantageous to choose growth techniques and solvents in accordance with the demands on and specific composition of the crystal needed. Fig. 5 shows some results of the methods which will be described below. Firstly, however, we will mention some general observations on growth and morphology, which are of importance for a proper choice of growth procedures.
5.1 Morphology and growth characteristics
The growth form of (C„H NH,)„CuCl, has been derived [55] using the Periodic Bond Chain analysis of Hartman and Perdok. This analysis is in good agreement with the observations on mono-arranonium compounds. The most important faces are the {001} platelet face and the {111} side faces (cf. fig. 5 ) . The {100} and {010} faces of the copper compounds are rarely observed, in contrast with those of the cadmium and manganese ones. Other faces can be present, but bear little importance.
The mono-ammonium compounds with carbon chains up to propyl and the diam-monium compounds can be grown rather thick (in particular the latter), and
show well developed side faces. The mono-ammonium compounds with longer chains grow only as very thin platelets. The longer the carbon chain, the worse the side faces will develop. Only very recently we succeeded in gro-wing (C.-H„jNH ) CuCl, single crystals of about 0.1 mm thickness and up to 8x15 mm2 size with well defined edges.
Microscopic observations on the growth of (C,H_NH ) CuCl, from aqueous solution [62] show a steep relationship between the growth rates of {111} faces and both crystal thickness and flow velocity along the crystal, i.e. volume diffusion plays a dominant part. The development and disturbance of strong concentration gradients gives easily rise to the formation of liquid inclusions and the outgrow of satellite platelets [59, 62].
As a consequence, growth should take place under well defined hydrodynamic conditions and low, well defined supersaturations. In practice, a slight lami-nar flow or stagnant solution and a slow evaporation or slight undercooling
(with relative supersaturations below 1%) under well thermostated conditions (eliminating fluctuations in the supersaturation of more than 0.1%) are re-quired to get large single crystals. Generally, spontaneous nucleation has to
Fig. 5(a-f) Macrophotograph of (CH^NH^J^CuCl^(b) and (C H NH J CuCl (a,c-f) crystals from aqueous (a-c, e-g) and nmpd (d) solution, respectively. The picture shows the spread in length to thickness ratio of the mono-ammonium compounds with short chain lengths (n<3). The crystals have been grown in situ by the following methods: (a,c,e) slow isothermal evaporation (% 5.2b);
(b) temperature gradient method ('§ 5.3d); (d) slow cooling f§ 5.2b); (f)
si-multaneous cooling and evaporation r§ 5.3e).
Fig. 5(g) Macrophotograph of a NH^-(CH^)^-NH^CuCl^ crystal grown from a seed
in aqueous solution by the oscillating temperature technique (§ 5.4b).
be preferred over seed growth, since seed crystals are easily damaged and deformed during the necessary handling procedures. This will exert a negative
influence on the first stages of growth.
5.2 Isothermal evaporation of unseeded solutions
3
a) Small untwinned crystals (up to 2x2x0,2 mm ) for, among others, struc-ture determinations can be obtained from slow evaporation of thin layers or droplets of aqueous, methanol or ethanol solution.
b) Thick platelets for nuclear magnetic resonance, neutron scattering, thermal conductivity, etc. can be obtained by very slow evaporation from high columns (2-4 cm) of aqueous or diluted aqueous hydrochloric acid
so-lutions at stable (better than 0.1 C) room or elevated temperature (e.g. a NH,(CH„)„NH,MnCl, solution yields the correct primary phase only above 40 C ) . Crystals have been grown up to 15x15x3 mm of the mono-ammonium
3
compounds with n<_3 and up to 25x15x10 mm of the diammonium compounds. Evaporation of an aqueous solution at 4 C takes more time, but yields crystals with a substantially reduced amount of liquid inclusions. It is also more economical in the case of expensive fully deuterated compounds, since it allows to work with less substance at the same volume of solu-tion.
c) Thin single crystalline films (up to 50 ym) with cross sections up to 30 mm (for optical and magneto-optical investigations) of the mono-ammo-nium compounds with long chains (n>3) can be obtained at the surface of an aqueous solution by slow isothermal evaporation.
d) Extremely thin (less than 5 ym) single crystalline films (for optical and magneto-optical research) with cross sections up to 5 mm of the mo-no-ammonium family with short chains (n£^3) can be obtained by rapid eva-poration of thin, undersaturated layers of methanol solution on the sur-face of, for example, very clean cover-glasses.
5.3 Unseeded cooling
a) Small platelets for optics etc. can be obtained easily by slow cooling of aqueous or nmpd solutions at a relative superaturation of less than
1%.
b) Large crystals for nuclear magnetic resonance and nuclear quadrupole resonance can be obtained by slow cooling to room temperature of aqueous or nmpd solutions, saturated at 50 C. The former yield good crystals up
3
to 5x5x0.4 mm , the latter up to 15 mm length and 4 mm thickness of the mono-ammonium compounds with short carbon chains and of the diammonium family.
c) The sensitivity to oxidation of the iron compounds (interesting for Mossbauer studies) makes it necessary to handle the solutions under
argon. The apparatus shown in fig. 3 and the procedure described before, allows the growth of the short-chain-compound crystals (up to 8x8x1 mm ) by slow cooling from solutions saturated of about 50 C.
d) Thick (up to 6 mm) mono-ammonium crystals with short carbon chains can be grown from an aqueous solution, applying a temperature gradient method in a 150 ml vessel of 5 cm cross section. The vessel is similar to the one of fig. 6 without the inner glass cylinder and with a glass table instead of a heating coil. The cooled "finger" at the top reaches down to 2 cm from the bottom. A thermostat keeps the outer wall of the vessel slightly above the saturation temperature of about 30 C. Seeds are for-med at the "cold finger" and drop, after switching off the cooling, on the glass table. Lowering the temperature of the "cold finger" to about one degree below the saturation temperature suffices to create a small supersaturated region above the seed, "forcing" growth in the c-direc-tion along the temperature gradient.
e) Thin (up to 0.2 mm) single crystalline films with a cross section up to 15 mm of the mono-ammonium compounds with short chains can be obtained by cooling and simultaneous evaporation at the surface of aqueous solu-tions (saturated at about 50 C ) .
4 Seeded cooling
a) For microscopic growth observations under well defined conditions a closed vessel and a flow system have been developed. Both systems (con-taining up to 1 and 4 liter of solution, respectively) are described in
[62]. Seeds are glued [53] on to a glass holder and dipped into the so-lution. The saturation temperature can be determined from growth and dissolution tests within + 0.02 C. The long term stability of the tem-perature is better than + 0.01 C or + 0.03 C, respectively. In the closed
3 vessel (C H NH,)„MC1, crystals with M = Cu, Mn up to 10x10x2 mm have been grown from aqueous solutions. The flow system is less suited for the growth of good crystals (cf. also § 5.1).
T2>T, solute supply growth flow - seed dissolution flow
b) Large crystals (up to 3
25x10x8 mm ) of the diammo-nium family were grown from an aqueous solution using an oscillating temperature tech-nique in the vessel shown in fig. 6. Its outer wall is thermostated at a higher tem-perature than the glass cooler at the top. Due to the inner glass cylinder a convective flow circulates,which can be reversed by the heating coil below the seed holder in the centre. Thus growth and dis-solution periods can be al-ternated .
I I
heating coil
Fig. 6 Apparatus for the oscillating tempera-ture technique.
5.5 Handling
The crystals are rather sensitive to stress and deformation (cf. also§ 6.2). Mechanical and thermal strain leads easily to twinning of the Cd, Fe, Mn and Pd compounds, in contrast to the Cu compounds.
Thicker crystals may be cleaved parallel to the platelet face. Cutting per-pendicular to it, however, leads to severe damage. Sawing with a water-wetted thread gives good results.
Storage at a cool, dark place in the presence of a strong drying agent is essential for maintaining the crystal quality over long periods.
Freely suspended, well dried crystals generally survive (however rarely untwinned) cooling down to liquid helium temperatures (cf. e.g. [28, 3 0 ] . If a glue has to be applied to the crystals an a-cyano-acrylate [63] can be used for both room and low temperatures and even in solutions, as it does neither react with the compounds nor does it dissolve them.
6 CHARACTERIZATION
6.1 Chemical Analysis
A control of the chemical composition of crystals and starting materials is easily provided by a combination of volumetric inorganic and elementary orga-nic micro-analysis. No deviation from stoichiometry could be found in twice recrystallized material within the accuracy of the analytical method: e.g. in (CH,NH ) CuCl, we found 23.52% Cu and 52.64% CI, whereas the calculated va-lues are 23.58% Cu and 52.63% CI (all quantities are given in weight % ) . This agrees with the analyses mentioned by Remy and Laves [4], Wliealy et.al. [56] and Daoud [9]. A systematic deviation is found in large crystals with liquid inclusions. These can be determined quantitatively by thermogravimetry to be 0.1-1% in bad crystals.
A flame spectroscopic analysis of (C,H_NH,)„MC1, with M = Cd, Cu showed no traces of Co, Cr, Fe, Mn, Pb and Zn above the detection limit of 0.0002%. The samples contained less than 0.001% K, 0.004% Ca and 0.004% Na. The Cu compound contained 0.004% Cd; the Cd compound contained 0.003% Cu.
6.2 Crystal perfection
Except for the Cu-serles of the mono-ammonium family, twinning occurs very easily, mostly due to mechanical or thermal strain. Polarizing microscopy of the birefringent crystals is an easy method for selection. Most twin bounda-ries intersect the platelet face. The diammonium compounds, however, contain
also many twin boundaries parallel to (001), as was found by polarized ortho-scopic and conoortho-scopic observations.
The main imperfections in single crystals are liquid inclusions. Crystals 3
up to 4x4x0.5 mm usually can be grown free from inclusions. Larger crystals exhibit large differences in inclusion density for different sectors. Many crystals contain large inclusion free areas.
Complementary X-ray techniques, including Lang topography, have been applied to (C H„ NH,)„CuCl, crystals with n=l,2,3 grown from aqueous solutions [58, 59]. A strong lattice deformation was found in the vicinity of liquid inclu-sions. Large strain free areas, however, could be seen in crystals up to
3
15x15x2 mm , the diagonal areas usually being more perfect than the rest of the crystal. Pealing effects were visible. In all crystals individual
dis-(020)
(a)
2 min
(b)
Fig. 7 Lang topographs of a (C Jl„M J JinCl. (a) and a part of a
NH JCHj ^NHJ4nCl. (b) crystal. The 1 mm thick specimens have been grown in situ and from a seed (SE), respectively, in aqueous solution. The trace of the reflecting plane (020) is indicated by a line on the topographs. D screw dislocations; E edge dislocations; G growth bands; S sector boundary.
locations and/or dislocation bundles could be discerned. In thin crystals most dislocations were raised '-y plastic deformation [59].
Fig. 7 gives an impression of the differences between mono- and diammonium compounds: it represents Lang topographs of a (C H_NH,) MnCl, (fig. 7a) and a NH,(CH„)„NH MnCl. (fig. 7b) platelet from aqueous solution, grown in situ and from a seed (SE) respectively. The former contains, just like the copper compounds, edge dislocations (E) with Burgersvector parallel to <110> and running along <110>. In contrast to the copper compounds, however, it also exhibits clearly sector boundaries (S) and growth bands (G). The latter shows a higher perfection; in particular the number of dislocations due to plastic deformation is negligible. Screw dislocations (D) with the Burgers vector parallel to <010> run in the <010> directions. This is striking, since in the mono-ammonium crystals neither edge dislocations running along the a- or b-direction, nor screw dislocations have been seen [58-60].
Further evidence on the crystal quality and on the suitability of the crystals for solid state research is provided by the physical measurements in the refe-rences quoted.
Acknowledgements
We are very much indebted to Dr. P. Bennema for stimulating comments, and to Dr. H. Klapper (RWTH Aachen) for making the Lang topographs and for fruit-ful discussions. This work was supported by the "Nederlandse Organisatie
voor Zuiver Wetenschappelijk Onderzoek" (Z.W.O.) and the Swiss National Science Foundation.
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[48] M. Vacatello and P. Corradini, Gaz.Chim.Ital., 103 (1973) 1027; ibid.
104 (1974) 771
[49] A.D. Bangham, Progr. in Biophys. and Mol.Biol. 18 (1968) 29
[50] H. Arend, H. von Kanel and P. Wachter, Helv.Phys.Acta 50 (1977) 150;
H. Arend, J. Roos and M.A.A. Arriandiaga, ibid. 50 {\')11) 150
[51] C. Carfagna, M. Vacatello, and P. Corradini, Gaz.Chim.Ital. 107 (1977) 131
[52] W. Wendlandt and R. Whealy, Tex.J.Sci. (1954) 475
[53] M.J. Tello, E.H. Bocanegra, M. Arriandiaga and H. Arend, Thermochim. Acta 11 (1975) 96
[54] W. Bachmann, H.R. Oswald and J.R.Gunter, J.Appl .Cryst. 9 (1976) 243; W. Bachmann, Diploma thesis (Inst,Inorg.Chem., Univ. Zurich, 1975, unpublished)
[55] P. Bennema, H.J. Human, F.H. Mischgofsky and C F . Woensdregt, to be published
[56] R.D. Whealy, D.H. Bier and B.J. McCormick, J.Am.Chem.Soc. 81 (1959) 5900
[57] R. Filler and L. Gorelic, J.Inorg.Nucl.Chem. 24 (1962) 1297
[58] R. Delhez, F.H. Mischgofsky and N.M. van der Pers, in "Third European Crystallographic Meeting (Zurich), Collected Abstracts" (1976, un-published) p 412
[59] F.H. Mischgofsky and R. Delhez, to be published [60] H. Klapper and F.H. Mischgofsky, unpublished
[51] The tabulated solubility data may be obtained from the authors [52] F.H. Mischgofsky, to be published
[53] For example: Cyanolit 201, 3M Company (Minnesota Nederland N.V.), Leiden The Netherlands
CHAPTER III:
OPTICAL AND X-RAY CHARACTERIZATION OF THE LAYER TYPE PEROVSKITES (C H, ,NH,),CuCl, IN RELATION TO THEIR GROWTH.
n 2 n + 1 3 2 4
F.H. Mischgofsky and R. Delhez ; Laboratory of Physical Chemistry, and Laboratory of Metallurgy; Delft University of Technology, Delft, The Netherlands.
Abstract
The lattice perfection of as grown specimens of the very anisotropically bound, plastic (C H, ,NH,),CuCl, crystals with n = 1, 2, 3 has been
inves-n 2inves-n+1 3 2 4 - ^
tigated. Polarizing microscopy, Laue diffraction. X-ray absorption and con-tinuous X-radiation topography were used, since they are very useful for quick qualification and for selection. The microstructure has been resolved in more detail by Lang topography.
The crystals were grown from aqueous solutions (i) from a seed by
control-led cooling, (ii) in situ by isothermal evaporation, and (iii) in situ by a
temperature gradient method. In the first method the growing crystal was recorded by time lapse microphotography. From this and from X-ray pictures it was found that the lattice perfection was mainly determined by unstable and irregular concentration profiles. These caused elevated and overhanging areas (satellites), splitting up of side faces and capturing of liquid. The main other volume defects were damage and deformation due to handling. In
general the diagonal areas exhibited the highest perfection. The sectors could be rather perfect. In many cases satellites showed no lattice mismatch with the parent crystal. Growth bands were absent. Occasionally contrasting
sector boundaries were observed on Lang topographs.
Thin crystals contained a high number of mixed dislocations due to plastic deformation. Growth dislocations appeared from liquid inclusions, in parti-cular from tiny ones. They were edge dislocations running in the <110> direc-tions with Burgers vectors parallel to <110>. Neither edge dislocadirec-tions along the a- or b-direction, nor screw dislocations have been found. The edge dis-locations seemed to stimulate "healing" of their sectors. Contrasts similar
to those of "sector boundaries" were raised by step bunches growing together along <100> or <010> directions.
1. INTRODUCTION
Single crystals of layer perovskites of the bis-(alkylammonium) metal(ll) tetrahalide family (C^H2^^|NH ) 2 M ^ with n = 1, 2 18; M = Cd, Cu, Fe, Mn or Pd, and X, = CI,, Br, or CI,Br, are studied as a model substance for
4 4 4 2 2
two-dimensional magnetism, structural phase transitions, lattice vibrations, ferroelasticity, ferroelectricity and crystal growth [l]. The lattice con-sists of layers of corner sharing MX, octahedra. A double layer of alkylammo-nium chains is sandwiched between these layers. The chains are connected to each other by van der Waals bonds and to the octahedra by N-H...X hydrogen bonds. The bromide crystals are brittle and the chloride crystals are very plastic. Thus sawing or cleavage easily may cause damage or deformation. Therefore it is advantageous to choose growth technique and solvent in accor-dance with the desired crystal dimensions [l].
Up to now little attention has been paid to the perfection of metal-organic and of plastic crystals. Also as grown crystals are rarely studied [2-4]. Yet many properties are investigated which may be influenced by strain and lattice perfection. For the (C H NH )„MX, crystals this applies to e.g. structural phase transitions [1,5,6], thermal conductivity [7], magnetic domain confi-gurations [5] and crystal growth [8].
In this paper we shall describe the lattice perfection of (C H, ,NH,)„CuCl, n 2n+l 3 2 4 crystals investigated as a whole, and grown by various methods from aqueous solutions. The copper compound has been chosen as it appeared to be the most stable against twinning [Ij. The chloride compound has been chosen since it also allows optical examination. If not stated otherwise n = 3. In those cases that we studied compounds with n = 1 or 2, no different results have been found.
At room temperature the crystals have the orthorhombic Pbca symmetry with a = 7.3 S, fc ~ 7.5 8 and c = 18.6, 21.2 and 24.7 8 for n = 1, 2, 3,
respec-tively [9-11]. The shortest translations in the (a,b)-plane are along <110>, <100> and <010> with lengths of 5.2, 7.3 and 7.5 A respectively. The crystals
grow as {001} platelets, bounded by {111} faces and, in some cases, also by {010} faces. Other forms are of minor importance [l2] . Many crystals
show unusual geometries (see fig. 1 ) : for example, the diagonal areas are elevated (at A ) , the central area is elevated (pyramidal shape) or satel-lites have appeared (at B in fig. 1; B and S in fig. 7a). Satellites are defined as crystalline parts which have only a part of a face in common with the parent crystal.
In the following we will focus our attention to:
Fig. 1. Scanning electron micrograph i) the overall perfection of these
of an evaporation grown very plastic crystals,
(CJlyNHr.)^CuCl crystal with protruding ii) the relation between perfection
diagonals (A), overhanging satellite (B ) and macros teps.
and growth conditions, and iii)the question whether a relation
exists between unusual geometries and microstructure.
Polarizing microscopy, Laue diffraction. X-ray absorption and continuous radiation topography have been applied as complementary characterization techniques. We want to emphasize their usefulness for quick qualification and for selection. Lang topography has been applied to the most interesting samples for further refinement of the microstructure.
GROWTH TECHNIQUES
Because of decomposition the crystals cannot be grown from the melt, but they can be grown from a variety of solutions. Solvents, solubilities, purity and simple growth techniques have been described elsewhere [1]. The crystals for this investigation have been grown from aqueous solutions by three methods: by controlled cooling of an unstirred seeded solution, and "in situ" by both
a temperature gradient method in a closed vessel and isothermal evaporation at constant air humidity. During controlled cooling the growing crystal has been recorded by time lapse microphotography.
2.1. Controlled cooling
The best hydrodynamic and thermostatic control has been obtained in an un-stirred solution in an air tight vessel, described in [8]. Seeds were glued on to a glass holder and dipped into the solution. The saturation temperature has been determined within + 0.02 C. The long term stability of the tempera-ture was better than + 0.01 C. The growing crystal could be observed and recorded by means of an inverted microscope placed under the glass thermo-stat.
2.2. Temperature gradient method
To obtain very thick crystals, a "cold finger" technique has been used, described in [1]. In a closed vessel seeds were grown at the "cold finger" and "placed" on a glass table by dissolution. There the "platelets" would grow in the c-direction along the gradient. In this way crystals up to 6 mm thickness have been grown within a day.
2.3. Isothermal evaporation
3
Large crystals (up to 15x15x3 mm ) grew within a week by slow evaporation in an air thermostat (with 45% air humidity at 35 C, and a long term drift of 0.1 C ) . Nearly closed glass vessels were filled with a 4-6 cm high column of unseeded solution.
The crystals with seriously deviating geometries (cf. fig. 1 and 10b, c) have been grown from open vessels at a temperature drift of 0.5-2 C, and from solution layers with a depth of only 1-5 mm.
2.4. Sample preparation
In order to diminish bending, the crystals were drawn obliquely out of the solution with a platinum grid. They were dried immediately with filtering paper and stored in a desiccator.
In order to prevent plastic deformation, we did not cleave or saw the crystals. So, contrary to most workers in the field, we investigated the crystals as grown. Before X-ray analysis the crystals were wrapped air tight into a Mylar sheet, which then was clamped into an empty slide frame for strain free handling.
3. X-RAY TECHNIQUES
3.1. X-ray absorption
Variations in thickness or density (e.g. due to liquid inclusions) yield different intensities in the transmission image (cf. fig. 6a, 9c and 10b). A microfocus X-ray tube shows details comparable to its focus size of 10 pm if high resolution film is used.
The divergence of the X-ray beam intercepted by the crystal did not exceed ten degrees in order to keep the variation of transmitted intensity, caused by an increase in apparent thickness, within reasonable limits. Exposure times were a few seconds for X-ray film and 1 to 50 minutes for high resolu-tion plates. The distance from focus to specimen was about 10 cm and the distance from specimen to film could be varied between 1 and 50 cm.
3.2. Laue diffraction
Heavily distorted, twinned or polycrystalline samples are easily and quickly identified with Laue diffraction. Less distorted crystals can be searched spotwise as the narrow X-ray beam provides information about small areas of 0.5 mm in diameter (fig. 2a). This holds even for thick, optically nearly opaque samples (fig. 2 b ) . Exposure times were as usual 1-20 min. for 40 kV X-rays from an 1 kW X-ray tube with an 0.5 mm (f pin hole.
