ST R U C T U R E A N D PR O PE R T IE S OF CeRhSn
— A VALENCE FLU C TU A TIN G SYSTEM *
K . Łą t k a, M . Ra m s
M. Smoluchowski Institute of Physics, Jagellonian University Reymonta 4, 30-059 Kraków, Poland
R . K m i e ć , R . K r u k , A .W . P a c y n a H. Niewodniczański Institute of Nuclear Physics
Radzikowskiego 152, 31-342 Kraków, Poland T . S c h m i d t , G . K o t z y b a , R . P o t t g e n
Institut fur Anorganische und Analytische Chemie, Universitat Munster Wilhelm-Klemm-Strasse 8, 48149 Munster, Germany
AND D . JOHRENDT
Institut fur Anorganische Chemie und Strukturchemie, Lehrstuhl II H. Heine Universitat Dusseldorf, Universitatsstr. 1, 40225, Dusseldorf, Germany
(Received July 10, 2002)
X ray diffraction studies have been performed on a CeRhSn single crystal and its anomalous unit-cell volume was confirmed. This, together with tem
perature dependence of magnetic susceptibility indicate valence-fluctuating behaviour of Ce ions. Band structure calculations support such a be
haviour. Anomalous value of the quadrupole interaction constant derived from 119Sn Mossbauer spectroscopy is observed. Ac and dc magnetic sus
ceptibility investigations as well as preliminary resistivity measurements ev
idence th at this compound does not order magnetically down to 2 K, but one of our samples is superconducting with a transition tem perature of 6.5 K.
PACS numbers: 75.20.Hr, 74.70.-b, 82.80.Ej
1. Introduction
C erium based te rn a ry com pounds C eT X (T tra n sitio n m etal, X p-elem ent) show m any interesting physical properties such as th e Kondo effect, valence fluctuations, heavy ferm ion behaviour or different types of m agnetic order
* Presented at the International Conference on Strongly Correlated Electron Systems, (SCES02), Cracow, Poland, July 10-13, 2002.
(1225)
ing, which depend on th e degree of th e hybridisation betw een 4 / electrons w ith th e conduction electrons
[1-4].
For exam ple, CeR hSb and CeR hA s were found to be Kondo sem iconductors [1,2] and sim ultaneously th e first comp ound has a m ixed valence character. Valence in stability was also discovered for C eR hP [2] and C eR hln [3].
T he title com pound was first rep o rted by R outsi et a l, [5], however, their lattice p aram eters are not correct and m agnetic susceptibility m easurem ents shown up to 120 K led only to a conclusion th a t Curie-Weiss behaviour is not observed. Recent m easurem ents showed th a t lattice p aram eters of CeRhSn differ significantly from th e plot of th e cell volumes for th e whole series of R E R hSn stannides [6] and a deviation of th e cerium com pound from th e usual lanthan ide contraction is clearly observed.
In th is work new X ray investigations carried out on a CeR hSn single crystal are presented to g ether w ith th e results of ac and dc m agnetic suscep
tib ility and 119Sn M ossbauer spectroscopy m easurem ents perform ed in the te m p e ra tu re range from 2 K to 300 K on a polycrvstalline sample.
2. Experimental results and discussion
C eR hSn was prepared by reaction of th e elem ents in a sealed tan ta lu m tu b e in a high-frequency furnace. T he stru c tu re was refined from single crys
ta l X ray diffraction d ata: ZrNiAl type, space group P 6 2 m , a = 745.8(1), c = 408.62(9) pm , w R 2 = 0.0379, for 354 F 2 values and 14 variable param eters. Refinem ent of th e occupancy p aram eters revealed full occupancy of all four atom ic positions w ithin one sta n d a rd deviation. No deviation from th e ideal com position is observed.
T he present single cry stal stu d y [6] confirmed th e values of th e lattice p aram eters derived in our recent investigations [7] on powder sam ples, re
vealing th a t th ey are indeed different from those given by R outsi et al., [5].
T h e course of th e cell volumes now unam biguously indicates th e m ixed va
lent behaviour of th e cerium atom s. T h e CeR hSn s tru c tu re contains two crystallographicallv different rhodium sites which b o th have a tricap p ed trigonal prism atic coordination: [R M S ^ C e s] and [Rh2SngCe3]. Together th e rhodium and tin atom s build a three-dim ensional polvanion in which th e cerium atom s fill d isto rted hexagonal channels.
T he M ossbauer spectroscopic investigations employing th e 119Sn 23.875 keV tran sitio n were carried out in th e tem p e ra tu re range 2 to 300 K. In accordance w ith th e non-cubic sym m etry (m 2 m ) of th e tin sites, all recorded sp e ctra show only pure q uadrupole splitting, and can be easily fitted w ith a single com ponent confirming high p u rity and hom ogeneity of our sam ple. T h e isom er shift 5;s(4.2iF ) = 1.82(1) m m /s (in respect to B a119SnC>3
source) is in th e lim it of error equal to th e value obtained a t room tem
p e ra tu re 5;S(2 9 3 K ) = 1.81(1) m m /s being typical and com parable to the values m easured in o th er R E R hSn stannides and m ost generally tin inter- m etallics (7] (and references therein). T h e quadrupole interaction constant A Eq = eQVzz(2.1 K ) = 1.10(1) m m /s does not change rem arkably as a func
tion of tem p e ra tu re b u t is clearly higher th a n observed in oth er m em bers of th e R E R hSn fam ily (7] indicating an anom alous ch aracter for CeRhSn.
T his resembles th e situ atio n found for th e antiferrom agnetic heavy ferm ion stan n id e CeAgSn against a background of th e REA gSn series [4].
Fig. 1 shows th e tem p e ra tu re dependence of th e inverse dc m agnetic sus
ceptibility m easured by m eans of SQUID m agnetom eter, in an applied field of 1 kOe. No phase tra n sitio n of m agnetic origin was detected down to 2 K.
Above 120 K th e tem p e ra tu re dependence of th e m agnetic susceptibility can be described by a m odified C urie-W eiss law in th e form: y = x o + C / ( T —&p) where y o is th e tem p e ra tu re independent term . T he least squares refinem ent
Fig. 1. Magnetic dc-susceptibility of CeRhSn in H = 1000 Oe. The solid line is the Curie-Weiss fit in the tem perature range 120-300K (see text).
results in p,eg = 1.30/rB/Ce ion, highly negative &p = ^ 7 0 K evidencing an
tiferrom agnetic correlations, and relatively big yo = 0.87 x l t T 3 em u/m ole which is com parable to th e values obtained for valence fluctuating C eR hP (1.30 x l t T 3 em u/m ole) (2] or C eR hln (1.13 x
10-3
em u/m ole) [3]. T he derived effective m agnetic m om ent is significantly reduced in com parison w ith th e theoretical value for a free Ce3+ ion = 2.54/t b, giving another hint for an interm ediate valence behaviour of cerium . Since th e m agnetic m om ent of tetravalent cerium is zero, so th en th e observed reduction of the m agnetic m om ent could be explained in a n a tu ra l way by fractional occup atio n of 4 / ° (Ce4+) and 4 / 1 (Ce3+) states. In this context th e Sales and W ohlleben (SW ) phenom enological IC F-m odel (8] was considered. However, it was found out th a t such a m odel does n ot fit properly our susceptibil
ity d a ta . T h e reason is th a t for a negative excitation energy E x , th e SW
form ula [8] gives only a small reduction of th e effective m agnetic m om ent of cerium a t high tem p e ra tu re i.e. n eg = 6 /72.54/ t b = 2.35/ t b while for positive E x th e resulting x (T ) function has a broad m axim um which is not observed experim entally. T he observed reduction of th e effective m agnetic m om ent is too strong to be explained by crystalline field effects.
Electronic ban d stru c tu re calculations, carried o ut using th e T B -LM TO [9]
and W IE N 2k [10] program packages, confirm th e m etallic ch aracter of CeR hSn. Spin polarized calculations resulted in no m agnetically ordered ground s ta te for th e Ce 4 / shell. Even forcing th e m agnetic Ce3+ con
figuration using th e fixed-spin m etho d showed no energy gain com pared w ith th e nonm agnetic Ce4+. T hus b o th sta te s ap pear to be energetically equivalent, which reflects th e valence fluctu ating behaviour of CeR hSn. T he chemical bonding analysis shows strong R h -S n bonding w ithin th e [RhSn]4- polvanion.
Below Tq = 6.5 K one of two our sam ples undergoes a superconducting tra n sitio n as docum ented in Fig. 2 by a sudden diam agnetic drop of th e dc susceptibility m easured in a m agnetic field of 3.6 Oe after cooling th e sam ple in zero m agnetic field (ZFC). A t th e sam e te m p e ra tu re a fall of resistivity is observed. T he resistivity does not decrease to zero, b u t it m ay be a result of weak superconducting links betw een superconducting grains, since th e m easurem ent was done using a pellet obtained by pressing powder m aterial.
Fig. 2. Left: dc-susceptibility of our CeRhSn sample measured in H = 3.6 Oe after zero field cooling. Resistivity measured on the pressed powder pellet is also shown.
Right: real y ' and imaginary y" parts of the zero field ac magnetic susceptibility for CeRhSn.
T he tran sitio n into th e superconducting s ta te is also confirmed by th e characteristic hysteresis loop below Tq (not shown) as well as by th e onset of th e diam agnetic signal in y ' a t T = 6.6 K associated w ith th e m axim um of y " (Fig. 2) and th e d istinct m axim um of th e th ird harm onic (not displayed
here) observed close to th e tra n sitio n tem p e ra tu re . T he superconducting fraction was estim ated from m agnetic m easurem ents (w ithout tak ing into account th e dem agnetization factor) to be ab o u t 10%.
3. Conclusions
X-rays, ac and dc m agnetic susceptibility m easurem ents, su p p o rt anom a
lous behaviour of CeR hSn associated w ith valence fluctuation. It was shown th a t th e Sales-W ohlleben IC F m odel is n o t adequate to describe th e tem p er
a tu re dependence of th e m agnetic susceptibility. A superconducting tra n sition was detected for one CeR hSn sam ple b u t fu rth e r investigations are needed to recognise th e origin of th is superconducting sta te .
R E FE R E N C E S
fl) S.K. Malik, D.T. Adroja, Phys. Rev. B43, 6277 (1991).
(2] T. Sasakawa et al., J. Phys.: Condens. Matter 14, L267 (2002).
(3] D.T. Adroja et al, Phys. Rev. B39, 4831 (1989).
(4] K. Łątka, R. Kmieć, J. Gurgul, J. Alloy. Compd. 319, 43 (2001).
(5] Ch.D. Routsi et al, J. Magn. Magn. Mater. 117, 79 (1992).
(6] T. Schmidt, R. Pottgen, K. Łątka, R. Kmieć, R. Kruk, AAV. Pacyna, M. Rams, to be published.
(7] R. Mishra et al, Z. Naturforsch. 56b, 589 (2001).
(8] B.C. Sales, D.K. Wohlleben, Phys. Rev. Lett. 35, 1240 (1975).
(9] O.K. Andersen, Tight-Binding LM TO Vers. 47, M ax-Planck-Institut fur Fes- tkorperforschung, Stuttgart 1994.
(10] P. Blaha, K. Schwarz, J. Luitz, WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program, Vienna University of Technology (2001).
