Magnetic resonance study of new compound Nb2SbVO10

Electron paramagnetic resonance (EPR) study of Nb VSbO 2 10

has been carried out on a conventional X-band continuous wave Bruker E 500 equipped with TE cavity with 100 kHz 102

modulation. The temperature variation studies in the 7-300 K range were performed using an Oxford ESP300 continous flow helium cryostat.

Magnetic resonance study of

new compound Nb SbVO

₂

₁₀

1

1

2

1

J. Typek , G. Zolnierkiewicz , A. Cyran , K. Wardal ,

3

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E. Filipek and M. Piz

1

Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland

Institute of Material Science, West Pomeranian University of Technology,Al. Piastow 19, 70-310 Szczecin, Poland

Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology,

Al. Piastow 42, 71-065 Szczecin, Poland

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 M ic ro w a w e p o w e r [m W ] F irs td er iv at iv e am pl itu de pe ak -t o-pe ak [a rb .u ni ts ] B = 3 3 6 9 G B = 3 2 6 8 G B = 3 0 8 9 G 2600 2650 2700 2750 2800 2850 2900 2950 3000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 110K 120K 130K 140K 90K 100K 150K 160K 170K 180K 190K 200K 210K 220K 230K 240K 250K 260K 270K 280K In te n s it y [a rb . u n it s ] Magnetic field [G] Component "C" 290K 160 180 200 220 240 260 280 300 90 100 110 120 130 140 "p e a k -t o -p e a k " L in e w id th o f c o m p o n e n t C [G ] Temperature [K] 160 180 200 220 240 260 280 300 2740 2750 2760 2770 2780 2790 2800 2810 2820 160 180 200 220 240 260 280 300 2.37 2.38 2.39 2.40 2.41 2.42 2.43 R e s o n a n c e fi e ld [G ] Temperature [K] g -f a c to r 160 180 200 220 240 260 280 300 7 8 9 10 11 12 13 14 15 16 In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K]

Parameter Unit SbVO5 Nb2VSbO10

g|| - 1.9311 1.902 g - 1.9425 1.93725 g0 - 1.9387 1.9255 Ä g|| / Ä g - 1.19 1.542 |A||| 10-4 cm-1 181 147.1 |A | 10-4 cm-1 54 38.61 P 10-4 cm-1 140 137.1 k - 0.6244 0.3866 P·k 10-4 cm-1 87.4 53.01

The observed spectrum of Nb VSbO (continous line) at T=290 K and its two main 2 10 4+

components: isolated V ions (dashed line - component A) and a broad line (dotted line - component B).

Temperature dependence of the reciprocal integrated intensivity of Nb SbVO . The straight lines are the least 2 11 squares best fits for the low and high temperature ranges.

SEM picture of Nb VSbO compound.2 10

EPR signal amplitude as a function of the microwawe power for three lines in EPR spectrum of Nb SbVO .2 10

Comparison of hfs parameters of two similar compounds: SbVO i 5 Nb VSbO .2 10

The component C in the EPR spectrum of Nb VSbO at different 2 10 temperatures.

Temperature dependence of peak-to-peak linewidth of the component C.

Temperature dependence of the resonance field (black) and g-factor (red) for the component C.

Temperature dependence of the integrated intensity of the component C.

In the last years, the research on multicomponent oxides systems has been intensified in order to obtain new materials of interesting electric, magnetic or catalytic properties. The basic studies on the ternary V O -Nb O -Sb O system have shown that the oxides formed in the 2 5 2 5 2 4

solid state in air contain an unknown compound with the formula Nb VSbO . This compound has been obtained by heating of the mixture _{2 10} V O /Sb O /Nb O (in molar ratio 1:1:2) in air at temperatures up to 750C according to the equation: 2 5 2 4 2 5

V O +2Nb O +Sb O +1/2O =2Nb SbVO2 5(s) 2 5(s) 2 4(s) 2(g) 2 10(s). The compound Nb VSbO can be also obtained by heating an equimolar mixture of 2 10

SbVO and T-Nb O according to the equation: Nb O +SbVO = Nb SbVO5 2 5 2 5(s) 5(s) 2 10(s). The new compound Nb VSbO is stable in air up to 880°C 2 10

and next melts incongruently with deposition of solid Nb VO . It crystallizes in the orthorhombic system with the following unit cell 9 25

parameters: a = 0.328143 nm, b = 0.458946 nm, c = 1.22476 nm, (Z = 1).

To estimate the number of spins in Nb VSbO sample participating in the resonance a comparative study with standard VOSO •5H O sample with a known number of spins has been performed. 2 10 4 2

Assuming that vanadium ions in the Nb VSbO compound could be only in 4+ (EPR active) or 5+ (EPR inactive) valence states it was calculated that only 5.2% of vanadium ion are magnetic and thus in 4+ 2 10

valence state.This concentration of magnetic ions is even smaller than in previously studied SbVO compounded. The knowledge of concentration of magnetic ions might be important in estimation of the 5

catalytic activity of the compound as often the minority magnetic ions play a dominant role in such activity.

The EPR spectrum of Nb SbVO in the high temperature range could be decomposed on three components: a broad signal in the range of g>_{2 10} 2 without hyperfine structure (hfs), a partially resolved hfs

4+ 5+

lines typical of isolated vanadium ions in an axial symmetry and a weak line near 2800 G. The broad line without hfs could be attributed to a mobile electron hopping along the V -O-V bond. The hfs could be substatailly suppresed or even disappear due to various interactions of electonic spins with their surroundings. One such interaction occurs via the super-exchage through oxygen brigde of an electron

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between aliovalent vanadium centers. The second component displays usuall two sets of eight lines, partially overlapping, attributed to the interaction of electron spin (S=1/2) with V nucleus (I=7/2, abundance 99.75%). This spectrum could be satisfactory described by an axial spin Hamiltonian of the form H = b[g||B S + g^z z (B S + B S )] + A|x x y y |S I + A^z z (S I + S I ), where the symbols have their usual x x y y

meaning. Fitting the registered spectrum by using the program SimFonia allowed to obtain the following values of the spin Hamiltonian: g||=1.907, g^=1.947 A||=164.6 G, A^=43.5 G. These values didn’t show any significant temperature dependence. The third component displays a very anisotropic behaviour. It shifts significantly with temperature, reaching the lowest resonance field of 2800 G at 200 K

To reveal magnetic interactions in Nb SbVO the temperature dependence of EPR integrated intensivity has been studied. In the temperature dependence of reciprocal of integrated intensivity two 2 10

temperature regimes are observed: in high temperature range (T>120 K) this dependence is described by a Curie-Weiss law, I =C/(T-T ), with T =-82.8 K, while in low temperature EPR range (below 120 0 0

K) the Curie law is registered. Negative value of T indicates on an effective antiferromagnetic interaction in the magnetic system of Nb SbVO at high temperature.0 2 10

5+ 5+ 4+ 5+ 5+

V O V O V O V O V

V O V O V O V O V

---e

--- V --- O --- V --- O --- V --- O --- V --- O --- V --- O --- V ---

Model of the component A - isolated V ions with hfs.

Model of the component B - electron jumping along the vanadium chains.

4+ 4+

Model of the component C - antiferromagnetic V - V dimers

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