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Crystal growth and characterizationofS = 1/2 1-D system y-LiV2O5 and the spin-Feierls system a'NaV2O5

Masahiko ISOBE* and Yutaka UEDA

Materials Design and Characterization Laboratories, Institute for Solid State Physics,The University ofTokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106-8666, Japan

ABSTRACT

The phase diagram of the Li-V-O system was investigated to fmd the best conditions for the crystal growth of S 1/2 1-1)system rLiV2O5. Basing on the obtained phase diagram a single ciystal with dimensions 18 x 3 x 2 mm was successfullygrown from a liquid phase using LiVO3 as a flux. A bigger single crystal of the spin-Peierls system a'-NaV2O5 withdimensions 18 x 6 x 2 mm was obtained by a similar method. The crystals of Li (Na) deficient samples were alsoprepared. Magnetic and electric properties of these crystals were investigated.

Keywords: Phase diagram, Crystal growth, y-LiV2O5, a'-NaV2O5, S 1/2 l-D system, Spin-Peierls transition,

1. INTRODUCTION

Since the discovery of the spin-Peierls compound CuGeO3 and the spin ladder compound SrCu2O3, more extensive studieshave been performed in low-dimensional quantum spin systems both experimentally and theoretically.

We have investigated vanadium oxides as a low-dimensional magnetic system with V ion (S 1/2). Until now we havereported the vanadate family of AV205 (A Na'6, Ca7, Cs8, Li8'9, Mg10). The structures of all these compoundsfundamentally consist of V05 square pyramids. In a '-NaV2O5, we discovered the spin-Peierls transition"4 and reported theeffect of Na-deficiency3 and crystal growth2. After our reports, many researches have investigated and reported theanomalous behaviors.11 Orthorhombic y-LiV2O5 has layer stmcture with lithium ions between the layers. In the intra-layer of y-LiV2O5 the VO5 (S = 1/2) square pyramids form an infinite linear zigzag chain by edge sharing (or an infinitedouble-linear chain by corner sharing) along the b-axis.'2 In y-LiV2O5, we have reported that the magnetic susceptibility ofpowder sample fits very well to a Bonner-Fischer curve, what indicates a good S 1/2 1-D system for y-LiV2O5.8 Howevera large single crystal with high quality is crucial for further investigation of microscopic structural and physical properties.We have tried to grow single crystals by a flux method similar to the method2 for a '-NaV2O5. In this paper we report thecrystal growth ofS = 1/2 1-D system, y-LiV2O5.

2. CRYSTAL GROWTHy-LiV2Os

Single crystals of y-LiV2O5 were grown from the liquid phase using LiVO3 as a flux. A single crystal of a '-NaV2O5 hasbeen successfully grown by a similar method.2 Figure 1(a) shows a part of the ternary Li-V-O phase diagram. There existmany compounds of LiV2O5 (the line between V205 and Li) which are the so-called vanadium bronze oxides with anmtercalant of Li. On the other hand no intermediate compound appears on the line between LiV2O5 and LiVO3, andtherefore LiV2O5-LiVO3 system is considered to be a pseudo binary system. At first, we examined the phase relationshipsin this pseudo binary system at high temperatures by thermogravimetric and differential thennal analysis (TG-DTA). TG-DTA curves were measured at a heating and cooling rates of 5°C/mm. The measurements were done under Ar atmosphereto prevent the oxidation of the compound. Powder samples of r-LiV2O5 were prepared by the solid-state reaction ofmixtures with appropriate molar ratios of LiVO3, V203 and V2O5. The weighed mixtures were pressed into pellets andheated to 600°C in an evacuated silica tube for several days with one intermediate grinding. The LiVO3 was prepared byheating a mixture of Li2CO3 and V205 in 02 gas at 550°C for 2 days.

*Corresponding author. e-mail : isobekodama.issp.u-tokyo.ac.jp

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The results are shown in Fig. 1(b) as the phase diagram. The"-LiV2O5 showed one endothermic peak on heating at around780°C, and on cooling a few very sharp exothermic peaksaround 550°C in DTA measurements. During both heating andcooling a significant weight change was not observed. Fromthis result and the previous study of a '-NaV2O5 it wasconcluded that rLiV2O5 melts mcongruently at 780°C to aliquid and solid V02. The coexistence of solid V02 and aliquid phase could hinder the growth of rLiV2O5 crystals by aslow cooling because of many V02 crystallites at the startingpoint. LiVO3 melts at 620°C, showing an endothermic peak inthe DTA curve. The mixture of rLiV2O5 and LiVO3 in themolar ratio of R=100LiV2O5/(LiVO3+LiV2O5)>20% shows twoendothermic peaks in the DTA curve around at 615°C and780°C on heating. This indicates the eutectic temperatureabout 6 15°C and the peritectic temperature about 780°C. Themixture with R=1O % shows only one endothermic peak aroundat 615°C on heating and an exotherrnic peak around at 690°C oncooling. The temperature 6 15°C on heating can be regarded asthe same eutectic temperature but the temperature 690°C oncooling has never been observed in the region of R>20% wherethe super-cooled peritectic temperature has been observedbelow 600°C. From these results it was concluded that thefollowing reaction takes place around 690°C at R 10 %,

L -÷LiV2O5 (S) +LConsequently we chose R=1O % as a composition for crystal

growth. The mixture with R=1O % was put into a plathiumcrucible, heated at 780°C in an evacuated silica tube, and thencooled slowly (cooling rate of 1°C fhour) down to 550°C. Bythis method we obtained rather large size (.40 x 0.5 x 0.5 nun3)black single crystals. We finally succeeded in the growth of much larger crystals using these crystals as seed crystals, asdescribed below. The powder mixture with R12.5% was put into a plalinum crucible together with a seed crystal, sealedinto an evacuated silica tube, heated at 730°C for 1 hour, and then held at 700°C for three days. Here we chose 700°C as thegrowth temperature because it could be considered that the temperature 690°C detected by DTA measurements is thesupercooled temperature for the reaction and the reaction in equilibrium occurs at a little higher temperature. The finalproduct after cooling to room temperature was a mixture of single crystals of j-LiV2O5 and either powder or very smallcrystals of LiVO3. Large crystals of rLiV2Os were found near upper zone of the crucible, what indicated that the seedcrystals floated in the liquid. By boiling the products with water we can isolate the single crystals of rLiV2O5 from Li VU3,because only LiVO3 is soluble in hot water. The largest crystal thus obtained in this experiment has a dimension of 18 x 3 x2 mm, as shown in Fig. 2. The crystals usually have extended growth along the b-axis, that is the 1-D chain direction.aNaV2O5

Previously we reported single crystal growth of d-NaV2U5 from liquid phase using NaVU3 as a flux.2 Recently weobtained a bigger single crystal of a'-NaV2U5 with dimensions 18 x 6 x2 mm than the previous one. It is shown in Fig. 3.

iETa

10 20

Fig. 2. LiV2O5 single crystal. Fig. 3. a'-NaV2U5 single crystal.

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(a)

L800 - L÷V02

(b)

00cEI- L+L1V205

700

600

L1VO3 + L1V205Iii Iii Iii04

L1VO3

'II50

(mol %)

II100

4

L'V205

4

v02

Fig. 1. Phase diagram of(a) Li-V-U system and (b)pseudo binary system Li V2U5-Li VU3.

b1t

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Li (Na) - deficiencySingle ciystals of '-LiV2O5 (a'-NaV2O5) were prepared by heating a small crystal of stoichiometric y'LiV2O5 (a'-

NaV2O5)embedded in a large quantity of the powder sample of yLiV2O5 (a'-NaV2O5) in an evacuated silica tube at 600°C(650°C) for a week. It was checked by the measurements of the magnetic susceptibility that the composition of singlecrystal is identical with that of powder samples.

3. CHARACTERIZATION

The magnetic susceptibility was measured using a Quantum Design SQUID magnetometer. The electric resistivitymeasurements were made by an ordinary four-probe method using single crystals. The specific heat measurements weredone with ac calorimetry using chopped light as a source of heat. Since the absorbed power is not known, we onlydetermined relative values of C, with ac calorimetry.

yLiV2O5The magnetic susceptibility of rLiV2Os single crystal has a broad maximum around 190 K in agreement with the

susceptibility of powder sample8 and approaches to the constant value at low temperatures. The easy axis of magnetizationis the a-axis. Any evidence for a magnetic order has never been observed in contrast to the other linear chain systems.

The composition range of the p-phase was in LiV2O5 in the present work, which was in agreement with theprevious rt3 The Curie-like increase of magnetic susceptibility which was proportional to Li-deficiency was observedin .LiV2O5 at low temperatures. Any evidence for a magnetic order, however, has never been observed. This indicatesthat y-LiV2O5 is an ideal 1-D magnetic system and the magnetic interchain interaction is very weak. The resistivity of '-LiV2O5 decreases with Li-deficiency at 300 K but is semiconductive in all samples. This suggests that the carriers arepossibly doped into j'LiV2O5 system, however the conducting behavior of doped samples is very complex.aNaV2O5

In the previous study, we reported that the magnetic susceptibility of single crystal a'-NaV2O5 rapidly decreases in eachdirection with decreasing temperature below the spin-Peierls transition temperature of 35 K2 and the easy axis ofmagnetization is the a-axis.2 The magnetic susceptibility of Nao.98V205 single crystal is shown in Fig. 4. The easy axis ofmagnetization is also the a-axis. The transitiontemperature decrease and the Curie-like increase ofmagnetic susceptibility is also observed at lowtemperatures in the powder samples3 of d-Na.98V2O5.The specific heat of a'-NaV2O5 was measured usingsingle crystals. Figure 5 shows the temperaturedependence of the specific heat of a'-NaV2O5 (x0.96-4.00) between 10 and 50 K. The each data for

Na 8V205

5T' : ' HI/a-axisV S H II b-axisH II c-axis

5

4

0E

Ea)

x

0

0.64C

30 •• NaV2O50.5

.2O:

10

;0.4

,O.5 0.90

C xinNay2O

0.2 . x=1 .00

:0 50 100 150 200 250 300 Temperature (K)

Temperature (K)

Fig. 4. Temperature dependence of magnetic susceptibilityalong a, b and Caxisin Na98V2O5 single crystal.

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Fig. 5. The temperature dependence of the specific heatof a'-NaV2O5. The inset shows compositionaldependence of T, for a'-NaV2O5

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x = O.97—1.OO is normalized at 15 K and shifted at vertical position. In a'-NaV2O5 (x 1.00), the peak of specific heat isclearly seen around the spin-Peierls transition of 35 K. Decreasing with x in cr'-NaV2O5, the peak is shifted to lowtemperature, becomes broad and vanishes around Na097V205. The inset shows the compositional dependence of Tp for a'-NaV2O5 measured by the specific heat. This results are in good agreement with those from the magnetic susceptibility.3The resistivity of a'-NaV2O5 decreased with Na-deficiency but was of semiconductive in all samples.3 The temperaturedependence of resistivity ofNa-deficient samples seems to be consistent with a variable range hopping in 1-D system ratherthan activation type.3 This suggests that the carriers are doped into 1-D chain by Na-deficiency but do not induce cleanmetallic behavior, because an arbitrary small concentration of defects often leads to localization in a 1-D material. Thispoint is significant, differing the results in doped-CuGeO3 and SrCu2O3.

4. CONCLUSIONS

In conclusion big single crystals of S = 1/2 1 D system y-LiV2O5 have been successfully grown from liquid phase usingLiVO3 as a flux. The largest crystal thus obtained has a dimension of 18 x 3 x 2 mm. The crystals usually have extendedgrowth along the b-axis, that is the 1-D chain direction. We obtained bigger single crystal of a'-NaV2O5 with dimensions 18x 6 x 2 mm3 than the previous one. By using these crystals, electhc and magnetic properties were measured.

5. REFERENCE

1 . M. Isobe and Y. Ueda, "Magnetic Susceptibility of Quasi-One-Dimensional Compound a'-NaV2O5 -Possible 5pm-Peierls Compound with High Critical Temperature of 34 K," J. Phys. Soc. Jpn 65, pp. 1178-1181, 1996.

2. M. Isobe, C. Kagami and Y. Ueda, "Crystal Growth ofNew Spin- Peierls Compound NaV2O5," J. Crystal Growth 181,pp. 314-217, 1997.

3. M. Isobe and Y. Ueda, "Effect ofNa-deficiency on the Spin-Peierls Transition in a'-NaV2O5," I Alloys and Compounds262-263, pp. 180-184, 1997.

4. Y. Fujii, H. Nakao, T Yoshihama, M. Nishi, K. Kakurai, M. Isobe and Y. Ueda, "New inorganic Spm-Peierls CompoundNaV2O5 Evidenced by X-ray and Neutron Scattering"J. Phys. Soc. Jpn 66, pp. 326-329, 1997.

5. T. Yoshihama et al.,"Spin Dynamics in NaV2O5 -Inelastic Neutron Scattering,"J. Phys. Soc. Jpn 67, pp.744-747, 1998.6. T Ohama, et a!., "23Na NMR Study of Spin-Peierls Transition in NaV2O5" J. Phys. Soc. Jpn 66, pp. 545-547, 1997.7. H. Iwase, M. Isobe, Y. Ueda and H. Yasuoka, "Observation of Spin Gap in CaV2O5 by NMR," J. Phys. Soc. Jpn 65, pp.

2397-2400, 1996.8. M. Isobe and Y Ueda, "Magnetic Susceptibilities ofAV2O5 (A Li and Cs) with Square Pyramidal V(IV)05," J. Phys.

Soc. Jpn 65, pp. 3142-3145, 1996.9. H. Fjiwara, H. Yasuoka, M. Isobe, Y. Ueda and S. Maegawa, "Spin fluctuation in 5=1/2 double-linear-chain rLiV2O5

studied by 7Li NMR" Phys. Rev. B 55, pp. R11945-R11948, 1997.10. M. Isobe, Y. Ueda, K. Takizawa and T Goto, "Observation of a Spin Gap in MgV2O5 from High Field Magnetization

Measurements," J. Phys. Soc. Jpn. 67, pp. 755-758, 1998.11. For example, A. N. Vasil'ev, VV. Pryadun, D. I. Khmoskii, G. Dhalenne, A. Revcolevschi, M. Isobe and Y. Ueda,

"Anomalous thermal conductivity of NaV2O5 as compared to conventional spm-Peierls system," Phys. Rev. Lett. 81,pp. 1949-1952, 1998, and references therein.

12. D. N. Anderson et al., "Refinement of the strucuture of LiV2O5," Acta Crystaiogr Sect. B 27, pp. 1476-1497, 1971.13. E. Takayama-Muromachi and K. Kato, "Phase Equilibrium Study of the LiV2O5-V204-V205 System at 923 K: A Series

of Lithium Vanadium Bronzes, LxV6nOi5nm," J. Solid State Chem. 71, pp. 274-277, 1987.

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