1b Ti3O5

Mat. Res. Bull., Vol. 15, pp. 171-176, 1980. Printed in the USA. 0025-5408/80/020171-06502.00/0 Copyright (c) 1980 Pergamon Press Ltd.

STUDIES ON THE SYSTEM V305 - Ti305 ~rjan S~vborg Department of Inorganic Chemistry Arrhenius Laboratory, University of Stockholm S-I06 91 Stockholm, Sweden

(Received December 4, 1979; Communicatedby A. Magn~lli)

ABSTRACT The system (V1_xTix) 305 has been studied by means of X-ray powder photography, DTA and magnetic susceptibility measurements. A continuous series of solid solutions has been found for samples prepared at 1175 K, while samples prepared at 1275 K showed the solid solubility in the V305 end phase to be limited by x < n.7. DTA studies have shown that the peak associated with the V305(low) - V305(high) transition disappears at x = 0.024. The DTA studies have also confirmed the existence of a phase transition in ¥-Ti305 (2 = I) at 227 K.

Introduction The occurrence of metal-insulator transitions in reduced vanadium oxides has attracted considerable interest. In V305, which may be regarded as a member of the homologous series VnO2n_1, a phase transition occurs at 420 K as demonstrated by means of electrical conductivity and magnetic susceptibility measurements, and by DTA (1,2,3). It has been reported that this transition, unlike those in other members of the series, is a semiconductor to semiconductor transition (2). The first structural study of V305 was made by Asbrink et al. (4). The monoclinic structure derived can be described in terms of VO6"~cTahedra. One layer of octahedra, perpendicular to ~I~, is shown in Fig. I (the dotted lines indicate part of the next layer). The octahedra form two types of chains running parallel to the ~-axis. One type (A in Fig. I) is formed by edge sharing between pairs of face sharing octahedra (called double octahedra below). The other type (B in Fig. I) is formed by corner sharing octahedra. The chains are mutually connected in the ac-plane by corner sharing and in the b-direction by edge sharing.

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In an effort to clarify the structural character of the phasetransition Asbrink (3) observed that the X-ray diffraction patterns of the two modifications are closely similar, but he found a large number of very weak reflexions for the low temperature phase, which indicated deviations from the previously assumed bodycentered space group. These extra reflexions were all absent for the high temperature phase, which thus actually possesses the centered structure. For this reason the structures of low- and high-V305 have been further studied by Asbri~k-and Hong from accurate single crystal data. They found that high-V305 has the previously reported structure, space group 12/c (5), and that the loss of centering in low-V305, space group P2/c, is caused by-the segregation of the chains of double octahedra into two slightly different types (6). The existence in V305 of a maximum in the magnetic susceptibility at approximately 135 K has been reported (7). It has been shown by NMR (8), M~ssbauer spectroscopy (9) and inelastic neutron scattering (10) that V305 is antiferromagnetically ordered below 76 K.

In studies of Ti305 Asbrink et al.~in addition to the previously known ~- and B-modifications, found a new p-hase, called ~f-Ti305, formed at temperatures below 1225 K (11). The powder pattern of this phase closely resembles that of V 305 and can be indexed on the basis of the same unit cell. The Japanese contributors to that article (11) also found evidence from magnetic susceptibility measurements of a phase transition around 250 K. No further information is at present available regarding the structural character and electrical properties of this material. Terukov et al. (12) have measured the electrical conductivity and Seebeck effect on monocrystals of (V1_xTix)305 with x = 0, 0.0049, 0.0081 and 0.04. They found that the sharp transition observed Tor x = 0.0081 was more gradual for ~ = 0.04.

Experimental Samples with gross composition (V1_xTi~)305 were prepared by heating intimate mixtures of V205 (Fisher, Sc. Co., ~.a.), V203 (prepared as in ref. 13) and TiO 2 (Baker, Chem. Co., ~.~.). For samples with 0.33 < x < 1.0 mixtures of V203, TiO 2 and Ti (Mackay, Inc., 99.5 % pure) were used. All samples were reacted at 800 K for 24 h. Two series of samples were prepared; one was further heat treated at 1275 K for one week, and the other at 1175 K for three weeks. The products were characterized by their X-ray powder patterns

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THE SYSTEM V 3 0 5 - T i 3 0 5

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recorded in a Guinier-H~gg focusing camera using CuK~tradiation and with KCI (a = 6.2919 A) added as an internal standard. Unit cell parameters were refTned with the aid of the program TETLIN (14) written at this Laboratory. The DTA studies were performed in an apparatus allowing five samples to be studied in the same run (15). The latent heat of transformation was calculated by comparing the area of the peak with that of VO 2, which was given the value AH t = 1.02 kcal/mol (16). The magnetic susceptibility measurements were performed in an automatized apparatus using the Faraday principle (17). Results Analysis of the powder patterns from samples heat treated at 1275 K showed that the range of homogeneity of the V305-type phase (V1_xTix) 305 extends from x = 0 to ~ = 0.7. In the region 0.7 < x < I the powdeF patterns showed the presence of a B-Ti305 phase together wTth V305. Thus vanadium does not, as iron and some other metals, stabilize the ~-Ti305 type of structure

(18).

The powder patterns from samples heat treated at 1175 K indicated that at this temperature the V305 phase extends from ~ = 0 to ~ = I (y-Ti305). It must, however, be pointed out that in the region 0.6 < x < I the powder patterns were very diffuse and did not improve after several weeks of heat treatment. Unit cell parameters of the 1175 K samples are plotted versus composition in Fig. 2. No significant difference in the unit cell parameters between samples of the same composition heat treated at 1175 K and 1275 K has been observed. The unit cell dimensions of V3 ~0 (3), V2TiO 5 (19) and y-Ti305 (11) agree well with those previously published. Samples with 0 < x < 0.1 and with x = I were studied by ~ A . For x = 0 a peak was observed at 423 K, in agreement with previous findings (3). The latent heat of transformation was found to be 180 cal/mol, in reasonable agreement with previous observations (20). Upon Ti substitution the peak broadens, and the area decreases rapidly so that no peak was observed for ~ = 0.024. The transition temperature decreases slightly with increasing x, so that T t - 415 K for x = 0.020. OkTnaka et al. (~I) found evidence of a magnetic transition in 7-Ti305. Therefore samples with x = I were studied by DTA; and a peak was observed at 227 K, in good agreement with magnetic susceptibility data from this study, but somewhat lower than the observations at 250 K mentioned above. The latent heat of transformation was found to be 270 cal/mol.

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FIG. 4. Inverse magnetic susceptibility versus temperature for V305 in an interval containing the transition point.

FIG. 3. Magnetic susceptibility versus temperature for (VI_xTix~305 .

In Fig. 3 the molar magnetic susceptibilities of some typical samples are plotted versus inverse temperature. The data for V30 ~ agree well with previously published results (7). A smooth maximum in th~ susceptibility exists at approximately 140 K, with XM= 0.80 10 -2 cgs emu mol -I. There is also a small but distinct jump in the susceptibility at 420 K (Fig. 4). For small values of x (<0.024) the shape of the susceptibility curves is quite similar to that fo-~ V305, except that the maximum in ×M is less pronounced. It is, however, noteworthy that the susceptibility for x = 0.008 and 0.016 is somewhat higher than for V305, the difference becoming negligible only "X:lO&/cgs e m u rook -1 at the highest temperatures applied (700 K). For values of x in the , region 0.024 < x < 0.33--(x = 0.064 and 0.33 given ~n Fig. 3)-the susceptibility curves are distinctly dif0 o 5ferent from that of V305. Thus there is no tendency towards a maximum in 0 ×M at low temperatures, and there is 0 no sharp jump at 420 K. In the region 0.33 < x < I the curves are nearly 0 3straight lines; the slopes decrease 000 0 with increasing x. 0 0 0 0

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Pure ¥-Ti305 shows a completely different behaviour (Fig. 5). In the low temperature region it is temperature independent paramagnetic with X M = 3.0 10 -4 cgs emu mol -I (after correction for the diamagnetic contribution). At the phase transition there is a sharp increase of the value, which then increases at a

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THE SYSTEM V305-Ti30 5

175

lower rate up to 500 K; then it appears to be constant. Our susceptibility data are in good agreement with that of Okinaka et al.(11), apart from the high temperature behaviour. Discussion The DTA studies clearly show a first order transition around 420 K in (V1-xTix)305 with ~ < 0.024, indicating that the low to high-V305 transition persists in this region. For x > 0.024, however, the picture ~s less clear. In the absence of single crystal X-ray data it is impossible to determine which phase is present at room temperature, as the very weak reflexions characteristic of low-V305 are unobservable in the powder photographs, and as there is no marked difference in magnetic behaviour between low- and high-V305. The previously mentioned observations by Terukov et al. (12) on the electrical properties of Ti doped crystals, together with observations by BrDckner et al. (21) on the thermal expansion of Ti doped crystals (x = 0.04), seem to indicate the existence of a higher order transition in this region. The susceptibility data from our study can be interpreted in the same fashion; they indicate that the transition might be present up to x = 0.33. The plots of unit cell parameters in Fig. 2 show very marked changes of slope at x = 0.33 and somewhat less pronounced ones around x = 0.67. This might ind]'cate that the substitution of Ti for V is not random, but the small difference in scattering power for X-rays between Ti and V makes it impossible to determine this from X-ray data. For this reason neutron powder diffraction data have been collected for a sample with x = 0.33, and the distribution of Ti between the two types of Me positions (in double or single octahedra) has been refined assuming that the compound has the high-V305 type of structure. The refinement showed that Ti exclusively enters the double octahedra with random half occupancy of these sites. A full report of this investigation will be published shortly (22). Acknowledgements I wish to thank Professor A. Magn~li for his advice and support. I am also indebted to Dr. M. Nygren and Dr. S. Asbrink for their kind interest and several valuable discussions. Mrs. R. Wilhelmi is cordially thanked for her skilful typing of this manuscript. This investigation Research Council.

has been supported by the Swedish Natural Science

References I. E.I. Terukov and F.A. Chudnovskii,

Fiz. Tekh. Polaprov. 8, 1226 (1974).

2. W. BrDckner, H. Wich, E.I. Terukov and F.A. Chudnovskii,

Fiz. Tverd. Tela

17, 2191 (1975). 3. S. Asbrink,

Mat. Res. Bull.

10, 861

4. S. Asbrink, S. Friberg, A. Magn@li 603 (1959).

(1975). and G. Andersson,

Acta Chem. Scand.

13,

5. S.-H. Hong and G. Asbrink. To be published. 6. S. Asbrink. To be published. 7. G. Grossmann, 9 (1960)

O.W. Proskurendo and S.M. Arizu, Z. anorg, allg. Chem. 305,

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8. A.C. Gossard, F.J. Di Salvo, L.C. Erich, J.D. Remeika, H. Yasuoka, K. Kosuge and S. Kachi, Phys. Rev. B10, 4178 (1974). 9. K. Kosuge, H. Okinaka and S. Kachi, IEEE Trans. Magnetics 8, 581 (1972). 10. A. Heidemann,

K. Kosuge and S. Kachi, Phys. Stat. Sol. (a) 35, 481 (1976).

11. G. Asbrink, S. Asbrink, A. Magn~li, H. Okinaka, K. Kosuge and S. Kachi, Acta Chem. Scand. 2_55, 3889 (1971). 12. E.I. Terukov, F.A. Chudnovskii, W. BrDckner, W. Reichelt, H.-D. BrUckner, W. Moldenhauer and H. Opperman, Phys. Stat. Sol. (a) 38, K23 (1976). 13. T. HSrlin, T. Niklewski and M. Nygren, Mat. Res. Bull. 8, 179 (1973). 14. T. H~rlin. To be published. 15. T. H~rlin, T. Niklewski and M. Nygren, Chem. Commun. Univ. Stockholm No. 9 (1975). 16. E.J. Ryder, F.S.L. Hsu, H.J. Guggenheim and J.E. Kimzler, quoted by C.N. Berglund and H.J. Guggenheim, Phys. Rev. 185, 1022 (1969). 17. B. Blom and T. H~rlin, Chem. Commun. Univ. Stockholm No. 5 (1977). 18. B. Blom. Private communication. 19. B. Brach, J.E. Grey and C. Li, J. Solid State Chem. 2__0_0,29 (1977). 20. E.I. Terukov, F.A. Chudnovskii, W. Reichelt, H. Oppermann, W. BrUckner, H.-D. BrDckner and W. Moldenhauer, Phys. Stat. Sol. (a) 3J_7, 541 (1976). 21. W. BrUckner, W. Moldenhauer, B. Thuss and G. FSrsterling, Phys. Star. Sol. (a) 3_~5, K99 (1976). 22. S. Asbrink and ~. S~vborg. To be published.