SOLIDSOLUTIONS
OB
1781
VANADIUM AND TITANIUM OXIDES
Crystallography and Defect Chemistry of Solid Solutions of Vanadium and Titanium Oxides1 by R. E. Loehman, C. N. R. Rao, and J. M. Honig Department of Chemistry, Purdue University, Lafayette, Indiana
47907
(Received December 9, 1968)
The solubility of VO, in TiO, (x = 0.8-1.2) and vice versa is about 10%. The concentration of defects is slightly lower in the solid solutions than in the pure compounds. Tiz03and VzOa form solid solutions over the entire range of compositions. The c parameter of Tiz03is extended about 3% by incorporation of about 20% V&, while the a parameter contracts about 1.5% with the addition of 20% VtOS. At higher percentages of v203 the variations in the a and c parameters are not very marked. Electrical resistivities of TiO, VO, and their solid solutions at 5 mol yo are also reported.
Introduction There has been considerable interest in recent years in the study of the electrical transitions in and V203. Vz03undergoes a sharp insulator to metal transition at 160°K. A crystallographic transformation accompanies the electrical transition; for T < 160°K the monoclinic structure is stable, while for T > 160°K the corundum structure is stable.*t3 Unlike VzOa, TizOa shows an electrical transition occurring over a wide range of temperatures (380-500°K) ;3-6 changes in lattice dimensions are found to accompany the transition but there is no change in crystal symmetry.6 A recent crystallographic study of Tiz03in this laboratory has provided precise data on the changes in lattice dimensions accompanying the electrical transition,’ We have examined here the crystallography of solid solutions of T h o 3 and Vs03 since such studies would be of value in the understanding of the mechanisms of the electrical transitions. A related study of the crystallography and defect chemistry of solid solutions of vanadium and titanium oxides of NaCl structure was prompted by the interesting defect chemistry of the parent oxides. It was decided that it also would be of interest to obtain data on the electrical resistivities of TiO-VO solid solutions. X-Ray investigations have shown that titanium oxides in the composition range TiOo.7-Ti01.3possess the NaCl structure, with the lattice dimension decreasing with increasing 0 :Ti ratio.8 The determination of the densities of these defect oxides has provided the degree of occupancy of the titanium and oxygen sites.8 Vanadium oxides in the composition range VOo.s-V01.26also possess the NaCl structure; the lattice dimension increases as the 0 : V ratio is increased.B The vacancy concentrations in the cationic and anionic lattices of these vanadium oxides also have been e ~ a m i n e d . ~The behavior of the defect vanadium and titanium oxides is quite similar in the oxygen-rich phase limits (-VOl.z or -TiOl.2) in that they show nearly complete occupancy
in the anion lattice. I n the metal-rich phase limit, on the 0ther hand, the Ti-0 system (-TiO0.J shows a greater occupancy of the cation lattice than the corresponding vanadium oxide ( ~ v 0 0 . where d ~ the &ion lattice has about 13% vacancies. Experimental Section Solid beads of V,Til-,O and (V,Til-,)203 were prepared by melting stoichiometric amounts of TiOz (99.99%, United Mineral and Chemical Corp.), Ti (99.995%, Gallard Schlesinger Corp.) , VzOs (99.995%, United Mineral and Chemical Corp.), and V (99.997%, Materials Research Corp,) in an arc furnace in an atmoyhere of argon gettered with hot titanium.1° X-Ray diffraction patterns of the powdered samples were recorded using a n’orth American Philips diffractometer (Cu Kar radiation), Portions of the same samples were analyzed for vanadium content by neutron activation methods as a check on the composition; some of the samples also were analyzed gravimetrically by combustion in air. Pycnometric densities were measured using dibromoethane. Four-probe dc electrical resistivity studies were carried out using standard techniques.
Results and Discussion TizO3-V2O3System. The products obtained by arc (1) This research was assisted by NASA Multidisciplinary Grant NGR-15-005-021. (2) J. Feinleib and W. Paul, Phys. Rev., 155, 841 (1967). (3) F. J. Morin, Phys. Rev. Lett., 3, 34 (1959). (4) J. Yahia and H. P. R. Frederikse, Phys. Rev., 123, 1257 (1961). (5) (a) J. M . Honig and T. B. Reed, ibid., 174, 1020 (1968); (b) L. L. Van Zandt, J. M. Honig, and J. B. Goodenough, J. A p p l . Phys., 39, 594 (1968). (6) A. D. Pearson, J . Phys. Chem. Solids, 5, 316 (1958). (7) C. N. R. Rao, R. E. Loehman, and J. M. Honig, Phys. Lett., 27A, 271 (1968). (8) S. Andersson, B. Collen, C . Kuylenstierna, and A. Magneli, Acta Chem. Scand., 11, 1641 (1957). (9) J. Stringer, J. Less-Common Metals, 8, 1 (1965). (10) T. B. Reed, Mater. Res. Bull., 2, 349 (1967). Volume 78. Number 6
June 1960
R. E. LOEHMAN, C. N. R. RAO,AND J, M,HONIC
1782 melting mixtures of Tiz03 and VZOSin different proportions gave well-defined X-ray patterns; the patterns could be readily indexed for a rhombohedral unit cell.11 There was no evidence for the presence of mixtures of phases or precipitation of either of the components. The X-ray data clearly indicate the existence of solid solutions in the entire range between VzOa and TizO3. The lattice parameters a, e, and a and the trigonal unit cell volumes of the solid solutions ( V,Til-,)zOa are shown graphically in Figure 1. The data clearly show that Ti203and VZO3do not form ideal solid solutions following Vegard’s law. The most marked changes in the c parameter are found up to x = 0-2; marked changes in the a parameter and the unit cell volume are found up to x = 0.5. The structural parameters on samples up to x = 0.1 have been reported by Kawakubo, Yamayi, and Namouri,12 and their results are in good agreement with those from the present study. The values of a, e, and a when x = 0.1 are nearly the same as the corresponding values for pure Tiz03after it undergoes the electrical transition.’ Correspondingly, it has been found that Tho3containing -10% VZOSdoes not exhibit any electrical transition; instead, it shows metallic behavior throughout the temperature range.lZ These results seem to be consistent with the bandbroadening model of Van Zandt, Honig, and Goodenough6 for the electrical transition in TizO3. Following similar arguments, it appears that the electrical transition of V2O3 may show a progressive variation in the transition temperature with incorporation of Ti208. Unfortunately, we have not been able t o make these measurements, owing to difficulties encountered in preparing cohesive Vzo3 crystals containing TizO3; work on this problem is still in progress. TiO-VO System. The crystallography of the pure vanadium oxides as well as titanium oxides of NaCl structure prepared by arc melting has been studied; the results agree with the literature (lines 1 and 4, Table I, and lines 1, 3, 5, and 7, Table 11). The X-ray lattice constant of VO0., was considerably lower than that reported by Westman and Nordmark13 but agreed fairly well with the dimensions reported by other worker^.^^^^^ It appears that VOo.8 does not belong to the same family of cubic vanadium oxides as the compounds in the range VOo.Q-V01.2. Generally speaking, the lattice dimensions of the cubic vanadium oxides were close to those given by Vol’f and coworker^'^ and Gel’d and coworkers. l 6 An examination of the products obtained by arc melting mixtures of T i 0 and VO in different proportions showed that the maximum solubility of VO in T i 0 and vice versa was about 10%. The other compositions showed evidence for the presence of mixtures of more than one phase. An examination of the crystallography of the solid solutions of the general formula V,Til-,O, (where x varied in the ranges 0.0-0.1 and 0.9-1.0, and y varied between 0.8 and 1.2) showed that The Journal of Physical Chemistry
>
X
Figure 1. Variation of a, e, R: (trigonal), and V (trigonal) in (V,Til-.)zOa as a function of x.
Table I : Crystallography and Defect Chemistry of the Solid Solutions of V,Til-,O (0.1 5 x 5 0.9, 0:M = 1.0) -Density, a,
Compn
b
Ti0 4.185 Tio,gsVo.obO 4.172 Tio.~~Vo.osO~ 4.155 vo 4.063 4.091 Vo.ssTio.oaO Vo.ggTio.o8ob 4.104
X-Ray
5.790 5.858 5.939 6.628 6.478 6.408
g/ornJ-
Pyonorn- --% disordereter Cation Anion
4.96 5.04 5.11 5.67 5.66 5.67
14.3 14.0 14.0 14.5 12.6 11.5
14.3 14.0 14.0 14.5 12.6 11.5
a The lattice dimension of Tio,goVo,loO is slightly smaller; the showed the presence of a mixture of bead from Tio.s+Vo,leO phases. The beads from V0.86Ti0.1r,0 showed the presence of a mixture of phases.
all these solid solutions were of NaC1 structure. The X-ray lattice constants and the measured densities of these cubic solid solutions are summarized in Tables I (11) R. E. Newnham and Y . M. De Haan, 2. Krist., 117, 234 (1962). (12) T. Kawakubo, T. Yamayi, and S. Namouri, J. Phz/s. SOC. Japan, 15, 2102 (1960). (13) S. Westman and C. Nordmark, Acta Chem. Scand., 14, 465 (1960). (14) E. Vol’f, S. S. Tolkachev, and I. I. Kozhina, Vestn. Leningr. Univ., Ser. Fiz. i Khim., 14 (lo), No. 2,87 (1969). (16) P.V. Gel’d, S. I. Alyamovskii, and I. I. Matveenko, Zh. Strukt. Khim., 2, 301 (1961).
1783
SOLIDSOLUTIONB OF VANADIUM AND TITANIUM OXIDES Table 11: Crystallography and Defect Chemistry of the Solid Solutions of V,Til-,O (0.1 5 .z 5 0.9, 0 : M = 0.8 or 1.2) -Density,
g/oms-
A
X-Ray
Pycnometer
4.193 4.179 4.I72 4.164 4.038 4.071 4.126 4.132
5.468 5.549 6.137 6.194 6.429 6.239 6.632 6 580
4.99 5.16 4.83 4.89 5.54 5.56 5.30 5.31
a.
Compn
I
of the beads. The results are shown in Table 111. AS noted in footnote a of Table 111, no meaningful data could be obtained on the silvery portion.
-7% diaorderCation
Anion
8.7 7.1 21.3 21.1 13.8 11.2 20.1 19.3
27.0 25.7 5.6 5.3 30.0 29.0 4.1 3.2
a The bead from Va.91Ti0.0900.8 shows the presence of a mixture of phases.
and 11, along with those of the parent oxides of the appropriate stoichiometry. The defect compositions calculated on the b s i s of the X-ray and measured densities are also shown in these tables. It can be seen from the results in Tables I and 11that incorporation of vanadium in the titanium oxides generally decreases the lattice dimension; similarly, the incorporation of titanium in the vanadium oxides increases the lattice dimension. The incorporation of titanium in vanadium oxides has a greater effect on the defect chemistry than the incorporation of vanadium in titanium oxides, the per cent disorder being generally lower in the solid solutions. This appears to be true both for the stoichiometric solid solutions with an 0 : h l (M = Ti 3- V) ratio of unity, as well as the nonstoichiometric solid solutions where the 0 :h1 ratio is 0.8 or 1.2. Special mention must be made of the solid solutions V,Til-,00,8 where x = 0.9-1.0. The composition corresponding to x = 0.9 proved to be a mixture of phases and not a solid solution. The maximum solubility of titanium in this nonstoichiometric vanadium oxide seemed to be about 57, (compared to -10% in the other solid solutions). It may be noted that VOo E does not belong to the same family of structures as the oxides in the range VOo.9-V01.2. The surprising feature about VOO.*is its high disorder in the cation sublattice; the cation disorder is reduced, however, by incorporation of as little as 5% titanium. When mixtures of T i 0 and VO (V,Til-,O) in the composition range x = 0.2-0.9 were arc melted, two distinct phases could be visually observed, a dark outer crust and a silvery inner portion. The dark outer portion, when subjected to X-ray examination, clearly indicated that it had the rhombohedral structure similar to V Z Oor ~ Ti20a; the lattice dimensions, however, were different from those for either of these two compounds, indicating that solid solutions of the type (VzTil-,j2O3 may have been formed. Making use of the X-ray data of the solid solutions (Figure l), it was possible to estimate the composition of the outer portions of some
Table 111: X-Ray Data and Approximate Compositions of the Major Phases Obtained by Rlelting the Composition V,Til-20 (z = 0.3-0.6) Lattice parameters of ---dark outer portiona--Compn of initial mixture
3
Tio,e.r6Vo.aza0 5.11 5.08 Ti0.5mV0.4410 5.03 Ti0.42eV0.6710
2
V,
Estd compn as
sb
(VPTii& gOaG
13.81
54.9
13.87 13.95
51.9 48.5
~ 0 . 0 7f 0.02 ~ 0 ~i 1 0.05 5 -0.27 i 0.07
* I t was not possible to get good X-ray data on the silvery inner portion of the beads ; their appearance and malleability seemed to indicate they were mainly metal-rich vanadium oxides, possibly containing some Ti. A crude separat,ion and weighing of the two phases formed in the melting of the mixture with initial composition Tio,mVo.4aa0supported this conclusion; it indicated that the inner portion had the approximate composition There is little possibility of YO. * Trigonal unit cell volume. nonstoichiometry in these solid solutions since the sesquioxides have narrow homogeneity ranges.
It seems quite remarkable that the mutual solubility of the TiO-VO system is so small, considering the identity in crystal symmetry of the parent structures and the fact that the lattice parameters change in the same direction whether TiO, is added t o VO, or vice versa. Here it must be kept in mind that X-ray data taken at room tempeature reflect the solid-liquid equilibrium of the Ti0,-VO, solution at its melting point, since the samples under study were quenched rapidly from the melt. As the X-ray diffraction patterns obtained for solid solutions of the general formula V,Til-,O (0.0 5 x 5 0.1 or 0.9 I x I 1.0) indicated the existence only of the NaC1 structure, it is quite likely that the quenching was sufficiently rapid in all cases to prevent significant formation of the low-temperature phase of TiO.6 Apparently, the disorder in the cation lattice is sufficiently different in the two compounds at elevated temperatures that a basic structural incompatibility precludes formation of a solid solution over the entire Ti-V compositional range. Figure 2 shows the results of standard four-probe measurements of electrical resistivity as a function of temperature for polycrystalline samples of Vo.06Tio,os0 and Tio.s5Vo.060 prepared in the arc furnace described above. Also shown in Figure 2 are literature values for the resistivity of VOI6 and TiO" as a function of temperature, as well as preliminary measurements on arccastlo polycrystalline VO. As can be seen from Figure (16) S. Kawano, K. Kosuge, and 9. Kachi, J. Phys. SOC.Japan, 21, 2744 (1966). (17) 8. P. Denker, J . A p p l . Phys., 37, 142 (1966). Volume '73, Number 6
June 1969
1784
R. E. LOEHMAN, C. X. R.RAO,AND J. M. HONIG
10-
c
E
g
10-
a
d
10e .
-------_
-------
f
Figure 2. p us. 1/T curves for TiO, VO, and their solid solutions at 5 mol %: a, Vo.s6Tio.ojO; b, VOl.oe; c, Tio.ssVo.orO;d, VOo.s7;lBe, arc-cast VO; f, TiO.1'
2 the slopes of the curves for VO and Vo,ssTio40s0 are essentially the same, the Vo.~~Tio.osO curve merely being shifted to higher resistivity than that for VO. Sim-
The Journal of Physical Chemistry
ilarly, the curve for Tio,s5Vo.o,0 has almost the same slope as that for T i 0 and is likewise shifted to higher resistivity values. The resistivity curve for VO reported by us is similar to that of Kawano, et al.,lB the shift to lower resistivity values probably being due to different methods of sample preparation. It should be noted that neither in this work nor in that of ref 16 did the VO under study exhibit the semiconduct,ing-metallic transition ascribed to it by Morin ; a it is possible that the samples investigated by him were heavily contaminated with VzO3. The upward shift without appreciable change in slope of the p us. T curve when T i 0 is added to VO or vice versa is quite reminiscent of similar shifts when Pd, Mn, or Pt is added in like amounts to pure Cu, Ag, or Au.'* It is likely that Ti in VO or V in Ti0 acts as an ionized impurity scattering center, thus raising the over-all resistivity of the samples without appreciably altering the band structure of the parent compound. The lattice of the oxide is stiffer than that of the group Ib metals; hence one would anticipate a somewhat larger cross section for and contribution from impurity scattering in the two oxides as compared with these metals, as is indeed observed. This explains in part why one obtains a greater resistivity even though charge carriers are injected into the parent oxide in amounts proportional to the impurity content. This effect also is consistent with an overlapping band structure model proposed by Ern and Switendi~k.'~
Acknowledgment. The authors are grateful to Dr. Margaret A. Wechter for performing the neutron activation analysis on the samples used in this work. (18) F. A. Otler, J. d p p l . Phya., 27, 197 (1956). (19) V. Ern and A. C. Switendick, Phys. Rev., 137A, 1927 (1966).
