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VANADIUM-VANADIUM BONDS IN THE
Ti1-xVxO2 SYSTEM
T. Hörlin, T. Niklewski, M. Nygren
To cite this version:
JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 37, Octobre 1976, page C4-69
VANADIUM-VANADIUM BONDS
IN
THE Ti,
-,Vx02 SYSTEM
T. HORLIN, T. NIKLEWSKI and M. NYGREN Department of Inorganic Chemistry, Arrhenius Laboratory,
University of Stockholm, S-104 05 Stockholm, Sweden
Resume. - Une ktude sur poudre du syst6me TiOpV02 a et6 conduite par ATD, diffraction X
et mesure de susceptibilite magnetique. Le diagramme de phase prQente indique la prksence de plusieurs phases de'structure rutile ou derivant de celle-ci. Les mesures magnetiques montrent que des liaisons vanadium-vanadium non magnetiques se forment a basse tempkrature sur l'ensemble du domaine de composition. La rupture de ces liaisons se produit dans un intervalle de temperature trop etroit pour 6tre explique uniquement par la presence &interactions antiferromagnetiques d'Heisenberg. L'interaction entre paires metal-mktal non magnktiques doit &tre envisagke pour expliquer 1'6volution de la susceptibilite magnetique avec la temperature.
Abstract. - Powder samples of the TiO2-V02 system have been studied by DTA, X-ray powder diffraction and magnetic susceptibility measurements. A phase diagram displaying several phases with rutile or rutile related structures is presented. The magnetic susceptibility measurements showed that nonmagnetic vanadium-vanadium bonds form at low temperatures over the entire range of composition. These bonds rupture in a temperature interval too narrow to be explained only
by antiferromagnetic Heisenberg interaction. Interaction between the nonmagnetic metal-metal pairs must be considered in order to expiain the temperature dependence of the magnetic suscepti- bility.
1. Introduction.
-
The system Ti0,-VO, has been studied since the mid-fifties by several investigators with respect to : (i) the metal-nonmetal transition of V 0 2 and how it is affected by substitution of vanadium ions by titanium ions, (ii) the magnetic properties and (iii) the relation between the c/a ratio and the number of valence electrons available for metal-metal bonds. In a previous paper [ l ] we treated only a narrow range of composition (Til-xV,O, with 1 x 0.94) and the experimental data were discussed in terms which are related to the first point of interest mentioned above. The results of DTA, X-ray powder diffraction and magnetic susceptibility measurements for the entire range of compositions are reported in this article with a particular emphasis paid to the second and third aspects.2. Experimental.
-
The starting materials were V205 (Fisher, Sc. Co., p. a.) and TiO, (Baker, Sc. Co., p. a.). VO, was prepared and characterized as described in ref. [l]. Powder samples of Ti,-,V,O, were obtained from thoroughly mixed powders of VO, and TiO, heated at 1 175 K for one week in evacuated silica tubes. The heat-treatment was terminated and the samples were allowed to cool inside the furnace with an approximate initial cooling rate of 10 K per minute. In order t o control the stoichiometry and to promote the rate of reaction a small amount of V 2 0 5 (< 0.5 weight percent) was added to the dioxide mixtures before the heat treatment. The excess of V,O, and anot too high cooling rate appear to be very important in order to achieve reproducible results. After the firing the dioxide phase was easily separated from the excess V 2 0 5 as the latter strongly adhered to the walls of the silica tubes.
The procedure described above assures the stoichio- metry to be a t the oxygen rich extreme of the dioxide phase area which is believed to be very close to the true dioxide.
2 . 1 DTA.
-
The measurements were performed in an apparatus which enables simultaneous analysis of five samples in the temperature range 100-600 K [2]. 2.2 X-RAY POWDER DIFFRACTION ANALYSIS.-
At ambient temperature the X-ray powder patterns were recorded in a Guinier-Hagg focusing camera, with CuKa, radiation. Above room temperature a powder diffractorneter with CuKa radiation was used. In both cases KC1 was used as internal standard. The thermal expansion of KC1 was taken account of using data from Glover [3]. The cell parameters were calculated with the least squares program Tetlin [4].2 . 3 MAGNETIC SUSCEPTIBILITY MEASUREMENT.
-
These measurements were performed according to the Faraday method using HgCo(SCN), as standard [ 5 ] . An equipment built a t this laboratory was used [6]. The measurements covered the temperature range 78-750 K. The magnetizing field was 0.7 X 106 A m-'(- 0.9 X 104 Oe).
C4-70 T. HORLIN, T. NIKLEW! SKI AND M. NYGREN
3. Phase analysis.
-
TiOz and VO, form solid solutions over the entire range of composition. The results of DTA and X-ray powder diffraction studies are presented as a phase diagram in figure 1. The fullFIG. 1.
-
Phase diagram of T ~ I - ~ V X O ~ .lines represent the starting points of endothermic reactions obtained for increasing temperatures and the broken lines represent the starting points of exothermic reactions when the temperature was decreased.
The M, + R, transition temperatures did not vary among different samples of the same composition but the M, + M, transition temperatures seemed to be very sensitive to even small changes of the procedure of preparation. Thus the presented data refer to six samples prepared at the same time and under the same conditions.
The heat exchange of the M, + R, transitions is of
comparable size to the latent heat of the metal-non metal transition of pure VO, while the heat exchange of the lower transition temperatures is approximately ten times less than that of the upper ones.
The transitions discussed so far are probably of the first order with well defined starting points and hyste- resis. The hatched area in figure I illustrates the increa- singly broader DTA peaks without well defined starting points observed for decreasing vanadium content. In the R, and R, phase areas the samples possess the rutile structure. Metallic behaviour is observed in the R, area whereas samples in R, area exhibit semi- conducting properties. Preliminary studies of the elec- trical conductivity indicate that the semiconducting R, phase continuously transform into a metallic RM phase around X = 0.75.
In the M, phase metal-metal pairs are found along the c axis of the pseudo rutile cell [7]. In the M, struc- ture half of the vanadium atoms form pairs while the other half are accommodated in zig-zag chains of equally spaced atoms also running parallel with the c, axis [S]. The M, phase area might be regarded as an extension of the M, phase area where some long range order has vanished. This interpretation is supported by : (i) the similarity of the X-ray powder patterns of the M, and M, phases, (ii) the magnetic susceptibility measurements described below and (iii) preliminary studies of the electrical conductivity.
Our cell parameter in the R, and M, phase areas are in agreement with those reported by Marin- der et al. [9, 101.
The temperature dependence of the cR/a, ratios of V 0 2 (from ref. [l l]), Tio.loV, .9002 and Tio.,,Vo.7502 respectively is outlined in figure 2. The corresponding
0 . 6 4 5
R X I R L R R T I O C / R V S . T E M P E R R T U R E / K
ratios of the distorted rutile phases are calculated as (C:/V,)'~~. As expected the curves of VO, and Ti,, ,,Vo ,,,O, exhibit discontinuities at the various phase transition temperatures. The sudden decrease of the axial ratio of Tio~,,Vo~,,O, around 380 K is, however, not associated with any appearent change in symmetry.
The decrease of the axial ratio of rutile related struc- ture of the M, type (e. g . VO,, MOO, and WOz) with the number of valence electrons available per metal- metal bond has been interpreted in terms of a streng- thening of the bonding force between the metal atoms within the pairs [93, [12]. From this point of view it is interesting to note that the axial ratios of the semi- conducting (Ti, V)O, samples with M, structure are larger than those for the metallic ones with rutile structures. MOO, and WO, are also metallic which suggests that the above discussed variation of the axial ratios partly can be associated to the occurrence of metallic bondings in these compounds.
4. Magnetic Properties. - The molar magnetic susceptibility data, converted to squared Bohr magne- ton numbers per metal ion according to the expression
are represented in figure 3a and b. Here p, = Bohr magneton and p. = l in cgs emu = 4 n X 10-' in
S. I. units.
For X > 0.90 in Ti, -,V,O, sudden jumps in the values are observed at the M, + M, and M2 + R, transition temperatures. A more continuous increase
of the ,U& values is observed when passing through the
VANADIUM-VANADIUM BONDS IN THE Til-xVx02 SYSTEM C4-71
/
SOURRED BOHR t l R i N E T D N NUMBERl
Rs phase area and an anomaly is still observed at a vanadium content as low as x = 0.10.
The plots of CL$f versus temperature are linear at lower temperatures. The values extrapolated to T = 0 K and those obtained at 700 K are plotted versus x in figure 4. The latter numbers are proportional to the vanadium content up to X N- 0.80. The contri-
bution from temperature independent paramagnetism (TIP) to the observed ,U& values a t 700 K can be
estimated from the slopes of the linear parts of the curves in figures 3a and b. The values thus corrected for TIP are in good agreement with the expected spin only values except for compositions close to x = 1 where the magnetic moment per metal atom is smaller than that for a spin independent doublet indicating the onset of Pauli paramagnetism.
The magnetic properties of the M, and M, phases are discussed in ref. [l]. In the
M ,
phase area spin pairing occurs at every second site between two adja- cent vanadium ions. The results described above3.0- 2.5. 2 . 0 - 1 . 5 . 3.0 - SQURRED BOHR M R G N E T O F TI 1 - ~ V ~ 0 2 V S : X 2.5 - AT 700 K
-
0 2 . 0 - .F c - < , -, c < - " <,,<. .-'c .. -SBURREO BOHR MRGNEION NUMBER ..?-
'
/- ,:
-
. . V X T ~ I - X O Z ,.'A? . ' _I .' .A<</ , " .&'. - -4
, - ' I ,- ,..
suggest that spin pairing also occurs between adjacent vanadium atoms at lower temperatures in the
R,
phase area.The 0 K curve in figure 4 may be discussed in terms of two models of spin pairing. The first model is based on the assumption that the metal atoms are randomly dispersed over the metal sites of the rutile structure and that antiferromagnetic interaction occurs only between nearest neighbour vanadium ions in the chains of vanadium ions running in the c, direction. The ground state of chains with an even number of vanadium ions, terminated at both ends by titanium ions, is a non- magnetic spin singlet whereas the ground state of chains with an odd number of vanadium ions is a paramagnetic spin doublet. The probability that a certain site is occupied by a vanadium ion, which is a member of a chain of n vanadium ions terminated by titanium ions, is n(l
-
x)2 xn where x is the concentra- tion of vanadium ions. The factor n is included as the vanadium ion can take any of the n positions within the chain and random dispersion is introduced by the multiplication of probabilities. The average number of spin doublets per metal atom is calculated by summing the probabilities of finding chains with odd numbers of vanadium ions. Each term in this summation is divided by n as there are only lln doublet states per vanadium atom in a chain containing n vanadium ions. Thus the average number of spin doublets per metal atom, according to this model is'SKI AND M. NYGREN
pairs. The second model is based on the occurrence of a potential, p, with a period twice that of the rutile c axis, inducing a dimerization of the randomly dispersed metal atoms as illustrated in figure 5. Chains with even
numbers of vanadium ions are in this case split into two types according to figure 5A and B. The energy diffe- rence between the spin singlet ground state and the first excited spin triplet state is larger for chains of type A
than that for type B. Furthermore it is assumed that cp
is strong enough to make the singlet and triplet states practically degenerate for chains of type B. Thus the latter type contains two paramagnetic spin doublets per chain while type A is non magnetic. If type A and B are present in equal proportions and as the number of spin doublets per chain with odd numbers of vanadium ions in this case is the same as in the first model one can assodate one spin doublet to each chain of vanadium ions. Thus the average number of spin doublets per metal atom according to this model is
The g factors are obtained from a linear combination of the mean values of the g factors of TiO, and V 0 2 [13].
The second model which represents the case when spin pairing only can occur at every second site between two adjacent vanadium ions as in the M, structure is obviously appropriate for higher vanadium contents. The first model, where spin pairing can occur between any pairs of adjacent vanadium ions, is applicable for low vanadium concentrations whereas in the inter- mediate range restrictions in the spin pairing seem to be present.
These results clearly show that non magnetic vana- dium-vanadium bonds are present at low temperatures over the entire range of composition. Figures 3a and b thus illustrate the rupture of these bonds with increas- ing temperature. If no interaction is present between the non magnetic metal-metal pairs the temperature dependence of the with respect to the rupture of these bonds is described by
where 2 J is the energy difference between the singlet and triplet state.
The presence of an interaction between the pairs of the type described in connection with the second model of spin pairing can be accounted for by allowing J to vary with the temperature as
The number of spin doublets per metal atom accord- ing to these two models of spin pairing is plotted versus the composition X in figure 6. The experimental num-
NUMBER OF S P I N DOUBLET GROUND S T R T E S P E R M E T A L I O N I N T l l _ x V X O 2 V S : X
3 x N
N - 1
+
expI
- 2 J o / k T I ' (7) The experimental data are least squares fitted to the following equationC
X M = 1 + C 2 .
N
T T
N
-
1+
expI
- 2 J,/kTI
+
Xo (8)bers of spin doublets per metal atom represented as rings in the figure are obtained from the ,u:ff values
at 0 K by
The temperature dependence of J must be non linear in order to prevent the occurence of ferromagnetic interaction at higher temperatures. In equation (6) J increases linearly with the temperature in the low temperature limit while at higher temperatures J approaches a constant value. The temperature depen- dence of p$f now becomss
where the first term is associated to the spin doublets, the second term as above and the third term accounts for TIP.
The fit of the experimental data to equation (8) is good for X < 0.30. At X = 0.30 and in the intermediary temperature region systematic deviations start to appear. These deviations increase with increasing x
and clearly approach the behaviour of the versus T curves in figure 3b where the rupture of the bonds takes place in two steps via the M, or M, phase area.
Log N and -, J,, are plotted versus x in figures 7
VANADIUM-VANADIUM BONDS I N THE Til-,VsOz SYSTEM C4-73
corresponding J, value is -- 300
k,
K. The latternumber is of the same order of magnitude as found in the M, phase [l]. This thus corresponds to the case when antiferromagnetic Heisenberg interaction is pre- sent within the pairs but no interaction between the pairs occurs. As N increases with x interactions between the pairs have to be present over the entire range of composition. The nature of this interaction has been discussed above in connection with second model of spin pairing.
Riidorf et al. [l41 and Ariya and Grossmann 1151
have also studied the Ti0,-VO, system by means of X-ray and magnetic susceptibility measurements. They
have not observed the M, and M4 phase areas and report the M, -+ RM transition temperature to decrease slightly with increasing titanium content. This is not in agreement with our findings. This discrepancy is most probably due to the different preparation techniques. Thus we have observed that samples, deliberately made substoichiometric, exhibit similar properties as those reported in ref. [l41 and [15].
3.0
2.5
2 . 0
Acknowledgements. - The authors wish to express their gratitude to Professor A. MagnCli and Dr. L. Kihl- borg for their interest in this work.
This investigation has been sponsored by the Swedish Natural Science Research Council.
FIG. S. FIG. 7. 0 - LOG [ N I V S . X - 0 0 1400 l000 BOO 600 1100 200 0 References 1.5 - ' 4 I I I I I 0.5 D 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 - - J K - ~ / K V S . X 0 - 0 0
-
.-
-
l I , I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7[l] HORLIN, T., NIKLEWSKI, T. and NYGREN, M., Acta Chem. [91 MARINDER, B. 0. and MAGNBLI, A., Acta Chem. Scand. 11
Scand., in press. (1957) 1635.
[2] HORLIN, T., NIKLEWSKI, % T. and NYGREN, M., Chem. L101 MARINDER, B. 0. and FRIBERG, M., Final ~echnical Report 1
Commun. Univ. Stockholm No. 9 (1975). (DA-91-591-EUC-1319) Univ. Stockholm 1960. [3] GLOVER, R. E., 2. Phys. 138 (1954) 222. [l11 RAO, K. V. K., NAIDA, S. V. N. and IYENGAR, L., J. Phys.
[4] HORLIN, T., to be published. Soc. Japan 23 (1967) 1380.
[l21 ROGERS, D. B., SHANNON, R. D., SLEIGH^, A. W. and [5] KIZHAEV, S. A., USACHEV, P. V. and YUDIN, V. M., Fiz.
Tverd. Tela 13 (1971) 2829. GILLSON, J. L., Inorg. Chem. 8 (1969) 841. 1131 SHIMIZU, T., J. Phys. Soc. Japan 23 (1967) 848. [6] BLOM, B. and HORLIN, T., to be published. [l41 R ~ O R F , W., WALTER, G. and STADLER, S., Z. Anorg. Allg. [7] ANDERSSON, G., Acta Chem. Scand. 10 (1956) 623. Chem. 297 (1958) 1.
181 MAREZIO, M., MCWHAN, D. B., REMEIKA, J. P. and DER- [l51 ARIYA, S. M. and GROSSMANN, G., SOV. P&. Solid State 2
