METAL-INSULATOR TRANSITION IN (Ti1-xVx)2O3 : IMPURITY BAND CONDUCTION AND SPIN GLASS PROPERTIES

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METAL-INSULATOR TRANSITION IN

(Ti1-xVx)2O3 : IMPURITY BAND CONDUCTION

AND SPIN GLASS PROPERTIES

J. Dumas, C. Schlenker

To cite this version:

J. Dumas, C. Schlenker. METAL-INSULATOR TRANSITION IN (Ti1-xVx)2O3 : IMPURITY

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JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 37, Octobre 1976, page C4-41

METAL-INSULATOR TRANSITION IN (Ti, -,V,),O,

:

IMPURITY BAND CONDUCTION AND SPIN GLASS PROPERTIES

J. DUMAS and C. SCHLENKER

Groupe des Transitions de Phases, Centre National de la Recherche Scientifique B. P. 166,38042 Grenoble Cedex, France

Resumb. - Des travaux anthieurs ont montre que l'incorporation de vanadium dans Ti209 induit la phase metallique pour des concentrations en V superieures B quelques % at. Nous prtsen- tons des mesures de resistivite electrique entre 4,2 K et 300 K pour 0 C X < 7,5 % et montrons qu'il y a a 4,2 K une transition a un Btat quasi metallique pour X

-

1 %. Pour X < 1 %, les proprietes de transport a basse temperature peuvent 6tre decrites par des lois en T-114 correspondant B

des sauts des 6lectrons a distance variable sur des niveaux Ti4+. On montre que la conductivitk metallique pour X

-

1 % est due a une bande d'impuretks construite sur ces niveaux Ti4+. La suscep- tibilite magnetique est paramagnktique dans la gamme de concentrations exploree avec un compor- tement de Curie-Weiss et un moment magnktique effectif allant de 3,8 @ B a 1,8 @B lorsque X aug-

mente de 0,l % B 7,s %. Une etude magnktique a basse tempkrature etablit que (Ti1-~V~)203 est un verre de spin avec des temperatures de blocage atteignant 2,8 K pour X = 7,s %. Nous demon- trons que la contribution a la chaleur specifique basse temp6rature ACp = ET, obtenue dans une etude anterieure, doit 6tre attribuke aux proprietes de verre de spin. Nous proposons aussi des schemas de bandes d'energie qui sont en accord avec les proprietes Blectriques et magnetiques.

Abstract. - Vanadium in TizO3 has been shown previously to induce the metallic phase for V concentrations larger than a few at %. We report electrical resistivity data obtained between 4.2 K

and 300 K for 0 i X C 7.5 % and show that at 4.2 K there is a transition to a quasi-metallic state

for X

-

1 %. For X < 1 %, the transport properties at low temperatures can be described by a

T-114 law related to avariable range hopping of the electrons on Ti4+ levels. The metallic conducti-

vity for X

-

1 % is shown to take place in the impurity band built on these Ti4+ levels. The magnetic

susceptibility is found to be paramagnetic in all the explored concentration range with a Curie-Weiss behaviour and effective magnetic moments per V ion decreasing from 3.8 LLB to 1.8 @B when X is increased. Magnetic measurements at low temperatures show a spin glass behaviour with freezing temperatures ranging from 0.3 K to 2.8 K depending on X. We establish that the contribution linear in temperature previously reported for the low temperature specific heat has to be attributed to the spin glass properties. We also propose energy band schemes which account for both the electrical and magnetic data.

1. Introduction.

-

Titanium sesquioxide Ti203 and

the mixed compounds (Til-,VX),O3 have been the object of a considerable amount of work these last years. Previous authors have established the existence of an insulator-metal transition versus temperature in pure Ti,O, and have proposed a band overlap mechanism for this tramition [l]. They have also shown that the incorporation of Vanadium in Ti203 stabilizes the metallic phase at all temperatures for V concentrations X larger than 2 to 4

%

[2] and have

proposed several mechanisms to account for these results [3,4]. Although a large number of data, such as thermoelectric power [5], elastic constants [6] Raman scattering [7], specific heat [3], etc., had been obtained, the magnetic properties of these compounds had been completely ignored by these authors.

We want to point out in this paper two main aspects unknown in the previous works. First, the presence of V gives rise not only to metallic conductivity but also to magnetic moments [8] and any reasonable model for (Ti,-,VX),O3 must take in.to account these properties.

Secondly, there is a position disorder for the V ions in the Ti,03 matrix which is responsible for several classes of data. For the magnetic properties, this disorder leads a t low temperatures, for x > 0.5

%

to remarkable spin glass properties [9], just as in dilute alloys of transition metals in noble metals [10]. The transport properties are also sensitive to this disorder and we shall show that a t low concentrations (X < 0.5

%)

and low temperatures, they are governed by a variable range hopping of the electrons which gives rise to the well-known exp

-

(T,/T)lJ4

law [l l]. We shall also establish in this work that the transport properties of (Ti, -,V,),O, versus V concentration must be described by two diffetent mechanisms. At low concentrations (X

-

1

%)

there is a first transition to a degenerate quasi-metallic state related to a n impurity band conduction, while the true band overlap transition takes place for larger V concentrations.

The properties of pure Ti,03 are mainly determined by the presence of the Ti3+ ions with a 3d1 electronic configuration. It is now well-known that in the transi-

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C4-42 J. DUMAS AND C. SCHLENKER

tion metal compounds with 3d1 ions, for example VO, (121, Ti40, [13], there is a strong tendency for the cations to form covalent bonds, such as Ti3'-Ti3+. The ground* state of these bonds with two paired 3d-electrons is a non-magnetic singlet spin state. In the case .of Ti,03, the formation of these bonds is stikl made easier by the rhomboedral corundum structure which contains pairs of ~i~ + in octahedral sites along the c-axis [14]. In the semiconducting low-temperature phase of Ti203, the covalency with the oxygens broa- dens this singlet ground state into a band and the electrical properties are well-described by a band structure including for the 3d states a lowest filled a,, bonding band, with 3d orbitals metal-metal bonding along the c-axis and an upper empty e, band, with orbitals lying mainly in the basal plane of the corundum structure [l]. The semiconductor to metal transition takes place in the temperature range of 400 K-500 K and is due to a gradual reduction of the bandgap between the a,, and e, bands. This band overlap has been shown to be related to an anomalous increase of the c/a ratio of the lattice parameters [IS].

The electrical conductivity of (Ti, -xVx)203 has been reported for 0 < X < 10

%

in the temperature range of 80 K to 600 K [2] ; the presence of V increases the conductivity below 400 K and reduces the size of the electrical transition without changing its temperature range ; no electrical transition is observed for

X > 5

%.

The c/a ratio of the lattice parameters

increases also with x ; these transport properties have therefore been related to a shrinking of the intrinsic Ti203 energy gap [2]. The specific heat, measured at low temperature for 0.02

<

X < 0.10, shows an ano- malously large linear term which saturates at temperatures of the order of a few degrees 131. These data have been attributed to electronic properties in terms of an extremely narrow V impurity band falling at the Fermi level E, giving rise to avery large density of states at E, and a very small Fermi temperature 141. The V ions were supposed to be non magnetic in that model. In section 2 of this paper, we describe the experi- mental procedure. In section 3, we report new electrical conductivity data in the temperature range 4.2 K to 300 K for 0 < X

<

7.5

%.

Section4 is devoted to the magnetic properties including spin glass data at low temperature. We shall show that the low temperature linear term in the specific heat must be attributed to the spin glass properties and that the model of reference [4] is incorrect. In section 5, we shall discuss all these results and present models and band schemes which account for both the electrical and magnetic properties.

2. Experimental procedure.

-

The single crystals used in this investigation were grown by the Czo- chralsky method in a commercial triarc furnace by means of a pulling rod equipped with a heat pipe. Ti203 powder was first prepared by mixing Ti metal and TiO, rutile powders, V 2 0 3 was obtained by,reduc-

tion under hydrogen from V,05 at 700 O C for three

days. The desired amounts of V,03 and Ti,O, powders

were then mixed, pressed and arc-melted. The V

concentration in the final products was determined by atomic spectroscopy analysis ; the accuracy attained

was 5

X.

The crystals were characterized by X-ray technique. The absence of any other compound such as V,03 or Ti305 was controlled by specific heat analysis with a commercial differential scanning calorimeter ; V203 and Ti305 show first order phase transitions at 150 K and 450 K respectively and are easily detected by thermal analysis.

The electrical conductivity measurements were -performed between 4.2 K and 300 K with the usual four probes techniques. The magnetic susceptibility was measured in the same temperature range with a vibrat- ing sample magnetometer in a magnetic field of 9.6 kG.

For the spin glass properties, the susceptibility and the remanent magnetizations were measured between 0.05 K and 4.2 K by the extraction method in a helium cryostat ; the lowest temperatures were obtained by adiabatic demagnetization [16].

3. Electrical properties. - 3.1 EXPERIMENTAL

RESULTS. - Figure 1 shows the resistivity versus tempe-

FIG. l.

-

Electrical resistivity vs temperature for pure and vanadium doped Tiz03.

rature for samples with 0

<

X

<

7.5

%,

in the temperature range 4.2 K-300 K. The data are roughly in agreement with those of reference [2], obtained for T > 80 K. In the case of pure Ti,03, a comparison with the data of reference [ l ] seems to indicate that the stoichiometry of our samples corresponds to the for-

mula TiO,.,,,. This stoichiometry defect shows that

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METAL-INSULATOR TRANSITION IN (Til,Vz)2 0 3 : IMPURITY BAND CONDUCTION C4-43

FIG. 2. - Resistivity vs vanadium concentration ; the insert shows the variation of the cla ratio of the lattice parameters vs

vanadium concentration (data obtained on powder).

cannot be interpreted in terms of a well-defined acti- vation energy. The presence of Ti vacancies in pure Ti203 and of defects related to the presence of V

in (Ti,-xV3,0, suggest that the low temperature

conductivity could be due to some hopping of the electrons between the defects centers ; the disorder of these centers could then lead to the law :

114

a = A exp -

g)

characteristic of a variable range hopping [l?]. Figure 3 shows the curves of the resistivity at low temperatures plotted versus T - ' / ~ . The T-'I4 law is found to be well obeyed for pure Ti,03 in the temperature interval 11 K-120 K and for X = 0.18

%

and 0.5

%

for 5 K

<

T < 30 to 40 K. The decrease of To

FIG. 3. - loglo p vs T-114 for pure and V- doped Ti203.

with X (Table I) is coherent with an increase of the

defects centers concentration with X.

Values of the parameter To of the variable range

114

hopping law a = A exp

(-2)

-

at low temperature and of the activation energy E, for the colzductivity

for 100 K

<

T

<

300 K X To (K) Ea(meV)

-

- - 0 60 000 18.1 0,18

%

6 600 14.1 0,s

%

800 7.0 0,7

%

4.9

In the high temperature range, figure 4 shows that for X

5

1

%,

the conductivity is thermally activated, with activation energies of the order of 10 to 20 meV.

FIG. 4. - log10 p vs lO3/T for pure and V- doped Tiz03.

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C444 J. DUMAS AND C. SCHLENKER

FIG. 5.

-

Energy level schemes for (Ti1-~V~)203. a) X < 0.5 %

at V. b) 1 % < x < 3 % . c ) 3 % < x < 1 0 % .

corresponds to the distance between the top of this band and the ~ i levels ~ ' ; the conductivity and the thermoelectric power are therefore p-type as experi- mentally observed [l]. Our model differs from that given in reference [5] ; we feel that below 300 K, the conductivity data do not give any accurate informa- tion on the intrinsic energy gap E, between the a,, and e, bands.

In (Ti, ...,V,),O3 for small x (c 0.5

X),

the magnetic data show that the V ions are in V2' states (see Sec- tion 4) ; for the electric neutrality, each V2' has to be associated with a Ti4+ ion ; the concentration of Ti4+ centers is therefore increased compared to pure Ti203 ;

the hopping

T-

'l4 law corresponds to a higher density of states, as suggested by the smaller experimental values of To. At higher temperatures, due to the increased dispersion of the Ti4+ levels, the activation energy is found smaller than in pure Ti,03.

For larger X

( X

l X), the system becomes quasi-

metallic as the Ti4' levels now form a partially filled band which merges into the a,, band (Fig. 5b). The conductivity then takes place essentially inside this band. Let us point out that in this concentration range, the conductivity is not related to any band overlap between the a,, and e, bands ; the energy gap between these bands should be approximately the same as in pure Ti203 at low temperatures. A progressive band overlap can only account for the soft increase of conductivity versus x at higher concentrations ( X

-

2

to 4 X), as shown by the variation of the c/a ratio versus X, which takes place essentially for large x

(Fig. 2, insert).

4. Magnetic properties.

-

4 . 1 SUSCEPTIBILITY DATA FOR 4 . 2 K

<

T < 300 K. - The magnetic susceptibility of pure Ti203 is small and temperature-

independent below 450 K (X

-

10-4 emu/mole). We

have previously reported that it is anisotropic, the value obtained with the magnetic field parallel to the c-axis being smaller than in the perpendiculer case [18].

These data showed that the susceptibility of pure Ti,O,

is due to Van Vleck paramagnetism, mainly through the matrix elements connecting the a,, states to the e, states.

The magnetic susceptibility of (Ti,

-

,V,),O3 powder is plotted versus temperature on figure 6 for several x.

FIG. 6. - Molar magnetic susceptibility X as a fonction of temperature for different concentration of vanadium (data

obtained on powder).

The data show a Curie-Weiss behaviour and may be fitted by the law :

xo

is a temperature-independent term obtained by the high temperature data ; it may include a Van Vleck and a Pauli contribution. Figure 7 shows the curves. of (X

-

xo)-' versus T, from which one can deduce the

T (K)

FIG. 7.

-

Inverse of molar susceptibility 1/(x -XO) as a fonc-

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METAL-INSULATOR TRANSITION IN ( T ~ I - Z V ~ ) ~ 0 3 : IMPURITY BAND CONDUCTION C4-45

Curie constants C. The effective magnetic moment

per V atom peff, calculated from the values of C is plotted versus X on figure 8. For x < 0.02, p,,, is

decreasing from 3.80 pB, very close to the spin-only value for S = 312 to 1.8 pB very close to the spin only value for S = 112. The curve shows an anomaly for 0.02

<

X < 0.04. At higher concentrations, p,, =ppB

keeps a constant value of 1.8 pB.

FIG. 8. - Effective magnetic moment per vanadium atom as a fonction of vanadium concentration (data obtained on powder).

N. M. R. measurements performed on a powder

with x = 0.10 on the V nuclei show a Knightshift decreasing with temperature from - 4.6

%

at 25 OC to

-

2.7

%

a t 100 OC [19]. This Curie-Weiss type behaviour indicates that the magnetic moments are related to the V ions. The N. M.

R.

linewidth was found to be of the order of 100 G, also decreasing with temperature, showing that the relaxation is mainly due to interactions between the nuclear spins and the V

moments are hrogressively frozen in random directions below a critical temperature TB. It is now well known that in this case, the initial reversible magnetic susceptibility (measured in small a. c. or d. c. fields), plotted versus temperature, shows a peak at the temperature TB

When a spin glass system is cooled in a magnetic field H from a temperature larger than the freezing temperature TB, down to a temperature T

<

TB, it acquires a socalled thermoremanent magnetization (T. R. M.), which depends on H. It also shows an isothermal remanent magnetization (I. R. M.) obtained when the field H is applied at the temperature T

<

TB and then suppressed [10]. One also knows that the magnetic contribution to the specific heat of a spin glass is linear versus temperature at low temperatures and shows a bump at a temperature

T,

usually larger than

TB

[20].

The initial reversible susceptibility of (Ti, -xVx)203 measured in low d. c. fields (< 20 Oe) roughly perpendicular to the c-axis, is plotted versus temperature on figure 9 for several V concentrations X > 0.7

%

[21].

magnetic moments.

One can conclude from these results that in FIG. 9. --Initial _{different V concentrations (single crystals).}reversible susceptibility vs temperature for (Ti, -,V,)203, there are magnetic moments associated

to the V ions. In the metallic phase, these moments may

be well described in terms of a V virtual bound state at The x ( ~ ) are characteristic of a glass-

the Fermi level, with a width small enough for the local Figure 10 shows typical data obtained for the T.

R.

M.

condition of magnetism to be fulfilled. versus the field H applied during the cooling process.

4.2 SPIN GLASS PROPERTIES.

-

We have shown in f i section 3 that for X

2

1

%,

the electrical behaviour is

quasi-metallic with a non-zero density of states at the Fermi level. One then expects exchange interactions to take place between the magnetic moments through the conduction electrons.

These exchange interactions are of the Rudermann-

₁₅

Kittel-Kasuya-Yoshida (RKKY) type. They decrease

"

as the third power of the distance r between the magne-

bL

tic impurities, if one omits oscillations of short wave length. In the case of dilute alloys of transition metals in noble metals, such as AuFe, these properties lead to

spin glass behaviour at low temperatures [IQ]. A spin

0

1 _H(kOe)

2

3

is a system as a 'Onsequence of the _FIG._10._-_{Thermoremanent (T.}_R._{M.) and isother&} _rema-

exchange interactions between the moments and of the _{nent ,ametization} _0._R. _{vs magnetic field, for}

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C4-46 J. DUMAS AND C. SCHLENKER The I. R. M. is also shown on figure 10. The T. R. M.

and I. R. M. depend on H only for low fields, as usually in spin glass systems ; they also reach the same saturation values for a field H of the order of 1 to

2 kOe. These last data corroborate that (Ti,-xVx)20,

at low temperature is a spin glass system.

The linear term in the low temperature specific heat of (Ti, -,Vx)203 should then include both a magnetic contribution related to the spin glass properties and a purely electronic contribution. The macroscopic properties of a spin glass system may be calculated in a random molecular field model [22]. The model involves

a distribution P(H,, T) for the molecular field H, on

an impurity site at the temperature T. The value of the reversible susceptibility x(T) is then found to be at 0 K and for small concentrations :

where Nx is the number of magnetic impurities, S their spin and g the Lande factor.

In the same model, the slope of the linear term of the specific heat is expected to be :

where k is the Boltzman constant.

In dilute alloys, conduction electrons are provided by the noble metal matrix and their density does not depend on the concentration of the magnetic impurities. In this case, due to the variation of the magnetic interactions with the third power of the distance between the impurities, xP(0,O) should not depend on X

at low concentrations. In the case of (Ti,-,Vx)203, both the density of states at the Fermi level and the V magneti moment depend on the concentration and this " scaling " law should not be obeyed. One also knows from the high temperature susceptibility data which show ferromagnetic Curie temperatures, that even for small X, ferromagnetic interactions are predominant in (Ti,-xV,)z03 ; the average ~nolecular field at an impurity site may be non zero and the previous model may be a crude approximation. Figure 11 shows our data obtained for ~ ( 0 ) versus X and the experimental

slopes a of the specific heat linear term previously obtained by Sjostrand and Keesom [3]. It is clear that both ~ ( 0 ) and a depend on x even for small X. One may

however extrapolate the value of ~ ( 0 ) a t zero concentration and calculate the expected value a, of a in that limit ; a, (shown on figure 11) is found to be very near

the experimental value obtained for small X. One may

then conclude that the linear term in the specific heat is mainly due to the spin glass properties, the electronic contribution being an order of magnitude smaller. One may also notice that the curves obtained by Sjostrand and Keesom for AC,(T) are very similar to the data recently reported for CuMn dilute alloys [20], where AC, shows a very round maximum a t a temperature larger than the freezing temperature.

FIG. l l. - Initial reversible susceptibility extrapolated a t

T = 0 K ~ ( 0 ) and linear term of the specific heat ACp/T as a

fonction of vanadium concentration. The value of AC,/T cal- culated in a random molecular field model from the value of ~ ( 0 )

extrapolated at x = 0 is also shown.

Therefore in the case of (Ti, -xVx)z03 the maximum in AC, has also to be attributed to the magnetic proper- ties and it is excluded that the temperature of this maximum is a Fermi temperature for the electron gas, as it had been proposed previously [4].

It is useful in that context to evaluate the electronic contribution to AC, from the high temperature magnetic susceptibility X,. Above 300 K, X, is temperature independent for X larger than a few percents ; it is due

both to a Van Vleck paramagnetism and to Pauli paramagnetism. As

X,

is found to be of the order of 10-4 emu/mole [8], one obtains an upper estimation of the electronic specific heat slope in a free electron

model :

y < 10 mJ/mole

.

KZ

which is much smaller than the experimental value of AC,/T.

One may therefore state that the electronic contribution to the low temperature excess specific heat has the usual order of magnitude (< 10 mJ/mole.K). The Fermi temperature cannot be obtained from the low temperature specific heat data and should also have a normal value, several orders of magnitude larger than that of the bump of the excess specific heat.

5. Discussion. - This discussion will try to correlate the magnetio with the transport properties and to account for the variation of the magnetic moment versus the V concentration.

5.1 SMALL VANADIUM CONCENTRATION (X

<

1

X).

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METAL-INSULATOR TRANSITION IN ( T ~ I - ~ V ~ ) ~ O3 : IMPURITY BAND CONDUCTION C4-47

observed in (Ti,-,V,)40, (X < 0.5

%)

an E. P. R.

signal which has to be attributed to V2+ centers. In a free atom model, it is clear that a V2+-Ti4+ configuration has an energy higher than a V3+ -Ti3+ one. One

does not know which mechanism stabilizes a V2 + -Ti4+

state in a crystal at small V concentration ; the local distortion around the V and a strong electron phonon coupling could be responsible for this effect.

The ground state of the V2+ ion is expected to be an orbital singlet as in other compounds with the corundum structure [23] and the Hund's rule leads to a spin 312 and an effective magnetic moment of 3.87 p,.

Figure 5a shows the energy diagram corresponding to this case. As already proposed in section 3, the Ti4+ levels are located in the gap between the a,, and e, bands. The

v2+

levels fall very likely in the filled a,, band.

5.2 INTERMEDIATE V CONCENTRATIONS (l

%

< X

3

%).

-

When X is increased the effective magnetic moment decreases and a t the same time the system goes to metallic conductivity. One may explain these results by the following picture : it becomes possible for the Ti4+ holes which are trapped near the V centers a t very low concentrations, to wander around the V at higher X, when the lattice distortion caused by the V is spread out. In this process, the V2+ ions are destabilized and the system has a tendency to form Ti3 + -V3+ pairs.

The bond of these molecules is due to the two paired electrons in a,, orbitals of the Ti and the V ions. The V e, orbital state is then responsible for the magnetic moment and the resulting spin is 112.

The band model corresponding to this picture is shown on figure 5b. In this situation, the magnetic moments are due to a V virtual bound state falling near the Fermi level and decoupled into two states with opposite spins through the Coulomb repulsion.

As the density of states at the Fermi level g(EF) is non zero, exchange interactions take place between the

magnetic moments ; at low temperatures, they become

predominant and lead to the spin glass regime. One should notice a remarkable property : the R. K. K. Y. interactions take place in this case in a nearly filled band and are related to a hole conductivity.

known from previous work that there is a progressive overlap of the a,, and e, bands of pure Ti203 [2]. We shall not discuss here in details the origin of this overlap, which could be due to a strong electron phonon coupling. At large V concentrations, the conductivity takes place in an unfilled a,,-e, band (figure 5c). In an ionic picture, the V ions are in V3 + (3d2) states,

with one electron which participates..to the chemical bond with the Ti3 + ions and which is described by the a,,-e, band ; the second electron of the V3 + is in an e,

orbital state and is responsible for the magnetic moment ; it gives rise, as in the case of intermediate V concentrations, to a decoupled virtual bound state and to R. K. K. Y. exchange interactions related to the non zero density of states at the Fermi level falling inside the a,,-e, band. It should be pointed out that this system is quite different from the usual dilute alloys where the conduction electrons are S electrons. I n our

case, the electrons responsible for both the transport and the magnetism are 3d electrons, but with partly different orbital states.

6. Conclusion. - One of the most interesting feature of (Ti, -,V,),O, is that Vanadium in this system induces both the metallic state and magnetic moments. We have shown that, for intermediate V concentrations, a model of an impurity band related to ~ iacceptor states accounts for both the electrical ~ + and magnetic data.

We have also established that this system is a t low temperature a spin glass and that the anomaly in the low temperature specific heat is due to this fact. As far as we are aware, it is the first time that such properties are reported for a transition metal oxide. This work should open the way to similar studies in other transition metal or rare earth chalcogenides.

Acknowledgments.

-

The authors wish to thank Sir N. F. Mott, $D,. Kaplan, A. Zylbersztjen and B. K. Chakraverty for extremely helpful discussions. They are specially grateful to J. L. Tholence and

R. Tournier for their arnica1 collaboration for the spin

glass work and to M. Minier for communicating the

NMR data prior to publication. They also thank

5.3 LARGE V CONCENTRATIONS (3

%

< X

<

10

X).

R. Buder for technical help, J. Devenyi and J. Mercier

- When the V concentration is increased, it is well for growing, orienting and cutting the crystals.

References

[l] HONIG, J. M. and REED, T. B., Phys. Rev. 174 (1968) 1020.

[2] CHANDRASHEKHAR, G. V., WON CHOI, Q., MOYO, J. and HONIG;J. M., Matev. Res. Bull. 5 (1970) 999.

[3] SJOSTRAND, M. E. and KEESOM, P. H., Phys. Rev. B 7 (1973) 3558.

[4] VAN ZANDT, L. L., Phys. Rev. Lett. 31 (1973) 598.

[5] SHIN, S. H., CHANDRASHEKHAR, G. V., LOEHMAN, R. E. and HONIG, J. M., Phys. Rev. B 8 (1973) 1364.

[6] BENNETT, J. G. and SLADEK, R. J., Solid State Commun. 18

(1976) 1055.

[7] SHIN, S. H., AGGARWAL, R. L., LAX, B. and HONIG, J. M.

Phys. Rev. B 9 (1974) 583.

[B] DUMAS, J., SCHLENKER, C. and NATOLI, R. C., Solid State

Commun. 16 (1975) 493.

[9] DUMAS, J., SCHLENKER, C., THOLENCE, J. L. and TOUR- NIER, R., (a) Solid State Commun. 17 (1975) 1215 and

(b) Conf. on Magn. and Magn. Mat., Philadelphia (dCc. 75), GIP Proc. 29 (1976) 43 1.

[l01 THOLENCE, J. L. and TOURNIER, R., J. Physique Colloq. 35

(9)

C4-48 J. DUMAS AND C. SCHLENKER [l11 MOTT, N. F., Metal-Insulator Transitions (Taylor and

Francis Ltd) 1974, p. 35.

[l31 LAKKIS, S., SCHLENKER, C., CHAKRAVERN, B. K., BUDER, R.

and MAREZIO, M., Phys. Rev. B 14 (1976) 1429. LAKKIS, S., T h b e de Doctorat d'Etat, Universite Scien- tifique et Medicale de Grenoble (1975).

[l41 NEWNHAM, R. E. and DE HAAN, Y., 2. Kristall. 117 (1962) 235.

[l51 RAO, C. N. R., LOEHMAN, R. E. and HONIG, J. M., Phys.

Lett. 27A (1968) 271.

1161 THOLENCE, J. L., These de Doctorat d7Etat, Universite Scientifique et MCdicale de Grenoble (1973).

1171 MOTT, N. F., Adv. Phys. 21 (1972) 785.

[l81 SCHLENKER, C., DUMLS, J., BUDER, R., WAKSMANN, B., ADLER, D., SHIN, S. H. and REED, T. B., Proc. ICM 73

(Nauka) Moscow (1974) Vol. V, p. 134. [l91 MINIER, M. (Private communication).

1201 WENGER, L. E., KEESOM, P. H., Phys. Rev. B 13 (1976) 4053. [211 A strong anisotropy is found for x(T) depending on the

orientation of the field parallel or perpendicular to the c axis. These data which are out of the scope of this paper are reported in reference [9b].

[22] KLEIN, M. W. and RRoWT, R., Phys. Rev. 132 (1963) 2412. 1231 ABRAGAM, A. and BLEANEY, B., Electron Paramagnetic

Resonance of Transition Ions (Clarendon Press, Oxford)