A. CONAN et al. : Electrical and Structural Properties of Tungsten Bronze 609

phys. stat. sol. (b) 155, 609 (1989)

Subject classification: 72.20; 61.60; 71.20; S10

Laboratoire de Physique Cristalline, I.P.C.M., associe' au C.N.R.S. 110 (a) and Laboratoire des Mate'riaux et Composants de I'Electronique, Faculte' des Xciences et des Techniques (b) , Universite' de Nantes')

Electrical and Structural Properties of the Tungsten Bronze Bi4W,,0,, BY 9. CONAN (a), A. BONNET (a), and &I. MORSLI (b)

Transport coefficient measurements (electrical conductivity and thermoelectric power) are per- formed on compact bars of a bismuth tungsten bronze compound Bi,W,,O,, (labelled as B-phase) in polycrystalline form over the temperature range 100 t o 500 K. Experimental results are discussed in comparison with those obtained recently on a compound, labelled as A-phase and which is pre- pared a t a lower temperature. A single band model accounts for all the experimental results. The main differences which are observed in the electrical properties of the two phases are a consequence of the less disordered HTB and perovskite intergrowth sequences for the high-temperature phase.

Des mesures de conductivitb Blectrique et de pouvoir thermoblectrique de poudres compactkes d'un compose de bronze de tungs the nu bismuth Bi,W,,O,, (phase B) ont 6tB effectubes entre 100 et 500 K. Les rBsultats experimentaux sont interpret& par comparaison avec ceux obtenus r6- comment sur un composB (phase A) prepare a plus basse tempkrature. Un modble une bande rend compte de tous les rbsultats expbrimentaux obtenus. Les diffbrences principales o bservbes entre les propri6ti.s Blectriques des deux phases rbsultent de la plus grande regularit6 dans les sequences d'intercroissance bronze hexagonal-structure perovskite prbsentee par la phase "haute temphrature".

1. Introduction

I n a recent paper [ 11, transport coefficient measurements (electrical conductivity and thermoelectric power) were performed on polycrystalline compact bars of a law- temperature phase of the bismuth tungsten bronze Bi,W,,O,,. These results were ex- plained on the basis of,a single band formed by the overlap of some broadened levels (d-orbitals). It was shown that variable range hopping (VRH) a t the Fermi level occurred a t low temperature, whereas nearest neighbour hopping (NNH) took place a t higher temperature. It is obvious that the strong localization of the electronic states is a consequence of the structural disorder.

More recently a high-temperature phase of this bismuth tungsten bronze has been synthetized in polycrystalline form using a new technique. The differences observed on the transport properties and the crystal structures of these two phases are reported in tthis paper. They are a consequence of the intrinsic structural disorder which depends strongly upon the temperature of preparat,ion.

Essentially, it is shown that all the transport measurement results which are ob- tained for the high-temperature phase (B-phase) may be explained on the basis of the single band model which has been used in [I] (A-phase). Nevertheless, the room- temperature electrical conductivity which is obtained on the B-phase is higher by

1) F-44072 Nantes, CBdex 03, France.

610 A. CONAN, A. BONNET, and M. MORSLI

one order of magnitude. Correlatively, the thermoelectric power (TEP) is two times smaller. All these results are in agreement with a weaker localization of the B-phase electronic states and a less disordered structure. This is justified by high resolution electron micrographs for the two phases.

2. Experimental

The high-temperature phase compounds are prepared by sealing evacuated silica am- poules containing homogeneous mixtures of the powdered elements weighted accurate- ly in suitable proportions according to the following reaction :

2 Bi,O, + WO, + W -+ Bi,W,,O,, . These ampoules have been heated by steps of 24 h a t 250, 350, 550, and 630 "C,

respectively. After this treatment no particular precaution is taken to quench the samples especially rapidly. The final reaction is carried out by annealing a t 630 "C for one day. It can be recalled that the A-phase had been prepared a t 600 "C instead of 630 "C. The resulting powder is pressed in 3 x 2 x 15 mm3 bars yielding samples having 80% of the theoretical density.

X-ray diffraction analysis which has been performed with a Guinier camera, has been carried out by reference to the Sb,,,,WO, cell [ 2 ] . Owing to the presence of diffuse rays in the Guinier diagram, the B-phase Bi,W,,O,, cell is difficult to solve accurately. It has not yet been possible to index completely the X-ray diagram on a superstructure cell 2a x 2b x 2c where a, 6, and c are the lattice parameters of Sb,W,,O,,. Moreover high resolution microscopy still reveals some structural disorder in spite of the new high-temperature preparation technique. This explains the difficulties encountered in the attempts to refine the structural model.

The experimental procedure, i.e. electrical conductivity and thermopower apparatus, has been described elsewhere [3]. Transport measurements have been performed on four different compact bars being issued from the same preparation technique. A t most 2 and 5 % variations are observed between the different samples for the elec- trical conductivity and the thermoelectric power, respectively.

Experimental conductivity values of the most representative sample are plotted as In CI versus 103/T (Fig. 1). The conductivity is a t first sight thermally activated with two slopes: 7 meV a t low temperature and 26 meV a t temperatures above 100 K

Fig. 1. Experimental variations of In (rvs. 103/T for the low-temperature (A) and high-temperature (B) Bi,W,,Oo, phases. The theoretical variations are drawn as a solid line

I ! I I -

Fig. 3. High resolution electronic micrographs: a) low-temperature Bi,W,,O,, phase, b) high-tem- perature Bi4W2,,Ps2 phase

Fig. 2. Experimental varia- tions of S vs. T for the A-

- q - a 2 5

-a4

-a6

(r,

-08

and B-phases of Bi4W,,06,. The theoretical variations are drawn as a solid line - -

-

-

612 A. CONAN, A. BONNET, and M. MORSLI

(respectively, 37 and 52 meV above 110 K for the A-phase). TheTEP variations which are obtained on the same sample as a function of the temperature are plotted in Fig. 2. The TEP which is negative, shows a quasi-linear T dependence of the form

A-phase: S(pV/K) = -1.3 x 10-lT + 8 , B-phase: X(pV/K) = -5.5 x 1OP2T + 6 .

Moreover, high resolution electronic micrographs (Fig. 3) have been performed along the a-axis of the two phases. A higher regularity in the intergrowth sequences for the phase which had been prepared a t 630 "C can be shown.

3. Discussion A key relation in the behaviour of the electronic conduction is that conductivity should depend on temperature as In c - (To/T)n with n = 1/2, 1/3, or 1/4. Following the same approach as [I], it appears that VRH (Mott's law) occurs a t low temperature,

c - exp - (TM/T)lI4 , whilst nearest neighbour hopping takes place a t higher temperature [4],

c = coexp( - =). WD

The theoretical temperature TZ a t which the hopping distance becomes equal to the nearest neighbour distance is such that

kTw(kT1)3 = ( 4 W D ) 4 ,

where WD is the activation energy measured in the high-temperature limit. One ob- tains TI = 124 K which is in good agreement with the experimental transition tem- perature 120 K. The last value is given by the intercept of the asymptotic transport coefficient variations in the two regimes. The results, i.e. values of the Mott tempera- ture TnI, the room-temperature conductivity O & ~ , R T in the VRH regime, the pre- exponential factor co, and the activation energy Wu of the conductivity in the NNH regime are given in Table 1.

The theoretical variations of the electrical conductivity as a function of the reciprocal temperature are drawn as a solid line in Fig. 1.

It can be recalled that the asymptotic behaviour of the thermoelectric power S has been calculated by Whall [5] when the Fermi level, the temperature variations of which are assumed to be the same as in a metal, lies in the middle or near the top or the bottom of a large band. According to the effective medium theory [5] we have performed a numerical integration for a symmetrical energy band with the width E, and for a density of states function varying as the square root of the energy. The TEP fit leads to a band width E, = 170 meV and a zero temperature Fermi energyEFO = = 85 meV.

Table 1

VRH NNH

'JM, RT TiVI 0 0 WD E B EFO ([I-1 cm-l) (K) (a-l cm-1) (meV) (meV) (meV)

A-phase 0.35 2.4 x 107 122 55 85 42 B-phase 38.60 1.1 x 106 571 26 170 85

Electrical and Structural Properties of the Tungsten Bronze Bi,W,,O,, 613

Fig. 4. Schematic density of states: a) low-temperature Bi4W2,06, phase, b) high-temperature Bi4W2,0B, phase

EE =85meV

I b EE =770meV

The thermopower had to be fitt,ed in two ways: S = I/r [C,] below T, and numerical integration above TI. I n fact, the numerical integration which has been done over the whole temperature rangeinvestigated leads to a very good agreement between the experimental and the theoretical results. To justify this, it is important to remember that the mean hopping distance remains close to the nearest neighbour distance down to 80 K. Moreover, in this region the thermopower is small and varies slowly with the temperature and could be fitted as well by the Zvyagiii --1/Tlaw.

The schematic band model which takes into account all the electrical properties is proposed in Fig. 4. It can be compared with that which had been obtained for the A- phase: the larger extent of the band (170 meV to be compared with 85 meV) must be attributed to the less disordered structure. Moreover, the term aR,, in which I[& and R, are the localization radius and the nearest neighbour distance, respectively, varies as (T , /WD)1/3 . It is found to be two times smaller for the B-phase. This confirms the predominant role played by the thermal treatment which affects the HTB and perov- skite intergrowth sequences.

Effectively, a careful examination of the high resolution electronic micrographs which have been performed on the A- and B-phases show a less disordered inter- growth sequences for the B-phase.

References [l] A. BONNET, A. CONAN, M. MORSLI, M. GANNE, and M. TOURNOUX, phys. stat. sol. (b) 150, 225

[2] M. DION, ThBse, Nantes 1984. 231 A. BONNET, P. SAID, and A. CONAN, Rev. Phys. appl. 17, 145 (1982). [4] N. F. MOTT and E. A. DAVIS, Electronic Processes in Noncrystalline Materials, 2nd ed.,

[ 5 ] T. E. WHALL, J. Phys. C 14, L887 (1981). [6] P. ZVYAGIN, phys. stat. sol. (b) 68, 443 (1973).

(1988).

Clarendon Press, Oxford 1979.

(Received June 12, 1989)