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and over a wide temperature range it is possible to get single activation energy. With these in view, the solid solutions (Y.•Yb,)) 2Mo20 7 were prepared and the systematic changes in the electrical resistivity (p=l/a), thermopower (S) and power factor (S2a) have been studied above RT to explore their potential as thermoelectric materials. Experimental

High pure chemicals Y 2 0 3 , Yb 2 0 3 and MoO 2 were mixed thoroughly and pelletized. MoO2 was prepared by hydrogen reduction of MoO 3 (480* C/8h); complete reduction of MoO 3 without any impurity was confirmed by powder X-ray diffraction (XRD). The pellets were heated in an evacuated sealed quartz tube in the temperature range 1613-1653K for 12h with intermittent grindings to ensure homogeneity; higher temperature was used for higher Yb content. To avoid contamination due to diffusion of oxygen through the quartz tube (due to phase transition of quartz at these temperatures), double walled fresh ampoules with annular space filled with Ti sponge were used frequently (3hrs). Single phasic nature of the materials was confirmed by powder XRD. Electrical resistivity was measured by van der Pauw method in the temperature range 300-900K in Ar atmosphere on thin pellets. Thermopower was measured in Ar atmosphere by differential technique maintaining a AT of 3K on thick pellets (ý3mm thick). Results and Discussion The powder X-ray diffiaction patterns show clear single phase nature of all compositions indicating the existence of complete solid solubility in the range x=0.0 to x=-1.0. The lattice parameters 'a' and 'c' are smaller for higher Yb31 content phases due to smaller Yb3' radius. A small tetragonality is observed for all the phases. This is possibly due to oxygen deficiency caused by the preparative method. Fig. 1 shows the linear variation of lattice parameters with x (Vegard's law). This means Y3' and Yb3' are randomly distributed and the Mo-O-Mo bond angle expected to vary linearly and hence a systematic variatinn in transport properties is expected across the compositions. 10.23 C

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250

Semiconducting behaviour is seen for all compositions with systematic increase in activation energy with increasing Yb content (Fig.2). Thermally activated conduction can occur due to two reasons viz., thermal activation of charge carriers and thermal activation of mobility. The data fit well in the equation cr =A/T exp(E3/kT) derived for polaron hopping in adiabatic limit. The temperature range (300-900K) is large enough to distinguish it from Arrhenius equation. In the intrinsic regime, excitation of charge carriers occur from lower band to higher band giving rise to multiband conduction.

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