Thermoelectric properties of Mo6Se8-based chevrel phase with semiconducting properties

Mo₆Se₈-based Chevrel phase compounds with inserted cations into cavities are interesting as a new candidate of PGEC (Phonon Glass Electron Crystal) thermoelectric materials. From band structure calculations, the compositions including TiMo₆Se₈ have been found to be semiconductor or semimetal, and therefore are expected to have good Seebeck coefficient besides low thermal conductivity. However it had been apparent that insertion of enough cations to the cavities in Mo₆Se₈ with simple process was difficult due to secondary phase and oxide appearance. We present in this paper an estimation of the effect of two inserted cations experimentally and theoretically in order to improve thermoelectric properties of Chevrel phases. The thermoelectric properties are significantly enhanced by Cu-addition, and the dimensionless figure of merit, ZT=0.22 is estimated for nominal Cu_0.3Ti_1.0Mo₆Se₈ sample at 800°C. It is found from band structure calculations by the self-consistent Korringa-Kohn-Rostoker (KKR) method that the Fermi level tends to shift to upper state by Cu-addition. Cu-addition can compensate a loss of Ti and control carrier concentration instead of Ti.