Influenceof phase on thermoelectric properties in lanthanum sesquisulfide doped withtitanium

M.Ohta¹, S. Hirai¹, S. Morita², T. Nishimura²,Y. Uemura²

¹Department of Materials Science and Engineering, Muroran Institute ofTechnology, 27-1 Mizumoto, Muroran, Hokkaido 050-8585, Japan

²Advanced Materials Laboratory, National Institute for Materials Science, 1-1Namiki, Tukuba, Ibaraki 305-0044, Japan

Itis well known that gamma-phase of lanthanum sesquisulfide (La₂S₃)exhibits high figure of merit due to heavy electron system at high temperature.

Thegamma-phase is obtained by transforming the beta-phase, which stabilizes at lowtemperature compared to gamma-phase and behaves like an insulator, at 1573 K.However, the dopant O atoms which are substituted for S atoms stabilize thebeta-phase which is La₁₀S_15-xO_x with 0 < x≤ 1 at high temperature in excess of 1573 K.

Wehave found that the transformation to gamma-phase is controlled by sinteringthe beta-phase which is La₁₀S_14-xO_x doped withTi.

We report measurement of thethermoelectric power and the electrical resistivity in lanthanum sesquisulfideof which the phase is controlled　by doping Ti. As the volume ratio of the gamma-phase to the beta-phase increases, the electrical resistivity and the absolute value of thethermoelectric power decrease.The facts suggest that the concentration of carrier increases with increasingthe volume ratio. Added to this, the dopant Ti has an effect on theconcentration of carrier. The power factor of the lanthanum sesquisulfide dopedwith Ti whichis mainly composed of the gamma-phase exceeds 300 mWm^-1K^-1 at roomtemperature.