EFFECT OF SUBSTITUTION OF LA BY ALKALINE-EARTH METAL ON THE THERMOELECTRIC PROPERTIES AND THE PHASE-STABILITY OF GAMMA-LA3S4

The Seebeck coefficient, electrical resistivity, and thermal conductiv ity of La(3-x)A(x)S(4) with the Th3P4-type structure (the gamma-phase) for A=Ca, Ba, and x=0-0.8 have been measured as a function of alkalin e earth metal content and temperature to determine the optimum dopant concentration and doping agent by calculating the corresponding figure of merit (Z). With an increase of x, the Seebeck coefficient and elec trical resistivity increase, while the thermal conductivity decreases. The decrease of thermal conductivity is ascribed to the decrease of t he electron concentration and the increase of the phonon scattering ac companying the substitution for La by an alkaline earth metal. As a re sult, La2.2Ca0.8S4 has a maximum value of Z of 2.3 x 10(-4) degrees C- 1 at 700 degrees C. We also have examined the long term stability of g amma-La(3-x)A(x)S(4). The substitution for La by alkaline earth metal of La3S4 stabilized the Th3P4-type structure at high temperature. (C) 1997 American Institute of Physics.