Abstract
The effectiveness of substitution for La by Ba or Ca on the physical properties of rare earth chalcogenide La3S4 was examined. Cubic γ -phase (Th3P4-type) was obtained in the range of 0≤x≤1 in La3-xAxS4 (A=Ba, Ca). The lattice parameter in La3-xBaxS4 system increased, while that in La3-xCaxS4 system decreased with an increase of x, which is ascribed to the difference of the radius among the ions. The thermoelectric power (α) and the electrical resistivity (ρ) of La3-xAxS4, which are functions of the electron concentration, increased with an increase of the temperature and the alkaline earth metal content x. A maximum value of the power factor (p=α2/ρ) at high temperature above 750K was obtained at x=0, i.e., La3S4. On the other hand, the thermal conductivity (κ) decreased with an increases of x. In result, a maximum value of the figure of merit (Z=α2/(ρκ)) was obtained at large value of x (x≈0.6 in La3-x, BaxS4 and x≈0.8 in La3-xCaxS4).
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