  In this study, the effects of sulfur substitution on the structural,
mechanical, electronic, optical, and thermodynamic properties of RbTaO3-xSx
have been investigated using the WIEN2k code in the framework of density
functional theory (DFT). The cubic phase of RbTaO3 transforms to tetragonal for
RbTaO2S and RbTaOS2, the later transforms again to a cubic phase with added
sulfur for RbTaS3. The results showed that substituting S for O anions in
RbTaO3 effectively decreased the band gap from 2.717 eV to 1.438 eV, 0.286 eV,
and 0.103 eV for the RbTaO3,RbTaO2S, RbTaOS2, and RbTaS3 compounds,
respectively. The optical constants such as dielectric constants, refractive
index, absorption coefficient, photoconductivity, reflectivity and loss
function have been calculated and analyzed. The elastic constants and moduli,
and their anisotropic nature were also investigated. Finally, the Debye
temperature, thermal conductivity, melting temperature, specific capacities and
thermal expansion coefficients were computed and analyzed using established
formalisms. The reduced band gap (1.438 eV) and high absorption coefficient
(~106 cm-1) of RbTaO2S makes it suitable for solar cell applications and for
other visible light devices. Reduction of the band gap and phonon thermal
conductivity owing to Ssubstitution is expected to enhance thermoelectric
performances of the S-containing phases
