New lead-free double perovskites (Rb₂GeCl/Br)₆; a promising materials for renewable energy applications

The vacancy ordered double perovskites are emerging compounds for optoelectronics, and transport properties. In this article, optical, thermoelectric, mechanical, and thermodynamic properties of Rb₂GeCl₆ and Rb₂GeBr₆ are investigated comprehensively. The phonon band structures, and tolerance factor confirm their thermodynamic, and structural stabilities. The direct band gaps 2.60 eV and 1.78 eV are reported at Γ-symmetry point which ensure the absorption of light from visible to UV regions. The less dispersion of light, and refractive index range 1.5–2 along with broad absorption band in visible to UV regions make them outstanding materials for optoelectronics, Moreover, the transport characteristics are addressed by electrical conductivity, thermal conductivity and Seebeck coefficient, and Figure of merit. The room temperature ZT values 0.78, and 0.74 also ensure their importance for thermoelectric applications. The mechanical properties confirm their ductile, ionic, and anisotropic nature. The high Debye, and melting temperatures increases their heating resistant capacity and stability.