It has been found that inorganic double perovskites are highly stable, flexible, and appropriate to utilize in photovoltaic, photocatalytic, and thermoelectric applications. In the current work, X2LaAuBr6 (X = K, Rb, Cs) compounds are investigated in the context of density functional theory. To determine the exchange-correlation effects, PBEsol GGA and TB-mBJ approximations are utilized in conjunction with the full potential scheme of a linearized augmented plane wave with local orbitals fitted in Wien2K. Formation energy, tolerance factor, and octahedral factor show the structural stability of these compounds. The elastic constants are calculated using the IR-elastic scheme. The calculation of elastic parameters confirms that the materials under study are mechanically stable. Poisson and Pugh ratios show that these materials have a ductile nature. The compounds X2LaAuBr6 (X = K, Rb, Cs) demonstrate anisotropy behavior with readings of 0.53, 0.28, and 1.08. Structural parameters were computed by applying the generalized gradient scheme and confirm the cubic phase of X2LaAuBr6 (X = K, Rb, Cs) compounds having the space group Fm \(\bar{3}m\) (225). The compounds show a wide bandgap nature (between 2.5 and 3.13 eV) calculated by TB-mBJ approximation, and band structure plots reveal the direct bandgap nature, except K2LaAuBr6, which has an indirect bandgap nature. The bandgap is also computed through HSE06 approximation; 2.77, 2.97, and 2.81 eV are the indirect bandgap values of K2LaAuBr6, Rb2LaAuBr6, and Cs2LaAuBr6, respectively. In the visible and ultraviolet energy range, the materials under investigation reveal good optical conduction and absorption coefficient. The peak values of optical conductivity are 4300 Ω−1 cm−1, 4460 Ω−1 cm−1, and 4205 Ω−1 cm−1 for K2LaAuBr6, Rb2LaAuBr6, and Cs2LaAuBr6, respectively. X2LaAuBr6 (X = K, Rb, Cs) demonstrate excellent absorption and optical conductivity in the UV and visible regimes of the spectrum, suggesting these materials as potential candidates for use as photoactive layers in tandem solar cells, and for photovoltaic applications. Moreover, additional optical parameters like dielectric constant, refractive index, reflectivity, and energy loss function are analyzed, which suggest the semiconductor nature of the under-studied compounds. Their potential employment as photocatalysts is inspected, which fulfills all requirements of a proficient photocatalyst. Finally, the BoltzTraP code has been used to compute thermoelectric parameters like Seebeck coefficient, electronic thermal conductivity, lattice thermal conductivity, and figure of merit ZT against temperature; a reasonable figure of merit makes these compounds appropriate for use in thermometric applications.
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