Revisiting lattice dynamics and stability of lead-free double perovskites Cs2InAgX6 (X = F, Br, Cl) from first-principles calculations

Halide double perovskites are promising candidates for optoelectronic and thermoelectric applications due to their tunable structural, electronic, and vibrational properties. However, achieving an accurate description of these properties remains a challenge within density functional theory. The structural, electronic, mechanical, and vibrational properties of Cs2InAgX6 (X = F, Cl, Br) are investigated using the PBEsol and HSE06 functionals. The computed lattice parameters increase systematically with halide ionic size, consistent with experimental trends, while elastic constants satisfy the Born stability criteria and decrease from fluoride to bromide, indicating progressively weaker bonding. Bandgap energies show an increasing trend with halogen electronegativity due to changes in orbital hybridization, and HSE06 results agree well with available experimental data. Born effective charges and dielectric constants reveal a strong halide dependence of polarization response, with the bromide compound exhibiting the largest values. Detailed phonon calculations, including symmetry-resolved mode assignment and dispersion relations, indicate that Cs2InAgCl6 and Cs2InAgF6 are dynamically stable, while Cs2InAgBr6 displays soft modes consistent with experimentally observed phase transitions; notably, the predicted stability of the fluoride compound contrasts with several previous density functional theory reports. These findings highlight the critical role of the halide element in the governance of bonding, electronic structure, and stability and identify Cs2InAgCl6 and Cs2InAgF6 as promising stable materials for optoelectronic applications.