Engineering the A-site cations in (Na/K/Rb)₃InBr₆ halides for enhanced optoelectronic and thermoelectric performance...

Perovskite halides are a subclass of perovskite materials that are widely known for their exceptional compositional tunability and structural flexibility. This manuscript reports first principles analysis of the (Na/K/Rb)₃InBr₆ perovskite halides. The structural analysis for all three halides is validated based on the optimization curves and formation energies. The obtained tolerance factors and octahedral titling indicate minimum distortions in the formation of the cubic structure of (Na/K/Rb)₃InBr₆. The stress-energy tensor matrix is employed to assess the elastic constants of A₃InBr₆ (A = Na, K, and Rb). The transverse rigidity, longitudinal stiffness and ductility decline when the A-site cation is exchanged from Na to Rb. The elastic anisotropy computed using the ELATE software indicates that among the investigated compounds, K₃InBr₆ displays comparatively reduced anisotropy and a more uniform elastic response, but Na₃InBr₆ and Rb₃InBr₆ show pronounced directional changes. This highlights the function of the alkali-ion size in modifying the mechanical properties of these halide systems. Direct bandgaps of 3.37 eV, 3.84 eV and 3.86 eV were determined for Na₃InBr_6, K₃InBr₆ and Rb₃InBr₆, respectively. The partial density of states plots indicate that for all studied halides, the Br-p states have a high contribution to the valence band. Optical analysis of Na₃InBr_6, K₃InBr₆ and Rb₃InBr₆ reveals a prominent peak and electronic excitations in the UV region. The Seebeck coefficient of the studied halides is high at low temperatures, while the electrical and thermal conductivities increase with temperature. The ZT values suggest that Na₃InBr_6, K₃InBr₆ and Rb₃InBr₆ are strong candidates for thermoelectric devices.