Exploring A-Site Chemistry of A2TlInF6 (A = Na, K) Double Perovskites: Structural, Optoelectronic, Elastic, and Thermoelectric Properties.

Abstract

First-principles density functional theory (DFT) calculations were performed to investigate the structural, mechanical, electronic, optical, thermodynamic, and thermoelectric properties of double perovskites A2TlInF6 (A = Na, K). The tolerance factor, formation energies, Born-Huang criteria, and Gibbs free energy confirm the structural stability and energetic favorability of the cubic phases. Ab initio molecular dynamics simulations at 300, 500, and 700 K further demonstrate their structural integrity and finite-temperature stability. Both compounds exhibit ductile behavior and elastic anisotropy. Without spin-orbit coupling (SOC), the indirect bandgaps are 2.41 eV for Na2TlInF6 and 2.68 eV for K2TlInF6. Including SOC, the PBE-GGA/TB-mBJ bandgaps are 2.37/5.05 and 2.64/4.75 eV, respectively. Optical spectra across 0-13.5 eV reveal strong ultraviolet-region absorption, with electronic transitions primarily involving Tl-s and F-p states. Thermodynamic analysis shows decreasing Gibbs free energy, increasing entropy, thermal expansion, and decreasing Debye temperature with increasing temperature, reflecting enhanced thermal vibrations. BoltzTraP calculations predict maximum thermoelectric figures of merit (ZT) of 0.65 for Na2TlInF6 and 0.58 for K2TlInF6 at 800 K. Overall, these findings highlight their potential for ultraviolet optoelectronics and high-temperature thermoelectric energy conversion and provide a basis for further theoretical and experimental investigation.

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