First-principles investigation of Cs2TlGaX6 (X = Cl, F) double perovskites: structural and dynamical stability, elastic properties, and optoelectronic characteristics for applications in semiconductor technology

Due to their stability, eco-friendly nature, lack of lead, and high performance, double perovskites are promising materials for solar cells, thermoelectric generators, and renewable energy applications. This study investigates the structural, dynamical, elastic, and optoelectronic properties of Cs 2 TlGaX 6 (X = Cl, F) compounds using first-principles calculations with the WIEN2k code. While the PBE-GGA method accurately predicts ground state properties, it underestimates band gaps. To address this, the TB-mBJ approach, known for precise semiconductor bandgap calculations, was used. Calculated formation energy confirmed the thermodynamic stability of both compounds, with Cs 2 TlGaF 6 (-2.81 eV) more stable than Cs 2 TlGaCl 6 (-1.79 eV). Imaginary frequencies absence indicates phonon stability. Structural optimization plots reveal stable unit cell configurations, and elastic property analysis confirms their ductile nature and resistance to cracking. Cs 2 TlGaCl 6 behaves as an indirect wide band gap semiconductor (3.78 eV), suitable for various applications, while Cs 2 TlGaF 6 exhibits insulating behavior with a larger band gap (6.23 eV). Optical properties were also calculated, suggesting potential applications in optoelectronics, photonics, and semiconductors within the ultraviolet energy ranges.