First-principles examination of hydrostatic pressure effects on the structural, mechanical, phonon, and optoelectronic properties of TlGeX3 (X = Br, I)

Significant developments have been made on the application of lead-free halide perovskites. Inspired by the outstanding properties of these compounds, the physical properties of two types of lead-free halide perovskites, TlGeX 3 where X = Br, I, under hydrostatic pressure are explored. Particular emphasis is given to the phase transition under pressure from semiconducting to metallic behavior and its impact on the optoelectronic and mechanical properties. Ab initio calculations based on density functional theory (DFT) are carried out to examine the evolution of the structural, electronic, optical, and mechanical properties under applied pressures up to 5 GPa. The structural and thermodynamic stabilities are determined through the Goldschmidt factor and the formation energies, respectively, while the mechanical stabilities are confirmed through the Born criteria. The dynamic stability of the crystal lattice of these materials is determined through the phonon dispersion curves within the entire range of applied pressure. The values of the lattice constants of TlGeBr 3 and TlGeI 3 are in excellent agreement with the reference values, and the increase in pressure causes the lattice and bond lengths to contract due to enhanced interatomic interactions. According to electronic analysis, TlGeX 3 (X = Br, I) compounds exhibit a pressure-induced transition from a semiconducting to a metal state. The transition is marked by a decrease in band gap, which eventually closes at 5 GPa for TlGeBr 3 and 4 GPa for TlGeI 3 , as determined via the TB-mBJ method. This change is supported by the analysis of the density of states (DOS) and the band structure. At the same time, the reduction in the band gap under pressure increases the spectrum of the conduction band, while the increase in the dielectric constants and the refractive indices reduces the recombination of the carriers and increases the interactions between the photons and the electrons. Furthermore, the mechanical studies indicate improved rigidity and ductility, with good machinability observed for TlGeBr 3 . The enhancement in ductility under pressure for TlGeX 3 (X = Br, I) suggests potential applications for flexible electronics and portable electronics. This study has revealed important findings for non-toxic halide perovskites and has opened doors for further studies.