Effects of metals (X = Be, Mg, Ca) encapsulation on the structural, electronic, phonon, and hydrogen storage properties of KXCl₃ halide perovskites: Perspective from density functional theory

To overcome the challenges surrounding toxic and unstable lead-based halide perovskites, it is important to develop promising non-toxic s-block halide perovskites that can serve as an alternative in optoelectronic applications. Therefore, based on density functional theory (DFT), the structural, electronics, optical, phonon, and thermodynamic properties of the potassium-based halide perovskites KBeCl₃, KCaCl₃, and KMgCl₃ have been examined through utilization of the PBE + GGA method to determine the stability, thermodynamics as well as the optoelectronic applications of these perovskites. The calculated lattice constants for KBeCl₃, KCaCl₃, and KMgCl₃ were 4.41 Å, 5.10 Å, and 4.74 Å respectively. The band structure and the density of state of the studied materials displayed narrow and indirect band gaps. Furthermore, optical property calculations such as the real part ε₁ (ω), and imaginary part ε₂ (ω) of the dielectric function ε (ω) along with the absorption coefficient α (ω), and the refractive index n (ω) of dielectric function were carried out. In terms of stability, the phonon frequency calculations show that dynamic stability increases as we move down the group on the periodic table for the s-block halide perovskites, indicating that KMgCl₃ and KCaCl₃ have the highest stability and are better candidates for optoelectronic applications. Thermodynamic parameters show an absolute dependence on temperature. The gravimetric hydrogen storage capacity was computed to be 5.866%, 4.516%, and 3.649% for KBeH₃, KMgH₃, and KCaH₃ respectively. It is observed that gravimetric density increases with decreasing cationic size: Be, Mg, and Ca. Furthermore, KBeH₃ perovskite is computed as the highest amongst the studied materials are suitable and effective long term hydrogen storage as a fuel due to its relatively higher gravimetric ratios.