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.