DFT Study of the Effect of A-Site and Halide Substitution on the Structural and Elastic Behavior of APbX₃ Perovskites

In this work, density functional theory within the generalized gradient approximation (GGA-PBE) was employed to examine the mechanical and structural characteristics of cubic APbX₃ (A = Li, Na, K, Rb, Cs; X = F, I) perovskites. Comparing the larger ionic size of iodides to fluorides, the optimized lattice parameters increase with the A-site ionic radius. The Born-Huang stability criteria are satisfied by all computed elastic constants, indicating mechanical stability in the cubic phase. Cubic APbI₃ perovskite compounds tend to have lower bulk moduli and are easily compressible, especially when the heavier A-site cations are present, while APbF₃ compounds show increased stiffness as bulk and Young's moduli increase from Li to Cs. Poisson ratios suggest predominantly ionic bonding across all compositions. Overall, fluoride perovskites are mechanically more robust than their iodide counterparts, highlighting the strong influence of both A-site and halide substitution on structure–mechanical property relationships.