Dual cation-anion substitution in phosphorus-based perovskites A3PX3 (A = Ca, Sr; X = Cl, F): structural, mechanical, electronic, and optical properties.

We present a comprehensive Density Functional Theory (DFT) investigation of cubic perovskites A3PX3 (A = Ca, Sr; X = Cl, F) to explore the influence of cation-anion substitution on their structural, mechanical, electronic, and optical properties. Structural optimization confirms lattice expansion when substituting Ca with Sr and F with Cl. Electronic band structure calculations reveal direct band gaps at the Γ point, with values 1.85 eV (2.90 eV) for Ca3PF3 , 2.06 eV (2.97 eV) for Sr3PF3 , 1.96 eV (2.77 eV) for Sr3PCl3 , using the PBE (HSE06) functional, respectively. These results demonstrate that substituting both the A-site cation and the X-site anion provides a straightforward route for tuning the band gap within the visible range. Optical analyses show that substitution also modulates dielectric response, absorption onset, and optical conductivity, while mechanical calculations confirm elastic stability with higher stiffness for Ca-F rich compounds compared to Sr-Cl systems. Overall, this dual substitutional strategy underscores the promise of A3PX3 perovskites as stable and tunable candidates for photovoltaic and optoelectronic applications.