Effect of Br Substitution on KBeCl ₃ : Optoelectronic and Mechanical Properties via First‐Principle Calculations

The structural, mechanical, and optoelectronic properties of mixed‐halide perovskites KBeCl 3− x Br x ( x = 0, 1, 2, and 3) have been investigated using first‐principles calculations. Interestingly, a partial substitution of Cl by Br induces a symmetry reduction, driving a phase transition from cubic Pm ‐3 m to tetragonal P4/mmm. The tolerance factor values validate the structural integrity, whereas the cohesive energy indicates thermal stability. Elastic constants validate the mechanical stability in both cubic and tetragonal structures under ambient pressure. The electronic band gap decreases from 4.01 eV ( x = 0) to 2.23 eV ( x = 3), tunable with Br content. Optical characteristics expose the prime absorption in the ultraviolet (UV) region, suggesting these substances as viable for UV‐based photovoltaics uses. Further, the values of , , and fall in respective range of 2–3, ∼0.1, and 1.5–1.7, suggesting nonmetallic (semiconducting) less reflector and optically transparent with moderate dielectric response—likely suitable for optical coatings, photonics, or sensor applications.