Symmetry‐Driven Band Gap Engineering in Lead Halide Perovskites

ABSTRACT The band gap tunability of halide perovskites underpins their exceptional photovoltaic and optoelectronic performance. However, rational band gap engineering remains challenging due to the complex interplay between multiple structural degrees of freedom. Conventional empirical descriptors like the tolerance factor fail to capture this complexity. Here, we report a universal symmetry‐based methodology to decode structure‐property relationships in lead halide perovskites. Integrating group theory with high‐accuracy DFT calculations, we deconstruct the Pnma structure into symmetry‐adapted distortions and quantify their individual impacts on the band gap, revealing opposing effects of octahedral tilts (band gap increase) and lattice strains (band gap decrease). These insights yield a tilt‐based structural descriptor that exhibits universal correlation with experimental band gaps across the lead halide perovskite family spanning diverse A‐site cations (inorganic/organic), crystal phases (orthorhombic/tetragonal), and halide compositions (I, Br, Cl) under compositional and pressure variations. The framework's predictive power is demonstrated through accurate forecasting of band gap evolution in pressure‐tuned CsPbI 3 and compositionally engineered solid solutions using a physical model that explicitly incorporates both tilt and strain mode contributions. Our approach transcends the limitations of empirical descriptors by establishing a rigorous, symmetry‐based foundation for rational perovskite design, thereby accelerating the development of next‐generation functional materials.