Wide-band-gap and thermal stability of NaCaX 3 (X = Cl, Br, I) lead-free halide perovskites: a first-principles study

Abstract Halide substitution offers a simple yet effective route to tune the performance of lead-free perovskites. Using density functional theory, we examined NaCaX 3 (X = Cl, Br, I) across their structural, electronic, thermodynamic, thermoelectric, and optical properties. All compounds are stable, wide-band-gap semiconductors with direct gaps narrowing from 3.76 eV (NaCaCl 3 ) to 2.45 eV (NaCaI 3 ). Mechanical and thermal analyses confirm robust bonding in NaCaCl 3 and enhanced lattice softness in NaCaI 3 , while NaCaBr 3 delivers the best thermoelectric response with ZT ≈2.4 at 1000 K. Optical spectra reveal strong ultraviolet absorption, low visible reflectivity, and plasma resonances near 12–13 eV. These results position NaCaX 3 (X = Cl, Br, I) as tunable, environmentally benign perovskites for ultraviolet optoelectronics, dielectric coatings, and sustainable energy conversion.