Electronic, dielectric, and optical properties of Cs2Hf1-xZrxCl6 double perovskites: first-principles insights with implications for scintillation

Cs 2 HfCl 6 and Cs 2 ZrCl 6 are vacancy-ordered double perovskites that have emerged as promising halide scintillators. We present a unified investigation of Cs 2 HfCl 6 , Cs 2 ZrCl 6 , and their mixed alloy Cs 2 Hf 0.5 Zr 0.5 Cl 6 using density functional theory and density functional perturbation theory. We systematically examine their electronic structure, dielectric response, and optical properties, complemented by lattice dynamics, mechanical stability, and bonding character analyses. The calculated band gaps decrease with Zr substitution. We separate the electronic (ε ∞ ) and lattice-driven ionic (ε ionic ) contributions to the dielectric response to clarify how cation substitution influences polarization behavior. Optical absorption spectra show direct band-edge transitions near the calculated gaps and intense near-UV absorption arising from inter-band transitions between Cl 3p-derived valence bands and Zr 4d- and Hf 5d-derived conduction bands. Phonon dispersions confirm dynamical stability with no imaginary frequencies, elastic tensors satisfy the Born mechanical-stability criteria, and electron localization function and Bader charge analyses support slightly greater ionicity of Zr–Cl than Hf–Cl bonding. Overall, the results demonstrate how Hf/Zr substitution influences the band gap, dielectric screening, optical response, lattice dynamics, mechanical stability, and bonding character, properties that may affect carrier relaxation and recombination processes relevant to scintillation, providing a first-principles basis for interpreting scintillation-related trends.