Unveiling the Role of Crystal Symmetry in the Electronic Structure and Optical Anisotropy of ZrO₂ Polymorphs: A First-Principles Study

This study presents a systematic first-principles investigation into the structural, electronic, and optical properties of zirconium dioxide (ZrO₂) in its cubic and monoclinic phases using density functional theory (DFT) within the GGA-PBE functional, as implemented in the CASTEP code. To overcome the standard DFT underestimation of the band gap, a scissors operator was applied to match the experimental values. Our electronic structure calculations reveal a direct band gap for the cubic phase (4.83 eV), whereas the monoclinic phase exhibits an indirect band gap nature (5.17 eV). This discrepancy in the gap nature and magnitude is critically attributed to the lower crystal symmetry and structural distortions inherent in the monoclinic lattice. Density of states (DOS) analysis indicates that the valence band maximum is dominated by O-2p states, while the conduction band minimum is primarily governed by Zr-4d orbitals, establishing that the fundamental optical transitions are (p      d)   in nature. A rigorous evaluation of the frequency-dependent dielectric function, absorption coefficient, refractive index, and optical conductivity was conducted. Our findings demonstrate a pronounced optical anisotropy in monoclinic ZrO₂ along the [100], [010], and [001] crystallographic directions, contrasting with the nearly isotropic response of the high-symmetry cubic ZrO₂ phase. Furthermore, the energy loss function (ELF) identifies a prominent bulk plasma resonance cantered at 37–38 eV. This work elucidates the fundamental link between crystal symmetry and anisotropic optical response, providing essential insights for the design of ZrO₂-based optoelectronic and ultraviolet (UV) devices.