Local-symmetry distortion, optical properties, and plasmonic states of monoclinic Hf_0.5Zr_0.5O₂ system: a density-functional study

We study structural, electronic, and optical properties of the monoclinic Hf_0.5Zr_0.5O₂ (space group: P2₁/c). Monoclinic ZrO₂ and HfO₂ systems are used as the references. This study employs the plane-wave density-functional method within the generalized gradient approximation. The structural properties emphasize the strongest local-symmetry distortion in Hf_0.5Zr_0.5O₂ system. The bandgap (3.67 eV) and valence bandwidth (∼4.94 eV) of the system are between that of ZrO₂ and HfO₂ systems. Furthermore, the negative average defect formation energy confirms the stability of Hf_0.5Z_0.5O₂ system. In all the systems, the significant differences between n _0x(n _0y) and n _0z imply the strong optical dichroism between xy plane and z axis. On the other hand, the small differences between n _0x and n _0y imply the weak optical dichroism along xy plane. In Hf_0.5Zr_0.5O₂ system, the optical properties show the zero-frequency refractive indices of (n _0x, n _0y, n _0z) = (2.039, 2.086, 1.851). The system also shows the highest intensity and weakest dichroism of plasmonic states along y axis and xy plane, respectively. Moreover, we find the lowest energy level of maximum transmittance (T ≈ 1), strong-reflectance width, absorption width, and saturation threshold energy (∼9.6 eV) of the effective number of valence electrons. Our result emphasizes the importance of the doping treatment to the significant tuning of optical properties of Hf_1−xZr_xO₂ system. This study presents the essential properties to guide future experiments to develop novel optical devices. (paper)