Structure, electronic, magnetic and optical properties of CrO2 at 128 GPa

Abstract

The present study investigates the structure, electronic, magnetic and optical properties of orthorhombic CrO₂ (Pnma CrO₂) at 128 GPa using ab initio first-principles calculations. The coordination number of Cr with O is found to be 7. Structural distortion in the crystal is caused by the variations in Cr-O distances, <O-Cr-O and <Cr-O-Cr angles. The system is found to be a nonmagnetic metal for U = 0 eV. The metallic behaviour is caused by the sharing of the available two Cr-3d electrons by all five Cr-3d states in both spin channels. This material undergoes metal-insulator transition (MIT), exhibiting ferromagnetism for U = 3 eV. The complete orbital ordering among the Cr-d_3z²_{- r}², d_xz, d_yz, d_x²_{- y}² and d_xy orbitals is responsible for the observed MIT. The cooperative effect of strong p-d hybridization and Cr-O antiferromagnetic coupling facilitates ferromagnetism in insulating Pnma CrO₂. The investigation of the optical properties of the present material is performed in terms of real [ε₁ (ω)] and imaginary [ε₂ (ω)] parts of the dielectric function and, the electron energy loss function [L (ω)]. The presence of several peaks in ${\epsilon }_{1\text{xx}}$, ${\epsilon }_{1\text{yy}}$ and ${\epsilon }_{1\text{zz}}$ for both U = 0 and 3 eV indicates anisotropy in the structure. The incident photons are primarily reflected along the [100], [010] and [001] directions at U = 0 eV, with a significant reduction in reflection across the MIT at U = 3 eV. The absorption is found to be minimal along the [100] direction, with higher absorption along the [010] and [001] directions. The absorption increases across the MIT in the present system. High energy dissipation is observed up to 10 eV, while transparency occurs beyond 10 eV. The occurrence of high energy dissipation is shifted in the UV region across the MIT.