Electronic And Optical Properties Of Orthorhombic And Hexagonal Phases Of CsMgH₃: First-principles Calculations

The electronic band structure, chemical bonding, optical and Raman properties of the low pressure orthorhombic (lpo) and high pressure hexagonal (hph) phases of CsMgH3 are extensively studied using first-principles calculations based on the density-functional theory. Total energy calculations are performed to understand the structural stability of the two polymorphs under pressure. The phase transformation of orthorhombic to hexagonal is calculated to occur at 0.8 GPa, in a satisfactory agreement with the experimental suggestion. Moreover, the current band structure calculation suggests that the lpo- and hph-CsMgH3 are insulators with indirect and direct energy gaps of 3.3 and 3.6 eV, respectively. A detailed study of the electronic density of states and the charge density redistribution reveals the ionic/covalent mixing bonding features between Mg and H atom for both phases, while Cs atom plays a major role as an electron donor. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. Furthermore, the real parts of the dielectric function, reflectivity R(ω), absorption coefficientα(ω), refractive index n(ω), and electron energy loss function L(ω) of the lpo- and hph-CsMgH3 are also presented as the theoretical database for the future experimental measurement. From the calculation of the zone-center (Γ) phonon frequencies, eigenvectors, and the dynamical matrix§(Γ), we obtained irreducible representations of allowed Raman-active and infrared-active phonons for both structures.