Two novel stable phases of polonium oxides from first-principles evolutionary algorithm

Abstract

Oxygen prominently displays a strong propensity to efficiently form stable molecules by establishing ionic/covalent bonds with a broad array of elements within the Mendeleev Table. That gives rise to a diverse spectrum of oxides that hold pivotal significance in contemporary optoelectronics, mineralogy, biological entities, and atmospheric constitution. We explore here the feasibility of polonium-oxygen compound formation, employing a first-principle evolutionary algorithm. The obtained structural predictions yield two distinct phases of PoO2, exhibiting respectively thermodynamic stability and metastability, specifically the cubic (Fm 3‾ m) and orthorhombic (Pmn21) crystal structures. Phonon calculations have unequivocally substantiated the dynamical stability inherent in these PoO2 structures. The Po–O system, characterized by its unusual physical-chemical attributes, presents noteworthy features in electronic behavior, bonding interactions, and dynamical properties. Both the Fm 3‾ m and the Pmn21 phases exhibit semiconductor behavior, each displaying a relatively substantial indirect bandgap of 2.11 eV and 2.50 eV, respectively, within the cubic and orthorhombic crystals. To unravel the bonding nature of PoO2, we employ a suite of analytical tools, including electronic density of states, Bader charge analysis, and electron localization function. These analyses collectively unveil a transfer of electrons from polonium to oxygen, thereby elucidating the coexistence of significant ionic and partial covalent Po–O bonds within the Po–O system.