Structural transformation and vibrational properties of BaO2 at high pressures

The tetragonal ambient-pressure phase of BaO2 (CaC2-type, space group I4/mmm) with sixfold coordination of Ba atoms and O-2 dumbbells was found to transform reversibly to an orthorhombic modification (space group Cmmm) near 33 GPa. The eight-coordinated high-pressure phase represents a new structure type. It is related to the CsCl-type structure but can also be viewed as a distorted variant of the hexagonal AlB2 type, suggesting possible polymerization of the isolated O-2 dumbbells to a two-dimensional network at higher pressure. In addition to in situ x-ray diffraction, Raman measurements were performed to study the lattice dynamics of the BaO2 phases under pressure. The experimental observations are compared to the results of ab initio calculations of the structural stability and dynamical properties. Raman spectra of barium monoxide BaO are reported in the Appendix; these results were found useful for the interpretation of the Raman scattering of BaO2.