Isostructural Second-Order Phase Transition of β-Bi2O3 at High Pressures: An Experimental and Theoretical Study

Abstract

We report a joint experimental and theoretical study of the structural and vibrational properties of synthetic sphaerobismoite (β-Bi2O3) at high pressures in which room-temperature angle-dispersive X-ray diffraction (XRD) and Raman scattering measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Striking changes in Raman spectra were observed around 2 GPa, whereas X-ray diffraction measurements evidence no change in the tetragonal symmetry of the compound up to 20 GPa; however, a significant change exists in the compressibility when increasing pressure above 2 GPa. These features have been understood by means of theoretical calculations, which show that β-Bi2O3 undergoes a pressure-induced isostructural phase transition near 2 GPa. In the new isostructural β′ phase, the Bi3+ and O2– environments become more regular than those in the original β phase because of the strong decrease in the activity of the lone electron pair of Bi above 2 GPa. Raman measurements and theoretical calculations provide evidence of the second-order nature of the pressure-induced isostructural transition. Above 20 GPa, XRD measurements suggest a partial amorphization of the sample despite Raman measurements still show weak peaks, probably related to a new unknown phase, which remains up to 27 GPa. On pressure release, XRD patterns and Raman spectra below 2 GPa correspond to elemental Bi–I, thus evidencing a pressure-induced decomposition of the sample during downstroke.