β-Bi2O3 under compression: Optical and elastic properties and electron density topology analysis

We report a joint experimental and theoretical study of the optical properties of tetragonal bismuth oxide (beta-Bi2O3) at high pressure by means of optical absorption measurements combined with ab initio electronic band structure calculations. Our results are consistent with previous results that show the presence of a second-order isostructural phase transition in Bi2O3 (from beta to beta') around 2 GPa and a phase transition above 15 GPa combined with a pressure-induced amorphization above 17-20 GPa. In order to further understand the pressure-induced phase transitions and amorphization occurring in beta-Bi2O3, we theoretically studied the mechanical and dynamical stability of the tetragonal structures of beta- and beta'-Bi2O3 at high pressure through calculations of their elastic constants, elastic stiffness coefficients, and phonon dispersion curves. The pressure dependence of the elastic stiffness coefficients and phonon dispersion curves confirms that the isostructural phase transition near 2 GPa is of ferroelastic nature. Furthermore, a topological study of the electron density shows that the ferroelastic transition is not caused by a change in number of critical points (cusp catastrophe), but by the equalization of the electron densities of both independent O atoms in the unit cell due to a local rise in symmetry. Finally, from theoretical simulations, beta'-Bi2O3 is found to be mechanically and dynamically stable at least up to 26.7 GPa under hydrostatic conditions; thus, the pressure-induced amorphization reported above 17-20 GPa in powder beta'-Bi2O3 using methanol-ethanol-water as pressure-transmitting medium could be related to the frustration of a reconstructive phase transition at room temperature and the presence of mechanical or dynamical instabilities under nonhydrostatic conditions.