Structural compression and vibrational properties of Bi12SiO20 sillenite from experiment and theory

The crystal structure of the bismuth silicon oxide Bi12SiO20 was determined by single-crystal x-ray diffraction at ambient conditions and at high pressure. Single-crystal intensity data between 0.0001 and 16.8(3) GPa were collected in house with Mo K alpha radiation and with synchrotron radiation (lambda = 0.45 angstrom) at HASYLAB (D3), while lattice parameters were measured up to 23.0(3) GPa. The large cavities which exist in the crystal structure and host the lone electron pairs of the Bi3+ ions are considerably compressed at high pressure. The crystal structure, however, remains stable and the lone electron pair is stereochemically active up to at least 16.8 GPa. A larger compression in the direction of the lone electron pairs by shear deformation was not observed. Raman spectra of Bi12SiO20 were measured on powder samples during pressure decrease from 39.1(1) GPa down to ambient pressure and on single crystals during pressure increase up to 12.50(3) GPa. Density functional perturbation theory was used to compute Raman frequencies and intensities at ambient pressure and to investigate pressure-induced changes up to 50 GPa.