The crystal structure of the bismuth silicon oxide Bi(12)SiO(20) 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 Kα radiation and with synchrotron radiation (λ = 0.45 Å) 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 Bi(3 + ) 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 Bi(12)SiO(20) 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.

Crystal structures of the NaLnF₄ materials (Ln = La, Ce, Pr, Nd, Sm or Gd) were studied with synchrotron single-crystal and powder diffraction. The materials with Ln = La, Ce, Nd, Sm and Gd have the average β structure (P, Z = 1) with partial ordering of the cations. A new type of a superstructure due to ordering of the cations and vacancies was found in NaPrF₄ (P3, Z = 6). It could be described using the group–subgroup relationships P ↔ P3. Our observations suggest that the β structure is unstable and that the ordering is a slow process at ambient conditions. Upon compression, β-NaNdF₄, β-NaGdF₄ and the superstructure NaPrF₄ are stable to at least 8 GPa with no evidence for any pressure-induced disorder–order phenomena.