Volkov Sergey, Bubnova Rimma, Bolotina Nadezhda, Krzhizhanovskaya Maria, Belousova Olga, Filatov Stanislav
Grebenshchikov Institute of Silicate Chemistry, Makarov Emb, St Petersburg 199053, Russian Federation.
Shubnikov Institute of Crystallography, Leninsky pr. 59, Moscow 119333, Russian Federation.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2015 Oct;71(Pt 5):489-97. doi: 10.1107/S2052520615011713. Epub 2015 Sep 19.
Crystal structures of Sr3B(2 + x)Si(1 - x)O(8 - x/2) solid solutions with nominal compositions x = 0.28, 0.53, 0.78 in the Sr3B2SiO8-Sr2B2O5 section of the SrO-B2O3-SiO2 system are refined using single-crystal X-ray diffraction data. Incommensurate structure modulations are mainly associated with various orientations of corner-sharing (B,Si)-polyhedra. Preference is given to the (3 + 2)-dimensional symmetry group Pnma(0βγ)000(0βγ)000 for a single crystal compared with an alternate model of a twin formed by monoclinic components, each of them corresponding to the (3 + 1)-dimensional symmetry group P2(1)/n(0βγ). Single-phase polycrystalline samples of solid solutions are investigated by high-temperature X-ray powder diffraction in air. Orientation preferences of the BO3 units lead to a strong anisotropy of thermal expansion. Negative expansion is observed along the a axis over the temperature range 303-753 K. Anisotropy decreases both on heating and decreasing of the boron content.
