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.
Keywords: incommensurately modulated structure; solid solution; strontium borosilicate; thermal expansion.