Synthesis, crystal structure and thermal behavior of Sr3B2SiO8 borosilicate

Single crystals of Sr3B2SiO8 were obtained by solid-state reaction of stoichiometric mixture at 1200 °C. The crystal structure of the compound has been solved by direct methods and refined to R1=0.064 (wR=0.133). It is orthorhombic, Pnma, a=12.361(4), b=3.927(1), c=5.419(1) Å, V=263.05(11) Å3. The structure contains zigzag pseudo-chains running along the b axis and built up from corner sharing (Si,B)−O polyhedra. Boron and silicon are statistically distributed over one site with their coordination strongly disordered. Sr atoms are located between the chains providing three-dimensional linkage of the structure. The formation of Sr3B2SiO8 has been studied using annealing series in air at 900–1200 °C. According powder XRD, the probe contains pure Sr3B2SiO8 over 1100 °C. The compound is not stable below 900 °C. In the pseudobinary Sr2B2O5–Sr3B2SiO8 system a new series of solid solutions Sr3−xB2Si1−xO8−3x (x=0–0.9) have been crystallized from melt. The thermal behavior of Sr3B2SiO8 was investigated using powder high-temperature X-ray diffraction (HTXRD) in the temperature range 20–900 °C. The anisotropic character of thermal expansion has been observed: αa= −1.3, αb=23.5, αc=13.9, and αV=36.1×10−6 °C−1 (25 °C); αa= −1.3, αb=23.2, αc=5.2, and αV=27.1×10−6 °C−1 (650 °C). Maximal thermal expansion of the structure along of the chain direction [0 1 0] is caused by the partial straightening of chain zigzag. Hinge mechanism of thermal expansion is discussed.