Synthesis, crystal structure and thermal behavior of Sr3B2SiO8 borosilicate

Single crystals of Sr3B2SiO8 were obtained by solid-state reaction of stoichiometric mixture at 1200 degrees 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) angstrom, V=263.05(11) angstrom(3). 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 degrees C. According powder XRD, the probe contains pure Sr3B2SiO8 over 1100 degrees C. The compound is not stable below 900 degrees 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 degrees C. The anisotropic character of thermal expansion has been observed: alpha(a) = -1.3, alpha(b)=23.5, alpha(c)=13.9, and alpha(v)=36.1 x 10(-6) degrees C-1 (25 degrees C): alpha(a) = -1.3, alpha(b)=23.2, alpha(c)= 5.2, and alpha(v)=27.1 x 10(-6) degrees C-1 (650 degrees 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. (C) 2010 Elsevier Inc. All rights reserved.