Crystal structure of parasibirskite (CaHBO3) and polymorphism in sibirskite and parasibirskite

Abstract

The crystal structure of parastbirskite (CaHBO3) obtained at Fuka, Okayama Prefecture, Japan, was determined by Monte Carlo simulation using powder X-ray diffraction data and confirmed by Rietveld refinement Parasibirskite is monoclinic with space group P2(1)/m and cell constants a = 6 722(4), b = 5 437(2), c = 3 555(2) angstrom, and beta = 93 00(5)degrees The calcium atom is surrounded by seven oxygen atoms with a monocapped distorted trigonal prism arrangement The edge-shared CaO7 polyhedra form a layer structure elongated along the crystallographic b- and c-axes These CaO7 layers are connected to each other by BO3 triangles, thus forming the parastbirskite structure The hydrogen atom was presumed to connect to the O1 atom from the bond-valence calculation Parasibirskite is a dimorph of sibirskite (CaHBO3), which consists of CaO6 double chains connected by BO3 triangles