Crystal structure and structural disorder of (Ba0.65Ca0.35)2SiO4

Crystal structure and structural disorder of (B0.65Ca0.35)(2)SiO4 were investigated by laboratory X-ray powder diffraction (CuK alpha(1)). The initial structural model with eleven independent atoms in the unit cell was determined using direct methods, and it was further modified to a split-atom model, in which the two types of Ba/Ca atoms and two types of SiO4 tetrahedra were, respectively, positionally and orientationally disordered. The crystal structure is trigonal (space group P (3) over bar ml, Z = 4) with lattice dimensions a= 0.57505(1) rim, c = 1.46706(2) nm and V = 0.42014(1) nm(3). The validity of the structural model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The final reliability indices calculated from the MPF were R-wp = 9.56% (S = 1.48), R-p = 7.29%, R-B = 1.82% and R-F = 0.88%. This compound is most probably homeotypic to glaserite. (C) 2007 Elsevier Inc. All rights reserved.