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

Crystal structure and structural disorder of (Ba0.65Ca0.35)2SiO4 were investigated by laboratory X-ray powder diffraction (CuKα1). 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 image, Z=4) with lattice dimensions a=0.57505(1) nm, c=1.46706(2) nm and V=0.42014(1) nm3. 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 Rwp=9.56% (S=1.48), Rp=7.29%, RB=1.82% and RF=0.88%. This compound is most probably homeotypic to glaserite.