The crystal structures of CeSiO4 and Ca2Ce8(SiO4)6O2

The crystal structure of the low-temperature oxidized form of Sr49.5Ca16.5Bi34O151 has been determined using a combination of neutron, synchrotron, and laboratory X-ray powder diffraction data. The structure is pseudo-orthorhombic; systematic absences and successful refinement indicated the true structure to be monoclinic, with space group P21/n. Structural refinement using only neutron powder data yielded the lattice parameters a=8.38 898(29) A, b=5.99 334(21) A, c=5.89 586(20) A, (beta)=89.997(8)°, and v-296.43(3) A^3 This compound is a distorted perovskite phase [described in the perovskite ABO3 formula as Sr(Bi0.7Ca0.3)O3] with ordering of the M-site cations, resulting in the formula A2MMʹO6. In this ordered structure, the A sites are solely occupied by Sr, the M sites mainly by Bi, while on the Mʹ sites Bi and Ca are distributed in an approximate ratio of 2:3. The MO6 and MʹO6 octahedra share corners, and are tilled with respect to the neighboring layers with an angle of ~15° around all three axes. The tilt system symbol is a^+ a^- a^- according to Glazer notation. All Bi ions are in the 5^+ oxidation state.