Crystal structure of the monoclinic perovskite Sr3.94Ca1.31Bi2.70O12

The crystal structure of the low-temperature oxidized form of Sr49.5Ca16.5B i34O151 has been determined using a combination of neutron, synchrotron, an d laboratory X-ray powder diffraction data. The structure is pseudo-orthorh ombic; systematic absences and successful refinement indicated the true str ucture to be monoclinic, with space group P2(1)/n. Structural refinement us ing only neutron powder data yielded the lattice parameters a=8.38 898(29) Angstrom, b=5.99 334(21) Angstrom, c=5.89586(20) Angstrom, beta =89.997(8)d egrees and V=296.43(3) Angstrom (3). This compound is a distorted perovskit e phase [described in the perovskite ABO(3) formula as ST(Bi0.7Ca0.3)O-3] W ith ordering of the M-site cations, resulting in the formula A(2)MM'O-6. In this ordered structure, the A sites are solely occupied by Sr, the M sites mainly by Bi, while on the M' sites Bi and Ca are distributed in an approx imate ratio of 2:3. The MO6 and M'O-6 octahedra share corners, and are tilt ed with respect to the neighboring layers with an angle of similar to 15 de grees around all three axes. The tilt system symbol is a(+)a(-)a(-) accordi ng to Glazer notation. All Pi ions are in the 5(+) oxidation state. (C) 200 0 International Centre for Diffraction Data. [S0885-7156(00)00304-3].