Crystal structure of the monoclinic perovskite Sr 3.94 Ca 1.31 Bi 2.70 O 12

The crystal structure of the low-temperature oxidized form of Sr 49.5 Ca 16.5 Bi 34 O 151 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 P 2 l / n . Structural refinement using only neutron powder data yielded the lattice parameters a =8.38 898(29) Å, b =5.99 334(21) Å, c =5.89 586(20) Å, β=89.997(8)°, and V =296.43(3) Å 3 . This compound is a distorted perovskite phase [described in the perovskite ABO 3 formula as Sr ( Bi 0.7 Ca 0.3 ) O 3 ] with 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 approximate ratio of 2:3. The MO 6 and M ′ O 6 octahedra share corners, and are tilted 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.