The Crystal Structure of Bi2PbMnO4(PO4)2, a Member of a New Solid Solution Series in the Bi–Pb–Mn–P Oxide System

Abstract

Single crystals of a new oxyphosphate were found while exploring the Bi–Pb–Mn–P oxide phase diagram. The reaction product from a mixture of Bi2O3, PbO, MnO2, and (NH4)2HPO4 in the ratio 1:1:2:2 displayed several phases including a Bi-rich phase with the approximate composition (Bi3−xPbx)MnP2Oy and a Pb-rich phase with the approximate composition (BixPb4−x)MnP3Oy. A single crystal of the Bi-rich phase was selected. The title compound is tetragonal, P421c, a=13.275(3) Å, c=5.500(2) Å, Z=4. The structure refinement converged to R1=0.0442. On the basis of the X-ray diffraction refinement, the formula is Bi2PbMnP2O12. The crystal is an inverse twin. Electron diffraction revealed that the X-ray diffraction lattice parameters are from a subcell and the unit cell is 2asub, 2bsub, 3csub. The detailed study of reciprocal space, reconstructed from observations made using single-crystal electron diffraction data, leads us to an apparent wave vector q*=12a*+12b*+13c*, compatible with the Bravais lattice 4/mmmP (1212γ). However, this choice leads to forbidden reflections, e.g. h00, h=2n+1, due to the presence of a 21 axis. An assumption that the crystal consists of a modulated, twinned orthorhombic structure with space group Ccc2 based on the cell√2a,√2a, c leads to a modulation vector q*=a*+γc*. The (3+1)-dimensional Bravais lattice becomes mmmC(10γ) and the super space group is Ccc2(10γ). The refinement of the single crystal X-ray diffraction data based on Ccc2 consisting of two 90° twin domains led, as expected, to the identical result as the refinement in P421c.The crystal structure was determined from the X-ray diffraction intensities based on the subcell and hence represents an averaged structure. The PO4 radical can be present in two orientations. Pb occupies only 12 of the crystallographic 8e positions. The Pb atoms bond through oxygen atoms from PO4 to form a ring, creating channels parallel to c. Manganese and oxygen are present within the channels but the average structure prevents the determination of their location with certainty. Valence bond sums show that lead is tetravalent. The oxygen content requires tetravalent manganese.