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

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 Birich 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, P 21c, a=13.275(3) ?, c=5.500(2) ?, 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*=a*+b*+c*, compatible with the Bravais lattice 4/mmmP (GAMMA). 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 cell2a,2a, c leads to a modulation vector q*=a*+c*. The (3+1)-dimensional Bravais lattice becomes mmmC(10) and the super space group is Ccc2(10). 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 P 21c.