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 Bi 2 O 3 , PbO, MnO 2 , and (NH 4 ) 2 HPO 4 in the ratio 1:1:2:2 displayed several phases including a Bi-rich phase with the approximate composition (Bi 3؊x Pb x )MnP 2 O y and a Pb-rich phase with the approximate composition (Bi x Pb 4؊x )MnP 3 O y . A single crystal of the Bi-rich phase was selected. The title compound is tetragonal, P4 2 1 c, a ؍ 13.275(3) A > , c ؍ 5.500(2) A > , Z ؍ 4. The structure re5nement converged to R1 ؍ 0.0442. On the basis of the X-ray di4raction re5nement, the formula is Bi 2 PbMnP 2 O 12 . The crystal is an inverse twin. Electron di4raction revealed that the X-ray di4raction lattice parameters are from a subcell and the unit cell is 2a sub , 2b sub , 3c sub . The detailed study of reciprocal space, reconstructed from observations made using single-crystal electron di4raction data, leads us to an apparent wave vector q* ؍ 1 2 a* ؉ 1 2 b* ؉ 1 3 c*, compatible with the Bravais lattice 4/mmmP ( 1 2 1 2 ). However, this choice leads to forbidden re6ections, e.g. h00, h ؍ 2n ؉ 1, due to the presence of a 2 1 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* ؉ c*. The (3 ؉ 1)-dimensional Bravais lattice becomes mmmC(10 ) and the super space group is Ccc2(10 ). The re5nement of the single crystal X-ray di4raction data based on Ccc2 consisting of two 903 twin domains led, as expected, to the identical result as the re5nement in P4 2 1 c.
The crystal structure was determined from the X-ray di4raction intensities based on the subcell and hence represents an averaged structure. The PO 4 radical can be present in two orientations. Pb occupies only 1 2 of the crystallographic 8e positions. The Pb atoms bond through oxygen atoms from PO 4 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.
2002 Elsevier Science (USA)