A reinvestigation of the crystal structure of α-Pb 2BiVO 6

Abstract

A previously reported β phase for Pb 2BiVO 6 has been characterized as a stable phase, and the phase transitions α→ β and β→ δ were identified in the mother phase as well as in Pb 2BiV 1− x M x O 6− y solid solutions (M=Cr, Mn); the high temperature form δ-Pb 2BiVO 6 eventually decomposes at 480 °C to a mixture of PbBiVO 5 and Pb 4BiVO 8 before showing recombination at 650 °C. The related substituted compositions behave the same way. The crystal structure of α-Pb 2BiVO 6 (I) is monoclinic, P2 1/ n, a=7.717(3) Å, b=5.845(3) Å, c=29.081(8) Å, β=94.27(1)°, Z=8. Oxygen atoms are in tetrahedral interstices formed by four Bi and Pb atoms. These tetrahedra articulate by BiPb edge sharing in two dimensions parallel to the b axis to form infinite chains. Mixed O(V Bi Pb) 4 tetrahedra bridge the O(Bi Pb) 4 ribbons by edge sharing to complete the three-dimensional articulation of the structure. α-Pb 2BiV 1− x Mn x O 6− y ( x=0.06) (II) is monoclinic, P2 1/ m, a=7.684(3) Å, b=5.822(3) Å, c=14.708(6) Å, β=100.92(1)°, Z=4. Tetrahedral units of O(2Bi 2Pb) are also present in (II). They form dimers O 2Bi 4Pb 4 by BiBi edge sharing. The dimers form a chain along the b axis by sharing BiPb edges. Two independent MO 4 tetrahedra (M=V, Mn) are present in which one has V/Mn mixed occupancy. Both tetrahedra show statistical disorder by rotation around a VO apex. The disordered tetrahedral oxygen atoms are part of the coordination sphere of Bi and Pb. The matrix {−1 0 0, 0 −1 0, 1 0 2} relates the structures and unit cells of [I], and [II].