The incommensurately modulated crystal structure of β-Pb2BiVO6: interpretation of the phase transition α → β → δ and conduction properties of related materials

A new polymorph of Pb2BiVO6 was prepared under ambient conditions and its crystal structure was determined by single-crystal X-ray diffraction. The phase transitions alpha -> beta and beta -> delta were identified in the mother phase; the high-temperature form delta-Pb2BiVO6 eventually decomposes at 753 K to a mixture of Pb4BiVO8 and the high-temperature form of PbBiVO5 before showing recombination at 923 K. beta-Pb2BiVO6 has an incommensurate monoclinic modulated structure. This crystal structure is twinned and complementary structural investigations of a powder sample of beta-Pb2BiVO6 by TEM diffraction studies confirmed the lattice and incommensurate modulation character. Log sigma = f(T-1) dependences for Pb2BiVO6-related materials (6% M-for-V-substituted compositions; M = Cr, Mn, P) are reported, which allow the characterization (E-a and isothermal sigma values) of the beta- as well as the delta- and alpha-Pb2BiVO6 varieties.