Phase Transitions in the BiVO4-nPbO (n = 1, 2) System: Structural–Electrical Properties Relationships
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In a general survey of nPbO-BiVO4 compounds, interesting phases corresponding to n = 1: PbBiVO5, and n = 2: Pb2BiVO6 are described. A phase transition has been unambiguously characterized for PbBiVO5. The crystal structures were solved from twinned crystals at room temperature (α phase, triclinic, S.G. P-1) and at 530°C (β phase, monoclinic, C2/m). Powder neutron diffraction experiments confirmed these settings and both room-temperature (RT) and high-temperature (HT) refinements corroborated space group choices, clearing up a literature controversy about the centrosymmetry of the α phase, and identifying structural modifications occurring under the α → β transition. Cationic substitutions for V were tested and PbBi(V1−xMx)O5 (M = P) solid solutions identified. Pb2BiVO6 (n = 2) is a compound showing several successive structural transitions, i.e., α → β → δ. Structures of α and δ forms have been previously described from powder diffraction data (x-ray and neutron). In this work, we have refined these structures from single-crystal data, and the resolution of the intermediate β form, so far unsolved, was possible through a stabilization thermal cycle; its complete structural understanding required a 4D formalism. Two new polymorphic phases, α′ and δ′, were obtained by substituting Mn or P for V; their structures are closely related to, respectively, the α phase at room temperature, and the δ phase at 680°C. Electrical conductivities of all structurally characterized compositions were investigated, and correlations were drawn between their conduction properties and structural characteristics. Conductivity properties measured under variable O2 partial pressures for Pb2Bi(V0.75P0.25)O6 were interpreted as a mixed ionic–electronic (p-type) conduction mechanism.
KeywordsPhase transitions bismuth lead oxy-vanadates oxide ion and mixed conductivity
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