Phase Transitions in the BiVO₄-nPbO (n = 1, 2) System: Structural–Electrical Properties Relationships

Abstract

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(V_1−x M _x )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(V_0.75P_0.25)O₆ were interpreted as a mixed ionic–electronic (p-type) conduction mechanism.