Structural Study of Lead Orthophosphovanadates: Role of the Electron Lone Pairs in the Phase Transitions

• Published in: Journal of solid state chemistry Vol. 103; no. 2; pp. 490 - 503
• Main Authors: Kiat, Jean-Michel, Garnier, Pierre, Calvarin, Gilbert, Pinot, Marcel
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.04.1993; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; Exact sciences and technology; Physics; Structure of solids and liquids; crystallography; Heteropolysalt; Phosphates; Temperature; Bond length; Neutron diffraction; Experimental study; Solid solution; X ray diffraction; Inorganic compound; Phase transformation; Vanadates; Chemical composition; Crystalline structure
• DOI: 10.1006/jssc.1993.1127
• ISSN: 0022-4596

A structural investigation of the lead orthophosphovanadate compounds Pb3P2xV2(1-x)O8 has been performed using neutron and X-ray powder diffraction. Results obtained by a Rietveld analysis are presented and compared with previous data on "pure" lead vanadate (x = 0) and lead phosphate (x = 1). The influence of the substitution of phosphorus for vanadium on the mechanism of the phase transitions of lead vanadate and lead phosphate is analyzed. For a given concentration x, a decrease of the distance between the layers of lead atoms is observed when the temperature diminishes. In the vanadium- and phosphorus-rich compounds a critical value is reached where the lone pairs of the lead atoms repel each other as well as the proximate oxygen atoms: this induces a phase transition from the high temperature phase. For the other compounds (middle part of the phase diagram) a critical value is not reached and the high temperature phase remains stable down to 5 K. The possibility of a monoclinic local order inside the high temperature rhombohedral phase is also pointed out.