NEUTRON AND X-RAY RIETVELD ANALYSIS OF THE 3 PHASES OF LEAD ORTHOVANADATE PB3V2O8 - IMPORTANCE OF THE ELECTRONIC LONE PAIRS IN THE MARTENSITIC TRANSITIONS

Neutron and X-ray powder diffraction investigations have been performed for the three phases of Pb3V2O8. The structural results from the Rietveld refinements are analyzed and compared with the available data of both phases of Pb3P2O8. The rhombohedral high temperature structures consist of layers of [PO4] or [VO4] tetrahedra. Inside this skeleton of tetrahedra the lead atoms have bonds of different lengths with the oxygen atoms in order to adapt themselves to the packings. The layers of the structures are connected by the electronic lone pairs of the lead ions inside the interlayers. The different phase transitions of both compounds mainly differ by the way the steric occupation of the electronic lone pairs is realized inside the interlayers. The microscopic interpretation of the large shears observed during the martensitic-like transitions is explained on the basis of the tilting of the lone pairs at the critical temperatures. © 1991.