Neutron powder diffraction structure and electrical properties of the defect pyrochlores Pb 1.5M2O 6.5 ( M = Nb, Ta)
Powder neutron diffraction and Rietveld analysis were used to investigate the crystal structures of the defective pyrochlores Pb 1.5Nb 2O 6.5 and Pb 1.5Ta 2O 6.5. Both materials crystallize with the symmetry of space group Fd3 m, with lattice parameters a = 10.5647(2) and a = 10.5558(2)Å, respectively. No evidence has been observed of oxygen or lead vacancy ordering in these compounds. This result is interpreted in terms of a model in which all lead present in the structure has sevenfold pyramidal coordination and forms domains separated by regions of lead vacancies with hexagonal or bipyramidal configurations of the oxygen atoms. This model, built on the assumption that the driving force in the formation of this type of defect pyrochlore is the coordination of lead, leads us to conclude that the system Pb 1+ xM2O 6+ x ( M = Nb, Ta) may exist over a range of compositions with 0.33 ≤ x ≤ 0.6, and may also explain results obtained in other studies of related materials. The electric measurements show that both compounds are predominantly electronic conductors and that the ionic contribution to the total conductivity is very small even at the highest temperatures used in the study.