Synthesis and structural study of Pb2Re2O7-x pyrochlores

Pb2Re2O7-x have been synthesized in sealed evacuated silica tubes by a high-pressure, high-temperature technique. The products were characterized by X-rag powder and single-crystal diffraction, electron diffraction, energy-dispersive X-ray analysis, and thermogravimetric analysis, Anion-deficient pyrochlore-type structures of Pb2Re2O6.65 and Pb2Re2O6.75 were refined from X-ray powder data (a = 10.4314(1) Angstrom, space group Fc (3) over bar m, R-I = 0.027, R-p = 0.055) and single-crystal data (a = 10.4239(3) Angstrom, space group Fd (3) over bar m, R-F = 0.025, R-w = 0.036), respectively. Electron diffraction of the Pb2Re2O6.65 oxide revealed a superstructure corresponding to a passible space group P (3) over bar m1 with cell parameters a(h) = a(c)I root 2 approximate to 7.39 Angstrom and c(h) = 2a(c)root 3 approximate to 36.20 Angstrom. A high-pressure-induced phase transition between the Pb2Re2O7-x pyrochlore and the Pb6Re6O19 compound was found. (C) 1998 Academic Press.