Preparation and X-ray diffraction data for the new pyrochlores Pb2[M0.33Sb1.67]O6.5(M = Mg, Ni, Cu, or Zn)

Abstract

From mixtures of PbO, Sb₂O₃, and MO (M = Mg, Ni, Cu, or Zn), four new oxide pyrochlores of stoicheiometry Pb₂[M_0.33Sb_1.67]O_6.50 have been synthesized. These compounds are coloured powders giving X-ray diffraction patterns typical of cubic pyrochlores, with space group Fdm(no. 227), Z= 8, and a values from 10.500 5(2)(M = Ni) to 10.512 9(2)Å(M = Zn). The lowest discrepancy factors R between the observed and calculated intensities were obtained for Pb in 16(c) positions, M and Sb (1 : 5) randomly distributed in 16(d), oxygen atoms in 48(f) and in half of the 8(a) sites, and x values for the oxygen positional parameter (origin at centre, m) of 0.430 (M = Cu) and 0.431 (M = Mg, Ni, or Zn). The co-ordination of M and Sb is nearly octahedral. For seven-co-ordinated Pb^II the stereochemical influence of the unshared electron pair is demonstrated. Observed interatomic distances are in agreement with those calculated.