Preparation and crystal data of the New Pyrochlores Pb2[M1.5Te0.5]O6.5 (M = Ti,Zr, Sn,Hf)

From mixtures of PbO, MO₂ (M = Ti, Zr, Hf), SnO, and TeO₂, four new oxides Pb₂[M_1.5Te_0.5]O_6.5 have been obtained as yellow powders giving X-ray diffraction patterns characteristic of cubic pyrochlores, S.G. Fd3 m (No. 227), Z = 8, and a/Å values from 10.3529(1) (M = Ti) to 10.7406(1) (M = Zr). The best R factors, from 0.0465 (M = Ti) to 0.0242 (M = Hf), were obtained for Pb in 16(c) positions, M and Te (3:1) randomly distributed in 16(d), oxygen atoms in 48(f) and in a half of the 8(a) sites, and x values for the oxygen positional parameter (origin at center, 3 m) from 0.436 (M = Ti) to 0.421 (M = Zr). For the compounds of Ti and Zr the angles of the coordination polyhedra around the metals are reported. For seven-coordinated Pb^II the stereochemical influence of the nonbonded electron pair is shown. Apparent interatomic distances agree with those calculated.