Study of the defect pyrochlores A(SbTe)O₆ (A=K, Rb, Cs, Tl)

Abstract

By solid-state reaction in air, four deficient pyrochlores A(SbTe)O₆ (A=K, Rb, Cs, Tl) have been prepared as pale polycrystalline powders. A(SbTe)O₆ are cubic with space group Fd¯3m (No. 227),Z=8, anda (nm) values varying from 1.01133(2) (A=K) to 1.01935(4) (A=Cs). The best discrepancyR factors were obtained for A at 32(e) positions andx positional parameters close to 1/8, origin at centre, ¯3m; antimony and tellurium distributed at random at 16(d); oxygen atoms at 48(f), withu positional parameters from 0.421 (A=Tl) to 0.430 (A=Cs). The coordination polyhedron of A has been considered as depending on thex positional parameter. The products of the thermal decomposition of A(SbTe)O₆ were examined by electron microscopy and could be identified as A₃Sb₅O₁₄ (A=Rb, Cs) and a pyrochlore phase that does not contain tellurium (A=Tl). The electric conductivity measurements led toσ (Ω ⁻¹ cm⁻¹) values between 1.0×10⁻⁹ (A=K) and 9.7×10⁻⁶ (A=Cs) at 773 K.