New pyrochlore phases in the ternary oxide Tl-Sb-O

A new series of pyrochlore phases in the ternary oxide Tl-Sb-O have been prepared at 650 °C in air and characterised by XRD, FT-IR, EDX, XPS, and density measurements. The unit cell parameters were continuously decreased from 10.3073(1) Å for Tl/Sb = 1/4 to 10.28204(8) Å for Tl/Sb = 1/2 as the content of Tl³⁺ cations was increased. The Rietveld analysis shows that Tl⁺, Tl³⁺, Sb³⁺, and Sb⁵⁺ cations were positioned in the 32e, 16d, 96g, and 16c sites, respectively. The Tl³⁺ cations progressively incorporated into the 16d sites in the space group Fd3m, substituting for Sb³⁺ cations in the 96g sites. The FT-IR absorption band at around 570 cm^-1, corresponding to the Sb³⁺-O' stretching vibrational mode, confirmed the existence of Sb³⁺ cations in the asymmetric 96g sites. These Tl-Sb-O phases are formulated with the composition (Tl⁺_2/3-2δ)[Sb³⁺_1-y Tl³⁺_y]_2/3Sb⁵⁺₂ O_6.33-δ (δ ∼ 0.5) and exhibit unique structural features with three types of A/A' cations in the 16d, 32e, and 96g sites.