α-Pbo₂-related phase appearing in the Sn^IV-Ta-O system transformed from cation-ordered fluorite-related phase

The cation-ordered α-PbO₂-related Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O _4.11 phase, which is a triclinic or monoclinic system with lattice parameters of a = 0.769 nm, b = 0.947 nm, c = 0.769 nm, α = 90°, β = 96.34° and γ = 90°, was synthesized by annealing of the cation-ordered fluorite-related Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O_4.11 phase at 973 ≤ T ≤ 1073K in O₂; the cation-ordered fluorite-related phase was prepared by the oxidation of the Sn^II_1.64(Ta_1.88Sn^IV_0.12)O_6.58 pyrochlore. The Sn^IV and [0.94Ta + 0.06Sn^IV] were respectively ordered similar to that of Sn^II and [0.94Ta + 0.06Sn^IV] as observed in the precursor pyrochlore phase. The superlattice diffraction due to the cation-ordering did not appear in its XRD because of the high density of the anti-phase domain boundaries parallel to {101} direction. The transformation temperature from the fluorite-related form to the α-PbO₂-related form for the Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O_4.11 was approximately 250K higher than that observed for the previous Sn^IV_0.81(Nb_0.93Sn^IV_0.07) O_4.085. The difference in the transformation behavior starting from the precursor pyrochlore phase between the present Sn-Ta-O and the previous SnNbO systems was discussed on the basis of the characteristics of the Ta- and Nb-ions.