α-PbO₂-related phase appearing in the Sn^IV–Ta–O system transformed from cation-ordered fluorite-related phase

Abstract

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 ≤ 1073 K 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 250 K 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 Sn–Nb–O systems was discussed on the basis of the characteristics of the Ta- and Nb-ions.