Pyrochlores. IX. Partially Oxidized Sn₂Nb₂O₇ and Sn₂Ta₂O₇: A Mössbauer Study of Sn(II,IV) Compounds

Abstract

Partially oxidized tin(II) niobate and tantalate pyrochlores are homogeneous cubic phases. They have been shown by Mössbauer 119 Sn spectroscopy to contain both Sn(II) and Sn(IV). Each valency state is segregated on its own, crystallographically distinct sublattice as a consequence of the size disparity of Sn 2+ and Sn 4+ atoms. The strong temperature dependence of the Mössbauer Sn 2+ :Sn 4+ area ratios between room temperature and 4.2 °K indicates that the recoil-free fractions of the Sn 2+ and Sn 4+ atoms in these structures are noticeably different. The values of the recoil-free fractions at 0 °K, which were estimated from simple Einstein and Debye models, were used to obtain Sn 2+ :Sn 4+ population ratios from the Mössbauer area ratios extrapolated to 0 °K. The area ratios were shown to differ from the population ratios by 15–20% at 0 °K and by as much as 70% at room temperature. The formulae of the two nonstoichiometric pyrochlores based on the population ratios are Sn 2−2u 2+ (Sn u 4+ B 2−u 5+ )O 7−(5/2)u (B = Nb, Ta; u = 0.18–0.20). These formulae are consistent with chemical and other evidence. Mössbauer parameters and the limitations of using a Mössbauer area ratio to determine the Sn 2+ :Sn 4+ population ratio in Sn(II,IV) compounds are discussed.