Characterization of novel cation-ordered compounds with fluorite and α-PbO2 related structures prepared by oxidation of Sn–Nb–O pyrochlore Original Research Article

Two cation-ordered phases were prepared by the oxidation of a pyrochlore phase as a precursor in the Sn–Nb–O system. One was a cation-ordered fluorite related phase, whose unit cell volume was eight times larger than that of the fluorite structure, similar to the κ-CeZrO4 phase. The other was a cation-ordered α-PbO2 phase, in which the ordered arrangement of cations was distinctly different from the well-known cation-ordered α-PbO2 structures such as wolflamite and columbite. The chemical compositions of the phases, including valence states of cations, local structure and electronic structure near the energy band gap were studied. The compositions of the precursor and its oxidized phases were evaluated, respectively, to be SnII1.62(NbV1.86SnIV0.14)O6.55 and SnIV0.81(NbV0.93SnIV0.07)O4.085, i.e. SnIV1.62(NbV1.86SnIV0.14)O8.17, by TG-DTA and ICP analyses, 119Sn Mössbauer spectroscopy and X-ray photoemission spectroscopy. It was ascertained using IR and Raman spectroscopies that the structural framework of the cation-ordered fluorite related phase was close to that of the precursor pyrochlore phase, and the structural framework of the cation-ordered α-PbO2 phase was distinct from that of the precursor pyrochlore. The Mössbauer parameters obtained showed a strong deviation of oxygen atoms contained in the SnO8 polyhedron in the cation-ordered fluorite related phase from the cubic arrangement for the ideal fluorite related structure, and a lower site symmetry of SnIV in the cation-ordered α-PbO2 than in rutile SnO2. An increase in the energy band gap from ∼2.5 eV for the precursor pyrochlore to ∼3.5 eV for the oxidized phases was attributed to the vanishing of Sn 5s2 lone-pair states upon oxidation of SnII in the precursor to SnIV in the oxidized phases.