Topotactic Oxidation of the Quadruple-Rutile-Type Chain Structure Na0.875Fe0.875Ti1.125O4

Abstract

Sodium removal from Na0.875Fe0.875Ti1.125O4by means of oxidizing agents leads to the formation of the defect solid solution Na0.875−δFe0.875−δTi1.125O4(0≤δ≤0.44). A systematic increase in theaparameter of the orthorhombic unit cell is observed as the sodium content is reduced, while thebparameter decreases only slightly. The cell volume remains almost constant as a consequence of the irregular change in thecparameter and the different behavior of both theaandbparameters. Structural changes have been followed by Rietveld powder diffraction analysis. Room- and low-temperature Mössbauer studies confirm the increase in the Fe4+state with decreased sodium content in the less extracted samples withδ=0.10 and 0.15, whereas a maximum Fe4+content (about 21%) is reached in a sample withδ=0.25. Surprisingly, the amount of Fe4+does not increase with further sodium extraction. This is thought to be related to the greater oxidative power of the more oxidized materials, which facilitates a subsequent partial backreduction of Fe4+formed during the oxidation procedure. Results of both Mössbauer spectroscopy and structure refinements indicate that the Fe4+cations randomly occupy both octahedral metal positions,M(1) andM(2), of each quadruple rutile unit.