Mössbauer spectroscopic investigations of metal incorporation within nasicon-related NbTiP3O12

Abstract

The reaction of Nasicon-related NbTiP₃O₁₂(Nasicon = Na₃Zr₂Si₂PO₁₂) with elemental iron to give materials of composition Fe_xNbTiP₃O₁₂(0.00 < x⩽ 0.33) resulted in the incorporation of Fe²⁺ in a distribution of similar environments within the type I octahedral interstitial sites. The decidedly positive ⁵⁷Fe Mössbauer chemical isomer shifts are associated with the longer distances between the highly ionic Fe²⁺ species and the oxygen atoms which define the interstitial sites. Heating of the materials in air induced the migration of Fe²⁺ from the type I sites to the surface where they were oxidised and formed a macroscopic iron(III) oxide phase. The reaction of NbTiP₃O₁₂ with elemental tin to give materials of composition Sn_xNbTiP₃O₁₂(0.00 < x⩽ 0.50) also resulted in the accommodation of Sn²⁺ within the type I sites. The highly positive Sn²⁺ Mössbauer chemical isomer shifts are interpreted in terms of the presence of highly ionic Sn²⁺ species. Thermal treatment in air induced the migration of the Sn²⁺ ions from the type I sites to the surface where they were oxidised to a discrete tin(IV) oxide phase. Compounds of composition Fe_0.25-xSn_xNbTiP₃O₁₂(0.00 ⩽x⩽ 0.25) have been prepared and the Fe²⁺ and Sn²⁺ ions found to occupy the same sites as in Fe_xNbTiP₃O₁₂ and Sn_xNbTiP₃O₁₂.