Preparation and characterization of metastable trigonal layered MSb2O6 phases (M = Co, Ni, Cu, Zn, and Mg) and considerations on FeSb2O6

Abstract

MSb₂O₆ compounds (M = Mg, Co, Ni, Cu, Zn) are known in the tetragonal trirutile forms, slightly distorted monoclinically with M = Cu due to the Jahn–Teller effect. In this study, using a low-temperature exchange reaction between ilmenite-type NaSbO₃ and molten MSO₄–KCl (or MgCl₂–KCl) mixtures, these five compositions were prepared for the first time as trigonal layered rosiaite (PbSb₂O₆)-type phases. Upon heating, they irreversibly transform to the known phases via amorphous intermediates, in contrast to previously studied isostructural MnSb₂O₆, where the stable phase is structurally related to the metastable phase. The same method was found to be applicable for preparing stable rosiaite-type CdSb₂O₆. The formula volumes of the new phases show an excellent correlation with the ionic radii (except for M = Cu, for which a Jahn–Teller distortion is suspected) and are 2–3% larger than those for the known forms although all coordination numbers are the same. The crystal structure of CoSb₂O₆ was refined via the Rietveld method: P1m, a = 5.1318(3) Å, and c = 4.5520(3) Å. Compounds with M = Co and Ni antiferromagnetically order at 11 and 15 K, respectively, whereas the copper compound does not show long-range magnetic order down to 1.5 K. A comparison between the magnetic behavior of the metastable and stable polymorphs was carried out. FeSb₂O₆ could not be prepared because of the 2Fe²⁺ + Sb⁵⁺ = 2Fe³⁺ + Sb³⁺ redox reaction. This electron transfer produces an additional 5s² shell for Sb and results in a volume increase. A comparison of the formula volume for the stable mixture FeSbO₄ + 0.5Sb₂O₄ with that extrapolated for FeSb₂O₆ predicted that the trirutile-type FeSb₂O₆ can be stabilized at high pressures.