Topotactic Fluorine Insertion into the Channels of FeSb2O4-Related Materials.

Benjamin P. de Laune,Hien-Yoong Hah,Frank J. Berry,Colin Greaves,José F. Marco,John V. Hanna,Jacqueline A. Johnson,Charles E. Johnson,Gregory J. Rees

doi:10.1021/acs.inorgchem.7b01613

Abstract

This paper discusses the fluorination characteristics of phases related to FeSb2O4, by reporting the results of a detailed study of Mg0.50Fe0.50Sb2O4 and Co0.50Fe0.50Sb2O4. Reaction with fluorine gas at low temperatures (typically 230 °C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb-F-Sb bridges. To date, these reactions have been observed only when Fe2+ ions are present within the chains of edge-linked octahedra (FeO6 in FeSb2O4) that form the structural channels. Oxidation of Fe2+ to Fe3+ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe3+ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents.

Highlights

Fluorine insertion into low-dimensional oxides is well understood and results in changes in structural properties, e.g., increased separation in layered materials, and electronic characteristics that are associated with oxidation of transition metal ions present in the parent material
The samples Co0.50Fe0.50Sb2O4 and Mg0.50Fe0.50Sb2O4 were selected for detailed study from a wide range of synthesized compounds, since they offered sufficient fluorine contents for reliable structure analysis while avoiding complications of broadened diffraction peaks seen for samples with higher iron contents; this occurs for oxygen-excess phases as previously discussed.[3]
Phases related to FeSb2O4, and which contain some Fe2+ cations, have been shown to undergo topotactic fluorine insertion when heated at low temperatures in 10% F2 in nitrogen

Summary

INTRODUCTION

Fluorine insertion into low-dimensional oxides is well understood and results in changes in structural properties, e.g., increased separation in layered materials, and electronic characteristics that are associated with oxidation of transition metal ions present in the parent material. These reactions have recently been reviewed,[1] and a classic example is provided by the conversion of semiconducting La2CuO4 to superconducting La2CuO4Fx by heating in ambient pressure fluorine gas.[2] Similar reactions may be observed for oxygen insertion, albeit generally at elevated pressures. High levels of oxygen insertion occur when the Fe2+ concentration is high, e.g., in FeSb2O4, and this causes structural strain and broadening of diffraction peaks This is not apparent for compounds such as Fe0.50Mg0.50Sb2O4, for which detailed investigations are possible. We show that similar reactions occur, but the Sb−F bonding preferences are different from Sb−O and result in significant differences to the structural characteristics, and to the nature of the redox processes involved

EXPERIMENTAL DETAILS

RESULTS

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES