EXAFS and XPS Study of Rutile-Type Difluorides of First-Row Transition Metals

Abstract

Although most rutile‐type difluorides (MnF2, CoF2 and NiF2) have a positive thermal expansion coefficient, FeF2 has a negative thermal expansion (NTE) along the c‐axis in the high temperature region. In this study, we give an explanation of that behavior with Extended X‐ray Absorption Fine Structure (EXAFS) and X‐ray Photoelectron Spectroscopy (XPS) techniques. From EXAFS results, it has become apparent that the length of the share‐edge (Fe‐Fe) of FeF6 octahedra increased with the rise of temperature in the high temperature region. We have revealed that the force constant between nearest neighbor atoms (Fe‐F) was much larger than that between second‐nearest neighbor atoms (Fe‐Fe) in FeF2. In XPS measurements, it was discovered that the peak of F 1s of FeF2 was located at the lowest binding energy position as compared to that of other difluorides. This means that the charge density around the F atom in FeF2 was higher than that in other difluorides. It follows from this that the share‐edge repulsive force in FeF2 is larger than that in other difluorides. On account of the large repulsive force and the large force constant between nearest neighbor atoms, Fe atoms are attracted to share‐edge with the rise of temperature.