Mössbauer studies of order and disorder in rutile and trirutile compounds derived from FeF2

Abstract

The occurrence or absence of cation ordering and the extent of magnetic ordering in complex phases derived from the rutile structure FeF₂ have been studied by Mössbauer spectroscopy over a wide range of temperature. The Fe²⁺ compounds FeCoNiF₆, Mg₂FeF₆, and MgFe₂F₆ are disordered rutile phases with Néel temperatures corresponding to the weighted average of those of the component metal difluorides. The local variation in nearest-neighbour cation environment leads to a variation in hyperfine magnetic field of ±30 kG about the mean for each compound. The altervalent fluoride LiFe₂F₆ shows separate spectra from Fe²⁺ and Fe³⁺ in both the paramagnetic and the magnetically ordered regions, thus demonstrating the integrity of the individual oxidation states and the absence of electron hopping. The narrow linewidths indicate complete geometric ordering of all cations in the trirutile structure and the repeating cation sequence in the c direction is shown to be Li⁺(Fe²⁺↑)(Fe³⁺↓)Li⁺(Fe²⁺↓)(Fe³⁺↑) leading to overall antiferromagnetic behaviour. By contrast the trirutile compound LiMgFeF₆ showed a range of quadrupole splittings consistent with the ordering of Fe³⁺ into every third layer along the c axis and a randomisation of Li⁺ and Mg²⁺ in the intervening two layers; this model was dictated by the absence of magnetic ordering even at 1·4 K, and the spectra at this temperature and at 4·2 K showed instead the effects of slow spin–spin relaxation characteristic of magnetically dilute Fe³⁺ ions.