Investigation of the K2NiF4,-type layer oxide fluorides Sr2FeO3+xF1−x (0 ≦x ≦ 0.20)

Preparation and characterization of the Sr2FeFe O3+xF1−x oxide fluorides which have a K2NiF4-type structure are reported for x = 0.00, 0.11, and 0.20. The compound containing only trivalent iron (x = 0.00) is an insulator, whereas x = 0 leads to semiconducting behaviour. The high value of the Néel temperature of Sr2FeIIIO3F (TN = 358 K) implies apical location of fluorine. With increasing x, TN decreases sharply because of the competition of the antiferromagnetic couplings with ferromagnetic superexchange interactions of FeIII–O-FeIV type. A Mossbauer spectroscopy investigation shows that for Sr2FIII,O3F, the spins of the FeIII ions lie in the (001) and (002) planes. For x ≠ 0, the Mössbauer spectra characterize two kinds of surrounding for FeIII and FeIV cations: one with only FeIII as nearest iron neighbours and one with at least one FeIV as nearest iron neighbour. At 4.2 K, a statistical distribution of high-spin FeIII and of low-spin FeIV accounts well for the experimental results. Above TN the spectra suggest that FeIV cations with only FeIII as nearest iron neighbours become high-spin.