LATTICE-DYNAMICS AND HYPERFINE INTERACTIONS IN M2FEO4 (M=K+,RB+,CS+) AND M'FEO4 (M'=SR-2+,BA-2+)

A number of alkali and alkaline earth ferrates [Fe(VI)] have been prepared and characterized by powder X-ray diffraction and temperature-dependent 57Fe Mössbauer effect studies. The isomer shift parameter (78 K) is only weakly dependent on the nature of the cation and reflects the large covalency in the Fe-O bond. The temperature dependence of the isomer shift yields, via the second-order Doppler shift formalism, a value for the effective vibrating mass, Meff, of~105 g mol-1, nearly independent of the nature of the cation and intermediate between the bare ion atom mass (57 g mol-1) and the formula weight of the FeO42- moiety (121 g mor1). The temperature dependence of the recoil-free fraction, when combined with the effective vibrating mass calculation, yields the “Mössbauer lattice temperature” θM, which is~197±5 K and is insensitive to the nature of the cation or the stoichiometry of the ferrate salt. Magnetic ordering has been observed for the K+, Rb+, Cs+, ad Ba2+ferrates and has been examined in detail for the potassium salt in the temperature range 0.015 ≤T ≤4.2 K. For the potassium salt, the magnetic ordering temperature is 4.2±0.1 K and the internal hyperfine field in the low-temperature limit is~147±1 kOe. These data are in reasonable agreement with the spontaneous magnetization temperature behavior expected for an S=1 (3d2) spin system. © 1979, American Chemical Society. All rights reserved.