An Improved X-Ray Method for Determining Cation Distribution in Ferrites

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Abstract
Cation distribution in many ferrites has been difficult to determine by x-ray diffraction since the scattering factors of such cations as Ni, Co, Mn, and Cu are close to that of Fe. By using radiation of a wavelength close to the absorption edge of one of the cations, as suggested by Bertaut, sufficient difference in x-ray scattering factors can be developed because of anomalous dispersion, thus allowing the cation distribution to be ascertained. This method was employed in the case of nickel ferrite. As a modification of Bertaut's technique, ratios of intensities of the diffracted peaks for FeKα and FeKβ radiation from nickel ferrite were used to eliminate certain factors in the intensity equation which are difficult to calculate, for example, the absorption and temperature factors. A least-squares technique was used to determine the best values of the cation distribution and the oxygen position for the intensity ratios obtained for seven reflections. It was found that the cation distribution coefficient was 0.48±0.02, in agreement with the results obtained by magnetic-moment measurements and neutron diffraction. The oxygen positional parameter was found to be 0.240±0.002 of the lattice parameter with an octahedral position at the origin, which is in good agreement with the value obtained using neutron diffraction. The method appears to be of general value in distinguishing between two elements distributed over nonequivalent positions in a crystal lattice when their x-ray scattering factors are almost equal.