The structure of chromium potassium alum

The magnetic moment density in the cubic compound K$_{2}$NaCrF$_{6}$ has been measured by polarized neutron diffraction. Since the orbital moment in the chromium ions is quenched, and since the crystal contains non-overlapping (CrF$_{6}$)$^{3-}$ ions, it is possible to interpret the data as the unpaired spin density of isolated clusters. A simple molecular orbital model of the cluster is used, with one adjustable covalency parameter $A_{\pi}$. This model, although it predicts the qualitative features of the observed deviations from free Cr$^{3-}$ ion behaviour, is not good enough to fit the intensities of all the reflexions. A Hartree-Fock calculation, of the type recently carried out on the octahedral (NiF$_{6}$)$^{4-}$ cluster, is needed but has not been attempted here.