Crystalline Field in Chrome Alum

Abstract

A calculation is made to test the detailed applicability of the crystalline field approximation for describing magnetic properties of chrome alum. The theoretical susceptibility formula of Schlapp and Penney represents the experimental susceptibility as a function of temperature reasonably well provided the parameter D_q, which gives a measure of the interaction between the crystalline field and the paramagnetic ion, is arbitrarily chosen to give the best fit. Here D_q is calculated; a model of the surroundings of the Cr⁺⁺⁺ ion is used to determine the crystalline field. The calculated D_q is in poor agreement with experimental values, from which it is inferred that the interaction between the Cr⁺⁺⁺ ion and its surroundings is so strong that the crystalline field approximation is inadequate to give detailed agreement with experiment. The results are expected to apply to other hydrated paramagnetic salts of the iron group.