Antiprismatic crystal field and paramagnetic susceptibility of U 4+ sulfates

Abstract

On the basis of the simplified model of the U 4+ ion in a distorted axial crystal field, the temperature dependence of the magnetic susceptibilities of three U 4+ sulfates, U(SO 4) 2 · 4H 2O, U 6O 4(OH) 4(SO 4) 6, and U(OH) 2SO 4, within the temperature range 4.2–300°K has been investigated and interpreted. This model proved to be successful, and the only parameters fitted empirically were the values of the crystal field splitting. The antiprismatic coordination ( D 4 d ) of the uranium ion in these compounds was confirmed and its electronic ground states were determined. The system of two singlets 1 2 1 2 | 3 > ± 1 2 1 2 | − 3 > originating from the doublet 1 ± 3 > is the ground state of the uranium ion in U(SO 4) 2 · 4H 2O. An analogous system of two singlets 1 2 1 2 | 2 > ± 1 2 1 2 | − 2 > is the ground state in U 6O 4(OH) 4(SO 4) 6. For U(OH) 2SO 4, the doublet | ± 2 > is the ground state above 21°K, whereas below this temperature it becomes split into the two singlets 1 2 1 2 | 2 > ± 1 2 1 2 | − 2 > , probably because of a crystallographic distortion induced by the cooperative Jahn-Teller effect. Deviations from the D 4 d symmetry of the uranium ion coordination occurring in these compounds are discussed.