E.P.R. Investigation of the incommensurate phase of ThBr₄ with the HELP of the Gd³⁺ Vn.n.n. Br⁻

Abstract

ThBr₄ exhibits a commensurate incommensurate phase transition at T_I = 95K From T_I to 4K, the phase remains incommensurate (I). In a preliminary E. P.R. investigation we have shown the efficiency of Gd³⁺ centres to investigate the incommensurate phase, through fourth order crystal field terms (2).

In the present work, we detail the properties of the particular probe which have been used, i.e. the Gd³⁺ ion substituted Th⁴⁺ and associated to a n.n.n. Br⁻ vacancy.

We focus our attention on the line shape for the ΔM_S = 5/2 + 7/2 transition. Indeed, the line shape does not obey the usual model for a plane wave modulation of displacement field which would induce “edge singularities”.

A recent investigation at 4.2K by luminescence on U⁴⁺ ions have evidenced phase pinning (3). With E. P.R. measurements, one is able to investigate the incommensurate phase in the whole temperature phase, and the plane wave modulation regime is not sufficient to interpretate the experimental results below T_I - 7K.