Spectroscopic study of the incommensurate phase of ThBr4 via the optical and magnetooptical properties of U4+

Abstract

U4+ is used to probe the incommensurate structure of ThBr 4 via absorption and magnetic circular dichroism (MCD) measurements. Below Tc = 95 K, the crystal field experienced by U 4+ ions is either of D2d or D2 symmetry. This can be simulated by varying a phase angle ϕ form 0(D 2d) to ± π/2 (D2). One very neat advantage of MCD over absorption is that the D2d centres show up very clearly in the former due to the occurrence of a derivative-like singularity. We present a model to simulate absorption, Zeeman and MCD profiles. It accounts satisfactorily for most of our data and we arrive at the following main conclusions : (i) it is probable that the phase is truly incommensurate since the lower limit of the number of sites is found to be 80; (ii) for a low concentration of U4+ (2 x 10-4), we found no evidence of a pinning of the incommensurate modulation by the impurities. MCD provides also a definite proof that the ground state of U 4+ in ThBr4 is Γ4 with a D2 d symmetry. Finally, we determine the Landé factors of five excited states, our findings being in fair agreement with the results of recent crystal field calculations