Spectra of U 4+ ion at a site of D 2d symmetry

Abstract

The optical spectra of U 4+ ion (5 f 2) at a site of D 2d symmetry were studied by different authors in various matrices as UCl 4 [1–5], ZrSiO 4 [6, 7]. HfSiO 4 [8], ThSiO 4 [8] and ThBr 4 [9, 4]. There also exist data on magnetic susceptibility of UCl 4 [10–13] and USiO 4 [14] but most results do not agree with those obtained from optical spectra. At this point, one can note the disparity between the previous results concerning UCl 4, which was the most studied of all these crystals and mention the necessity of performing both optical and magnetic studies on single crystals of the same origin. A recent study of these issues leads to a better agreement [5, 15]. ▪ Therefore, it seems of interest of grow some of these crystals as single crystals and study again the optical spectra of U 4+ in the D 2d symmetry. This way we could compare the spectra to one another in order to assign the absorption and emission lines more accurately. So we grew single crystals of UCl 4 and ThCl 4: U 4+ by the Bridgman method [16] and ThSiO 4: U 4+ by the flux method following Chase and Osmer [17]. Then we recorded polarized spectra of U 4+ in ThCl 4 and ThSiO 4 (Fig. 1) and unpolarized spectra for UCl 4 (Fig. 2). In all these spectra the main features are quite similar but the more clear ones are obtained with ThSiO 4:U 4+. In fact, the U 4+ absorption bands in UCl 4 are very broad, probably due to the interaction between the U 4+ ions and in ThCl 4: U 4+ they are affected by the incommensurate and modulated structure of the low temperature phase of ThCl 4. This interesting last point needs more explanations. In ThCl 4 a phase transition occurs at 40 K and the dynamic of the structure alteration was studied in the isomorphic crystal ThBr 4, mainly by neutron diffraction [18]. It was shown that the matrix loses its periodic structure in one direction (the z axis corresponding to the S 4 optical axis). The halide displacements are different in each until cell and can be described by a sinus function of the distance to the origin of the unic cell. This result in having the Th 4+ ion and therefore its substitute U 4+ ▪ ▪ in an affinity of D 2 symmetry sites and some of D 2d symmetry (each D 2 site being very close to the D 2d symmetry). This continuous modulation of the ▪ ▪ D 2 symmetry between two maximum distortions gives rise to special features which have not been seen up to now in any other optical spectra. The absorption and emission lines of U 4+ in ThCl 4, rise and fall very steeply due to edge singularities which ▪ are connected by a continuum of lines corresponding to U 4+ in the intermediate D 2 symmetries [19–21] (Fig. 3). Selective excitation of the different sites produces fluorescence lines whose energy varies continuously between the system singularities (Fig. 4). Thus in ThCl 4, The U 4+ doping ion acts as an indicator of the incommensurate structure of the low temperature phase. One question that arises then is: would a lanthanide give the same type of information? Though the phenomenon seems much smaller as one would expect with the 4f electrons which are more protected against the crystal field interaction, some special features were recorded in the spectra of Pr 3+ (4f 2) in ThCl 4 (Figs. 5, 6) and are under study [22]. In conclusion, ThSiO 4:U 4+ spectra give the clearest set of data concerning U 4+ in the D 2d site symmetry. The spectra are well polarized with only two allowed dipolar electric transitions: Γ 4 → Γ 5 (σ polarization) and Γ 4 → Γ 2 (π polarization). So the results on ThSiO 4:U 4+ appear as essential to improve the calculations of the usual spectroscopic parameters (the Slater's parameters F k, the spin-orbit constant ξ and the crystal field parameters B k q for the two other crystals ThCl 4:U 4+ and UCl 4. These calculations are underway.