Jean-Claude Krupa

New parametric analysis of the optical spectra of U4+ in ThSiO4

Crystals of ThSiO4–U4+ were grown by the flux method. Absorption and emission spectra were recorded from 4000 to 30 000 cm−1 at liquid helium and various higher temperatures. The transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least squares procedure. The crystal field parameters for U4+ in ThSiO4 were compared to those for U4+ in other hosts.

Synthesis and properties of a new fluorescent single crystal. Applications in radiophotovoltaic energy conversion.

We prepared ThBr4 on a large scale and grew a single crystal by the Bridgman method. The volume obtained is around few cubic centimeters. ThBr4 is fluorescent under UV (λ<300 nm) excitation. The maximum of the excitation function is about 270 nm. The emission of ThBr4 takes place at 405 nm in a single broad band. ThBr4 is also radioluminescent and as thorium is radioactive, ThBr4 crystal is a self-excited scintillator. ThBr4 crystal also acts as a nuclear radiation detector and nuclear energy converter.

Reinterpretation of the absorption spectrum of UCl4 single crystal

Abstract The absorption spectra of UCl 4 single crystals have been measured from 4000 to 30 000 cm −1 at temperatures ranging from 4.2 K to room temperature. The transitions were assigned by comparing the strong absorption features of UCl 4 to those of ThCl 4 :U 4+ . The energy levels were fitted to a semiempirical Hamiltonian in D 2d symmetry. A r.m.s. deviation of 60 cm −1 in the fit to 26 assigned levels was achieved using the following set of parameters (cm −1 ): F 2 = 42 561 ± 235; F 4 = 39 440 ± 634; F 6 = 24 174 ± 185; ξ = 1805 ± 8; α = 31 ± 1; β = −576 ± 168; (γ = 1200); B 2 0 = −903 ± 151; B 4 0 = 766 ± 220; B 4 4 = −3091 ± 185; B 6 0 = −1619 ± 482; B 6 4 = −308 ± 280.

6d-5f transitions of Pa4+ in ThBr4

Abstract Optical absorption and luminescence spectra resulting from 6d–5f transitions in Pa4+ : ThBr4 are reported. Anti-Stockes fluorescence and I.R. emission observed in selective laser excitation are discussed.

Energy levels of Pr3+ in the modulated incommensurate structure of ThCl4

Abstract The matrix β-ThCl4 which presents an incommensurate modulated structure at low temperature is doped with the lanthanide ion Pr3+. The strong features in the absorption and the laser-excited fluorescence spectra have been interpreted as arising from levels of Pr3+ in the site of D2d symmetry. In a least- squares fit calculation, the best parameters are determined.

Luminescence of tetravalent uranium in the incommensurate phase of ThBr4 and ThCl4

Abstract Luminescence properties of U4+ through lineshape and lifetime are directly affected by the modulated structure of the incommensurate phase of the host matrix : ThBr4 and ThCl4.

Lifetime variations of U4+ excited states in the incommensurate phase of β-ThCl4

Abstract The incommensurate phase of thorium tetrachloride (β-ThCl 4 ) is characterized by a continuous distribution of sites of slightly different symmetries induced by a continuous and weak displacement of the Cl - ions around the thorium (uranium) ions. The lifetimes of the radiative levels of tetravalent uranium diluted in the incommensurate phase of β-ThCl 4 have been measured, at low temperature, as a function of the occupied site symmetries. It is shown that they are governed mainly by the radiative process probability.

Spectra of U4+ ion at a site of D2d 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 Slaters 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.