First-principles estimation of electronic structure of uranium oxychalcogenides UOY, Y = S, Se, Te. Application to the INS spectra of UOS

Abstract

A consistent description of the electronic structure of the U4+(5f2) ion in the UOY (Y = S, Se, Te) compounds derived on the basis of a model first-principles calculation is presented. The crystal field potential is discussed in detail. Special attention is paid to contributions of non-equivalent ligand groups. Their competition and variation along the series explain apparently random total values of the crystal field parameters (CFPs). Discussion of an interplay of factors dependent on the coordination geometry and so called `intrinsic parameters' describing the separated metal-ligand (ML) linear ligators points to presumably rational ranges of actual values of CFP. Contrary to some earlier findings, the calculations evidence an approximate axial character of the crystal field potential. A dependence of the intrinsic parameters on the ML distance is examined thoroughly. The new numerical data show a dependence weaker than that reported before. At small ML distances, the intrinsic parameters behave in a manner characteristic of the metallic state. Some simplifications of the common phenomenological models suggested on the basis of the ab initio calculations open new possibilities of interpretation of complex magnetic and other properties of UOY. The obtained eigenstates of the uranium ion and simulated temperature characteristics of such quantities as the magnetic susceptibility or heat capacity may serve as good reference data. The crystal field (CF) parameters estimated from first principles have been used as starting data in the conventional phenomenological description of the recent inelastic neutron scattering (INS) data reported for UOS by Amoretti et al. In contrast to the earlier phenomenological approaches the effect of the term mixing has been taken into account. In initial steps of the fitting of the INS transition energies, a variation of the CF parameters has been restricted by using the angular overlap model. Then, the CF parameters have been refined to reproduce not only the observed energies of the INS transitions but also their relative intensities and the magnitude of the ordered magnetic moment. Other measurable quantities such as the temperature dependences of magnetic susceptibility or the Schottky contribution to the heat capacity restored according to the proposed CF model have been shown to agree satisfactorily with the corresponding experimental data. The CF scheme inferred here for UOS differs essentially from that proposed by Amoretti et al. However, the latter, recalculated in an extended function basis allowing for the term mixing, has been demonstrated to be not convergent with the original findings.