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

A consistent description of the electronic structure of the U4+(5f(2)) 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 discuss ed in detail. Special attention is paid to contributions of nonequivalent l igand groups. Their competition and variation along the series explain appa rently random total values of the crystal field parameters (CFPs). Discussi on of an interplay of factors dependent on the coordination geometry and so called 'intrinsic parameters' describing the separated metal-ligand (ML) l inear ligators points to presumably rational ranges of actual values of CFP . Contrary to some earlier findings, the calculations evidence an approxima te axial character of the crystal field potential. A dependence of the intrinsic parameters on the ML distance is examined tho roughly. 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 interpretati on of complex magnetic and other properties of UOY. The obtained eigenstate s of the uranium ion and simulated temperature characteristics of such quan tities as the magnetic susceptibility or heat capacity may serve as good re ference data. The crystal field (CF) parameters estimated from first principles hale been used as starting data in the conventional phenomenological description of the recent inelastic neutron scattering (INS) data reported for UOS by Amor etti et al. In contrast to the earlier phenomenological approaches the effe ct 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 ha s been restricted by using the angular overlap model. Then, the CF paramete rs have been refined to reproduce not only he observed energies of the INS transitions but also their relative intensities and the magnitude of the or dered 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 t o agree satisfactorily with the corresponding experimental data. The CF sch eme inferred here for UOS differs essentially from that proposed by Amorett i er al. However, the latter, recalculated in an extended function basis al lowing for the term mixing, has been demonstrated to be not convergent with the original findings.