Orbital Hybridization in Uranium Compounds and its Influence on Electronic Properties
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1104-NN04-08
Orbital Hybridization in Uranium Compounds and its Influence on Electronic Properties Wei Wang, Hong Zhang, and Guokui Liu Argonne National Laboratory, Argonne, IL, 60439 ABSTRACT Computational analysis and modeling of spectroscopic properties of trivalent uranium in crystals of hexagonal symmetry have been conducted with inclusion of the crystal-field induced orbital hybridization between the 5f3 and 5f26d configurations. It is shown that, in the absorption spectrum with energy above 20,000 cm-1, the mixing of 5f3 and 5f26d states is significant. The spectrum in this region cannot be interpreted by the conventional model of crystal field theory. The Judd-Ofelt theory fails completely in predicating the intensities of optical absorption from the ground state to the configuration mixed excited states. A new Hamiltonian including the odd ranks of crystal field interaction is diagonalized on the bases of all 5f3 and 5f26d states. A simulation of absorption spectrum is optimized in comparison with the experimental spectrum for determination of the Hamiltonian parameters. INTRODUCTION Optical spectroscopy is an effective and primary method to probe the electronic properties of atoms, molecules and the solid-state materials, therefore capable of revealing the macroscopic properties of materials. Spectroscopic studies on lanthanide and actinide ions in solids have significant accomplishment in last 50 years not only in developing advanced luminescent materials [1] but also in advancing new physics, such as energy up-conversion and electron-phonon interactions [2-5]. In general, the combination of spectroscopic experiments and theoretical analysis offers a unique methodology that can provide information for the desired on-site properties of materials, chemical products and biotechnology process at the atomic level, which is essential to understand the exact mechanisms involved in local behaviors. With the cutting edge spectroscopy technology, the localized 4f electrons of lanthanides and 5f electrons of actinides are ideal probes for in-situ environment monitoring. Based on the crystal field theory (CFT) [6-8], electronic energy level structure and local symmetry in solid-state materials can be identified precisely with sharp line featured f-f intra-configuration transitions. Since the experimental results from optical spectroscopic measurements were limited in the low energy region of the 4fn (5fn) configurations in many cases, the conventional CFT was established primarily for analyzing the energy level structure of lanthanide (actinide) ions in crystals in infrared-visible region. Moreover, with the difficulties in theoretical analysis and computational modeling, the influence of orbital hybridization, namely configuration mixing, and particularly the mechanism for the parity-forbidden electric dipole transitions within the 4fn (5fn) configurations was treated as a perturbation and evaluated by the effective operator method known as Judd-Ofelt theory that involves the fn configurations only [10,11]. However
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