Belén Ordejón

Transferability of core potentials to f and d states of lanthanide and actinide ions

An analysis is made of the transferability of frozen core potentials of neutral lanthanide and actinide elements to 4f/5f and 5d/6d states of their respective 3+ and 4+ ions. A good description of the orbital spin—orbit coupling constants ζ4f/ζ5f and ζ5d/ζ6d of the Ln3+/An4+ ions and of the 4f → 5d/5f ζ 6d transition energies is achieved by using [Kr]/[Xe, 4f] cores. The corresponding relativistic core ab initio model potentials (AIMPs) based on Cowan—Griffin atomic calculations, together with optimized Gaussian valence basis sets, are produced for the lanthanide elements Ce to Lu and for the actinide elements Th to Lr. Results are given for the Ce3+ and Pa4+ free ions and in Ce3+-and Pr3+-doped Cs2NaYCl6 and Pa4+-and U4+-doped Cs2ZrCl6 materials.

Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold.

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U(4+)-doped Cs(2)GeF(6). Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF(6))(2-) and (UF(6)Cs(8))(6+) clusters embedded in Cs(2)GeF(6), in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF(6) volume and their U-F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f(1)7s(1) configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.

The 5f2-->5f16d1 absorption spectrum of Cs2GeF6:U4+ crystals: A quantum chemical and experimental study.

Single crystals of U(4+)-doped Cs2GeF6 with 1% U4+ concentration have been obtained by the modified Bridgman-Stockbarger method in spite of the large difference in ionic radii between Ge4+ and U4+ in octahedral coordination. Their UV absorption spectrum has been recorded at 7 K, between 190 and 350 nm; it consists of a first broad and intense band peaking at about 38,000 cm(-1) followed by a number of broad bands of lower intensity from 39,000 to 45,000 cm(-1). None of the bands observed shows appreciable fine vibronic structure, so that the energies of experimental electronic origins cannot be deduced and the assignment of the experimental spectrum using empirical methods based on crystal field theory cannot be attempted. Alternatively, the profile of the absorption spectrum has been obtained theoretically using the U-F bond lengths and totally symmetric vibrational frequencies of the ground 5f2 - 1A(1g) and 5f16d(t(2g))1 - iT(1u) excited states, their energy differences, and their corresponding electric dipole transition moments calculated using the relativistic ab initio model potential embedded cluster method. The calculations suggest that the observed bands are associated with the lowest five 5f2 - 1A(1g)-->5f16d(t(2g))1 - iT(1u) (i = 1-5) dipole allowed electronic origins and their vibrational progressions. In particular, the first broad and intense band peaking at about 38,000 cm(-1) can be safely assigned to the 0-0 and 0-1 members of the a(1g) progression of the 5f2 - 1A(1g)-->5f16d(t(2g))1 - 1T(1u) electronic origin. The electronic structure of all the states with main configurational character 5f16d(t(2g))1 has been calculated as well. The results show that the lowest crystal level of this manifold is 5f16d(t(2g))1 - 1E(u) and lies about 6200 cm(-1) above the 5f2 level closest in energy, which amounts to some 11 vibrational quanta. This large energy gap could result in low nonradiative decay and efficient UV emission, which suggest the interest of investigating further this new material as a potential UV solid state laser.

Bond lengths of 4f1 and 5d1 states of Ce3+ hexahalides

Ligand and solvent effects on the bond length shift experienced by complexes of lanthanide ions upon 4f->5d excitation, addressed by means of ab initio embedded cluster calculations, are presented on the clusters (CeF{sub 6}){sup 3-}, (CeCl{sub 6}){sup 3-}, and (CeBr{sub 6}){sup 3-}in solid elpasolites, in liquid acetonitrile and in vacuo. Previous predictions of bond length shortening upon 4f->5d(t{sub 2g}) excitation seem to be general and, in particular, chloride and bromide compounds in liquid solution are predicted to be good candidates for excited-state EXAFS measurements of the distortion signs. A quantitative analysis of contributions to the bond length shifts is presented, which shows the importance of ligand field effects and points out insufficiencies in the Judd-Morrison model proposed to account for 4f->5d transitions in crystals.

5f-->5f transitions of U4+ ions in high-field, octahedral fluoride coordination: the Cs2GeF6:U4+ crystal.

The U-F bond length, totally symmetric vibrational frequency, and 5f(2) energy levels of the Cs(2)GeF(6):U(4+) crystal are predicted through quantum-chemical calculations on the embedded (UF(6))(2-) cluster. The U(4+) ions substitute for much smaller Ge(4+) retaining octahedral site symmetry, which is useful to interpret the electronic transitions. The structure of the 5f(2) manifold: its energy range, the crystal splitting of the 5f(2) levels, their parentage with free-ion levels, and the energy gaps appearing within the manifold, is presented and discussed, which allows to suggest which are the possible 5f(2) luminescent levels. The effects of Cl-to-F chemical substitution are discussed by comparison with isostructural Cs(2)ZrCl(6):U(4+). The energy range of the 5f(2) manifold increases by some 6000 cm(-1) and all levels shift to higher energies, but the shift is not uniform, so that noticeable changes of order are observed from Cs(2)ZrCl(6):U(4+) to Cs(2)GeF(6):U(4+). The comparison also reveals that the green-to-blue up-conversion luminescence, which has been experimentally detected and theoretically discussed on Cs(2)ZrCl(6):U(4+), is quenched in the fluoride host. The results of the Cs(2)GeF(6):U(4+) are used as a high-symmetry model to try to understand why efficient radiative cascade emissions in the visible do not occur for charged U(4+) defects in low-symmetry YF(3) crystals. The results presented here suggest that theoretical and experimental investigations of 4f5f ions doped in octahedral, high-symmetry fluoride crystals may be conducted even when the mismatch of ionic radii between the lanthanide/actinide ions and the substituted cations of the host is considerably large. Investigations of these new materials should reveal interesting spectroscopic features without the difficulties associated with more commonly used low-symmetry fluoride hosts.

Quantum chemical study of the lanthanide bond length contraction on Ln3+-doped Cs2NaYCl6 crystals (Ln=Ce to Lu)

The lanthanide–chlorine bond length, Re, and the frequency of the symmetric stretching mode, νa1g, of the (LnCl6)3− octahedral defect clusters embedded in Cs2NaYCl6 have been calculated for all 14 Ce3+ to Lu3+ impurities in their ground 4fn electronic state using wave-function-based ab initio methods of solid state quantum chemistry which include relativistic effects and electron correlation within the (LnCl6)3− defect clusters and quantum mechanical interactions between the (LnCl6)3− electronic group and the Cs2NaYCl6 embedding host ions. The bond distance values obtained provide useful data to improve the Judd–Morrison model of the 4f→5d energy separation by explicitly including the local distortions the Ln3+ ions produce in the Cs2NaYCl6 host. The values of the structural parameters Re and νa1g, and their variation across the series have also been studied using simpler models of the embedding host (in vacuo and Madelung embeddings), which has revealed that host effects, particularly those associated ...

Bond lengths of and states of hexahalides

Ligand and solvent effects on the bond length shift experienced by complexes of lanthanide ions upon 4f->5d excitation, addressed by means of ab initio embedded cluster calculations, are presented on the clusters (CeF{sub 6}){sup 3-}, (CeCl{sub 6}){sup 3-}, and (CeBr{sub 6}){sup 3-}in solid elpasolites, in liquid acetonitrile and in vacuo. Previous predictions of bond length shortening upon 4f->5d(t{sub 2g}) excitation seem to be general and, in particular, chloride and bromide compounds in liquid solution are predicted to be good candidates for excited-state EXAFS measurements of the distortion signs. A quantitative analysis of contributions to the bond length shifts is presented, which shows the importance of ligand field effects and points out insufficiencies in the Judd-Morrison model proposed to account for 4f->5d transitions in crystals.

Bond lengths of 4f{sup 1} and 5d{sup 1} states of Ce{sup 3+} hexahalides

Ligand and solvent effects on the bond length shift experienced by complexes of lanthanide ions upon 4f->5d excitation, addressed by means of ab initio embedded cluster calculations, are presented on the clusters (CeF{sub 6}){sup 3-}, (CeCl{sub 6}){sup 3-}, and (CeBr{sub 6}){sup 3-}in solid elpasolites, in liquid acetonitrile and in vacuo. Previous predictions of bond length shortening upon 4f->5d(t{sub 2g}) excitation seem to be general and, in particular, chloride and bromide compounds in liquid solution are predicted to be good candidates for excited-state EXAFS measurements of the distortion signs. A quantitative analysis of contributions to the bond length shifts is presented, which shows the importance of ligand field effects and points out insufficiencies in the Judd-Morrison model proposed to account for 4f->5d transitions in crystals.

Bond lengths of 4f14f1 and 5d15d1 states of Ce3+Ce3+ hexahalides

Ligand and solvent effects on the bond length shift experienced by complexes of lanthanide ions upon 4f->5d excitation, addressed by means of ab initio embedded cluster calculations, are presented on the clusters (CeF{sub 6}){sup 3-}, (CeCl{sub 6}){sup 3-}, and (CeBr{sub 6}){sup 3-}in solid elpasolites, in liquid acetonitrile and in vacuo. Previous predictions of bond length shortening upon 4f->5d(t{sub 2g}) excitation seem to be general and, in particular, chloride and bromide compounds in liquid solution are predicted to be good candidates for excited-state EXAFS measurements of the distortion signs. A quantitative analysis of contributions to the bond length shifts is presented, which shows the importance of ligand field effects and points out insufficiencies in the Judd-Morrison model proposed to account for 4f->5d transitions in crystals.