UV luminescence of U4+ ions in LiYF4 single crystal: Observation of 5f16d1→5f2 transitions

Single crystals of U 4+ -doped Cs 2 GeF 6 with 1% U 4+ concentration have been obtained by the modified Bridgman-Stockbarger method in spite of the large difference in ionic radii between Ge 4+ and U 4+ 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 5f 2 −1A 1g and 5f 1 6d͑t 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 5f 2 −1A 1g → 5f 1 6d͑t 2g ͒ 1 − iT 1u ͑i =1-5͒ 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 5f 2 −1A 1g → 5f 1 6d͑t 2g ͒ 1 −1T 1u electronic origin. The electronic structure of all the states with main configurational character 5f 1 6d͑t 2g ͒ 1 has been calculated as well. The results show that the lowest crystal level of this manifold is 5f 1 6d͑t 2g ͒ 1 −1E u and lies about 6200 cm −1 above the 5f 2 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.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Ordejón

Karbowiak

Seijo

et al. 2006

“…Also, the idea that the 5 f nϪ1 6d 1 manifolds of the actinide series could have analogous value and potentiality is leading new research on crystals doped with actinide ions. [12][13][14][15] The absorption and emission electronic transition energies observed with high resolution spectroscopies, and their interpretation, are providing much knowledge on the factors governing the energy differences between the f n manifolds and the higher, partially overlapping f nϪ1 d 1 manifolds, and on the energy transfer mechanisms. In clear contrast to this, very little quantitative information is available on the local geometry of the defects around the lanthanide ͑Ln͒ and actinide ͑An͒ impurities.…”

Section: Introductionmentioning

confidence: 99%

Quantum chemical analysis of the bond lengths in fn and fn−1d1 states of Ce3+, Pr3+, Pa4+, and U4+ defects in chloride hosts

Barandiarán¹,

Seijo²

2003

It is widely believed that impurity-ligand bond distances in lanthanide ͑Ln͒ and actinide ͑An͒ doped crystals, are larger in the f nϪ1 d 1 energy levels than in the f n ones. This idea, which was not justified and is probably based on the fact that Ln 5d (An 6d) orbitals have a radial extent much larger than Ln 4 f (An 5 f ) orbitals, has been neither confirmed nor rejected experimentally in spite of the fact that a very large number of absorption/emission spectroscopic studies on f -element doped hosts exist, because the band shapes depend on the square of the bond length offsets between initial and final electronic states. Recent quantum chemical calculations on Ln and An impurities in fluoride and chloride cubic hosts, which considered host embedding, dynamic electron correlation, and relativistic spin-free and spin-orbit coupling effects, have shown that impurity-ligand bond distances are classified in three sets according to their configuration, with the following trend:) 1 ͔, in contradiction with the assumed expectations. In this paper we give an interpretation of this, on the basis of a constrained space orbital variation analysis of the chemical bond in states of the f n , f nϪ1 d(t 2g ) 1 , and f nϪ1 d(e g ) 1 configurations of four model systems: Cs 2 NaYCl 6 :Ce 3ϩ , Cs 2 NaYCl 6 :Pr 3ϩ , Cs 2 ZrCl 6 :Pa 4ϩ , and Cs 2 ZrCl 6 :U 4ϩ . The analysis shows that the basic difference between f n and f nϪ1 d 1 configurations regarding bond effects which are responsible for the bond distance is that, in the former, all the open-shell electrons are shielded from the ligands by the 5p (6p) filled shell and the bond length is determined by closed-shell interactions between the outermost Ln 5p 6 (An 6p 6 ) shell and the ligands, whereas in the latter one electron has crossed the 5p (6p) barrier and is much more exposed to bonding interactions with the ligands, at the same time that an internal 4 f (5 f ) hole has been created which induces ligand to Ln ͑An͒ charge transfer, all of it resulting in the shown trends.

“…[1][2][3][4] It is related, on the one hand, to the wide transparency window of the fluorides that allows for very high-energy excitation and, on the other hand, to the potentiality of U 4+ -doped fluorides as either tunable UV solid-state lasers or as phosphor systems based on quantum cutting or cascade luminescence in the visible range. [1][2][3][4] When the highest 5f 2 -1 S 0 level of U 4+ defects is immersed in the 5f 1 6d 1 band and the lowest 5f 1 6d 1 level is well separated from the rest of lower f 2 states, strong, broad, and fast UV 5f 1 6d 1 → 5f 2 luminescence may be observed after high-energy excitation, 1,3 whereas if the 5f 2 -1 S 0 level is lower in energy and close to the 5f 1 6d 1 band, nonradiative decay to the 1 S 0 may occur and be followed by a number of 1 S 0 emissions ͑in the visible range͒ that may be followed, on a second step, by a series of lower-frequency emissions ͑also in the visible͒ from lower f 2 levels. 3 Whether one or the other occurs depends on the relative position of the 5f 2 and 5f 1 6d 1 manifolds, but also on the energy range of both manifolds and the number and size of energy gaps appearing between the numerous excited states, all of which result from the interplay of host effects and spin-orbit coupling on the U 4+ free-ion levels and are controlled by the choice of host crystal.…”

Section: Introductionmentioning

confidence: 99%

“…8,9 Vibrational frequencies of fluoride complexes are much higher due to the fluorine small mass, which favors nonradiative decay. Furthermore, it has been recently shown that different combinations of these features, through the choice of different fluoride hosts, may result in strong and fast 5f 1 6d 1 → 5f 2 emission, such as in LiYF 4 :U 4+ , 1,3 or slow 5f 2 → 5f 2 luminescence, such as in YF 3 :U 4+ . 4 However, the fluoride hosts studied so far, of which LiYF 4 and YF 3 are good examples, accomodate the U 4+ impurities in low-symmetry sites and lead to charged defects, all of which make it very difficult to get detailed descriptions of the defects actually formed and of their electronic structure.…”

Section: Introductionmentioning

confidence: 99%

5 f → 5 f transitions of U4+ ions in high-field, octahedral fluoride coordination: The Cs2GeF6:U4+ crystal

Ordejón

Seijo

Barandiarán

2005

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 4f /5f 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.