6d5f and 5f2 configurations of U4+ doped into LiYF4 and YF3 crystals

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

“…III D to help understand the quenching of the second step in photon cascade emission in YF 3 :U 4+ . 4 A. The electronic structure of the 5f 2 manifold of Cs 2 GeF 6 :U 4+ : Luminescent levels The 5f 2 manifold is formed by parallel potential-energy surfaces whose U-F bond lengths average to 2.174± 0.005 Å and a 1g vibrational frequencies to 563± 6 cm −1 ͑see Table II͒.…”

“…[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.…”

“…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.…”

“…Also, the need for charge compensation leads to several U 4+ defects whose luminescence properties sum up into a complex emission spectra. In effect, variation of temperature has revealed the existence of two different crystallographic sites in YLiF 4 and YF 3 hosts, which differ in the way U 4+ excess charge is compensated; the emission of these two sites sums up and leads to two groups of 5f 1 6d 1 → 5f 2 emissions in LiYF 4 :U 4+ and to 5f 2 → 5f 2 and 5f 1 6d 1 → 5f 2 emissions in YF 3 :U 4+ . 4 Taking all this into account, the study of U 4+ defects in highly symmetric ͑in particular, centrosymmetric͒ fluoride hosts is timely and interesting because of two reasons: ͑i͒ their potentiality as either UV solid-state lasers or as quantum cutters in the visible and ͑ii͒ their use as highly symmetric models for more complex, low-symmetry U 4+ -doped fluorides, such as those referred above.…”

“…In effect, variation of temperature has revealed the existence of two different crystallographic sites in YLiF 4 and YF 3 hosts, which differ in the way U 4+ excess charge is compensated; the emission of these two sites sums up and leads to two groups of 5f 1 6d 1 → 5f 2 emissions in LiYF 4 :U 4+ and to 5f 2 → 5f 2 and 5f 1 6d 1 → 5f 2 emissions in YF 3 :U 4+ . 4 Taking all this into account, the study of U 4+ defects in highly symmetric ͑in particular, centrosymmetric͒ fluoride hosts is timely and interesting because of two reasons: ͑i͒ their potentiality as either UV solid-state lasers or as quantum cutters in the visible and ͑ii͒ their use as highly symmetric models for more complex, low-symmetry U 4+ -doped fluorides, such as those referred above. Consequently, we have decided to study the structure and 5f 2 → 5f 2 and 5f 2 → 5f 1 6d 1 transitions of U 4+ defects in cubic Cs 2 GeF 6 host using quantum-chemical methods that, making explicit use of wave functions, incorporate electron correlation, relativistic effects including spin-orbit coupling, and host embedding effects in Cs 2 GeF 6 : ͑UF 6 ͒ 2− embedded-cluster calculations, which are similar to the ones performed on U 3+ and U 4+ defects in the Cs 2 NaYCl 6 and Cs 2 ZrCl 6 hosts.…”