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

Abstract

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.

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

“…13 The embedding potentials that represent the quantum mechanical effects of the Cs 2 GeF 6 host onto the ͑UF 6 ͒ 2− cluster in the Cs 2 GeF 6 : ͑UF 6 ͒ 2− calculations presented here were obtained elsewhere 14 and were used in the calculations of the 5f 2 manifold of Cs 2 GeF 6 :U 4+ . 6 Within the ͑UF 6 ͒ 2− cluster, relativistic core AIMPs were used to represent the ͓Xe, 4f͔ core of U ͑Ref. 15͒ and the ͓He͔ cores of F ͑Ref.…”

“…Whether such a large local distortion could result in difficulties in the growth of Cs 2 GeF 6 :U 4+ single crystals was not addressed in Ref. 6, leaving this question open to experimental confirmation. Together with their local structure, the energies of the 5f 2 levels were obtained and analyzed, which allowed to suggest which are the possible 5f 2 luminescent levels.…”

“…In particular, Cs 2 GeF 6 :U 4+ was ruled out as a phosphor material based on cascade emission initiating in the 5f 2 crystal level emparented with the U 4+ 1 S 0 state. 6 This conclusion shifted the interest towards a detailed study of the electronic structure of the 5f 1 6d 1 manifold, whose lowest levels could be responsible for intense UV absorption and laser emission. Taking all this into account, we decided to conduct an experimental and theoretical work in order to investigate ͑i͒ whether U 4+ -doped Cs 2 GeF 6 single crystals can be grown in spite of the large difference in ionic sizes and ͑ii͒ whether intense UV absorption could be experimentally detected and theoretically interpreted and assigned, which would encourage further research of this new material as a potential UV solid state laser.…”

“…The U-F bond lengths of all 5f 2 states were calculated and found to be quite constant, so that the average bond length over the whole manifold and the standard deviation of the individual values relative to the average were found to be 2.174± 0.005 Å. 6 When compared with the Ge-F bond length in the perfect host ͓1.80 Å ͑Ref. 9͔͒ this result showed a large outward distortion by some 0.37 Å caused by the much larger U 4+ impurity ͓the estimated ionic radius of U 4+ is larger than that of Ge 4+ in sixfold coordination by some 0.44 Å ͑Refs.…”

“…[1][2][3][4][5] The need for charge compensation results in the occurrence of several local defects whose structure and distribution across the crystal are difficult to determine and whose spectroscopic properties sum up leading to complex spectra. In this context, the interest of studying U 4+ neutral defects in highly symmetric ͑in particular, centrosymmetric͒ fluoride hosts was pointed out as an alternative to the complications associated with multisites in low symmetry, 6 and a theoretical study of the 5f 2 manifold of U 4+ defects in cubic Cs 2 GeF 6 crystals was done 6 using the relativistic ab initio model potential ͑AIMP͒ embedded cluster method. 7,8 In the Cs 2 GeF 6 crystal, U 4+ ions substitute for Ge 4+ host ions creating neutral, octahedral ͑UF 6 ͒ 2− defects.…”