Absorption and Emission Spectra of Ce³⁺ in Elpasolite Lattices

Abstract

The experimental determination of the electronic energy levels for Ce 3+ in some chloro-elpasolite hosts for both the ground 4f 1 and the excited 5d 1 configurations are described. High resolution f -f absorption and f -2 T 2g d absorption and emission spectra have been recorded at low temperatures for Ce 3+ diluted into various hexachloroelpasolite lattices. A fluorescence spectrum at ~ 50,000 cm -1 is tentatively assigned to the emission from the highest 5d crystal field level , 2 E g , of a Ce 3+ impurity in Cs 2 NaErCl 6 , enabling the values of all the energy levels of both the 4f 1 and 5d 1 configurations to be given for 2 Ce 3+ in elpasolite hosts. Vibronic structure superimposed on the electronic transitions is analyzed in terms of a simple configurational coordinate model involving the ground and excited configurations. It is found that the difference in the Ce-Cl bondlength between the 4f 1 and 5d 1 configurations is ~0.04Å. Ab initio model potential calculations on the (CeCl 6 ) 3-cluster embedded in a reliable representation of the Cs 2 NaYCl 6 host support these conclusions. IntroductionAt the beginning of the 4f and the 5f transition series, the f and d electronic configurations of the di-, tri-, and tetravalent free ions are very close in energy. 1,2 This is also true for these ions in compounds. For example, the lowest energy configuration for the Ce 2+ free ion is [Xe core]4f 2 with the beginning of the opposite parity 4f5d configuration at 3277 cm -1 . However in a CaF 2 host the ground configuration is [Xe core]4f5d. 3 The Ce 3+ free ion has a 4f 1 ground configuration with the lowest 5d state at 49737 cm -1 . 1 When this ion is placed in a crystal the lowest 4f to 5d electric dipole allowed transition has been reported to be in the range of 20,000 to 40,000 cm -1 depending on the particular compound or matrix investigated. 4 Similarly, for the actinide or 5f series, the lowest 5f -6d transition for the Pa 4+ ion diluted in Cs 2 ZrCl 6 is at 20,000 cm -1 , but the free ion separation between ground 5f level and the first 6d level is ~ 50,000 cm -1 . 5 In the few structurally characterized Th 3+ organometallic compounds the 6d 1 configuration has been found to be the ground configuration, 6,7 but the ground term is from the 5f configuration and the first 6d level is 10,000 cm -1 higher in the Th 3+ free ion.The U 3+ ion in the LaCl 3 host has the first 5f 3 to 5f 2 6d 1 transition at approximately 22,000 cm -1 although in the U 3+ /RbY 2 Cl 7 system this transition is found at ~ 14,000 cm -1 . 8 Thus, as the degree of ionization increases and/or the atomic number is increased (for both the lanthanide and the actinide series) the f n configuration becomes increasingly stabilized so that the interconfigurational transitions are not observed in the visible or near ultraviolet region. Only at the beginning of both the lanthanide and actinide series is it possible to observe these two electronic configurations in this energy range. 3For the lanthanide series the Ce 3+ ion has been extensivel...

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

“…17,19 The agreement of our theoretical results with these values ͑see Table II and Fig. 4͒ is very satisfactory and points out the accuracy of the methods that have been used.…”

“…8 The remaining experimental values of vibrational frequencies do not correspond to doped crystals but to Cs 2 NaLnCl 6 perfect crystals. The comparison of the experimental data on Ce 3ϩ (300 cm Ϫ1 ) 19 and Pr 3ϩ (298 cm Ϫ1 ) 17 in Cs 2 NaYCl 6 with the perfect crystal values measured on Cs 2 NaCeCl 6 (279 cm Ϫ1 ) 44 -47 and Cs 2 NaPrCl 6 (282 cm Ϫ1 ), 46,47 respectively ͑Table II, Fig. 4͒, makes it reasonable to expect that the vibrational frequencies measured on the rest of Cs 2 NaLnCl 6 crystals should be smaller than those corresponding to the Cs 2 NaYCl 6 :Ln 3ϩ defect crystals across the series.…”

Quantum chemical study of 4f→5d excitations of trivalent lanthanide ions doped in the cubic elpasolite Cs2NaYCl6. Ce3+ to Tb3+

“…Ab initio calculations are able to provide reliable data on the structure of the lanthanide ion defects in ionic crystals in their 4f n−1 5d 1 excited manifolds, as well as other informations and analyses that complement the experiments, 8 but it is only recently that they are beginning to appear in the literature. [8][9][10][11][12][13] In addition to having given interpretations of spectroscopic measurements in solid hosts doped with lanthanide and actinide ions, 8,[14][15][16] and of energy-transfer mechanisms, 15 ab initio calculations have predicted the distance between f-element ions and their first coordination shell to shorten in the lowest 4f → 5d and 5f → 6d excitations of Ce 3+ ,Pr 3+ ,Pa 4+ ,U 3+ , and U 4+ in sixfold chloride octahedral coordination 8,[14][15][16] and of Ce 3+ in eightfold fluoride cubic coordination.…”

“…[8][9][10][11][12][13] In addition to having given interpretations of spectroscopic measurements in solid hosts doped with lanthanide and actinide ions, 8,[14][15][16] and of energy-transfer mechanisms, 15 ab initio calculations have predicted the distance between f-element ions and their first coordination shell to shorten in the lowest 4f → 5d and 5f → 6d excitations of Ce 3+ ,Pr 3+ ,Pa 4+ ,U 3+ , and U 4+ in sixfold chloride octahedral coordination 8,[14][15][16] and of Ce 3+ in eightfold fluoride cubic coordination. 17 This behavior, which contradicted the widespread assumption that bond length increases upon 4f → 5d and 5f → 6d excitations, has been found to be present in fluorides, chlorides, and bromides of Ce 3+ in solid and liquid solutions.…”

“…The reliability of the calculations of this kind has been shown before in specific studies on ionic hosts doped with Ce 3+ ,Pr 3+ ,Pa 4+ ,U 3+ , and U 4+ . 8,[14][15][16] We present results on the local structure of the defects in the ground 4f n state and in the lowest states of the 4f n−1 5d͑t 2g ͒ 1 and 4f n−1 5d͑e g ͒ 1 configurations of high and low spins. We calculated bond lengths, breathing mode vibrational frequencies, minimumto-minimum energy differences, and vertical energy differences ͑including Stokes shifts͒.…”