Charge-transfer spectra of tetravalent lanthanide ions in oxides

Abstract

Abstract--The charge-transfer spectra of Ce 4~, Pr "+ and Yb "+ in a number of oxides are reported. It is noted that the position of the first charge-transfer band is fixed for the metal ion in an oxygen coordination of V1, but varies in VIII coordination as a function of the host lattice. It is argued that this variation is inherent to the VIII coordination itself.INTRODUCTION IN THE past decade quite some work has been done on charge transfer (c.t.) transitions in molecular or ionic groups in which an electron is excited from a delocalized ligand molecular orbital (m.o.) to an empty or incompletely filled orbital mainly localized on the central ion of the chromophore. The position of the first c.t. band of a complex is determined by the oxidizing character of the central ion and by the properties of the set of ligands. Thus not only the reducing character of a particular ligand is of importance, but also effects due to ligand-ligand repulsion and to destabilization of the metal ion orbitals due to interaction with ligand m.o. 's[l--4].Although most data refer to c.t. spectra of transition metal ions, c.t. transitions have also been reported for the trivalent lanthanides Eu 3+, Sm 3÷, Tm 3+ and Yb 3+ in complexes with halogen ligands [5] and in oxides [6], in which the first band in the absorption spectra is ascribed to such a transition, in contrast with Ce 3+, Pr 3+ and Tb 3+ where the first band is assigned to a 4[--, 5d transition, a markedly different feature being the bandwidth, which is almost twice as large for a c.t. band compared with an f--, d band [7].Since for a given element the c.t. bands shift to lower energies with increasing oxidation state, and transitions between different cationic subshells (Rydberg transitions) to higher energies, one might expect to find the c.t. bands for complexes of the tetravalent lanthanide (Ln) ions Ce 4+, Pr 4+ and Tb 4+ in an easily accessible region of the spectrum. In fact some years ago Ryan and Je~rgensen [5] have reported the absorption spectra of CeCI6 z--and CeBr6'-and Je~rgensen and Rittershaus[8] described the diffuse reflection spectra of Pr '+ and Tb '+ in TbO2 and Y:O3 in which the broad bands in the visible region of the spectrum were ascribed to c.t. transitions. Recently the spectra of these ions in monoclinic ZrO2 have been reported [9]. It was noted that in ZrO2 and Y203 the bands of Pr 4+ and Tb 4+ are situated at markedly higher energies than in ThO2, a fact which cannot readily be understood.Most complexes have been studied in solution, but in this case, since it is not possible to obtain Pr '+ and Tb 4~ complexes in solution, it was necessary to work in solid samples. Solutions have the advantage that absorption spectra can be obtained, whereas solid samples can only be studied by reflection spectroscopy, because it is usually difficult to obtain single crystals. On the other hand solid samples enable a systematic study of c.t. bands in several crystal lattices, with the central metal ion in a wide

Photostimulated luminescence and thermoluminescence of LSO scintillators

“…The position and the width of this band are similar to those observed for Ce4+ and other 4+ charged rare earth ions in several oxide lattices [5,6]. The fact that extra absorption at wavelengths c350 nm appears after annealing in an 0 2 atmosphere supports the assignment of the 276 nm absorption band to Ce4+.…”

A comparison between “second-sphere effects” in the excited state properties of coordination compounds and nonmolecular solids

“…A clear example of this is the position of the charge-transfer absorption band of the Eu3+ ion in six coordination in mixed-metal oxides. On experimental and theoretical grounds this position is expected to be independent of the Eu3+-02-distance (32). From spectroscopic measurements it is observed at (42 +-1)103 cm-i.…”

Luminescence properties of BaLaGa3O7

“…5) the band at 3351x0 Is probably due to the 4f-5d transition on Ce3+. It may be suggested that the broad absorption band is due to a Ce 4+ charge-transfer transition, lut these are usually situated at rmch higher energy [18]. Hanver, there is a more obvious possibility.…”