Lanthanide elpasolites

Abstract A brief review of my work with Carl Ballhausen in 1967-1968 and subsequent work. The assignments of the vibronic sidebands in the emission spectra of chromium ammine complexes are given with some comments on the Jahn-Teller effect in the emissive state. Energy transfer and cross relaxation phenomena are discussed and the shell model for this processes in lanthanide elpasolites is presented... 

We therefore developed a much simplified, but mathematically exact, version of a shell model which is particularly suitable for the analysis of cross relaxation in high symmetry systems such as the lanthanide elpasolites. Since the vibronic structure in the emission and absorption spectra of these compounds is both intense and broad (relative to the electronic origins), there are many cases where the vibronic structure of one electronic transition in emission overlaps with the vibronic structure of another electronic transition of a chemically identical ion in absorption. The emission of the excited ion may then be partially quenched by energy transfer. Implicit in this formulation is the assumption that the interionic coupling is weak compared with the vibronic coupling this is certainly true for the lanthanide elpasolites where the lanthanide ions are separated by distances of more than 0.7 nm. We refer to this process as cross relaxation by the electric dipole vibronic-electric dipole vibronic (EDVEDV) mechanism. 

At room temperature, the Raman spectra of cubic lanthanide elpasolites comprise four bands, corresponding to the Si, S2, S4 and S5 modes of vibration. On cooling, the very broad, unresolved features due to electronic transitions between CF levels of Ln3+, sharpen and reveal the locations of 4fN excited states. The first studies of the electronic Raman spectra therefore focused upon the determination of the CF levels of the lower multiplet terms... 

The HLO model has generally been tested by observing the temperature-dependence of the transfer rate, but no detailed calculations of ET rates, by including the evaluation of electronic factors, have been made for Ln3+ systems. In principle, such calculations are tractable for the nondiagonal case but have not yet been forthcoming. The general approach of the data analysis for nonresonant ET processes in lanthanide elpasolite systems has been to utilize a R 6 dependence of the transfer rate (i.e. for EDV EDV processes) and to allow for transfer to successive shells of acceptor neighbours (without detailed consideration of the electronic matrix elements or the phonons involved, Sect. 13.4.1), or to employ a spectral overlap model (Sect. 13.4.2). Some of these studies are now reviewed. 

This review has summarised and commented upon the literature up to the end of 2002. The electronic spectra of elpasolite systems are complex and mainly vibronic in character. Whereas the major features can be interpreted in terms of localized moiety-mode vibrations, our understanding of the fine structure requires a more detailed investigation of the lattice dynamics of these systems in the future. One- and two-photon studies of certain lanthanide elpasolite systems have recently enabled extensive energy level datasets to be obtained, and the parametrization of these has revealed the need for the incorporation of other interacting configurations into the calculation. 

The ligand field splitting energies of /-levels derived from optical spectra are available for lanthanide cations in various elpasolite crystals145. This data represents... 

Another set of cubic compounds are the elpasolites which have the typical formula Cs2NaRCl6, in which the lanthanide ion is six-coordinated to the chlorine ions. EPR measurements on trivalent ions of Ce, Dy, Yb and Er in Cs2NaYCl6 (Schwartz and Hill 1974, O Connor et al. 1977) have confirmed the presence of cubic symmetry, but for a number of other ions, and for some undiluted compounds, phase transitions have been observed at temperatures of 100 to 160 K. EPR measurements (Bleaney et al. 1982b) on the compounds with Pr, Tb andTm, indicate the presence at low temperatures of cubic symmetry with a small tetragonal distortion. 

A clear shift of Na and Cs NMR lines, proportional to the effective y-factor of the Ho + ion, has been observed in Cs2NaHoCl6 by Bleaney et al. (1981b). After subtracting contributions of the Lorentz and demagnetizing fields, there remains a small paramagnetic shift Ay ( 0.006 for Na and even less for Cs), which for Na and Cs atoms, placed in the cubic positions of elpasolite, can occur only due to the transferred hyperfine interaction. ENDOR experiments directly testify to the presence of the above interaction in elpasolites with lanthanide ions (Fish and Stapleton 1978). 

Fig. 6. Enthalpies of solution of rare-earth (open symbols) and actinide (shaded symbols) sesquioxides, trichlorides and elpasolites CsjNaMCl as a function of molar volume (representing ionic size). The molar volumes were calculated from crystallographic unit cell parameters, normalized to one mole of M Smooth curves have been drawn through each class of compounds to guide the eye. In all cases the enthalpies of solution of actinide (III) compounds are less exothermic than those of structurally similar lanthanide (III) compounds.

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