Site symmetry crystal-field analysis

Lochhead and Bray (1995) studied Eu3+ doped sodium disilicate glass with a high-pressure fluorescence line-narrowing technique. This technique was used to characterize the local structure of the Eu3+ ions up to a pressure of 21 GPa. For the crystal-field analysis they assumed a C2v site symmetry which allowed for a complete splitting of the crystal-field components. The crystal-field strength was determined according to eq. (11). The effect of pressure... 

Ideally, lanthanide ions occupy site with C3V symmetry when incorporated into the GaN lattice by substituting the Ga3+. However the actual site symmetry is often found lower than C3V due to strain and defects. So far no crystal field analysis has ever been reported for lanthanide-doped III-V QDs, due to the co-existence of multi-sites which complicates the energy level structure and makes crystal field analysis difficult. Site selective spectroscopy is a very useful tool thus proposed to investigate the different crystal field environments of lanthanide ions doped in III-V QDs. 

Crystal-Field Analysis of Ho3+-LaF3 and Er3+-LaF3 in C2V-Site Symmetry, W.T. Car-nail, G.L. Goodman, R.S. Rana, R Vandevelde, L. Fluyt, and C. Gorller-Walrand, J. Less-Common Met. 116, 17-29 (1986). 

Site Symmetry Through Crystal-Field Analysis... 

The crystal-field parameters introduced in sect. 4.1 still contain all the structural information about the local environment. Therefore, a direct comparison of crystal-field parameters derived from different hosts, even with the same site symmetry, is not reasonable. In addition, the crystal-field parameters cannot be directly related to the distance and angle variations induced by the high-pressure application. Widely used models which extract the structural information from the crystal-field parameters are the angular-overlap (Jprgensen et al., 1963) and superposition model (Bradbury and Newman, 1967). In the case of f elements, the superposition model has been employed widely for the analysis of crystal-field parameters. 

In solid-state laser materials, such as ruby (chromium doped alumina, AljOjiCr " ) (1) and emerald (chromium doped beryl, Be,Al,(Si03)5 Cr ) (2), transitions between multiplets of impurity states are utilized. These states mainly consist of 3d orbitals of the impurity chromium ions. For the analysis of these multiplet structures, the semi-empirical ligand-field theory (LFT) has been frequently used (3). However, this theory can be applied only to the high symmetry systems such as O, (or T ). Therefore, the effect of low symmetry is always ignored in the analysis based on the LFT, although most of the practical solid-state laser materials actually possess more or less distorted local structures. For example, in ruby and emerald, the impurity chromium ions are substituted for the aluminum ions in the host crystals and the site symmetry of the aluminum ions are C, in alumina and D, in beryl. Therefore, it is important to clarify the effect of low symmetry on the multiplet structure, in order to understand the electronic structure of ruby and emerald. 

Crystallographic data are invaluable in the interpretation of experimental optical spectra, both in identifying the symmetry of the lanthanide ion site and in providing input data to a superposition analysis (Newman, 1970) or to an ab initio crystal-field calculation. Crystallographic data are presented in a short table for each host that lists the following information ... 

The fact that 4f-electrons are well shielded from the environment by filled 5s and 5p shells (Wybourne, 1965), results in there being a close similarity in the level structure derived from the analysis of lanthanide spectra in crystalline solids such as R iLaCla and that observed for the R (aquo) ion. Indeed evidence was presented at an early stage to show that the spectroscopic properties of Eu ions in solution at reduced temperature were very similar to those of the ions in crystals (Freed and Weissman, 1938). Subsequent experiments with mixed component solutions containing Eu " (Sayre et al., 1957), and Nd " and Sm " (Freed and Hochanadel, 1950), demonstrated the continuity between the numerous sharp lines which could be identified as crystal-field components in a microfield of clearly defined site symmetry in solution at low temperature, and the band envelope of these lines that developed as the temperature was raised. 

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