Site symmetry in crystals

Fig. 7 Use of Do — Fj Eu luminescence as a probe of site symmetry in crystals. The transitions p3, are forbidden under Judd selection rules and are usually very weak so they are not included. The scheme refers to forced electric dipole-allowed transitions except for the case of the transition, which is allowed by the magnetic dipole mechanism (adapted from [102])...

R.A. Evarestov, V.P. Smirnov, Site Symmetry in Crystals Theory and Applications, 2nd edn. Springer Series in Sohd State Sciences, 108 (Springer, Berlin Heidelberg New York, 1997 )... 

The best conclusion one can draw, then, seems to be that comparisons between mineral and glass Mossbauer parameters are qualitative as far as coordination number and site symmetry in the glass are concerned. However, the crystal spectra are interesting for their own sake. 

For a definition of space group spectra see Section 10.5. Spectra for all space groups are arranged below (Tables A2.1-A2.10) in order of decreasing symmetry. The multiplicities of the Wyckoff positions, m, are given across the top of each table. The symmetries of sites with multiplicity 1 are given at the end of each line followed by structure types that crystallize in that space group (site symmetries in parentheses refer to positions of multiplicity 2). 

Comparison of glass and crystals spectra suggests that the site symmetry in fluorozirconates is close to the 8-coordinate bicapped trigonal prism in EuZrF7. The similarity is confirmed by lifetime measurements [80] and by photoluminescence... 

Excitation of these ions using 40,000 cm i energy results in various excited states for each ion. Each state relaxes by phonon processes until the terminal excited state is reached, from which emission of a visible photon results. But we need to remember that any time we put a trivalent rare earth ion into a host crystal to form a phosphor, we will modify its free-ion energy levels, depending upon the site symmetry and crystal field strength. 

Eu Eu -" is a typical and efficient activator for red-emitting phosphor due to its transitions from the excited Dq level to the Fj (J = 0-4) levels of the 4/ configuration. Its photoluminescence emission strongly depends on the symmetry of the crystal stmcture of Eu " occupied site in the host. The optical transitions of Eu " ions originating from the electronic dipole and magnetic dipole interaction of the internal 4/electrons are deeply affected by the crystal environment. If the Eu " ions occupy the sites with inversion symmetry, the emission will peak at 590-600 nm from the Dq Fi magnetic-dipole transition. This will dominate the emission, which is not affected much by the site symmetry. In contrast, the emission peaks at approximately 610-630 nm, due to the Dq p2 electronic dipole transition, will dominate the emission if the Eu " ion substitutes the site with no inversion symmetry [56], Moreover, Eu -doped phosphors usually have intense intrinsic... 

Further interesting and intensively studied systems are provided by A YX compounds. In particular, the crystal-field parameters have been determined for ROY(R = La, Gd Y = Br, Cl), BOCl Pr + (R = La, Pr, Gd) and MFCl Sm2+ (M = Ba, Ca, Sr). For the corresponding references see table 1. As an example for these compoimds, fig. 7 shows the crystal-field parameter shifts up to 16 GPa for BOCl Pr + (R = La, Pr, Gd) obtained by Bungenstock et al. (2000b). The Pr + ion in BOCl is surrormdedby four 0 and five Cl ions. According to the site symmetry five crystal-field parameters must be taken into account ... 

Very simple methods can be used to obtain useful information on the optical behaviour of impurity doped condensed matter. The first step in the characterization of such a material is usually to measure its absorption spectrum in the region of about 0.3-1 pm. Such measurements are often sufficient to identify the active impurities and to provide some information on their environment. For crystalline hosts, the polarization of absorption spectra can be analysed to draw conclusions on site symmetries and crystal field parameters (see Figure 4). 

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. 

Macroscopic morphology, crystal system information Unit cell and space group Site symmetry in crystalline and amorphous materials Position of atoms, thermal vibration amplitudes Imperfections... 

It is also apparent that there remains much to be done in developing our understanding of the model interactions that best characterize the spectra of higher-valent Pu compounds and indeed of the actinides in general. The synthesis of new compounds with a variety of different site symmetries could be of particular value in developing more detailed crystal-field... 

The vibrations of the free molecule can be correlated with the vibrations of the crystal by group theoretical methods. Starting with the point group of the molecule Did)> the irreducible representations (the symmetry classes) have to be correlated with those of the site symmetry (C2) in the crystal and, as a second step, the representations of the site have to be correlated with those of the crystal factor group (D2h) [89, 90]. Since the C2 point group is not a direct subgroup of of the molecule and of D211 of the crystal, the correlation has to be carried out in successive steps, for example ... 

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