Chrysoberyl structure

The host crystal of chrysoberyl has a hexagonal-close-packed structure. The space group is orthorhombic Pnma with four molecules per unit cell. The AP ions are octahedrally coordinated by the oxygen ions and occur in two not equivalent crystal field sites in the lattice. The AP" sites lying in the mirror-... 

Two types of Cr + luminescence centers have been found in steady-state natural alexandrite, characterized by J -lines at approximately 680 and 692 nm, accompanied by very many M-lines of Cr-Cr pairs (Tarashchan 1978). Those centers have been identified as connected with substitutions of AP" in different structural sites. It was found that natural alexandrites with very rare exceptions are characterized by very low CL intensities (Ponahlo 2000). Pulse CL study revealed that the spectrum consists of a relatively broad red band peaking at 685-695 nm, accompanied by narrow lines with the strongest one at 679 nm and the weaker ones at 650,655,664,700,707 and 716 nm. All lines and bands have been ascribed to several Cr + centers (Solomonov et al. 2002). The natural chrysoberyl and alexandrite in our study consisted of six samples. The laser-induced time-resolved technique enables us to detect two different Cr and possibly Mn + and V emission centers (Figs. 4.54-4.55). 

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

CHRYSOBERYL. The mineral chrysoberyl. an aluminaie of hery Ilium corresponds to the formula BeALO. , crystallizes in the orthorhombic system with both contact and penetration twins common, often repeated resulting in ro.setted structures. Hardness. 8.5 specific gravity. 3.75 luster vitreous color various shades of green sometimes yellow. A variety which is red by transmitted light is known as alexandrite. Streak colorless transparent to translucent, occasionally opalescent. Chrysoberyl also is known as cymopbane and golden beryl. 

Chrysoberyl, BeAl204, has an orthorhombic structure, D j, Pnma, a = 9.407, b = 5.4781, and c = 4.4285A, with four molecules in the unit cell. The structure is similar to that of olivine (Section 10.3.1). Ali... 

Chrysoberyl, AyJeO which is iso-structural with olivine, exists as the gems alexandrite, (Al,Cr)2Be04, and sinhalite, (Al,Fe)2Be04. Structure refinements of these gem minerals (Farrell et al., 1963), as well as EPR measurements... 

Example 4 BeO/Al203 Two components and P fixed (Figure 8.13). Notice that all the phases can be regarded as combinations of BeO and AI2O3, so we can denote them as B3A, BA, and BA3. From a chemical point of view, the system looks quite similar to Mg0/Al203, but clearly it is very different three separate eutectics are shown none of the compounds has the spinel structure. BeAl204 is the mineral chrysoberyl and has a structure similar to olivine, which is not unrelated to spinel. 

Polycrystalline-alumina-based fibres can at present not compete with silicon-carbide-ba.sed fibres when low creep rates are required. Fibres with higher resistance to creep by dislocation motion could be provided by oxides with high melting point and complex crystal structure, a tendency to order over long distances and the maintenance of this order to high fractions of the melting temperatures (Kelly, 1996). Experimental development of monocrystalline fibres by Czochralski-derived techniques from chrysoberyl... 

Alexandrite is the common name for Cr-doped chrysoberyl, a crystal whose unit cell contains four formula tmits BeAl204, forming an orthorhombic structure of... 

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