Garnet structure

See p. 500 of ref. 24 for a description of Ihe garnet structure which is also adopted by inany. synthetic and non-silicaie coniponnds these have been much stndied recently becan.se of their important optical and magnetic properties, e.g. ferrimagnetic vitriuni iron garnet (YIG), y> Fe (Al" b4),. 

The garnet structure has high overall symmetry (cubic) but a complex structure (Bragg and et al., 1965). The prototype is the mineral Grossularite... 

YjAlsOn)—YAG Most garnets are silicates, whereas yttrium aluminum garnet (YAG) is an aluminate. In YAG, both the tetrahedral and the octahedral holes of the garnet structure are occupied by Al-ions and the quasi-cubic holes are occupied by Y-ions. 

Finally, the particular relationship between the Cr3Si-type structure and the garnet structure (see 7.6.8) may be pointed out. The position of the cations, indeed, in the garnet structure correspond to the Cr3Si arrangement. 

The variety of symmetries in the garnet structure (coordinations 4, 6, and 8) allows considerable compositional range. Table 5.16 lists the elements commonly present in positions X, X and Z. The diadochy of Al, Ti" ", and Fe in the tetrahedral site has been confirmed by Mossbauer spectroscopy on natural Fe-Ti-bearing garnets (Schwartz and Burns, 1978), and the presence of phosphorus in these sites, observed in upper mantle garnet, is attributable, according to Bishop et al. (1976), to coupled substitutions of the type... 

Hawthorne F. C. (1981b). Some systematics of the garnet structure. J. Solid State Chem., 37 157-164. 

The octahedral sites in the garnet structure are of the point symmetry... 

This structure is very common, and its structural parameters have been the subject of considerable discussion We shall see later (Sect. 3.3) that (very much as in the example of the spinel structure, Sect. 3.2) the parameters of the garnet structure can be simply understood as a result of the oxygen atoms finding the appropriate positions within the cation array to produce the correct bond lengths. 

Fig. 33. The (A,nCi/2)3B array of cations in the garnet structure, A3B2C3Xi2, projected on 100. Compare Fig. 34...

From the beginning it was noted that the structure was dominated by large columnar elements of the garnet structure type The doubt has been about the remaining parts (between the garnet columns). The garnet structure itself is rather complicated it has only recently been given a satisfactory description in terms of articulated, cation-centred polyhedra The vesuvianite structure is even more complex. 

The second group consists of those structures that have the same number of parameters as independent bond lengths. In this case the structural parameters are determined by requiring the bond lengths to have their normal values. For oxides this can be done by using appropriate tables of bond lengths, such as those of Shannon and Prewitt. Conspicuous and important examples of this second class are the spinel and garnet structures. 

CT-VPP-REDOR) or the pulse duration fp (CT-VPD-REDOR) then produces CT-REDOR curves, from which the second moment may be evaluated with distinctively superior accuracy as compared to the values obtained from a parabolic fit to the conventional REDOR data. When restricting the experiment to short dipolar evolution times, the two-spin approximation may be applied for the data analysis, which proves to be especially attractive for amorphous solids, for which the exact spin geometry is unknovm. The data presented on the model compoimds illustrate the various facets of CT-REDOR NMR spectroscopy. First application examples, namely, the evaluation of the heteronuclear Li-Ti dipolar couplings within the garnet structure of Li5La3Nb20i2, the determi-nation of the intemuclear B- P distance in frustrated Lewis pairs, the analysis of Na- F dipolar interaction in fluormica or Na- P... 

Yudintsev, S. V. 2001. Incorporation of U, Th, Zr, and Gd into the garnet-structured host. In Proceedings of the 8th International Conference on Radioactive Waste Management and Environmental Remediation. The American Society of Mechanical Engineers, New York, CD-ROM. 

Yudintsev, S. V., Lapina, M. I., Ptashkin, A. G., Iou-DINTSEVA, T. S., Utsonomiya, S., Wang, L. M. Ewing, R. C. 2002. Accommodation of uranium into the garnet structure. Materials Research Society Symposium Proceedings, 713, 477-480. 

Carnets. Both YAG, yttrium aluminum garnet. YjAIsOi3, and GGG, gadolinium gallium garnet. GdiGa 0,2. have the garnet structure and were used al one time as diamond imitations. These have been supplanted by cubic zirconia. 

The crystal field spectra and derived A0 and CFSE parameters for several garnets containing octahedrally coordinated trivalent transition metal ions are summarized in table 5.3. The values of A0 and CFSE reflect the variations of metal-oxygen distances in the garnet structures. 

Miij AjGcj Oj2 [Mn06] Octahedra [MnOs] Dodecahedra Garnet structure (A = Al, V)... 

Garnet-structure ferrites. Garnet, ideally A3 B2[T04]3, is isometrie (Ia3d) with a straeture built of TO4 tetrahedra and BOe oetahedra linked by shared vertiees that form AOg dodeeahedra. The A-sites aeeommodate divalent (Ca, Mn, Fe) and trivalent (Y, REE) elements. The B-site may contain various di-, tri- (Fe, Al, Ga, Cr, Mn, In, Sc, Co), tetra- (Zr, Ti, Sn, Ru) and even pentavalent elements (Nb, Ta, Sb). The T-sites are filled with tetravalent (Si, Ge, Sn) but may be also occupied by trivalent (Al, Ga, Fe) and pentavalent (P, V, As) elements. Synthetic garnets form multicomponent solid solutions that provide opportunities for incorporating of HEW elements into the structure [149]. 

Although most materials grown by LPE have the garnet structure, spinel ferrites can be grown similarly, and films of hexagonal ferrites are also prepared ". Only small quantities are required, and the crystallographic quality of the film can be high, but a suitable substrate is required and the measurement or device must be made on a thin layer rather than a bulk crystal. 

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