Crystal spinel-based

Oxides. Although not widespread commercially, glass-ceramics consisting of various oxide crystals in a matrix of siUceous residual glass offer properties not available with mote common siUcate crystals. In particular, glass-ceramics based on spinels and perovskites can be quite refractory and can yield useful optical and electrical properties. 

Structure types have been established. Similar to Al, the M2X3 crystals (M = Ga, In, Tl X = S, Se, Te) are mostly based on M-defect tetrahedral structures, namely W (Ga2S3, In2Se3) and ZB (Ga2Se3, Ga2Te3, In2Te3). At atmospheric pressure, 283 can be present in three modifications. The low-temperature a-form is a cubic close-packed structure of S atoms, where 70% of the In atoms are randomly distributed on octahedral sites and the rest remain on tetrahedral sites. The P-form is related to the spinel structure, and the y-modification is hexagonal. 

Fabry-Perot cavity, 14 849, 850 Fabry-Perot etalons, 11 151, 152 Face-centered cube lattice, 8 114t Face-centered cubic (FCC) crystal structure in Ni-base alloys, 13 512 of spinel ferrites, 11 60 Facial makup, 7 846-847 Facial preparations, 7 842t Facial tridendate ligand, 7 578 Facihtated transport, 15 826-827 carrier, 15 845-846... 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

The displacement of atoms in passing from the spinel structure to the superlattice structure may introduce a considerable error into any calculation based on the atomic coordinates of the spinel structure. The determination of these displacements, considering the large number of position parameters, is out of the question except by Fourier analysis of measurements made on a single crystal. 

Cells based on this process (Eq. (47)) show lower cycling performance with respect to those based on the 4 V process because charge-discharge reactions are accompanied by an asymmetric lattice contraction-expansion of the Lii+.vMn204 electrode. This lattice distortion mainly results from the Jahn-Teller effect of the Mn + ion which transforms the crystal symmetry of the spinel oxide [118],... 

The reverse situation is also true, for compositions on the 10% plane and in the primary phase field of spinel the secondary phase would be expected to comprise components of the base ternary systems. This behaviour was observed with the compositions studied in this investigation with the exceptions of those samples which crystallized anorthite only. The behaviour of these glasses can be explained in terms of the viscosity of the melt hindering further crystallization. This generalization of crystallization of quaternary systenns must be used with care especially when the precipitating phase may be a solid solution. 

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