Crystal structures spinel

Figure 1.41 The spinel crystal structure of MgAl204. Reprinted, by permission, from C. Kittel, Introduction to Solid State Physics, p. 447. Copyright 1957 by John Wiley Sons, Inc.

Figure 4.18 The spinel crystal structure adopted by magnetite, Fe2+Fe3+204. Note the three-dimensional infinite chains of edge-shared [FeOe] octahedra extending along [110] directions, which accommode Fe2+ and Fe3+ ions separated by 297 pm. Fe3+ ions also occur in isolated tetrahedra linking the octahedral chains.

Chromia Chromite Cr2Fe04 is the most commonly used chromium-containing mineral for ceramic formulations. This mineral has a spinel crystal structure, where the iron may be replaced by magnesium and aluminum. Chromite is used in ceramics largely as a refractory in the form of burned and chemically bonded bricks. For this purpose, a low-silica material is desired. When low silica is desired, chromic oxide is extracted from chromite by dissolution in add, removal of the iron impiu-ity by liquid—liquid extraction, and precipitation of the hydroxide, which is subsequently calcined to the oxide. Chromic oxide is used as a color additive to azes and enamels and in ferrite production to give magnetic materials. 

The cubic spinel crystal structure (Fd3m) is a close-packed array of oxygen ions, which has the general form AB204. A is a divalent cation and B trivalent [60, 71]. 

In high radiation fields, the spinel crystal structure has been shown to change. The structure, while still cubic, becomes disordered with a reduction in lattice parameter. The disordered rock-salt structure has a smaller unit cell reflecting the more random occupation of the octahedral sites by both trivalent and divalent ions. Increased radiation damage results in the formation of completely amorphous spinels. Radial distribution functions (g(r)) of these amorphous phases have Al-0 and Mg-O radial distances that are different from equivalent crystalline phases. The Al-0 distance in the amorphous form is reduced from Al-O of 0.194nm in the crystalline phase to 0.18nm in the amorphous phase, while the Mg-O distance is increased (0.19nm in the crystal to 0.21 nm in the amorphous phase). Differences between the Al-O distances of crystalline and amorphous phases are a characteristic of both calcium and rare earth aluminates. 

The optimal material for magnetic recording media was, for many years, y-FejOs (maghemite). This form of FejOj, which has a defect-spinel crystal structure, can be prepared as small, elongated particles. The coercive field is produced within the required range by shape anisotropy (see Section 4.5.1). The process used to prepare small particles of... 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

CICPs that contain aluminum oxide, AI2O3, or alumina as a colorless base oxide are called aluminates. These almost always employ cobalt(II) oxide, CoO, as one of the coloring oxides. All of these pigments adopt the spinel crystal structure, and all have color hues in the blue to teal range. 

FIGURE 7.1 (a-c) The spinel crystal structure. The 32 aniens in the unit cell form eight slightly distorted foe oxygen lattices. The cations are then distributed with one tetrahedron occupied in each subcell (rather like CujO). The 16 octahedral sites are then distributed in rows along one <110> direction or the orthogonal one depending on the layer [V,R or T,P in (b)]. 

Cubic ferrites have the inverse spinel crystal structure, which is cubic in symmetry and is similar to the spinel structure (Section 12.2). The inverse spinel crystal structure might be thought of as having been generated by the stacking of close-packed planes... 

Concept Check 20.2 What is the difference between the spinel and inverse spinel crystal structures Hint You may want to consnlt Section 12.2. 

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