Spinel-type cubic crystal

Spinels have a crystal structure in which there is a face-centered cubic arrangement of O2 ions. There are two types of structures in which cations have octahedral or tetrahedral arrangements of anions surrounding them. In the spinel structure, it is found that the +3 ions are located in octahedral holes and the tetrahedral holes are occupied by the +2 ions. A different structure is possible for these ions. That structure has half of the +3 metal ions located in the tetrahedral holes while the other half of these ions and the +2 ions are located in the octahedral holes. In order to indicate the population of the two types of lattice sites, the formula for the compound is grouped with the tetrahedral hole population indicated first (the position normally occupied by the +2 ion, A) followed by the groups populating the octahedral holes. Thus, the formula AB204 becomes B(AB)04 in order to correctly... 

Pleonaste (syn. ceylonite, magnesioferrite) (ICSD 49551 and PDF 36-398) MgFe O M = 200.00 12.15 wt.% Mg 55.85 wt.% Fe 32.00 wt.% 0 Coordinence Mg(4), Fe(6) (Oxides and hydroxides) Cubic a = 836.6 pm Hl cF56 (Z=8) P.G. m3m S.G. Fd3m Spinel type (Ferrite series) Isotropic ip= 2.38 R = % 6-6.5 4650 Habit massive granular and also as well-formed fine sized crystals. Color brownish black, black. Diaphaneity opaque. Luster metallic. Streak dark red. Fracture uneven. Occurrence igneous volcanic rocks, Vesuvius and StromboU, Italy. 

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. 

Still more surprising are certain crystals in which a set of equivalent positions is only partially occupied, some sites here and there at random being empty. The spinel group also provides examples of this type of structure. The cubic (y) form of Fe2Oa, for instance, gives an X-ray... 

Cadmium chromium(III) selenide is a ferromagnetic semiconductor with a Curie temperature of 130 K. Crystals grown by this method are p type. Cadmium chromium(III) selenide has the spinel structure with a cubic cell edge of 10.75 A. 

Figure 10.1.10 shows the crystal structure of spinel. The unit cell (represented by solid lines) is divided into eight small cubic octants (represented by broken lines) of two different types, as shown in (a) the structure of type I and type n octants and the connections of atoms are shown in (b). There are four O2- in each octant. The number of O2- in the unit cell is 32 (= 8x 4). The Mg2+ ions are located at the centers of type I octants and half of the vertices of type I and type II... 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

If without additives, a carrier such as A3 consists of a mixture of 8-, 0-, and a-aluminas between 900° and 1000°C. The presence of the metal oxides, introduced by impregnation, effectively maintains a cubic type structure at a calcining temperature of 900°-1000°C. With alumina, these oxides form spinel-structured compounds which are more or less well crystallized. Because of the insertion of alumina, the lattice parameter of these compounds is expanded with respect to that of the stoichiometric spinel. The properties of the carrier are thus maintained up to a temperature which depends on the considered mixed oxide. At this temperature—about 1000 °C with magnesium aluminate and 900°C with zinc and copper aluminates—the stoichiometric spinel recrystallizes while a-alumina is rejected (6). The carrier then suddenly loses its mechanical and structural properties. None of the mentioned additives could improve the stability of the Cl carrier above 1000°C. 

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