Corundum-type hexagonal structure

Figure 9. Simplified model of the (111) surface of the corundum-type structure, (a) A view of the surface from a direction slightly shifted from <111>. Only metal ions of the zeroth, first, and second layers are shown, (b) A section of the surface along the arrows depicted in part a. Hexagonally close-packed oxide ion layers are shown with lines. Surface protons are not shown, (c) A divalent Co-57 or pentavalent Sb-119 ion on the zeroth metal ion layer, (d) Aquo or hydroxyl complex of divalent Co-57 or pentavalent Sb-119 hydrogen-bonded to the surface oxide ion layers of hematite.

Green hexagonal crystal system corundum type structure density 5.22 g/cm3 melts at 2,330°C vaporizes above 3,000°C insoluble in water and alcohol. 

When or-Fe203 was used as the positive electrode in high-temperature lithium cells, the introduction of a small amount of lithium into the corundum-type structure caused the hexagonal-close-packed oxygen array to shear irreversibly to cubic-close packing which generated a defect /-Li FCjOj (spinel-type) structure. Further lithiation resulted in the formation of LiFe,Ojj thereafter, the reaction followed the same sequence as that shown in reactions (4), (5) and (6) [100]. The stability of the spinel structures at elevated temperatures, as well as the ability of the cubic close-packed oxygen array to accommodate lithium at the expense of... 

There are two well-known oxide-hydroxides (AlOOH) with closely related structures diaspore and boehmite. Diaspore occurs in some types of clay and bauxite. It has been produced by the hydrothermal treatment of corundum, a-Al203. Whereas boehmite is characterized by cubic close-packing of the anions, diaspore has a hexagonal close-packed structure. This difference probably accounts for the direct thermal transformation of diaspore to corundum at relatively low temperatures (450-600°C). 

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. 

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