Corundum-type structure

Corundum-type Magnetic Oxide Surfaces. The substrate hematite with the corundum-type crystal structure is an antiferromagnet below 963 K. In the corundum-type structure of hematite, pairs of ferric ions are in a row spaced by single vacant sites along the <111> direction. The positions of ferric ions in each pair are shifted slightly upward or downward in the <111> direction. We denote these lattice positions as up and down sites (Au and A ), respectively. 

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. 

Gray crystalline sohd or amorphous powder corundum-type structure density 8.20 g/cm decomposes at about 1,100 to 1,150°C insoluble in water, acids, or aqua regia. 

Fig. 2.12 Ideal structure of VgOjj viewed along the a-axis of rutile, derived by the shear operation (121) [0il] on rutile. The marks and show the metals at x = 0 and leading to face-sharing of adjacent octahedra, similar to a Corundum-type structure.

Corundum is one of the many polimorphs of alumina (A1203). The corundum-type structure is the structural shape of hematite (aFe203). In Figure 2.12, layers A and B of a close-packed arrangement of spheres, and the formation of the corresponding octahedral and tetrahedral sites are shown [51,52]. 

The corundum-type structure of aFe203 is formed by a HCP array of O2- anions with Fe3+ cations filling 2/3 of the octahedral sites located between the contiguous A, B sequence of layers (see Figure 2.13). 

Iron oxide with a corundum-type structure, that is, aFe203, is a good catalyst for reactions such as the catalytic oxidation of S02 [53], Studies carried out on this reaction show that S02 adsorption is the rate-determining step it contributes electrons to the conduction band and the adsorption of 02 withdraws the conduction electrons from an oxygen vacancy [53],... 

In a fluorite-type oxide, whose chemical formula is expressed as MO2, the structure type is named afler Cap2, in which the cation/anion radius ratio is such that the anions achieve a simple cubic packing with the cations occupying half the available sites with 8-fold coordination. The packing of the anions is not as compact as those found in other oxides such as spinel or corundum-type structures, which have closest packing of oxide ions. 

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... 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

The structure of platinum dioxide and its reactions with some di, tri, and tetravalent metal oxides have been investigated. Ternary platinum oxides were synthesized at high pressure (40 kUobars) and temperature (to 1600°C). Properties of the systems were studied by x-ray, thermal analysis, and infrared methods. Complete miscibility is observed in most PtO2-rutile-type oxide systems, but no miscibility or compound formation is found with fluorite dioxides. Lead dioxide reacts with Pt02 to form cubic Pb2Pt207. Several corundum-type sesquioxides exhibit measurable solubility in PtOz. Two series of compounds are formed with metal monoxides M2PtOh (where M is Mg, Zn, Cd) and MPt306 (where M is Mg, Co, Ni, Cu, Zn, Cd, and Hg). 

Fig. 15. Experimental data on the structure of corundum type depolarized (0001) surfaces (side and top views).

The electronic structure of corundum type oxide surfaces have been studied with photoelectron spectroscopy [102, 103] as well as with electron energy loss spectroscopy [104]. Henderson and Chambers [103] have reeently reviewed the photoelectron spectroscopic evidences. Still, there seems to be an ambiguity about the oxidation state of the chromium ions in the immediate surface layer. XPS Cr 2p data may be to a large extent reconciled as being due to Cr(III) in the surface, because there is obviously no change in the spectra as a function of... 

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. 

The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga, the density is ca. 10% less than for the a (corundum-type) form. This preference of Ga for fourfold coordination, despite the fact that it is larger than may indicate the polarizing... 

Because the Madelung constant has been computed by a summation over all lattice sites, it adopts characteristic values for all structure types [5,8]. To give a few examples, M arrives at (dimensionless) values of 1.6381 (zinc-blende-type), 1.7476 (sodium chloride-type), 1.7627 (caesium chloride-type), 5.0388 (fluorite-type), and 25.0312 (corundum-type) and does not scale with (= is independent of) the interionic distances. For the case of NaCl, the Madelung constant shows that the three-dimensional lattice surpasses the ionic pair in energy by almost 75%. This is what has made the formation of solid NaCl possible, a collective stabilization. 

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