33654_07b - Previous Page Ternary and more complex oxide...

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§7.3.4 Ternary and more complex oxide phases 247 Figure 7.13 Chart illustrating transformation relationships among the forms of gallium oxide and its hydrates. Conversion (wet) of the phase designated as Ga2-,A1,03 to b-Ga2O3 occurs only where x < 1.3; where x > 1.3 an a-A1203 structure forms. the hydrated oxide T1203.1kH20 and desiccation; single crystals have a very low electrical resistivity (e.g. 7 x lop5 ohmcm at room temperature). A mixed oxide Tho3 (black) is known and also a violet peroxide Tl'O2 made by electrolysis of an aqueous solution of T12S04 and oxalic acid between Pt electrodes. TlOH has been mentioned previously (p. 226). 7.3.4 Ternary and more complex oxide phases This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgA1204) and related compounds, Na-B-alumina (NaA111017) and related phases, and trkalcium aluminate (Ca3A1206) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to modem technology. For aluminosilicates see p. 351 and for aluminophosphates see p. 526. Spinels and related compounds (56) Spinels form a large class of compounds whose crystal structure is related to that of the mineral spinel itself, MgA1204. The general formula is AB2X4 and the unit cell contains 32 oxygen atoms in almost perfect ccp array, i.e. AgB16032. In the normal spinel structure (Fig. 7.14) 8 metal atoms (A) occupy tetrahedral sites and 16 metal atoms (B) occupy octahedral sites, and the structure can be regarded as being built up of alternating cubelets of ZnS-type and NaCl- type structures. The two factors that determine which combinations of atoms can form a spinel- type structure are (a) the total formal cation charge, and (b) the relative sizes of the 2 cations with respect both to each other and to the 56 N. N. GREENWOOD, Ionic Crysfals, Lattice Defects and Nonsfoichiomefry, Buttenvorths. London, 1968, 194 pp. See also J. K. BURDETT, G. D. PNCE and S. L. PRICE, J. Am. Chem. SOC. 104, 92-5 (1982). Previous Page
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248 Aluminium, Gallium, Indium and Thallium Ch. 7 Figure 7.14 Spinel structure AB204. The structure can be thought of as 8 octants of alternating A04 tetrahedra and B404 cubes as shown in the left-hand diagram; the 40 have the same orientation in all 8 octants and so build up into a fcc lattice of 32 ions which coordinate A tetrahedrally and B octahedrally. The 4 A octants contain 4 A ions and the 4 B octants contain 16 B ions. The unit cell is completed by an encompassing fcc of ions (0) as shown in the right-hand diagram; this is shared with adjacent unit cells and comprises the remaining 4 A ions in the complete unit cell AgB16032.
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33654_07b - Previous Page Ternary and more complex oxide...

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