C-type M2O3 structures

In203 has the C-type M2O3 structure (p. 1238) and InO(OH) (prepared hydrothermal ly from In(OH)3 at 250-400°C and 100-1500 atm) has a deformed rutile structure (p. 961) rather than the layer lattice structure of AIO(OH) and GaO(OH). Crystalline In(OH)3 is best prepared by addition of NH3 to aqueous InCl3 at 100° and ageing the precipitate for a few hours at this temperature it has the simple Re03-type structure distorted somewhat by multiple H bonds. 

From Pu onwards, sesquioxides become increasingly stable with structures analogous to those of Lu203 (p. 1238) Bk02 is out-of-sequence but this is presumably due to the stability of the f configuration in Bk. For each actinide the C-type M2O3 structure (metal CN = 6) is the most common but A and B types (metal CN = 7) are often also obtainable. 

The other oxides listed in Table 9.1 are included because their mechanical properties have received some attention (especially quartz). Quartz is by no means close-packed because of the stability of the [Si04] tetrahedron. Yttria and the rare-earth sesquioxides and the mixed oxide garnet structures are compact, in spite of their complexity, but cannot be described as close-packed in the traditional sense. In fact, the C-type M2O3 structure of yttria can be thought of as having the fluorite structure of urania with one-quarter of the anion sites vacant. 

E112O3 3TCL2O5. — The rare earths from La to Er and also Y have been found [330] to form ternary oxides with Ta Os of the type M2O3 3 Ta Os having a perovskite structure. The other rare earths (Tm-Lu) form a mixture of MTaC>4 and Ta Os. Eu20s-3 Ta Os has the following lattice constants a = 3.871, b = 3.885 and c — 7.792 A. 

The C-M2O3 structure is related to that of Cap2, from which it may be derived by removing one-quarter of the anions (shown as dotted circles in Fig. 12.6) and then rearranging the atoms somewhat. The 6-coordinated M atoms are of two types. 

The carbides of the lanthanoids and actinoids can be prepared by heating M2O3 with C in an electric furnace or by arc-melting compressed pellets of the elements in an inert atmosphere. They contain the C2 unit and have a stoichiometry MC2 or M4(C2)3. MC2 have the CaC2 structure or a related one of lower symmetry in which the C2 units lie at right-angles to the c-axis of an orthogonal NaCl-type cell. They are more reactive than the alkaline-earth metal... 

CoO and NiO all take the NaCl-type structure and the difference in nonstoichiometry relates to the relative stability of the formal di- and trivalent oxidation states. The stability of the trivalent state and the degree of non-stoichiometry decreases from Fe3+ to Ni2+. Hence the non-stoichiometric nature of Fcj yO is made possible by the relatively high stability of Fe3+ that is reflected in the fact that Fe2C>3 is a stable compound in the Fe-0 system, whereas M2O3 is not in the Ni-O system. This relative stability of the different oxidation states is also reflected in Figure 7.11(c). 

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