Vacancy-ordered phases

There exist vacancy-ordered phases in the Ni-Al system (see below). 

Anion-deficient fluorite oxides are also present, for example, U02- c, Ce02-x The presence of anion vacancies in reduced fluorites has been confirmed by diffraction studies. In reduced ceria for example, some well-ordered phases has been reported (Sharma et al 1999). The defective compounds show very high anion mobilities and are useful as conductors and as catalytic materials as will be described later. However, the structures of many anion-deficient fluorite oxides remain unknown because of the shear complexity of the disordered phases. There are, therefore, many opportunities for EM studies to obtain a better understanding of the defect structures and properties of these complex materials which are used in catalysis. 

As Jettinek (147) pointed out there is a pronounced tendency in these subtractive structures to keep the anions crystallographically equivalent. This can be achieved by a statistical arrangement of the cation vacancies (e.g. Feo.92S (182)) or by removing metal atoms statistically from each second layer (e.g. C Ss (160) ). Since in the NiAs structure each anion is surrounded by six octahedral holes, crystallographic equivalence of the anions in an ordered structure is possible only for the compositions T Xe. Ordered phases of other stoichiometry will have a distorted structure of a symmetry lower than trigonal. 

Vacancy ordering in anion deficient LaNi03 was investigated by Gai and Rao59. Thermogravimetry in air and oxygen provided evidence for the formation of a series of phases pf the general formula La Ni 03n with n = 7, 9, 13 and 30 (Fig. 11). An electron diffraction study of the anion deficient samples... 

Ca2Fe205 and oxides of the CaMn03 v family are good examples of such vacancy-ordered structures. Complex intergrowth phases (ordered as well as disordered) involving brownmillerite and other related phases are commonly found in some of the anion-deficient oxides. Some of them also show polytypism due to different modes of stacking of the hexagonal and cubic layers. 

Theoretically, the discussion must turn on the relative energies of complex ordered structures and of defective, random structures. Bertaut (5) (1953) attempted to compute the electrostatic lattice energy of the pyrrhotite phase FeSj.,4 (Fe+2o. 25, Fe+30.25 n + ) (S 2 n ) or (Fe+25, Fe+3, ) (S-28),for several alternative cases a completely disordered structure with higher-valent cations and vacancies randomly distributed over all cation sites and structures with the vacancies ordered into alternate cation sheets, with various hypotheses... 

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