Lattice defects, nonstoichiometry

N. N. Greenwood, Ionic Crystals. Lattice Defects and Nonstoichiometry, Butterworths, London, 1968, 194 pp. 

N. N. Greenword, in Ionic Crystals Lattice Defects and Nonstoichiometry, Chemical Publishing, New York (1970). 

This suggests the need to look critically at what is really known about the nature of lattice defects. Here Roth s paper on ferrous oxide (1960) (37), Fe0i87O, marks an important stage. Methods of diagnosing how nonstoichiometry arises—e.g., that ferrous oxide is cation-deficient, Fe O—strictly go no further than an enumeration of the number of atoms of each kind per unit cell. That the changes in unit cell contents signify a corresponding number of simple vacancies or interstitials is purely inferential and, as now appears, questionable. 

The nonstoichiometry of the hydrides, therefore, must be attributed to lattice defects such as vacancies or interstitials. The reason for the large homogeneity ranges in these hydrides is the low interaction energy between defects. 

Uranium forms a trihydride which does not deviate from stoichiometry to any measurable degree at room temperature, but does so to a significant degree at elevated temperatures (29). For example, at 650° C., the deviation is larger than 5% to give UH2>84. The relation between lattice defects and nonstoichiometry in this compound is discussed below. 

Data from Greenwood, N.N., Ionic Crystals Lattice Defects and Nonstoichiometry, Butlerworths, London, 1968. With permission. 

FIGURE 3.33. Structure of spinel. The structure is composed of alternating octants of AO4 tetrahedra and B4O4 cubes (a) to build the fee unit cell (b). (From Greenwood, N.N., Ionic Crystals Lattice Defects and Nonstoichiometry, Butterworths, London, 1968. With permission.)... 

Electrical methods are veiy sensitive to the properties of solids, especially ionic soUds. Even changes of composition on the ppm level may result in substantial changes in semiconducting properties, such as n-p-type transitions, which may be determined by both electrical conductivity and thermopower. The WF is sensitive to surface properties on an atomic level. This is the reason the electrical methods are finding increasing applications in studies of defect-related properties of materials, based on nonstoichiometric compounds, such as transport properties. Electrical methods have been widely used with high accuracy in studies of nonstoichiometry and related concentrations of lattice defects at elevated temperatures and under gas phase of controlled composition. ... 

G. G. Libowitz, Nonstoichiometry and lattice defects in transition metal hydrides, J. Appl. Phys. 33, 399-405 (1962) G. G. Libowitz and J. G. Pack, The gadolinium-hydrogen system at elevated temperatures. Vacancy interactions in gadolinium dihydride, J. Phys. Chem. 73, 2352-2356 (1969). 

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