Variable oxides with different lattices

For metals exhibiting variable oxidation states, the relative thermodynamic stabilities of two ionic halides that contain a common halide ion but differ in the oxidation state of the metal (e.g. AgF and AgF2) can be assessed using Bom Haber cycles. In such a reaction as 17.19, if the increase in ionization energies (e.g. M — M versus M— M +) is approximately offset by the difference in lattice energies of the compounds, the two metal halides will be of about equal stability. This commonly happens with block metal halides. 

Proust s conclusions were immediately challenged by Berthollet, who maintained that many compounds could have a variable composition. Berthollet quoted the example of the metal copper, which appeared to form a wide range of oxides. Proust pointed out that this was due to the formation of different mixtures of two oxides, each of definite composition. Berthollet s concept of continuously variable composition would have been hard to reconcile with the chemical atomic theory, but by about 1808 Proust s views were generally accepted. However, many years later it was discovered that some compounds, such as the oxides and sulphides of iron, could indeed have a variable composition. In the case of iron(ll) sulphide, a compound corresponding to the formula FeS is rarely encountered, and samples are usually deficient in iron to a variable extent. This is due to some of the lattice sites of iron(ll) ions being vacant, while others are occupied by iron(lll) ions to maintain electrical neutrality. These non-stoichiometric compounds are sometimes called Berthollide compounds. 

Non-stoichiometric. A chemical compound is said to be nonstoichiometric if the ratio of its constituents differs from that demanded by the chemical formula. This may happen with oxides that are readily reducible, or with compounds containing an element of variable valency, or when interstitial atoms are present in the lattice. Some nonstoichiometric ceramics are of interest as being semi-conducting. 

In the corresponding crystalline oxides, the polyhedra can have common summits, edges, or faces. In its different crystalline forms, silica (quartz, cristobalite, tridymite, etc.) exhibits a lattice built with Si04 tetrahedra linked together by their summits. In the case of vitreous (glass) silica, the glass lattice is made of the same tetrahedral Si04 units, linked by their summits, but the mutual orientation of the constitutive tetrahedra is variable. 

Differential thermal curves for some common chlorites are reproduced in Figure 28, from which it will be observed that particle size can markedly affect the appearance of the curve. The variability among different species is also noteworthy, and it is impossible to define a mean curve for chlorite. Curves for some pseudochlorites (Figure 29), the synthetic ones with incomplete brucite layers, show some similarities to the curves for the chlorite minerals themselves. Some clay-sized chlorites in soils do not, however, give pronounced peaks (Mackenzie [1956]), apparently because of oxidation of Fe to Fe in the lattice (Bain [1972]). 

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