Nonstoichiometry
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص99-100
2025-08-23
422
Nonstoichiometry
Key point: Deviations from ideal stoichiometry are common in the solid-state compounds of the d-, f-, and later p-block elements. A nonstoichiometric compound is a substance that exhibits variable composition but retains the same structure type. For example, at 1000°C the composition of ‘iron monoxide’, which is sometimes referred to as wüstite, Fe1-x O, varies from Fe0.89 O to Fe0.96 O. Gradual changes in the size of the unit cell occur as the composition is varied, but all the features of the rock-salt structure are retained throughout this composition range. The fact that the lattice parameter of the compound varies smoothly with composition is a defining criterion of a nonstoichiometric compound because a discontinuity in the value of the lattice parameter indicates the formation of a new crystal phase. Moreover, the thermodynamic properties of nonstoichiometric compounds also vary continuously as the composition changes. For example, as the partial pressure of oxygen above a metal oxide is varied, both the lattice parameter and the equilibrium composition of the oxide change continuously (Figs 3.58 and 3.59). The gradual change in the lattice parameter of a solid as a function of its composition is known as Vegard’s rule. Table 3.12 lists some representative nonstoichiometric hydrides, oxides, and sulfides. Note that as the formation of a nonstoichiometric compound requires overall changes in
composition, it also requires at least one element to exist in more than one oxidation state. Thus in wüstite, Fe1 x O, as x increases some iron(II) must be oxidized to iron(III) in the structure. Hence deviations from stoichiometry are usual only for d- and f-block elements, which commonly adopt two or more oxidation states, and for some heavy p-block metals that have two accessible oxidation states.
Fig. 3.58 Schematic representation of the variation of the partial pressure of oxygen with composition at constant pressure for (a) a nonstoichiometric oxide MO1+x and (b) a stoichiometric pair of metal oxides MO and MO2 . The x-axis is the atom ratio in MO1+x
Fig. 3.59 Schematic representation of the variation of one lattice parameter with composition for (a) a nonstoichiometric oxide MO1+x and (b) a stoichiometric pair of metal oxides MO and MO2 with no intermediate stoichiometric phases (which would produce a two-phase mixture for 0<x<1, each phase in the mixture having the lattice parameter of the end member.
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