Non-stoichiometric Compound

Non-stoichiometric Compound

Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by a ratio of well-defined natural numbers, and therefore violate the law of definite proportions. Often, they are solids that contain crystallographic point defects, such as interstitial atoms and vacancies, which result in excess or deficiency of an element, respectively. Since solids are overall electrically neutral, the defect in an ionic compound is compensated by a change in the charge of other atoms in the solid, either by changing their oxidation state, or by replacing them with atoms of different elements with a different charge.

Nonstoichiometry is pervasive for transition metal oxides, especially when the metal is not in its highest oxidation state. For example, although wüstite (ferrous oxide) has an ideal (stoichiometric) formula FeO, the actual stoichiometry is closer to Fe_0.95O. The non-stoichiometry occurs because of the ease of oxidation of Fe2+ to Fe3+ effectively replacing a small portion of Fe2+ with two thirds their number of Fe3+. Thus for every three "missing" Fe2+ ions, the crystal contains two Fe3+ ions to balance the charge. The composition of a non-stoichiometric compound usually varies in a continuous manner over a narrow range. Thus, the formula for wüstite is written as Fe_1-xO, where x is a small number (0.05 in the previous example) representing the deviation from the "ideal" formula. Nonstoichiometry is especially important in solid, three-dimensional polymers that can tolerate mistakes. To some extent, entropy drives all solids to be non-stoichiometric. But for practical purposes, the term describes materials where the non-stoichiometry is measurable, usually at least 1% of the ideal composition.

Non-stoichiometric compounds are also known as berthollides (as opposed to the stoichiometric compounds or daltonides). The names come from Claude Louis Berthollet and John Dalton, respectively, who in the 19th century advocated rival theories of the composition of substances. Although Dalton "won" for the most part, it was later recognized that the law of definite proportions did have important exceptions.

However, transition metal oxides are not the only examples of deviations from stoichiometry. It is expected to be a general feature of crystalline compounds. Some examples are provided by eighty binary systems which form a single, essentially ordered, crystalline compound near fifty atomic percent in which the deviation from stoichiometry(a strict 50 at% composition)are important, clearly evident, but, with the exception of SnTe(c), are well below one atomic percent. The temperature-atom fraction phase diagrams of these systems show what appears to be a line compound at 50 at% with the normal resolution used on the composition axis. These are N:(8-N) compounds such as I-VIIs(e.g. NaCl, KCl,etc.), the II-VIs(CdSe,CdTe), IIB-VIs(PbS,PbSe,PbTe), and II-Vs(GaAs,InAs,GaSb). At high temperatures, but below the compound melting point the AB(c) compound is in equilibrium with either an A-rich or B-rich liquid and a gas phase. In either case, at equilibrium the chemical potential of A must be the same in all three phases and similarly for element B. The difference in the chemical potential of say element B for the liquid phase in equilibrium with AB(c)at a given temperature would not appear to be remarkable because of the significant difference in composition of the liquids. It appears striking for AB(c) because of the narrow homogeneity range of AB(c). The partial pressures of diatomic selenium and of tellurium have been determined for a number of selenides and tellurides by measuring the UV-VIS absorption of the gas phase. The arsenic pressure over a number of arsenides has been measured also. The results are usually shown on a log pressure-reciprocal temperature plot and show a parabola-like three phase curve ending at the melting point on the high temperature side. Along this curve AB(c), liquid, and gas phase coexist. Sufficiently below the AB(c) melting point this loop is typically a few decades wide in pressure. Many of the compounds from the II-VI, IIB-VI, and II-V compounds are semiconductors whose electrical properties are significantly affected by the deviations from stoichiometry. The A-saturated compound is usually n-type indicating an excess of element A through the incorporation of vacant sites in the B sublattice or A atoms in interstitial sites acting as donors in the electronic energy band structure. Similarly the B saturated AB(c) is B rich and p-type through the incorporation of vacant sites in the A sublattice or B atoms in interstitial sites acting as acceptors. Control of the electrical properties requires not only control of the concentration of foreign atoms but also control of the deviation from stoichiometry through equilibration with vapor phase.