Non-Stoichiometric Compounds Derived From Point Defects

In this chapter, we discuss ‘classical’ non-stoichiometry derived from various kinds of point defects. To derive the phase rule, which is indispensable for the understanding of non-stoichiometry, the key points of thermodynamics are reviewed, and then the relationship between the phase rule, Gibbs’ free energy, and non-stoichiometry is discussed. The concentrations of point defects in thermal equilibrium for many types of defect structure are calculated by simple statistical thermodynamics. In Section 1.4 examples of non-stoichiometric compounds are shown referred to published papers. The technical term ‘non-stoichiometric compounds’ has been used for a long time, in contradiction to the term ‘stoichiometric compounds’. The existence of non-stoichiometric compounds, which have also been called Bertholides compounds, cannot be explained from the law of definite proportion in its simplest meaning. Proust insisted that only stoichiometric compounds (also named Daltonide compounds) existed, whereas Bertholet maintained the existence of not only stoichiometric compounds but also non-stoichiometric compounds. This is a very famous argument in the history of chemistry. In the early years of the twentieth century, Kurnakov investigated the physical and chemical properties of intermetallic compounds in detail and found that the maximum or minimum in melting point, electrical resistivity, and also in the ordering temperature of lattices does not necessarily appear at the stoichiometric composition. An important discovery of Dingman was that stoichiometric FeO1.00 is non-existent under ordinary conditions. (At present, we can synthesize stoichiometric FeO1.00 under high pressure.) Non-stoichiometry, which originates from various kinds of lattice defect, can be derived from the phase rule. As an introduction, let us consider a trial experiment to understand non-stoichiometry (this experiment is, in principle, analogous to the one described in Section 1.4.8). Figure 1.1 shows a reaction vessel equipped with a vacuum pump, pressure gauge for oxygen gas, pressure controller for oxygen gas, thermometer, and chemical balance. The temperature of the vessel is controlled by an outer-furnace and the vessel has a special window for in-situ X-ray diffraction. A quantity of metal powder is placed on the chemical balance, and then the vessel is evacuated at room temperature.