Examples of the Practical Use of Non-Stoichiometric Compounds

In this chapter, we describe four kinds of non-stoichiometric compound, which are or will be in practical use, from the viewpoint of preparation methods or utility. As a first example, the solid electrolyte (ZrO2)0.85(CaO)0.15 is described, which are discussed in Sections 1.4.6–1.4.8 from the viewpoint of basic characteristics. The second example is the magnetic material Mn–Zn ferrite, for which the control of non-stoichiometry and the manufacturing process will be described. Then the metal hydrides or hydrogen absorbing alloys, which are one of the most promising materials for storing and transporting hydrogen in the solid state, are described, mainly focusing on the phase relation. Finally, we describe the relation between the control of composition and the growth of a single crystal of the semiconductive compound GaAs, which is expected to give electronic materials for 1C and LSI etc. Solid electrolytes, which show ionic conductivity in the solid state, are considered to be potential materials for practical use, some are already used as mentioned below. Solid electrolytes have characteristic functions, such as electromotive force, ion selective transmission, and ion omnipresence. Here we describe the practical use of calcia stabilized zirconia (CSZ), (ZrO2)0.85(CaO)0.15, the structure and basic properties of which are discussed in detail in Sections 1.4.5–1.4.8. The most simple practical application of CSZ is for the gauge of oxygen partial pressure, as mentioned in Sections 1.4.7 and 1.4.8. The oxygen partial pressure P2o2 in the closed system as shown in Fig. 3.1 can be measured, taking the air as the standard oxygen pressure P1o2. The electromotive force (EMF) of this concentration cell is expressed as . . . E = (RT/4F)ln(P1o2/ P2o2) . . . This principle is applied in the measurement of oxygen partial pressure in laboratory experiments and of the oxygen activity of slag in refineries. Based on the principle of coulometric titration (see Section 1.4.8), the oxygen partial pressure of a closed system can be kept constant by feedback of the EMF, in the oxygen pressure range 1 to 10−7 atm. By use of this closed system, investigations on redox reactions of metals and also enzyme reactions have been carried out.

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.

Non-Stoichiometric Compounds Derived From Extended Defects

The non-stoichiometric compounds that we describe in this chapter are closely correlated with the classical non-stoichiometric compounds derived from point defects discussed in Chapter 1. For the past twenty years precise structural analyses on complex binary and ternary compounds have been carried out using X-ray and neutron diffraction techniques. Moreover, owing to the striking development of the resolving power of the electron microscope crystal structures can be seen directly as structure images. As a result, it has been shown that most complex structures can be derived by introducing extended defects regularly into a mother structure. A typical example is a ‘shear structure’, which is derived by introducing planar defects of anion rows into the mother lattice. A ‘block structure’ is derived by introducing two groups of planar defects. ‘Vernier structures’, ‘micro-twin structures’, ‘intergrowth structures’, and ‘adaptive structures’ are also described in detail in this chapter. At the beginning of 1950, Professor A. Magnéli’s group in Sweden started a systematic study of the crystal structures of the oxides of transition metal elements such as Ti, V, Mo, and W, mainly by X-ray diffraction techniques. As a result, they confirmed the existence of the homologous compounds expressed by VnO2n–1; TinO2n–1 etc. (n = 2, 3, 4, . . .) and also predicted that the crystal structure of these compounds could be derived from a mother structure, ‘rutile’. Figure 2.1 shows the X-ray powder diffraction patterns (CuKα) of compounds TiOx between Ti2O3 (x = 1.5) and TiO2 (x = 2.0).3 This clearly indicates the convergence of the diffraction patterns to that of TiO2 (rutile) with increasing x, which is why the Magnéli school predicted the mother structure to be rutile. This prediction was verified by the structure determinations of Ti5O95 and VnO2n–1.6 These compounds are called Magnéli phases after the main investigator, and similar compounds have been discovered.