Using the relevant thermodynamic data, calculate the chemical stability domain (in terms of oxygen partial pressure) of FeO and NiO. Plot to scale a figure such as Fig. 6.7 for each compound, using the data given in Table 6.1. Which of these two oxides would you consider the more stoichiometric? Why? Stability domain Nonstoichiometric oxide +ve Deviation from stoichiometry Ar Stoichiometric oxide -ve log Poz Figure 6.7 Distinction between a stoichiometric and a nonstoichiometric MOhata oxide. where the functional dependence of the changes in stoichiometry on the oxygen partial pressure for two hypothetical compounds having the same range of chemical stability is compared. From the foregoing discussion, it follows that the oxide for which Ar varies widely over the stability domain will be labeled nonstoichiometric. and vice versa. Table 6.1 Range of stoichiometry and existence domains of a number of binary oxides at 1000 K Oxides Deviation from stoichiometry Stability or existance regiont - log Po, Xmin Xmax Art Min Max 44.28 41.5 25.7 35.9 41.5 30.1 0.5 0.011 0.008 0.5 0.18 0.155 0.045 0.012 0.001 0.0016 34.5$ 21.65 33.2 10.7 17.9 10.9 2.5 Nonstoichiometric oxides TiO 0.8 1.3 Ti 03 1.501 1.512 TiO2 1.992 2.00 VO 0.8 1.3 MnO 1.00 1.18 Feo 1.045 1.2 Fe304 1.336 1.381 CoO 1.00 1.012 NiO 1.00 1.001 Cu 0.500 0.5016 Stoichiometric oxides AL03 1.5000 1.5000 MgO 1.00000 1.0000 For an M,On, oxide, x =b/a 58. 1 See Sec. 5.4 for more details. In equilibrium with the parent metal. 17.9 17.18 16.5$ 9.978 7.0 71.3 51.5 77 T. B. Reed, The Chemistry of Extended Defects in Non-Metallic Solids, L. Eyring and M. O'Keeffe, eds., North-Holland, Amsterdam, 1970