Non-stoichiometry and redox energetics

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO, 

Another classical example of demixing in the liquid state occurs in the system CaO-Si02 and other binary silicate systems where Si02 is mixed with basic oxides. While the solubility of Si02(l) in the basic oxide is high, there is a 

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