Stoichiometry of Oxide Crystals

The essential concept of stoichiometry is discussed. The discussion is based on a simple consideration-whether the activities of the constituent elements of a material can be unity. A comparison with nonstoichiometry characterized by solute mixing reveals that the unity of activity for all constituent elements is an essential attribute of a stoichiometric material because it leads to a zero mixing term in the chemical potential. For the activity of an element to be unity, which results when the standard-state chemical potential is equal to the chemical potential itself, at least one degree of freedom is required in each crystal site containing an element. On the basis of the extended concept of stoichiometry, new LiNbO₃ and LiTaO₃ crystals have been developed by introducing Mg and vacancy, both of which are simultaneously congruent and stoichiometric. They are homogeneous in composition and show excellent nonlinear optical properties.