Theory of morphotropic transformations in vanadium oxides

A theory of the morphotropic transformations occurring between stoichiometric phases in the vanadium-oxide system is proposed. The interconnections between the different structures are shown to result from definite symmetry-breaking mechanisms from a common parent structure. The homologous series VₙO_2n-1 and VₙO₂ₙ₊₁ are interpreted as sequences of lock-in commensurate phases. Identification of the order-parameter symmetries provides us with the form of the thermodynamic functions which allow for construction of the theoretical phase diagrams in which the different phases are inserted. The general procedure allowing for the description of morphotropic transformations is outlined.