Landau, Lifshitz, and weak Lifshitz conditions in the Landau theory of phase transitions in solids.

Landau theory provides a group-theoretical method for determining which symmetry changes can occur in second-order phase transitions in solids. The irreducible representations of the space group of the higher-symmetry phase must satisfy certain conditions. For transitions to commensurate crystalline structures, the Landau and Lifshitz conditions must be satisfied. For transitions to incommensurate structures, the Landau and "weak Lifshitz" conditions must be satisfied. The irreducible representations of each of the 230 crystallographic space groups that violate these conditions are listed.