k‐q Functions and the Landau Theory of Second‐Order Phase Transitions

The use of k‐q functions is introduced into the Landau theory of second order phase transitions. The expansion of an arbitrary density function in a crystal is given in terms of basis functions of irreducible representations of a space group which are linear combinations of k‐q functions. This leads to a new condition on the irreducible representations which can be associated with second order phase transitions.