Phenomenological Theory on Incommensurate and Commensurate Phases and Electric Field-Temperature Phase Diagram in Thiourea

The well-known type of the free energy to explain the incommensurate-commensurate transition in thiourea is reinvestigated by taking account of higher harmonics of the primary order parameter. The coupling between the seventh or nineth order harmonics and a quantity belonging to the identical representation can stabilize the sevenfold or ninefold superstructure against the incommensurate phase. It is also shown that an electric field induces the eightfold superstructure and that the obtained E-T phase diagram is consistent with experiments. An anomaly of the dielectric susceptibility at the tricritical point on the disordered-incommensurate transition line is presented.