PHASE-DIAGRAM OF THIOUREA AT ATMOSPHERIC-PRESSURE UNDER ELECTRIC-FIELD - A THEORETICAL-ANALYSIS

The phase diagram of thiourea in the (E, T) plane at atmospheric pressure P//o is discussed in terms of a single harmonic approximation for the modulated phase. A Landau-Ginzburg approach is used - the type used to discuss Lifshitz points: in this approach the temperature dependence of the square of the modulation wave vector is accounted for by the temperature dependence of the negative exchange stiffness constant. This theory allows qualitative as well as semi-quantitative understanding of the phase diagram, which is shown to exhibit a tricritical point in the close neighbourhood of an isolated point.