Extension of the Lifshitz-point concept to first-order phase transitions: Incommensurate NaNO2 in a transverse electric field.

Abstract

The theory of the Lifshitz point, a triple point in the phase diagram of systems with incommensurate phases, is extended to systems in which the direct para-ferro transition is first order. The structure of the phase diagram is shown to be considerably more complex than in the usual case of a second-order direct transition. There is a virtual Lifshitz point at which the ${T}_{\mathrm{I}}$ and ${T}_{0}$ phase-transition lines cross, but it is not a triple point. A new tricritical point is found at which the para-to-incommensurate transition changes from second to first order, and a new triple point is found, distinct from the virtual Lifshitz point. We present the results of an elastic neutron scattering study of the incommensurate ferroelectric ${\mathrm{NaNO}}_{2}$ in transverse electric fields which exhibits behavior resembling that found in the theoretical analysis. We therefore extrapolate the experimental phase diagrams to construct a plausible picture of the region of the virtual Lifshitz point in ${\mathrm{NaNO}}_{2}$.