Incommensurate phase of (NH4)2BeF4in an applied electric field

The Landau theory developed in this paper predicts a narrower incommensurate phase of (${\mathrm{N}{\mathrm{H}}_4)}_2$Be${\mathrm{F}}_4$ in the presence of an applied bias electric field and allows the existence of a Lifshitz point in the phase diagram. The measurements of the dielectric constant $\epsilon$, performed in the range of bias fields up to $E\sim 90$ kV/cm, show a nonlinear variation of the incommensurate-commensurate transition temperature with $E$ and also a large reduction of the incommensurate phase at highest $E$, at which the Lifshitz point has still not been reached.