Incommensurate phase of (NH 4 ) 2 BeF 4 in an applied electric field

Abstract
The Landau theory developed predicts a narrower incommensurate phase of (NH4)2BeF4 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 ε, performed in the range of bias fields up to E ∼ 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.