Infinite Lifshitz point in incommensurate type-I dielectrics

The phenomenological analysis of the pressure-temperature (P-T) phase diagram of [N(CH₃)₄]₂CuCl₄ (TMATC-Cu) crystals is presented. It is shown that serious disagreement between the experimental results and theory may be removed assuming that the coefficient of the Landau free energy expansion κ at the gradient term (dq/dz)(dq*/dz) changes the sign in the experimental range of pressures. The phase diagram in the α-κ phase plane is characterized by infinite Lifshitz point (at →κ∞) in which both lines of the phase transitions into the incommensurate phase merge together, whereas the incommensurate wave vector approaches its zero value. The triple point (PK≈55MPa, TK≈304K) observed in these crystals may be considered as an artificial point, which results from the limitation of experimental resolution. Therefore, even above PK there still exist two very close (unresolved) lines of the incommensurate phase transitions.