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Free-energy amplitudes and a critical invariant for uniaxial ferroelectrics

1992; American Physical Society; Volume: 45; Issue: 6 Linguagem: Inglês

10.1103/physrevb.45.2704

1095-3795

B. Westwański, B. Fugiel,

Solid-state spectroscopy and crystallography

A singular free energy consisting of three terms ${\mathit{P}}^{2}$, ${\mathrm{P}}^{4}$, and ${\mathit{P}}^{6}$ that contribute simultaneously to the susceptibility scaling function is proposed for the description of triglycine sulfate (TGS) and triglycine selenate (TGSe) ferroelectrics, where P is the polarization. It is shown that the combination ${\mathit{C}}_{2}$${\mathit{C}}_{6}$/${\mathit{C}}_{4}^{2}$ of the nonuniversal critical amplitudes ${\mathit{C}}_{2}$,${\mathit{C}}_{4}$, and ${\mathit{C}}_{6}$ of the singular free energy is a nonzero-field critical invariant. This invariant is measured experimentally for TGS and TGSe ferroelectrics. It is proved theoretically and demonstrated experimentally that this invariant is proportional to another invariant which represents the parabolic approximation of the equation for the susceptibility scaling function for a Landau critical point in the case of TGS and TGSe ferroelectrics.