The equation-of-state parameters of triglycine sulphate (TGS) ferroelectric for both phases near the critical point

The experimental electric susceptibility isotherms (that is, the electric suscept ibility versus electric field dependences) were obtained for temperatures over the interval -4 K < T - T-c < 7 K. The constant coefficients A, B and C of the equation of state E = A tau P + BP3 + CP5 were found separately for the paraelectric and ferroelectric phases on the basis of precise susceptibility measurements. The validity of the singular part of the Landau equation of state E = A tau P + BP3 was checked. The parameter s, where s = 1 for tau > 0 and s approximate to 4/3 for tau < 0, which determines the modifted equation of state E/s = A tau P + BP3 for temperatures not too close to T-c, has been introduced.