THE EQUATION-OF-STATE PARAMETERS OF TRIGLYCINE SULFATE (TGS) FERROELECTRIC FOR BOTH PHASES NEAR THE CRITICAL-POINT

The experimental electric susceptibility isotherms (that is, the elect ric suscept ibility versus electric field dependences) were obtained f or temperatures over the interval -4 K < T - T-c < 7 K. The constant c oefficients A, B and C of the equation of state E = A tau P + BP3 + CP 5 were found separately for the paraelectric and ferroelectric phases on the basis of precise susceptibility measurements. The validity of t he 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 t o 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 intro duced.