THE EFFECT OF UNIAXIAL PRESSURE ON THE FERROELECTRIC PHASE-TRANSITIONOF TGSE CRYSTALS

Isobaric studies of the influence of uniaxial pressure on the transiti ons of pure TGSe crystals for the three directions x, y, z (a, b, c) are reported. The critical exponents, beta, delta and gamma, from diel ectric constant and hysteresis loop measurements, as well as the coeff icients xi and zeta of the fourth and sixth powers of P in the free-en ergy expansion under uniaxial pressure were determined. It has been fo und that uniaxial pressure applied along the a-axis keeps the transit ion near a tricritical point for 0 < X < 200 bar, but that when it is applied along the band c-axes it strengthens the second-order phase tr ansition behaviour. The tricritical behaviour of TGSe crystals is disc ussed and a possible driving mechanism for the change in order of the phase transition is proposed.