Evidence of Landau tricritical behaviour in TGSe by calorimetric measurements: effect of a weak uniaxial stress

The paraelectric–ferroelectric phase transition of a single crystal of triglycine selenate TGSe, whose first- or second-order character is controversial, has been studied using a high sensitivity calorimetric technique. The specific heat of a highly pure TGSe has been measured and no evidence of latent heat has been found. The anomalous part of the specific heat shows Landau tricritical behaviour. Experimental data have been fitted to a 2–6 Landau potential, whose coefficients have also been obtained. A weak uniaxial stress applied along the ferroelectric axis (10 bar) decreases the anomalous part of the specific heat. This effect is different for cooling and heating runs. The equilibrium data are those obtained on heating that fit to a 2–4–6 Landau potential, which indicates that the transition shifts to second order with the effect of uniaxial stress.