Study on the phase transition of LiNH4SO4:T1 at 459 K

Abstract

Study on the effect of thallium (T1+)-dopant with different concentration ratios n (from n = 0% up to n = 2%) on the phase transition of LiNH4SO4, system has been carried out between 400 up to 500 K, using specific heat measurements. The measured temperature dependence of the specific heat CP(T) of the pure system exhibits phase transition at T1 = 459.5K, shifted to lower temperatures for enhanced T1+ dopant. For the system with low T1+ dopant ratios (from n = 0% up to n = 0.7%), the Cp(T)-behavior around the phase transition, shows quantitative change, while for that with high T1 + dopant ratios (from n > 0.7% up to 2 %), critical change seems to characterize the Cp(T)-behavior. Phase transitions of the pure and doped system, in the early range of dopant concentration, exhibit first order transition changed to a continuous phase transition in the high dopant concentration range. The study gives information on the dipole excitation energy which contribute to the specific heat in the ferroelectric phase near the continuous transition. Decrease of the dipole excitation energy, depress of (ΔCp)max, change of the thermodynamic parameters and broadening of the temperature range of transition appear clearly in the system with high dopant concentrations.