Phase transition in lithium ammonium sulphate doped with cesium metal ions

Effects of doped cesium (Cs+) metal ions (with different molar ratios n) on the phase transition of lithium ammonium sulphate LiNH4SO4 system have been studied by measuring the specific heat Cp(T) of the doped systems in the temperature range from 400 to 480 K. The study shows a peculiar phase transition of the pure system (n=0) characterized by double distinct peaks, changed to a single sharp and narrow one as a result of the doping process. The measurements exhibit different effects of enhanced molar ratios of dopants on the phase transition behaviour of this system. At low dopant content (n≤3%), the excess specific heat (ΔCp)max at the transition temperature T1 decreases till a minimum value at n=0.8%, then it increases gradually. In this case, ΔCp(T) behaviour is varied quantitatively and not modified. Enhanced dopant content (n>3%) has a pronounced effect on the critical behaviour, which is significantly changed and considerably modified relative to the pure system. In addition, broadening of the critical temperature region, and decrease of (ΔCp)max associated with changes of the Landau expansion coefficients are obtained and discussed. The study deals with the contribution of the thermally excited dipoles to the specific heat in the ferroelectric region and shows that their energy depends on doping.