Effect of doped rubidium metal ions on the phase transition of LiNH4SO4

Measurements of Specific heat Cp(T) have been performed to study the effect of doped rubidium metal ions on the phase transition of lithium ammonium sulphate LiNH4SO4 within the temperature range from 400 up to 480K. A pronounced feature of this study is a phase transition of the pure system at 459.5 K, characterized by double distinict peak, changed to a single sharp and narrow one at lower transition temperatures T1, as a result of doping process. Various effects of enhanced molar ratios of dopant n (from n = 0%up to n = 10%) on the behaviour of the phase transition are observed. For small dopant ratios (up to n = 5%), quantitative changes in the temperature dependence of the specific heat in the vicinity of the transition temperature T1 are observed. At higher dopant ratios (n > 5%), the critical behaviour of the phase transition is considerably changed. Decrease of the excess specific heat at T1, (ΔCp)max, associated with changes of thermodynamical parameters, shift of T1 to lower temperatures, broadening of critical temperature region, are obtained and discussed. Contributions of thermally excited dipoles to specific heat in the ferroelectric region were estimated, and their excitation energies appear to change with increasing dopant ratios.