EFFECT OF DOPED RUBIDIUM METAL IONS ON THE PHASE TRANSITION OF LINH_4SO_4

摘要

Measurements of Specific heat C_p(T) have been performed to study the effect of doped rubidium metal ions on the phase transition of lithium ammonium sulphate LiNH_4SO_4 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 distinct peak, changed to a single sharp and narrow one at lower transition temperatures T_1, as a result of doping process. Various effects of enhanced molar ratios of dopant n (from n = 0 percent up to n = 10 percent) on the behaviour of the phase transition are observed. For small dopant ratios (up to n = 5 percent), quantitative changes in the temperature dependence of the specific heat in the vicinity of the transition temperature T_1 are observed. At higher dopant ratios (n> 5 percent), the critical behaviour of the phase transition is considerably changed. Decrease of the excess specific heat at T_1, (#DELTA#C_p)_(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.