STOICHIOMETRIC RATIO AND DOPING EFFECTS ON THE THERMAL-PROPERTIES OF SODIUM-POTASSIUM SULFATE CRYSTALS

The influence of stoichiometric ratio on the phase transition of sodium potassium sulphate (NaxK1-x)2SO4 Crystals is investigated in the temperature range 300-500 K. The stoichiometric ratio x is chosen to be x = 0.2, 0.3, 0.4, 0.8 and 0.9. The study has been carried out by using differential scanning calorimetry, DSC technique. The value of the specific heat. C(p), at the transition temperature, T(c), increases as x is increased up to x = 0.4 and then decreases for higher values of x. The change in the specific heat, DELTAC(p), is following a relation of the form DELTAC(p) alpha(T-T(c)/T(c))-alpha with alpha = 0.12 which is in a fair agreement with the 3d Ising model. Insertion of Cu2+ ions into the lattice of (NaxK1-x)2SO4 crystals leads to a multiple peak in the temperature dependence of C(p). The results are discussed from thermodynamical point of view.