Stoichiometric ratio and doping effects on the thermal properties of sodium potassium sulphate crystals

Abstract

The influence of stoichiometric ratio on the phase transition of sodium potassium sulphate (NaxKl-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 carded out by using differential scanning calorimetry, DSC technique. The value of the specific heat, Cp, at the transition temperature, To, increases as x is increased up to x = 0.4 and then decreases for higher values of x. The change in the specific heat, ACp, is following a relation of the form AC~ o~(T-Tc/Tc) "~ with o~ = 0.12 which is in a fair agreement with the 3d Ising model. Insertion of Cu ions into the lattice of (NaxKl-x)2SO4 crystals leads to a multiple peak in the temperature dependence of Cr,. The results are discussed from thermodynamical point of view.

Novel High-Temperature Modification of Belomarinaite KNaSO4: Crystal Structure and Thermal Order–Disorder Phase Transitions

“…The novel translationsgleiche phase transition of belomarinaite KNaSO4 (P3m1 → 𝑃3 𝑚1) was investigated. The presence of the 𝑃3 𝑚1 → P-3m1 phase transition is confirmed by the existence of a broad peak in the DSC curve, first discovered in [31], where the abrupt change in heat capacity was also studied, indicating a second-order transition. The symmetry increases with the increasing temperature in the trigonal-trigonal P3m1→ P-3m1 and trigonal-hexagonal 𝑃3 𝑚1 → P63/mmc transformations, which is in agreement with the well-known tendency [38,[46][47][48].…”

“…New data on the belomarinaite crystal structure obtained in this work at 300 • C indicate that with increasing temperature, the belomarinaite crystal structure changes slightly and translationsgleiche phase transition occurs. The belomarinaite crystallizes in the P3m1 space group at 300 • C. The presence of this transition is confirmed by the DSC data of synthetic KNaSO 4 , e.g., [10,30,31], and those of belomarinaite [5]; there are three thermal effects at about 123, 446 and 840 • C. It was suggested in [30] for synthetic KNaSO 4 that the first broad endothermic peak corresponds to a second-order phase transition. Thus, according to the high-temperature single crystal X-ray diffraction data, the first peak can be attributed to a P3m1↔ P-3m1 phase transition.…”