Detection of a ferroelastic phase transition in Csx(NH 4)1-xLiSO4 with the use of the DSC method

Abstract

In this paper the technology of producing solid solutions of Cs x(NH4)1-xLiSO4 using the slow evaporation method is presented. Appropriate conditions were chosen to grow large samples. The ammonium ion content in the solid solutions was determined using the Kjeldahl method. It was found that the real ammonium ion concentration is twice lower than the one applied in the initial substances. At room temperature, the base crystal, lithium cesium sulfate (CsLiSO4), is paraelastic, whereas lithium ammonium sulfate (NH4LiSO4) is ferroelectric. It is expected that as a result of substituting Cs+ ions with N{\text{H}}-{4}^{ + } N H 4 + ions, instead of the Cs+ ions, the modification of the ferroic properties of solid solutions of Cs x(NH4)1-xLiSO4 will take place. Tests conducted with the use of the differential scanning calorimetry method (DSC) allowed the detection of the ferroelastic phase transition which takes place in these compounds. A gradual increase of temperature transition was observed from 202 K for the pure CsLiSO4 to 203.8 K for Cs 0.90(NH4)0.10LiSO4 and 230.1 K for Cs0.85(NH4)0.15LiSO4 with the increase of N{\text{H}}-{4}^{ + } N H 4 + ions concentration. Using polarized light microscopy, a ferroelastic domain structure was detected in the examined solid solutions, which appeared below the structural phase transition temperature.