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Anhydrous double salts of rubidium sulphate and cesium sulphate with lithium sulphate have been prepared, and polymorphous transformations at 4-60 ~ and 4-200 ~ for Rb~SO4 9 Li2SO4 and at --75 ~ for Cs2SO 4 9 Li2SO4 have been disclosed by means of differential thermal analysis. These findings were confirmed by dilatometric measurements and X-ray phase analysis at various temperatures.Differential thermal analysis of the double sulphates Rb2SO4" Li2SO4 and Cs2SO4 " Li2SO 4 indicated the presence of solid state phase-transitions which are presumably due to polymorphous transformations. To confirm this assumption, additional methods, viz. dilatometry and high-temperature X-ray phase analysis, were employed to investigate the compounds in question.The double salt RbzSO 4 9 Li2SO 4 was obtained by fusion of the corresponding sulphates, or by crystallization from an aqueous solution containing both sulphates in equimolar quantities. The crystals of this compound are biaxial and slightly birefringent (Np = 1.482, Nm = 1.483, N O = 1.485). The density d~5 = 2.940 g/cm 3 [1 ]. DTA curves with a heating rate of 9--10 ~ were recorded on a Kurnakov DTA apparatus. The sample was placed in a corundum crucible. A Pt--Pt/Rh thermocouple was used to measure temperatures. The reference material was calcined alumina.On the DTA curves of the double salt obtained by fusion, endothermic peaks were observed at 200 ~ and 740 ~ corresponding to a polymorphous transformation and to the fusion of the compound, respectively. On the DTA curves of the double salt obtained by crystallization, a third, less distinct endothermic peak appears at N60 ~ indicating another transformation (Fig. 1).To confirm the existence of these phase transitions, coefficients of linear thermal expansion were calculated for various temperatures from linear extention measurements performed during the heating of Rb2SO4 " Li2SO~. For these measurements, three polycrystalline samples were used which were pressed into cylindrical shapes (diameter 4.08 mm, length 5.73, 7.88 and 8.2 ram) employing a pressure of 4500 g/cm 2. The experimental results are presented in Table 1. Fig. 2 shows the dependence of the coefficient of linear thermal expansion, 7, on temperature. The curve has two peaks, at ,-~60 ~ and ~200 ~ The change in the curve at ~60 ~ confirms that the endothermic peak observed at this temperature

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