Langbeinite

Abstract: Phase transitions were searched in five langbeinite-type crystals in the temperature range 20∼-196°C. Dielectric and elastic anomalies were found at -72°C in a single crystal of K 2 Mn 2 (SO 4 ) 3 . Pyroelectric measurements showed that the low temperature phase of K 2 Mn 2 (SO 4 ) 3 is not polar. Dielectric and pyroelectric measurements were made on the polycrystalline samples of the solidified molten K 2 Zn 2 (SO 4 ) 3 and K 2 Cd 2 (SO 4 ) 3 . It was found that K 2 Zn 2 (SO 4 ) 3 undergoes pyroelectric phase transition at -137°C, but K 2 Cd 2 (SO 4 ) 3 does not show any phase transition. It was also found that K 2 Mg 2 (SO 4 ) 3 and Rb 2 Mg 2 (SO 4 ) 3 of pressed powder crystals do not show any phase transition signal.

Abstract: K2Cd2(SO4)3 is orthorhombic below 432°K, with space group P212121, and cubic above with space group P213. A structural study has been completed at 298, 351, 390.5, 417.5, and 443.5°K based, respectively, on 3383, 1377, 1803, 1724, and 1022 symmetry‐independent structure factors. Measurement of integrated intensities was made using a CAD‐4 diffractometer with Nb‐filtered MoKα radiation. An air‐flow microfurnace with temperature controller provided a thermal stability better than ±1°. The a, b, c lattice constants in the orthorhombic phase are 10.2084(8), 10.2813(8), 10.1684(8) A at 298°K; 10.2239(8), 10.2897(8), 10.1821(8) A at 351°K; 10.2386(8), 10.2951(8), 10.1981(8) A at 390.5°K; and 10.2501(10), 10.2972(8),10.2132(9) A at 417.5°K. In the cubic phase, a=10.2704(10) A at 443.5°K. The model previously obtained at room temperature [J. Chem. Phys. 67, 2146(1977)] was refined by the method of least squares using the new sets of measurements. The final agreement factors are R=0.028 (298°K), 0.032(351°K), 0.02...