An anomalous sharp decrease in D.C. resistivity associated with an increase in both dielectric constant and dielectric loss values and the values of the pyrocurrent for both mono-and polycrystalline potassium sulphate samples were detected in the high temperature region. The pyroelectric behaviour of the high temperature K 2 SO" phase I suggested that this phase belongs to a polar class having a resultant vector in the crystal. Such polar nature might explain the excessive increase in the dielectric constant s' and dielectric lo~s s" with temperature. Differential thermal analysis (DTA) showed a pronounced peak at 558°C, indicating that the endothermic phase transition II -I of K 2 SO, was of the order -disorder type. The area under the DTA peak was found to be greater in the case of single crystals than that obtained for polycrystalline samples. Also, the rate of increase of the expansion coefficient during the phase transition II-I, as obtained by thermomechanical analysis (TMA), was found to be higher for single crystals due to the ordering and unique orientation of unit cells (summation rather than averaging). The decrease of D.C. resistivity associated with the increase of dielectric constant might be attributed to an orientational motion of the tetrahedral ions SO~. Such orientational motion occurring with high frequency ~10 2£, 1047). The existing structural information is incomplete and ambiguous as to the choice of a and b axes. In order to understand the nature of the reorientations associated with the phase transition, a combined single-crystal X-ray and neutron diffraction study at room temperature has been carried out. The space group is P63 with a= 5.127(2), c = 8.618(3) A and z = 2.The X-ray data set consisting of 855 tndependent reflections within sin97~ = 1.1 A-1(Mo~) was recorded using the on-line TDC-312 comnutercontrolled diffractometer at Trombay. Anisotropic least-squares refinement of the structure using this data set converged to a conventional R(on F) value of 0.077 and indicated partial disorder for the sulphate groups (~15%). The second minimum corresponded to a rotation of the sulphate ion by 60° about the S-0 bond along the 3-fold axis, but the S-0 bond distances for this position were somewhat less reasonable.The refinement based on the neutron data set (181 tndependent reflections within sinQ/,\ =: 0.6!-) which converged to a R(on F) value of 0.068 does not, however, seem to confirm the disorder in the structure at this stage of the investigation. The tetrahedrally coordinated Li ion is tightly bound with an average Li-0 distance of 1.90(2)!. The detailed results of the two refinements will be compared and the difficulties encountered in searching for possible disorder in this structure will be discussed. Both the X-ray and neutron data suffered from severe ~xtinction (minimum F~/~ valu.es being 0.095 and 0.25 respectively).. 05.1-32THE ELECTRICAL PROPERTIES OF SINGLE-CRYSTAL KLiSO". By Zhu Yong, Zhang Dao-fan, Xu Zheng-yi, Institute of Physics, Chinese Academy of Sciences, Beijing...

The absolute configuration of a-LilO3 crystals has been reinvestigated. The redetermination of the absolute configuration is in agreement with a report by Stadnicka, Glazer & Moxon [J. Appl. Cryst. (1985), 18, 237-240]. Special attention should be paid to the selection of Bijvoet pairs in these and similar crystals. The signs of the piezoelectric and pyroelectric coefficients have been verified and correlated with the spontaneous polarization. The pyroelectric coefficient P3 was --4"4x 10 -5 C m -2 K -~ at 180 K. The correlation between the optical activity and the morphology of the crystals is discussed.