Complementary neutron and X-ray single crystal diffraction studies of novel molecular ferroelectrics

Complete text of publication follows. Three different stoichiometries of 4,4,-bipyridine and pyromellitic acid were crystallised. The 1:1, 1:2 and 2:1 phases were examined with X-rays at room temperature, the positions of the acidic hydrogens were undetermined. The 1:1 and 1:2 phases were examined on the I.L.L diffractometer D19. The 1:1 phase at 215 K due to a phase transition at 200 K. The structure refined in P-1 to RI = 4.33%. The 2:1 phase caused problems for the X-rays. The space group was uncertain, either Cc or C2/c; Cc refinement was unstable, C2/c refined to only 12%. In contrast the neutron data, at 20 K, completed the structure and resolved the pseudo symmetry, refining to R1 = 3.38% in Cc. This is in agreement with a later 20 K X-ray structure recently recorded in Durham. The data set for the 2:1 phase has been collected during thermal tests on the neutron Laue diffractometer LADI at the I.L.L. This data set will yield interesting results and demonstrate the potential of neutron Laue diffractometry for the study of small molecules. (author)