CRYSTAL-STRUCTURE OF DEUTERATED TRIGLYCINE SULFATE BY NEUTRON-DIFFRACTION AT 180 AND 40 K

The crystal structure determination of a deuterated triglycine sulfate single crystal has been carried out at 180 and 40 K by neutron diffra ction (lambda = 0.8332 Angstrom). The space group P2(1) is not modifie d by lowering the temperature. Thermal parameters are well defined in the anisotropic model in the case of the experiment at 180K but they a re very small al 40 K, and only obtained in the isotropic model for al l atoms, except for hydrogen and deuterium. The general structural fea tures and most of the atomic coordinates are comparable to those previ ously obtained on a TGS single crystal in the ferroelectric phase at r oom temperature. Bond length differences between the structures at roo m temperature and at 180 and 40 K, do nat exceed 0.13 Angstrom for one H atom of the Gly III methyl group, 0.07 Angstrom for about ten dista nces, involving hydrogen atoms and one SO bond, less for the remaining ones. All intramolecular distances are statistically equal in the two low temperature studies. The same behaviour is observed for the three glycine least squares planes and for the shortest intermolecular dist ances which involve the sulfate ion and the adjacent glycine molecules except for three sulfate oxygen-deuterium distances where the bond le ngths differ by about 5 sigma.