Structural characterization of crystals of alpha-glycine during anomalous electrical behaviour.

The crystal structure of alpha-glycine has been investigated in the temperature range 288-427 K using neutron diffraction. The molecular structure does not change significantly and the putative crystallographic phase transition associated with anomalous electrical behaviour in this temperature range is not observed. The unit cell expands anisotropically with increasing temperature, with the unique monoclinic b axis, corresponding to the stacking direction of molecular layers, changing the most. The increasing separation of antiferroelectric molecular layers with increasing temperature is driven by an increase in molecular libration about an axis that lies perpendicular to the b axis. There is also a weakening of the interlayer hydrogen bonds with temperature. These structural and dynamic changes will affect the response of molecular dipoles to an applied electric field and provide a possible mechanism for the anomalous electrical behaviour.