A neutron diffraction determination of the crystal structures of thiourea and deuterated thiourea above and below the ferroelectric transition

The structures of thiourea, SC(NH₂)₂, and deuterated thiourea, SC(ND₂)₂, have been determined at room and liquid nitrogen temperatures from three-dimensional neutron diffraction data. No significant structural change on deuteration has been found. N—H...S hydrogen bonds occur in both materials at both temperatures with N—S distances of 3.35–3.43 Å and N—H—S angles of 169–171° and, apart from these hydrogen atoms, the molecules are planar to within 0.010 Å. An analysis of the thermal parameters of the atoms in each molecule in terms of rigid vibration parameters shows that at liquid nitrogen temperature the molecules are fairly rigid whereas at room temperature there are serious deviations from rigidity. Excellent agreement has been found between the thermal vibrations of the molecules at room temperature and the observed structure change to the lower ferroelectric state. A qualitative theory of the ferroelectric nature of thiourea is proposed which explains the observed temperature variation of the spontaneous polarization and coercive field in the lower ferroelectric region, in terms of a variable molecular orientation and a single hydrogen bond which is switched from one sublattice to the other during ferroelectric reversal.