Neutron Diffraction Study of Crystal Structures of Glycinium Phosphite H₃NCH₂COOH·H₂PO₃in Paraelectric and Ferroelectric Phases

Crystal structure of H 3 NCH 2 COOH·H 2 PO 3 was studied in the paraelectric phase (P phase) at 298 K and in the ferroelectric phase (F phase) at 170 K by means of the single crystal neutron diffraction. Two independent protons in the hydrogen bonds forming the H(HPO 3 ) - chains are disordered over two sites with the separations of 0.486(3) and 0.474(5) Å in the P phase. In the F phase, they are ordered at a position nearly equal to one of two sites related by the disorder. With the proton order, lengths of the P–O bonds with the oxygens involved in the hydrogen bonds forming the chain deviate from their values determined in the P phase. A possible mechanism for the appearance of the spontaneous polarization is discussed in connection with the deformation of the HPO 3 2- anions in the F phase.