Structural investigations of the ferroelectric glycinium hydrogenphosphite (GPI) and its deuterated analogue (DGPI) I. X-ray diffraction studies of the crystal structure of paraelectric and ferroelectric phases

The crystal structures of the glycinium hydrogenphosphite (GPI) and its deuterated analogue (DGPI) were determined by the X-ray diffraction method in the paraelectric (T = 295(2) and 360(2) K) and ferroelectric (T = 180(2) and 295(2) K) phases. The crystal structures of the paraelectric phase are consistent with the data published by Averbuch-Pouchot [Acta Cryst. C49 (1993) 815]. In the deuterated crystal, the interphosphite hydrogen bonds are longer (by ca. 0.03 Angstrom) than those in the normal crystal. In the ferroelectric phase crystals remain monoclinic with space group P2(1). Due to symmetry lowering, two kinds of glycinium cations (GA and GB) and two kinds of hydrogenphosphite anions (PA and PB) appear in the ferroelectric phase. The protons in the interphosphite hydrogen bonds are ordered. The P-H distances in PA (1.314(10) Angstrom) and PB (1.222(9) Angstrom) hydrogenphosphite anions differ significantly for GPI. Their P-D analogues (1.310(10) and 1.279(9) Angstrom) in the DGPI crystal are slightly less different. Molecular mechanism of the ferroelectric phase transition is proposed. (C) 2002 Elsevier Science B.V. All rights reserved.