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

Abstract

Crystal structure of H 3 NCH 2 COOHÁH 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 ) À chains are disordered over two sites with the separations of 0.486(3) and 0.474 (5) A 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 2À 3 anions in the F phase.

Proton Ordering Model of Phase Transitions in Hydrogen Bonded Ferrielectric Type Systems: The Gpi Crystal

“…The hydrogen bonds linking the phosphite groups are symmetric (with hydrogens equally disordered over two sites). Investigation of symmetry and structure of crystal in the ferroelectric phase (below temperature of the second order phase transition T c = 224.7 K) was recently performed by means of X-ray structure analysis [2] and neutron diffraction study [3]. The space group of this phase is P2 1 .…”

“…The estimates of the corresponding activation energy and relaxation time imply that para-ferroelectric phase transition in GPI does not seem to be strictly connected only with the movement of protons [9,10]. A reorientation and deformation of the ionic group (phosphite anions) could also play an important role (the corresponding structure changes can be extracted from the neutron scattering data [3]). …”

“…Such a model is proposed in this work due to the information obtained last year about the changes in the structure (including the changes in distribution of protons on hydrogen bonds) at the paraelectric-ferroelectric phase transition [2,3]. The symmetry analysis of the order parameters is performed.…”

“…We start from the structural data for localization of hydrogen bonds and equilibrium positions of protons in paraelectric and ferroelectric phases [2,3]. There are four translationally nonequivalent bonds in the unit cell of crystal (the 011−021 and 013−023 bonds as well as other two bonds connected with the mentioned ones by the point symmetry operations).…”

“…Such an interaction can be quite large in GPI, since redistribution of protons on the bonds is accompanied by deformation and reorientation of ionic groups connected by the bonds [3]. The procedure applied below is well known and accepted for the quasione-dimensional systems (see [17,18]) and corresponds to the two-particle cluster approximation [19,20].…”