The effect of hydrostatic compression on the crystal structure of glycinium phosphite

The crystal structure of glycinium phosphite (GPI) was studied in the pressure range from ambient to 6.5 GPa at 293 K using single-crystal X-ray diffraction. The changes in the unit-cell volume and parameters were continuous and anisotropic. The major compression was observed normal to the direction of the spontaneous polarization that occurs in this structure during a ferroelectric phase transition on cooling, whereas the structural compression along the b axis coinciding with the 21 axis was almost zero. The effect of pressure on the hydrogen bonds linking the H2PO3 tetrahedra into zigzag chains along the c axis was different from that on the hydrogen bonds connecting the glycinium cations with the H2PO3 tetrahedra in the (b × c) plane. The discontinuous changes in the geometries of selected hydrogen bonds at about 1.21 GPa may be evidence of a phase transition, e.g. into an ordered ferroelectric phase (with ordered positions of protons). The anisotropy of compression of GPI in the ferroelectric state (at 0 K) was studied using DFT calculations taking into account dispersive van der Waals interactions. The calculations predicted negative linear compressibility along the b axis.