Crystal Structure of the Ferroelectric Phase of (Glycine)₃·H₂SO₄

Ferroelectric (glycine)₃·H₂SO₄ crystallizes at room temperature in the monoclinic system with a=9.41₇ A, b=12.64₃ A, c=5.73₅ A, β=110°23^′; the space group is P2₁, and the polar direction is along the two-fold screw axis. Above 47°C the spontaneous polarization disappears as the space group becomes P2₁/m. The crystal structure was determined from full three-dimensional x-ray diffraction data, using Cu Kα radiation. Out of the three glycine molecules in the crystal, one has the usual zwitter-ion configuration, with the -NH₃⁺ group out of the plane of the other atoms; the remaining two glycines are mono-protonated, and planar within experimental error, and are designated as glycinium ions. Thus the chemical formula is properly written as (NH₃⁺CH₂COO^-) · (NH₃⁺CH₂COOH)₂ · SO₄^--, and the compound is best described by the chemical name glycine diglycinium sulfate. One of the planar glycinium ions lies near but not in the plane y=1/4, which becomes the mirror plane in the high-temperature phase.

The nitrogen atoms form N-H··O hydrogen bonds of the usual strength, whereas a quite strong O-H··O hydrogen bond of a distance of 2.43₈ A is found between the oxygen atom of the carboxyl group of the zwitter-ion glycine and that of the planar glycinium ion which lies near the plane y=1/2.

Above the Curie point, at 47°C, mirror symmetry is attained by statistical arrangement of atoms around the mirror planes at y=1/4 and 3/4. The disorder of the glycinium ions near the mirror planes, and the above-mentioned strong O-H··O bond, are of particular importance for the ferroelectric behavior of the crystal.