Crystal Structure of the Ferroelectric Phase of ${\mathrm{}\left(\mathrm{Glycine}\right)\mathrm{}}_3$·${\mathrm{H}}_2$S${\mathrm{O}}_4$

Ferroelectric ${\mathrm{}\left(\mathrm{glycine}\right)\mathrm{}}_3$·${\mathrm{H}}_2$S${\mathrm{O}}_4$ crystallizes at room temperature in the monoclinic system with $a={9.41}_7$ A, $b={12.64}_3$ A, $c={5.73}_5$ A, $\beta =110\mathrm{}°{23}^{\prime }$; the space group is $P{2}_1$, and the polar direction is along the two-fold screw axis. Above 47°C the spontaneous polarization disappears as the space group becomes $\frac{P{2}_1}{m}$. The crystal structure was determined from full three-dimensional x-ray diffraction data, using Cu $K\alpha$ radiation. Out of the three glycine molecules in the crystal, one has the usual zwitter-ion configuration, with the -N${\mathrm{H}}_3^{+}$ 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 (N${\mathrm{H}}_3^{+}$C${\mathrm{H}}_2$CO${\mathrm{O}}^{-}$) · ${\left(\mathrm{N}{\mathrm{H}}_3^{+}\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\right)}_2$ · S${\mathrm{O}}_4^{--}$, 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 $y=\frac{1}{4}$, which becomes the mirror plane in the high-temperature phase.