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

Abstract

Ferroelectric ${(\mathrm{glycine})}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{H}}_{2}$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, $\ensuremath{\beta}=110\ifmmode^\circ\else\textdegree\fi{}{23}^{\ensuremath{'}}$; the space group is $P{2}_{1}$, and the polar direction is along the two-fold screw axis. Above 47\ifmmode^\circ\else\textdegree\fi{}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 $K\ensuremath{\alpha}$ 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${\mathrm{O}}^{\ensuremath{-}}$) \ifmmode\cdot\else\textperiodcentered\fi{} ${(\mathrm{N}{\mathrm{H}}_{3}^{+}\mathrm{C}{\mathrm{H}}_{2}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H})}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} S${\mathrm{O}}_{4}^{\ensuremath{-}\ensuremath{-}}$, 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.The nitrogen atoms form N-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}O hydrogen bonds of the usual strength, whereas a quite strong O-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}O hydrogen bond of a distance of ${2.43}_{8}$ 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 $y=\frac{1}{2}$.Above the Curie point, at 47\ifmmode^\circ\else\textdegree\fi{}C, mirror symmetry is attained by statistical arrangement of atoms around the mirror planes at $y=\frac{1}{4} \mathrm{and} \frac{3}{4}$. The disorder of the glycinium ions near the mirror planes, and the above-mentioned strong O-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}O bond, are of particular importance for the ferroelectric behavior of the crystal.