New Room-Temperature Ferroelectric

Abstract

The neutral-salt complex with glycine, di-glycine manganous chloride dihydrate, (N${\mathrm{H}}_2$C${\mathrm{H}}_2$ ${\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H})}_2$ ·Mn${\mathrm{Cl}}_2$·${2\mathrm{H}}_2$O, is found to be ferroelectric from low temperatures to +55°C. Above the latter temperature the conductivity suddenly becomes excessive. At room temperature the spontaneous polarization is 1.3×${10}^{-6\mathrm{}}$ coulomb/${\mathrm{cm}}^2$ and the coercive field is 5.6 kv/cm. A nonreversible surface change is observable optically above 72°C, probably due to loss of ${\mathrm{H}}_2$O. Thermal measurements indicate onset of water loss above 35°C; electrical conductivity increases sharply and irreversibly above 55°C, and care must be taken to prevent dehydration even at room temperature. No Curie temperature can be observed. X-ray measurements of this hitherto unreported salt show space group $P{2}_1$, $a=9.96\mathrm{}$ A, $b=8.53\mathrm{}$ A, $c=6.86\mathrm{}$ A, $\beta =107\mathrm{}\pm 0.5°$, ${\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=1.875\mathrm{}}_0$ g/cc, and two formula units per cell. The ferroelectric axis is along $b$. The dielectric constants at room temperature are ${\epsilon }_a=6.6\mathrm{}$, ${\epsilon }_b=8.1\mathrm{}$, ${\epsilon }_c=7.4\mathrm{}$; and these decrease linearly and only very slightly as the temperature is lowered to 77°K. The corresponding neutral-salt complex of two glycines with Mn${\mathrm{Br}}_2$ is not isomorphous, occurring as a tri-hydrate, and is not ferroelectric. It has not been possible to find any metal which will replace manganese and form an isomorphous crystal.