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}$ \ifmmode\cdot\else\textperiodcentered\fi{}Mn${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}${2\mathrm{H}}_{2}$O, is found to be ferroelectric from low temperatures to +55\ifmmode^\circ\else\textdegree\fi{}C. Above the latter temperature the conductivity suddenly becomes excessive. At room temperature the spontaneous polarization is 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ coulomb/${\mathrm{cm}}^{2}$ and the coercive field is 5.6 kv/cm. A nonreversible surface change is observable optically above 72\ifmmode^\circ\else\textdegree\fi{}C, probably due to loss of ${\mathrm{H}}_{2}$O. Thermal measurements indicate onset of water loss above 35\ifmmode^\circ\else\textdegree\fi{}C; electrical conductivity increases sharply and irreversibly above 55\ifmmode^\circ\else\textdegree\fi{}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 $P{2}_{1}$, $a=9.96$ A, $b=8.53$ A, $c=6.86$ A, $\ensuremath{\beta}=107\ifmmode\pm\else\textpm\fi{}0.5\ifmmode^\circ\else\textdegree\fi{}$, ${\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}\phantom{\rule{0ex}{0ex}}=1.875}_{0}$ g/cc, and two formula units per cell. The ferroelectric axis is along $b$. The dielectric constants at room temperature are ${\ensuremath{\epsilon}}_{a}=6.6$, ${\ensuremath{\epsilon}}_{b}=8.1$, ${\ensuremath{\epsilon}}_{c}=7.4$; and these decrease linearly and only very slightly as the temperature is lowered to 77\ifmmode^\circ\else\textdegree\fi{}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.