Ferroelectricity in Di-Glycine Nitrate (NH2CH2COOH)2·HNO3

Abstract

${\mathrm{}\left(\mathrm{Glycine}\right)\mathrm{}}_2$·${\mathrm{HNO}}_3$ is ferroelectric below -67°C. The room-temperature phase is monoclinic, space group $\frac{P{2}_1}{a}$, with $a={9.49}_6$ A, $b={5.10}_7$ A, $c={9.35}_0$ A, $\beta =98.8\mathrm{}°$, ${\rho }_{\mathrm{obs}}={1.581}_0$ g/cc, and $Z=2\mathrm{}$. The nitrate groups must be disordered or rotating in this phase, since they are at centers of symmetry. The symmetry of the ferroelectric phase is $\mathrm{Pa}$, as established from systematic absences and the fact that spontaneous polarization appears along the room-temperature [101] direction below -67°C. The dielectric constant ${\epsilon }_{\mathrm{}\left[101\right]\mathrm{}}$, at 10 kc/sec and 5 v/cm, is 14 at room temperature, rises to a sharp peak of ∼600 at -67°C, and falls to ∼12 at -180°C. The transition appears to be of second order. At -77°C the spontaneous polarization is 0.60 μcoul/${\mathrm{cm}}^2$, and the coercive field is 400 volts/cm. A specific heat anomaly similar to that in ${\mathrm{}\left(\mathrm{glycine}\right)\mathrm{}}_3$·${\mathrm{H}}_2$S${\mathrm{O}}_4$ and ${\mathrm{}\left(\mathrm{glycine}\right)\mathrm{}}_3$·${\mathrm{H}}_2$Be${\mathrm{F}}_4$ is observed, with $\Delta E=0.15\mathrm{}$ kcal/mole and $\Delta S=0.74\mathrm{}$ cal/mole deg.