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

(Glycine)₂·HNO₃ is ferroelectric below -67°C. The room-temperature phase is monoclinic, space group P2₁/a, with a=9.49₆ A, b=5.10₇ A, c=9.35₀ A, β=98.8°, ρ_obs=1.581₀ g/cc, and Z=2. 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 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 ε_[101], 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/cm², and the coercive field is 400 volts/cm. A specific heat anomaly similar to that in (glycine)₃·H₂SO₄ and (glycine)₃·H₂BeF₄ is observed, with ΔE=0.15 kcal/mole and ΔS=0.74 cal/mole deg.