Nuclear Magnetic Resonance of Ferroelectric Lithium Ammonium Tartarate

Abstract

Lithium‐7, proton, and deuterium NMR have been studied in lithium ammonium tartarate ${\mathrm{LiNH}}_4{\mathrm{C}}_4{\mathrm{O}}_6{\mathrm{H}}_4·{\mathrm{H}}_2\mathrm{O}$ (LAT) both above and below the ferroelectric‐paraelectric transition temperature T_c (98°K). Complete rotation patterns have been obtained near 300°K for the ⁷Li, H, and D (of D₂O) resonance. The hydrogen atoms of the water of hydration have been located in the unit cell, and are observed not to change position as the transition temperature is traversed. Likewise the ⁷Li NMR does not change significantly above and below T_c. The deuteron resonances of the ${\mathrm{ND}}_4^{+}$ ion, however, change abruptly on traversing T_c in direct analogy to the behavior found in (ND₄)₂SO₄. The spontaneous polarization computed from the distortion of the ${\mathrm{ND}}_4^{+}$ ion determined by NMR accounts for 60% of the total polarization of LAT. Proton relaxation times are interpreted in terms of a relatively fast reorientation of the ${\mathrm{NH}}_4^{+}$ tetrahedron about a twofold axis and relatively slow reorientation about a threefold axis. Evidence is presented to show that the rotation is coherent at low temperatures corresponding to quantum mechanical tunneling. It is proposed that the hydroxyl protons of the tartarate ions are undergoing motion between several inequivalent positions in the unit cell.