NUCLEAR MAGNETIC RESONANCE STUDY OF FERROELECTRIC LITHIUM HYDRAZINIUM SULPHATE, Li(N2H5)SO4

Abstract

The proton and Li⁷ nuclear magnetic resonance signals have been investigated in powdered samples and single crystals of lithium hydrazinium sulphate, Li(N₂H₅)SO₄, which is known to be ferroelectric from −15 °C to above 80 °C.The quadrupolar splitting of the Li⁷ resonance, which was examined over the temperature range from −70 °C to +205 °C, undergoes rapid but continuous change between 80 °C and 160 °C indicating a crystal phase transition from a low to a high temperature polymorph. This transition is of the second-order type and completely reversible. The Li⁷ spectrum was examined in detail in the low and high temperature polymorphs at room temperature and +205 °C, respectively. The results have been analyzed and the quadrupole coupling constants, the asymmetry parameters, and the orientations of the principal axes of the electric field gradient tensors at the Li⁷ sites are reported.The second moment of the proton resonance in powdered Li(N₂H₅)SO₄ at −183 °C was found to have a value of 39 ± 2 gauss² which is consistent with the N₂H₅⁺ group existing as the hydrazinium ion NH₂—NH₃⁺ in an effectively rigid state. In the region of −130 °C, the second moment decreased rapidly to a little more than a third of its value at the temperature of liquid air. This effect is interpreted as resulting from the onset of rotation of the —NH₃⁺ group about the N—N axis. As the crystal temperature was increased from −70 °C, the second moment remained virtually constant at 16.5 ± 0.5 gauss² over a wide temperature range, began to decrease at +50 °C, paused slightly at a value of 8.8 ± 0.5 gauss² at 150 °C, and again decreased rapidly to 0.74 ± 0.14 gauss² at 210 °C. Our interpretation of the rapid decrease of the second moment between 50 °C and 150 °C is that it is caused by the onset of rotation of the —NH₂ group about the N—N axis, which triggers the transition observed in the Li⁷ study. The very low value of the second moment at high temperatures indicates that the protons become highly mobile, probably diffusing by transfer from one hydrazinium ion to another along the c axis.The fine structure of the proton resonance obtained with a single crystal at room temperature was studied at selected crystal orientations. From these measurements, the H—H distances were inferred to be 1.67 ± 0.01 Å in the —NH₃⁺ group and 1.64 ± 0.02 Å in the —NH₃⁺ group. Assuming tetrahedral bond angles, this implies N—H distances of 1.03 ± 0.01 Å and 1.01 ± 0.01 Å in the —NH₂ and —NH₃⁺ groups, respectively.A possible mechanism for the ferroelectric behavior of lithium hydrazinium sulphate is suggested.