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.