Nuclear Magnetic Resonance Studies of Ferroelectric Ammonium Sulfate and Fluoroberyllate

Abstract

<p> The temperature dependences of the deuteron spin-lattice relaxation times, T1, in ferroelectric ammonium sulfate, (ND4)2SO4 , and ammonium fluoroberyllate, (ND4)2BeF4, have been studied by transient methods over the range 80 to 47S°K. The ability to resolve the individual correlations of the two independent ND4 groups in the deuteron experiment has thrown new light on the reorientation of the ammonium groups and their behaviour in the phase transition. It has been proposed that the mechanism of the phase transition in ammonium sulfate is a disordering, with respect to the a b plane in the paraelectric phase, of the ND4 dipoles which also make the dominant contribution to the spontaneous polarization. The present N.M.R. results show that in (ND4)2so4 ,T1 at both non-equivalent ND4 groups is affected at the phase transition, whereas in (ND4)2BeF4 , it is affected at only one ND4 group. This suggests that the phase transitions in these two materials are rather different and that in ammonium sulfate both types of ammonium ions are involved whereas in ammonium fluoroberyllate only one type is involved in the transition. Such a conclusion is supported by evidence from measurements of thermal and dielectric properties. An earlier study of the proton spectrum in (NH4)2so4 reported a value of 33 gauss^2 for the second moment at 20°K. This is considerably smaller than the expected rigid lattice value of 5O gauss^2 and it was concluded that while some of the NH4+ groups are effectively rigid at 20°K others are still reorienting. Our new results for the proton second moment confirm the low value down to 4.2°K but our results for T1 indicate that the proton line is not motionally narrowed. </p>