A study of the paraelectric-ferroelectric phase transition of triglycine sulphate by deuteron nuclear magnetic resonance and relaxation

Abstract

The quadrupole coupling (QC) tensors of the deuteron in all hydrogen positions are reported for the paraelectric (p) phase of triglycine sulphate (TGS), while for the ferroelectric (f) phase the authors restrict themselves to the deuterons in the 'long' and 'short' hydrogen bonds and to the ND3 deuterons of glycine GI. In addition, the orientation dependence of the spin-lattice relaxation rate 1/T1 of the ND3 deuterons of GI and the temperature dependence of 1/T1 of the CD2 deuterons are reported for the p phase. It is shown that the QC tensors and the relaxation rates of the ND3 and CD2 groups of GI can be described well by a dynamic order-disorder model of the phase transition. Any displacive-type model can be ruled out. The temperature dependence of the order parameter p is determined. The CD2-ND3 group of GI flips as a rigid unit between the two sides with a correlation time tau f. The ND3 group reorients in addition about the C-N bond with a correlation time tau r. At T=Tc+6 K, tau r=(2.1+or-0.2)*10-11 s, tau f=(1.3+or-0.2)*10-11 s; tau f does not vary measurably in the temperature range from Tc to Tc+40 K. At the phase transition there is no critical slowing down to the flip rate. The QC tensors of the CD2 deuterons are used for a significant refinement of the neutron diffraction structure determination of TGS.