17O and 14N quadrupole coupling and the mechanism of the ferroelectric transition in diglycine nitrate

Abstract

The I4N and 170 quadrupole couplings in diglycine nitrate have been determined by proton- 14N and proton-170 double resonance. The O─H──O bonded proton is moving between two off-centre sites in the H─bond above TC and is frozen out in an off-centre site below TC. The results show that the basic phase transition mechanism is a biasing of the flipping of the ─NH3 groups around the C─N bondbetween two equilibrium orientations separated by ∼ 60° connected with a corresponding rotation of the NO3 −; ions, an exchange of the ionic character of the two glycine molecules and a transfer of the O─H──O bonded proton.