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Abstract
35Cl NQR relaxation measurements are used to derive the correlation times τ for the reorientation of both HCl and DCl dipoles in the ferroelectric phase of HCl-DCl mixed crystals. The τ's for DCl are reasonably consistent with a theoretical picture based on a dynamical Ising model in a transverse field which includes damping in an heuristic way. For the HCl dipoles this picture does not account for the τ's in the dilute limit of almost complete deuteration, where persistence of a quantum tunnelling contribution to the reorientations, namely short τ's, is observed, in spite of the coupling to the DCl dipoles with long correlation times.