Quantum tunneling effects in dipolar ferroelectrics: A nuclear-quadrupole-resonance study in HCl-DCl mixed crystals

Static and dynamical effects from a quantum tunneling contribution to the dipole reorientations in the ferroelectric phases of the mixed crystals HCl-DCl have been studied through Cl35 nuclearquadrupole-resonance pulse Fourier-transform measurements. Experimental data for the transition temperatures Tc and the local correlation times, of both the HCl and DCl dipoles, for several percentages of deuteration are analyzed in the light of a dynamical Ising model in a transverse field which includes damping in a heuristic way. The virtual-crystal approximation (VCA) is used in deriving both the static properties (local order parameters and transition temperatures) and the reorientational dynamics (local correlation functions). Estimates of the interaction energy and the tunneling integrals are obtained from the dependence of Tc on deuteration. The behavior of the correlation times for the DCl dipoles as a function of deuteration, predicted within the framework of the VCA approach, is consistent with the experimental results. However, for the HCl dipoles this theoretical picture does not account for the correlation times in the dilute limit of almost-complete deuteration. Instead, the persistence of quantum tunneling (namely, short correlation times) is observed in spite of the interaction with the "classical" DC1 dipoles (long correlation times). Possible sources for this phenomenon are discussed. The only hypothesis that appears suitable involves the effect of local phonon modes on the tunneling of the HCl dipoles.