Quantum Effects in Isotopically Mixed Crystals of Hydrogen Chloride and Deuterium Chloride in the Ferroelectric Phase.
An investigation of quantum effects in hydrogen chloride (HCl), deuterium chloride (DCl) and isotopically mixed crystals in the ferroelectric phase has been performed through nuclear quadrupole resonance (NQR) experiments. The NQR spin-lattice relaxation time, T(,1), has been measured at 55 K in pure HCl, pure DCl, and for both HCl and DCl in the mixed crystals. The mixed crystals were 75% HCl - 25% DCl, 50% HCl - 50% DCl, 25% HCl - 75% DCl, and 10% HCl - 90% DCl. The transition temperatures of the same samples were measured by warming the crystals from the ferroelectric phase into the disordered phase, and noting the temperature where the NQR signal vanished. The experimental results are analyzed theoretically in the light of an Ising model which incorporates the presence of quantum tunnelling in a double well potential. From the transition temperatures the interaction energy of a dipole with the rest of the crystal is calculated to be 417 K. The energy contribution from the tunnelling of a proton in HCl is 82 K, while that of deuterium in DCl is negligible. T(,1)('H,D) is shown to be directly related to the single particle correlation time, (tau)('H,D). The behavior of (tau)('H,D) as a function of DCl concentration as predicted in the theoretical model is not consistent with the experimental results. The theory does not explain the divergence of (tau)('H) from (tau)('D) for DCl concentration, above 50%. Possible explanations for the discrepancy are given.
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- Physics: Condensed Matter