Study of Slow Motion of Impurity-Vacancy Complexes in NaCl Crystals by Nuclear Magnetic Resonance

Abstract

The nuclear spin relaxation times T 1 and T 1ρ for Na in doped NaCl crystals, along the static and rf field, respectively, have been measured below 110°C to study slow thermal motion of the bound positive ion vacancy around the divalent impurity ions Cd ++ , Ca ++ , and Sr ++ Below 110°C almost all vacancies form complexes with divalent ions. Slichter's formula for T 1ρ is applicable to the present case where the electric quadrupole interaction determines the local field. Activation energies for the jump of a vacancy in a nearest-neighbor Na site to a next nearest-neighbor site have been measured accurately; 0.64±0.03 eV for NaCl:Cd, 0.71±0.05 eV for NaCl:Ca, and 0.74±0.03 eV for NaCl:Sr. These values agree with the results of dielectric relaxation and ionic thermocurrent measurements. Motion of vacancy-impurity dipoles in a higher complex (probably the pentamer) causes complicated temperature dependence of relaxation times in Ca-doped NaCl.