Physical properties of mixed-valence Tl ^I₃ Tl^IIICl₆ investigated by conductivity and positron annihilation lifetime measurements: Effect of crushing the crystals

Abstract

For a better understanding of isotope exchange in solid Tl₄Cl₆ the effects of crushing the crystals were investigated by conductivity and by positron annihilation lifetime measurements. As in untreated Tl₄Cl₆, the conductivity variation with temperature shows a break at about 450 K. The activation energies, 0.53 eV and 0.70 eV, respectively, below and above 450 K, are very close to those in the untreated material but the absolute values of conductivity are lower after crushing which is attributed to the trapping of the mobile defects of dislocation. The positron lifetime variation with increasing temperature shows some contribution of extrinsic defects, annealing at about 413 K. On further heating or cooling cycles, the lifetime changes are controlled by the production of intrinsic cation vacancies, whose formation enthalpy, 0.39 eV, is close to that derived for untreated Tl₄Cl₆. The shape of the initial part of the curves would indicate that crushing does not directly create appreciable concentrations of cation vacancies but would rather produce annealable defects, possibly dislocations, favouring the formation and/or trapping of such vacancies.