Studies of the defects in single crystals of sodium chloride

Measurements of ionic conductance in divalent cation-doped single crystals of NaCl have been used to investigate the impurity-cation vacancy association in these crystals. The measurements were made over the temperature range 150° - 650°C on crystals of NaCl doped with Mg 2+, Mn 2+, Ca 2+, Sr 2+, Pb 2+ and Ba 2+ impurity ions. Pour regions of ionic conductance have been observed; these correspond to an intrinsic region (i), an impurity-induced free vacancy region (ii), an association region (iii) and an impurity precipitation region (IV).

The conductivity results in regions II and III have been analysed in terms of the Lidiard association theory, and enthalpies of association of 0.31 eV, 0.28 eV, 0.30 eV, 0.53 eV, 0.56 eV and 0.79 eV have been obtained for Mg2+, Mn2+, Ca2+, Sr2+, Pb2+ and Ba2+ impurity-cation vacancy complexes respectively. A theoretical calculation of the binding energy of the 2+ Ba -cation vacancy complex has been made and this result, along with the results of previous theoretical calculations, indicates that the enthalpy of association should increase as the impurity ion size increases.

The enthalpies of association deduced from the conductance measurements are in agreement with this theoretical prediction for impurity ions which are larger than the Na ion. With the smaller impurity ions however, there is little change in the enthalpy of association with change in impurity ion size. It is suggested that in the case of large impurity ions a cation vacancy situated in the nearest-neighbour position to the impurity will substantially reduce the lattice distortion and hence form the more stable complex. On the other hand, the contraction of the lattice surrounding a small impurity ion may favour the cation vacancy in the next-nearest-neighbour position. Therefore, for small impurity ions, next-nearest neighbour association may become significant.

A detailed study of the association reactions in the NaCl-MnCl2 system has been made from ionic conductance, Mnst impurity-diffusion and electron spin resonance measurements. The enthalpy of association, as determined from the conductance and impurity diffusion studies, is 0.28 eV and 0.33 eV respectively. The agreement of the values obtained by the different methods is much better than has previously been observed for other impurity ions, and it is suggested that the discrepancies observed in earlier work are due to the effect of next-nearest-neighbour interactions.

The electron spin resonance measurements on the NaCl-MnCl2 system indicate that the nearest-neighbour complex is slightly more stable than the next-nearest-neighbour complex, and the conductance and impurity-diffusion measurements confirm this. The entropy of formation of complexes has been examined from the ionic conductance and impurity diffusion data. The inference is that the entropy of formation of the larger impurity ion complexes is positive while for the smaller impurity ion complexes it is negative. A possible explanation, in terms of close range exponential repulsions and lattice dilations, is proposed assuming that the entropy of formation depends only on the lattice frequencies before and after formation of the complex.

The conductance measurements have also been used to study the solubility of the impurities in the NaCl lattice. Energies of solution of I.I44 eV, 0.92 eV, 1.28 eV, 1.80 eV, 1.90 eV and 2.00 eV have been obtained for the solubilities of Mg2+, Mn2+, Ca2+, Sr2+, Fb2+ and 2+ Ba impurity ions respectively. The results show that impurity ion size is a more important factor than Impurity ion polarizability in determining the solubility. The relatively low solubility of ions which are appreciably different in size from the Na+ ion can be explained in terms of the lattice distortion due to the misfit of these impurity ions. The energy for formation of a Schottky defect and the energy for mobility of a cation vacancy have also been determined from the measurements of ionic conductance. A value of 2.20 eV was obtained for the energy for formation of a Schottky defect, and this is in good agreement with the results of other workers. The energy for mobility of a cation vacancy was found to be 0.76 eV, a value which compares favourably with the results reported here of some studies of Na+ ion 2+ self-diffusion on ’pure' and Ca -doped crystals of NaCl.