The temperature dependence of the dislocation charge in potassium chloride

Abstract

Using the piezoelectric defect technique (Robinson 1970, 1972 a), the charge on edge dislocations in bent single crystals of KCl has been measured from 300 to 1039 K at 40 and 80 kHz with strain amplitudes from 2·7 × 10⁻⁷ to 7·2 × 10-⁶. At room temperature the dislocation charge, for crystals oriented in < 100 > < 110 > and < 111 >, was found to be negative and reached a thermal equilibrium value at ∼ 550 K. Extrinsic isoelectric temperatures occurred in the range 730 to 800 K depending on the crystal, and for any one crystal were reproducible to within 2 K. Above 1000 K the measured dislocation charge fell rapidly towards zero suggesting that near the melting point the relaxation time θ of the charge cloud because shorter than the period ω⁻¹ of the applied stress (ωθ < 1). The temperature dependence of the restoring force acting on the dislocations at breakaway, together with the temperature for ωθ = 1, suggest that the enthalpy and entropy of motion of an isolated Ca²⁺ ion are 2·3 (+ 0·1, −0·3) eV and 7 (± 2) k, respectively. A theory is developed for the dislocation charge which is applicable for values of |eV_∞/kT | <l and ∞ 1, where V _∞ is the potential of the crystal bulk relative to the dislocation core. Applying this theory we conclude that, for KCl at high temperatures, the thermal jogs have a greater contribution to the charge than plastic jogs, but at temperatures below ∼ 750 K plastic jogs become increasingly important. A detailed matching of the theory to the experimental data is carried out allowing only thermal jogs and this requires that the intrinsic isoelectric temperature equals 1490 K, i.e. a value greatly exceeding the melting temperature of 1044 K, and the enthalpy and entropy of formation of a free cation vacancy are 1·19 eV and 4·01 k, respectively. The calculated impurity concentration of 2·50 × 10⁻⁷ agrees with the value of 2·7 (+ 0·3, −0·6) × 10⁻⁷ obtained from electrical conductivity measurements.