The temperature dependence of the dislocation charge in potassium chloride

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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 | 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.