Ionic Conductivity and Time-Dependent Polarization in NaCl Crystals

Abstract

When a constant electric field is applied to a dielectric, the current density per unit field decreases with time from an ``initial conductivity'' to a ``final (or steady-state) conductivity.'' A study is made of this time-dependent polarization effect for ``pure'' NaCl crystals in the range from 50° to 200°C. The validity of Ohm's law and the superposition principle is demonstrated for these crystals; this establishes the linearity of the formal equation which relates polarization, electric field, and their time derivatives. Also studied are the effect of prolonged current flow, the effect of deliberate introduction of an air gap between the crystal and one of its electrodes, and the effects of impurities, deformation, x-ray irradiation, and annealing. It is concluded that the results are not consistent with the well-known space-charge polarization theory of Joffé, according to which the buildup of space charge occurs because of blocking of the current carriers at one or both electrodes. Rather, it is necessary to regard the time-dependent polarization as a dielectric relaxation phenomenon. According to this viewpoint, the final conductivity, and not the initial value, represents the true ionic conductivity. Possible relaxation mechanisms are discussed in terms of defect clusters and charged jogs on dislocations.

Ionic Conductivity of NaCl: SrCl2 Crystals

“…The final value of the current was recorded as recommended by Sutter. 19 The temperature of measurement was increased at a rate of 60 0 /h to the melting point of NaCl. The crystal temperature was determined to a precision better than ±0.5° by means of a calibrated Chromel Alumel thermocouple placed within 1 mm of the electrodes.…”

Anomalous polarisation current in alkali halide crystals

“…The present work shows that much low-temperature dc polarisation has little fundamental significance in terms of bulk crystal properties. This probably explains why, as has been previously observed [9], there is no general agreement on what causes time-dependent polarisation in alkali halides.…”

Low frequency polarization effects in thin evaporated dielectric films

“…INTRODUCTION DURING recent years there has been an increasing realization that polarization effects due to blocking of ionic charge-carriers a t a dielectric-electrode interface can provide a mechanism for a.c. dielectric dispersion. Reviews have been given by Volger (1960) and Croitoru (1965) and several recent articles have given evidence of the existence of this type of mechanism in alkali halides (Allnatt and Jacobs 1961, Jacobs and Maycock 1963, Wintle and Rolfe 1966 and other ionic materials (Friauf 1954, Isard 1963, Sutton 1964. Similar effects have also been observed in thin film capacitors of NaCl and NaBr with silver electrodes (Macfarlane and Weaver 1966).…”

“…On the other hand, some authors claim to have demonstrated that electrode polarization effects are not generally observable in alkali halide specimens (Hamon 1953, Sutter andNowick 1963). The latter, in particular, made a careful study of d.c. polarization effects in pure NaCl crystals and found no evidence of electrode polarization except in cases where poor electrode cont)act with the crystal was deliberately introduced.…”

“…Theories of space-charge polarization, presented by Jaffk (1933), Macdonald (1953), Sutton (1964 and others, have been reviewed by Jacobs and Maycock (1963) and by Wintle and Rolfe (1966). Certain limitations as t o their validity have been noted (Friauf 1954, Allnatt and Jacobs 1961, Skov and Pearlstein 1965, Wintle and Rolfe 1966 and in a few recent cases results have been obtained which do not seem to be in agreement with existing space-charge polarization theories (Solomon, Sher andMuller 1966, Wimmer andTallan 1966) even though pronounced dielectric dispersion effects have been observed.…”