Thermally stimulated discharge of electrets

Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role. Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF2 structures, semiconductors and ice.

Charged dislocations in ionic crystals

Field-induced thermally stimulated currents

Transport of Matter in Simple Ionic Crystals (Cubic Halides)

In this paper it is argued that the generality of the Meyer-Neldel rule compels one to adopt a phenomenological approach if a universally valid model is aimed at. It is shown that there exists only one possible phenomenological model. This model is based on an exponential probability distribution of energy barriers. The model predicts a power-law frequency dependence of the AC conductivity with the exponent s given by s=1-T/T0 where T0 is the characteristic temperature of the Meyer-Neldel rule. It is conjectured that the exponential energy barrier distribution derives from a 'glass transition' at T0. The generalisation of the model to account for the compensation effect in other contexts is briefly discussed, using the case of heterogeneous catalysis as an example.

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A phenomenological model for the Meyer-Neldel rule

The experimental study of electrical transport in GeSx glasses (1.3 ⩽ x ⩽ 1.5) was performed by means of a thermo-stimulated depolarization technique (TSD), charging- and discharging-current and dc-conductivity measurements. It is shown that the results obtained point to non-electronic dielectric polarization phenomena. The nature of the mobile ions is briefly discussed.

Dielectric-polarization phenomena in glasses of the system Ge-S

Conductivity measurements were carried out on NaCl∶ NaOH and NaCl∶ Na2CO3. In both cases a decrease in conductivity with an increasing anion content and a conductivity minimum were found. In the case of NaCl∶ NaOH the conductivity of heavy doped crystals dropped even below the intrinsic conductivity value and conductivity hysteresis occurred.

The conductivity of NaCl∶ NaOH and NaCl∶ Na 2 CO 3 crystals

Thermally stimulated depolarization (TSD) of phosphate glass of the composition 72·9% P2O5, 8·5% Al2O3, 18·6% CaO, 0·23% MnO has confirmed the existence of two types of “slow” relaxation. Migration polarization leads to TSD maximum near 0 °C, while the high temperature TSD maximum at 113 °C probably corresponds to space charge polarization. An analysis of the high temperature maximum shows that it is formed by a spectrum of relaxations with a single activation energy 1·0 eV, distributed over the dipole frequency factors with the most probable value α0 ∼ 1010 sec−1. Strong dispersion of permittivity points to the space charge polarization, though it is found that the effect is linear and independent of the sample geometry.

Thermally stimulated depolarization of a phosphate glass

Conductivity and dielectric loss measurements were carried out on NaCl:CaCl2 + NaOH crystals. An evaluation of the experimental data, based on the assumption that a reaction takes place between the admixtures in the crystal which diminishes the number of charge carriers and complexes and leads to reaction yield and mass action constant values which are in good accord with theory. The results exclude any possibility of a mobility change.

Conductivity and dielectric loss maxima of NACl:CaCl2 + NaOH crystals

The temperature dependence of the d.c. conductivity, the dielectric loss and the permittivity of NaNO3 single crystals prepared by epitaxial accretion of the (111) plane of NaNO3 on the (001) plane of mica were investigated. The measurements were performed in a temperature range from 20–300‡C. Below the melting point a region of intrinsic conductivity (UI=3·08 eV), caused by Na+ ions which are in interstitial sites, was observed. At lower temperature a region II (UII= 0·87 eV), where the conductivity is due to the presence of divalent impurities, probably Ca+ + followed (structurally sensitive region). Up to 250‡C polarization is observed, most probably due to ion and electron shifting only. Above that temperature the influence of the relaxed orientation of the NO3− groups becomes evident. At 275‡C the NO3s- groups behave in a completely disordered way as indicated by a sudden increase of the permittivity plotted against temperature.

E. Mariani

Papers

Dielectric-polarization phenomena in glasses of the system Ge-S

The conductivity of NaCl∶ NaOH and NaCl∶ Na 2 CO 3 crystals

Thermally stimulated depolarization of a phosphate glass

Conductivity and dielectric loss maxima of NACl:CaCl2 + NaOH crystals

Permittivity, dielectric loss and dc conductivity of NaNO3 crystals grown by epitaxy