Pure NaCl Single Crystals Research Articles

Low-temperature creation of Frenkel defects via hot electron-hole recombination in highly pure NaCl single crystals

The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value EFD, which is sufficient for the creation of Frenkel defects at low temperature.

In-situ study of deformation-enhanced atomic diffusion in NaCl by nuclear magnetic resonance

Abstract Enhancements in atomic diffusion during deformation are investigated by 23Na rotating frame nuclear spin relaxation measurements on pure NaCl single crystals. The enhanced diffusion is shown to arise from deformation-induced excess concentration of vacancies. Evaluation of the data yields the concentration of these vacancies as a function of temperature and strain rate.

In-Situ NMR Investigation of Dynamical Behavior of Point and Line Defects During Deformation of NaCl

ABSTRACTSpin lattice relaxation time in the rotating frame [T1ρ] is investigated in pure NaCl single crystals as a function of temperature in-situ during deformation. Transition from 2- phonon Raman process to atomic diffusion was noted at around 500K in the undeformed material and the activation energy was determined to be that for the diffusion of extrinsic vacancies. Enhanced spin relaxation rates were noted during constant strain-rate deformation at temperatures from ambient to about 750K. These enhancements were identified to arise from dislocation motion at lower temperatures while enhanced diffusion due to excess vacancies at higher temperatures. This excess concentration increased with increased strain-rate and in-situ annealing of deforma-tion induced excess vacancies is noted at high temperatures.

Measurement of surface and boundary charge on NaCℓ

The potential on freshly cleaved surfaces of nominally pure NaCℓ single crystals has been measured and found to be negative at room temperature, whereas the potential near grain boundaries in bicrystals is found to be positive. The difference is attributed to grain boundary precipitation, a boundary isoelectric point being predicted at higher temperatures.

Electrical Signals in Dynamically Stressed Ionic Crystals: A Dislocation Model

A summary of new and previously published data on shock-induced electrical signals observed in nominally pure NaCl single crystals is presented. A dislocation dynamics model is developed to account for the microscopic origin of these signals. This model, although only qualitatively applied so far, is shown to be able to explain many of the interesting features of the observed data, notably the change of signal polarity at ∼10 and ∼110 kbar and the distinct signal profiles observed when the sample is shocked along different crystallographic directions. New experimental results for NaCl single crystals doped with Ca2+, Ba2+, Mg2+, and OH− impurity ions are also presented. These results are consistent with the predictions of the proposed dislocation dynamics model.

The Urbach Tails and Reflection Spectra of NaCl Single Crystals

In the tail part region of the first exciton band, the absorption constant (10 -1 to 10 2 cm -1 ) was measured on zone-refined and Harshaw NaCl single crystals from 10°K to 573°K. Fundamental absorption tails of pure NaCl single crystals are represented by the modified Urbach rule, which describes the influence of zero-point lattice vibrations at low temperature. Analysing a temperature dependence of Urbach constant σ=σ 0 F , it was found that σ 0 =0.757 and the temperature of zero-point phonon is \(\omega\hbar/2{=}58.2\)°K. Reflection spectra of single crystals were measured from 2112Å to 1060Å in the same temperature range. The peak shift of the first reflection band is investigated as a function of temperature to clarify the mode of phonons interacting with the exciton in the main part of the band.