eV In NaCl Research Articles

I<sup>–</sup> Ions as Obstacles to Dislocation Motion in NaCl:I<sup>–</sup> Single Crystals

Strain-rate cycling tests associated with the ultrasonic oscillation were conducted for the purpose of investigation on the interaction between dislocation and I– ions during plastic deformation of NaCl:I– (0.5 mol% in the melt) at 77 K to room temperature. The relative curves of stress decrement (Δt) due to the oscillation and strain-rate sensitivity ( ) have stair-like shape for NaCl single crystals doped with I– at low temperatures. There are two bending points and two plateau regions. λ decreases with Δt between the two bending points. τp at Δt of first bending point and λp between λ at first plateau place and at second one depend on the dopant ions as weak obstacles to dislocation motion. Not only temperature dependence of τp and λp but also τp versus V (activation volume) reflects the interaction between dislocation and I– ions. On the basis of the data (i.e. τp and λp) analyzed in terms of the relative curves of Δt and λ, the activation energy, G0, for the overcoming of dislocation from the dopant ion is found to be 0.47 and 0.53 eV for NaCl:Br– and NaCl:I–, respectively. This result that G0 for NaCl:I– is somewhat larger than for NaCl:Br– leads to the phenomenon that I– ions are slightly stronger than Br– ones as weak obstacles to dislocation motion because of the difference between isotropic strains around I– ion and around Br– in NaCl single crystal. Furthermore, the values of τp0 and Tc are also obtained for the two kinds of specimens. τp0 and Tc are the value of τp at absolute zero and critical temperature at which τp becomes zero.

Interaction between dislocation and defects induced by X-irradiation in alkali halide crystals

Abstract Interaction between dislocation and defect induced by X-irradiation has been investigated in NaCl and KCl single crystals by strain rate cycling tests under superimposition of ultrasonic oscillation during plastic deformation. The interaction energy between dislocation and radiation-induced defect has been obtained by fitting Barnett model to experimental results, assuming that the defect is tetragonal. The interaction energies between dislocation and defect were determined to be 0.39 and 0.87 eV for NaCl and KCl, respectively. The larger interaction energy for KCl presents its larger tetragonality of defect induced by X-irradiation than NaCl.

Na K PHOTOABSORPTION AND RESONANT KLL AUGER SPECTRA IN NaF AND NaCl

The nature of Na 1s photoabsorption in NaF and NaCl is investigated using the resonant Auger decay of Na 1s core excitations. The appearance of new peaks in Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that the core excitons are created in the threshold region of the Na 1s photoabsorption spectrum. The core excitons around 1074.2 eV originate from the dipole-forbidden 1s -1 3s final state, the population of which is made possible by the symmetry disruption at the photoabsorption site. Another core exciton, excited at photon energies of 1076.8 eV in NaF and 1076.3 eV in NaCl, originates from 1s -1 3p states similar to atoms of Ne and Na. Resonant Auger spectra show also that in NaCl the first sharp photoabsorption maximum at 1076.6 eV has only partially excitonic character.

Role of Shallow Electron Traps in the Fast Transient Optical Phenomena of Alkali Halide Crystals

We present additional evidences that the same shallow electron traps-atomic alkali impurity centres [M + ] c 0 e - are responsible for both classes (A and B) of the transient IR-absorption bands: (A) bands with maximum at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl (due to shallow electron traps or bound polarons) and (B) bands with maximum at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, KBr:I, RbCl:I and RbBr:I (due to on-centre STE or on-centre STE localised at iodine dimer). Both classes of the IR bands have the same location, similar shape (both exactly coincide for KCl:I and KCl at 10 or 80 K), half-width, vibration structure. It is established that the same Mollwo-Ivey plot curves E o =a/d n (d is the nn anion-cation distance, n is the exponent, a is the constant) take place for both IR band classes, if we plot instead of the IR band peak energy values the more definite values of the IR band zero-phonon line energy E 0 (for NaCl, KCl, KBr, RbCl and KCl:I) and/or the IR band low-energy edge E 0 (±0.03 eV) values (for NaBr, NaI, NaCl:I, KBr:I, RbCl:I and RbBr:I). Two types of the [M + ] c 0 e - centres are predominant: (i) [Na + ] c 0 e - in KX and RbX host crystals, for which the relation E 0 ≃6.15/d 2.74 is valid, and (ii) [Li + ] c 0 e - in NaX host crystals for which the relation E 0 ≃29.4/d 4.72 is valid. The Mollwo-Ivey relation E 0 ≃18.36/d 2.70 is valid as well for the F' band in NaCl, KCl, KBr, KI, RbCl, RbI, if we use the F' centre optical binding energy E o values. Under irradiation at 77 or 4.2 K the shallow trapped electron centres ([M + ] c 0 e - and F' centres) and their associations with the V k -type centres, i.e., dipolar pairs {[M + ] c 0 e - …V k } and {F'…V k } are created. Such a quasi-molecular electronic transients have a very wide lifetime spectrum (τ ≥ 1 ns) and can play a great role in the intense pulse excitation experiments.

Mollwo–Ivey relations for optical absorption bands of the atomic and F′ centres in alkali halides

Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27– 0.36 eV in NaCl, KCl, KBr, KI and RbCl and due to shallow electron traps or bound polarons according to Jacobs (Phys. Stat. Sol. B 129 (1985) 755) and Korovkin and Lebedkina (Fiz. Tverd. Tela (Russian) 35 (1993) 642), and (b) with maxima at 0.15– 0.36 eV in NaI, NaBr, NaCl : I, KCl : I, RbCl : I and RbBr : I, due to on-centre STE localised at iodine-dimer according to Hirota et al. (J. Phys. Soc. Japan 63 (1994) 2774, Phys. Rev. B 52 (1995) 7779) and Edamatsu and Hirai (Mater. Sci. Forum 239–241 (1997) 525), are caused by the same defect. We propose that the defect is an atomic alkali impurity centre [M +] c 0e −, i.e. an electron e − trapped by a smaller size substitutional alkali cation impurity [M +] c 0. The Mollwo–Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E 0 for NaCl, KCl, KBr, RbCl and NaBr, KCl : I and/or the low-energy edge values E 0 for NaI, RbCl : I and RbBr : I versus anion–cation distance, d, are obtained for the first time. These data suggest that two types of the [M +] c 0e − centres are predominant: (i) [Na +] c 0e − in KX and RbX host crystals with the relation E 0≈6.15/ d 2.74 and (ii) [Li +] c 0e − in NaX host crystals with E 0≈29.4/ d 4.72. The Mollwo–Ivey relation E 0≈18.36/ d 2.70 is fulfilled as well for the F′ band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F′ centre optical binding energy values for E 0.

Temperature Dependence of the Nuclear Quadrupole Relaxation Rate in Silver and Sodium Halide Crystals

The results of the temperature dependence of nuclear relaxation rate, 1/ T 1 measured for 23 Na, 35 Cl and 79 Br in AgX and NaX with X being Cl and Br are interpreted in terms of the quadrupole relaxation due to lattice vibrations and defect motions. The data of 1/ T 1 in the low temperature region are compared with a model calculation based on an ionic model, in which the degree of covalency is estimated. The result is 24.5% in AgBr and 20.5% in AgCl, which are found to be an order of magnitude larger than the values in sodium halides. On the other hand, from an analysis of 1/ T 1 data in the high temperature region, the migration and formation energies of defects are evaluated and compared with the results of previous conductivity measurements. The obtained activation energies for migration are 0.27 eV in AgBr, 0.23 eV in AgCl and 0.60 eV in NaCl.

Diffusion of heterovalent ions in ionic crystals

Diffusion of divalent and trivalent cations is investigated in NaCl and KCl under conditions of heterodiffusion at infinite dilution. This purpose is achieved by letting radiotracers of very strong specific activity diffuse in the intrinsic range of highly pure single crystals. The importance of the experimental procedures in yielding reliable results is emphasized. Thus, Co ++, although supposedly characterized by an anomalous diffusion in NaCl and KCl, gives very good diffusion profiles with the carrier-free isotope Co-58. Also Fe +++, whose diffusion was considered impossible, gives good diffusion profiles with certain experimental precautions. Analysis of the data gives, for the diffusion of Co ++, the migration enthalpy of 0.76 eV in NaCl and 0.66 eV in KCl and the migration entropy of −2.84 k in NaCl and −1.45 k in KCl. These results are in good agreement with the values for the full set of divalent cations.

Electrical conductivity and dielectric properties of NaCl, KCl, and KBr single crystals near their melting temperatures

The dielectric constant (K′) and loss (K″) of single crystals of NaCl, KCl, and KBr have been measured in the frequency region 104 to 107 Hz and in the temperature range 25 to 8 °C below their melting temperatures. The electrical conductivity of these crystals has also been measured at 104 Hz near their melting temperatures. The activation energy values for conduction in the temperature vicinity of the melting points of these solids are calculated to be 2.15, 2.35, and 2.55 eV for NaCl, KCl, and KBr, respectively; these values are larger than those reported in literature. Two points are emphasized: 1) the electrical conduction in these crystals near their melting temperatures appears to be mainly due to the generation and movement of negative ion vacancies and 2) these crystals do not show any dipole relaxation effects up to a frequency of 107 Hz.

Theory of positron trapping by F‐ and F′‐colour centres in alkali halides

AbstractThe trapping energies of the positron to the F‐ and F′‐colour centres are calculated for NaCl, KCl, and KBr crystals. For the field of F′‐centre the Hultéh potential approximation is used. The values of the positron affinity of the negatively charged F′‐centre are found to lie near 3 eV. This is about twice as much as the positron affinity of the neutral F‐centre which is found to be 1.4 eV for NaCl, 1.6 eV for KCl, and 1.7 eV for KBr.

Electrical and optical properties of zinc-doped alkali halides

The peak positions and half-widths of the bands characteristic of zinc-doped NaCl, KC1 and KBr are reported. Ionic-conductivity studies have been made in the temperature range (150/650) ℴC in NaCl, KC1 and KBr. The values of the energies of association of Zn2+ with a cation vacancy are found to be 0.49 eV, 0.43 eV and 0.35 eV in NaCl, KC1 and KBr respectively. X-irradiated Zn-doped KC1 and KBr crystals show theD1 bands at 250 and 245 nm respectively andD2 banda at 285 and 290 nm respectively. Long irradiation (more than 24 h) produces another band at 350 nm in both KC1 and KBr. This band has a broad Gaussian electron paramagnetic resonance line associated with it. Additively coloured zinc-doped KC1 and KBr show two bands in the ultraviolet region besides theF band. Thermal annealing from 550 ℴC reveals a band at 250 nm in KC1 and 300 nm in KBr.

Observed Fine Structure in X Rays Incoherently Scattered by Alkali Halides: Contribution from Color Centers. II

The spectra of Cu $K{\ensuremath{\alpha}}_{1}$, $K{\ensuremath{\alpha}}_{2}$ and Cr $K{\ensuremath{\alpha}}_{1}$, $K{\ensuremath{\alpha}}_{2}$ radiations scattered by LiF and NaCl single crystals and powders at several angles have been investigated. The spectra of Cr $K{\ensuremath{\alpha}}_{1}$, $K{\ensuremath{\alpha}}_{2}$ scattered radiation were studied for two different color-center concentrations. The spectra of scattered radiation by samples of high color-center concentrations include a new line on the long-wavelength side of the primary spectrum. The line is located at a distance of 5 eV from the primary for LiF, and 2 eV for NaCl. These distances of the new line from the coherent line are independent of the incident radiation energy and scattering angles, and are equal to the $F$-center excitation energies in the respective crystals. Judging from the position of the new line, and its absence from spectra in which the scatterer had lower $F$-center concentration, it is concluded that the new line is caused by "x-ray discrete Raman" scattering from the $F$ center. However, because the $F$-center concentration is at least three orders of magnitude lower than the concentration of the valence electrons, the observed intensity of the line cannot be explained by a simple model in which the isolated $F$ center behaves as a scatterer.

Erratum: “Localized Excitons and Band Excitons in Alkali Chloride-Alkali Iodide Solid Solutions”

The optical absorption spectra of KCl-KI and NaCl-NaI solid solutions in the whole concentration range are presented. Measurements are made in the range from 5.5 eV to 7.5 eV at 80°K. The changes in the profile, the positions of the resolved structures and their intensities in the absorption spectra in changing the concentrations of iodide are observed. With increases of the concentration of I - ions, the localized exciton bands due to cluster of I - ions appear on the low energy tail of those due to an isolated I - ion. Above certain concentration of I - ions, the exciton can be regarded as the band exciton rather than localized one. The percolation theory seems useful for explaining this. Taking account of the band structures in KI and NaI, it is proposed that the absorption bands at 6.69 eV and 7.49 eV in KCl : I and at 6.90 eV and 7.60 eV in NaCl : I are assigned as the spin-orbit pair of the localized exciton band.

Localized Excitons and Band Excitons in Alkali Chloride-Alkali Iodide Solid Solutions

The optical absorption spectra of KCl-KI and NaCl-NaI solid solutions in the whole concentration range are presented. Measurements are made in the range from 5.5 eV to 7.5 eV at 80°K. The changes in the profile, the positions of the resolved structures and their intensities in the absorption spectra in changing the concentrations of iodide are observed. With increases of the concentration of I - ions, the localized exciton bands due to cluster of I - ions appear on the low energy tail of those due to an isolated I - ion. Above certain concentration of I - ions, the exciton can be regarded as the band exciton rather than localized one. The percolation theory seems useful for explaining this. Taking account of the band structures in KI and NaI, it is proposed that the absorption bands at 6.69 eV and 7.49 eV in KCl : I and at 6.90 eV and 7.60 eV in NaCl : I are assigned as the spin-orbit pair of the localized exciton band.

Fundamental Absorption in Alkali Chlorides in the Extreme Ultraviolet

The absorption measurements were made on thin films of solid solutions of alkali chlorides at room temperature. The differences in the spectra of NaCl, KCl, and RbCl are discussed in connection with the nature of the alkali metals. From the measurements in KCl-NaCl and Kcl-RbCl, additional evidences were obtained for the assignment of the peaks at 20.3 eV and 21.5 eV in KCl being due to the transitions from K + 3 p level. The peaks at 11.4 eV and at 12.3 eV in NaCl are proposed to be attributed to Na + ion through the results obtained in KCl-NaCl system. These peaks in NaCl were found to behave consistently also in the system NaCl-NaBr.

Study of Slow Motion of Impurity-Vacancy Complexes in NaCl Crystals by Nuclear Magnetic Resonance

The nuclear spin relaxation times T 1 and T 1ρ for Na in doped NaCl crystals, along the static and rf field, respectively, have been measured below 110°C to study slow thermal motion of the bound positive ion vacancy around the divalent impurity ions Cd ++ , Ca ++ , and Sr ++ Below 110°C almost all vacancies form complexes with divalent ions. Slichter's formula for T 1ρ is applicable to the present case where the electric quadrupole interaction determines the local field. Activation energies for the jump of a vacancy in a nearest-neighbor Na site to a next nearest-neighbor site have been measured accurately; 0.64±0.03 eV for NaCl:Cd, 0.71±0.05 eV for NaCl:Ca, and 0.74±0.03 eV for NaCl:Sr. These values agree with the results of dielectric relaxation and ionic thermocurrent measurements. Motion of vacancy-impurity dipoles in a higher complex (probably the pentamer) causes complicated temperature dependence of relaxation times in Ca-doped NaCl.

Effect of Ion Size on Diffusion in Alkali Halides

An experimental determination of the effect of ion size on diffusion has been made by measuring the diffusivity of ${\mathrm{Rb}}^{86}$ in NaCl, KCl, and RbCl. In the temperature region near the melting point the results follow the Arrhenius relation $D={D}_{0} \mathrm{exp}(\frac{\ensuremath{-}E}{\mathrm{kT}})$. The Arrhenius parameters are ${D}_{0}=205\ifmmode\pm\else\textpm\fi{}47,26.8\ifmmode\pm\else\textpm\fi{}8.7,33.3\ifmmode\pm\else\textpm\fi{}7.1$ ${\mathrm{cm}}^{2}$/sec and $E=2.11\ifmmode\pm\else\textpm\fi{}0.02,2.04\ifmmode\pm\else\textpm\fi{}0.03,1.99\ifmmode\pm\else\textpm\fi{}0.02$ eV for NaCl, KCl, and RbCl host crystals, respectively. We conclude that the effect of ion size is important with regard to both the frequency factor and activation energy. The results are in general agreement with the theory proposed in the previous paper.

Vacancy Pairs and Dislocations in Ionic Crystals

(1) We have calculated the binding energy of vacancy pairs in NaCl and the other five alkali halide crystals. The calculations is improved at several points compared with that by Reitz and Gammel. The calculated values of the binding energy are considerably smaller than e2/ka and differ from the result by Reitz and Gammel; for instance, 0.44 ev in NaCl, 0.37 ev in NaBr, and 0.58 ev in KCl crystals. (2) As the first step of the systematic investigations of dislocation problems, we have calculated the core energy and the Peierls-Nabarro critical yield stress of the edge dislocation for slip in the (110) plane in NaCl type crystal [the Burgers vector is a (110)]. If we write the energy of the dislocation at the center of a cylinder with radius R as K ln (R/R0), R0 is about 1 A in most of the alkali halide crystals. The results of the calculation concerning the Peierls-Nabarro yield stress permit us to understand the well-known fact that LiF has much larger critical yield stress compared with the other alkali halide crystals.

αandβBands in NaF, NaCl, and KCl

By means of a vacuum spectrophotometer, $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ bands have been observed in NaF, NaCl, and KCl irradiated by soft x rays. At liquid nitrogen temperature, the positions of $\ensuremath{\alpha}$ band are ${9.4}_{3}$ ev in NaF, ${7.1}_{6}$ ev in NaCl and ${6.9}_{5}$ ev in KCl, and the positions of $\ensuremath{\beta}$ band are ${9.7}_{5}$ ev in NaF, ${7.3}_{8}$ ev in NaCl, and ${7.3}_{1}$ ev in KCl. The doublet structure of $\ensuremath{\beta}$ band in NaCl, expected by Fuchs' theory has not been detected. This may be due to the fairly wide band width of the component bands.