Efficiency of formation of quasi-metallic defects in alkali-halide crystals

The formation of quasi-metallic defects on the basis of the F _L center (F center with a metal atom) has been investigated with the aim of increasing the efficiency and decreasing the pump threshold of laser elements based on alkali-halide crystals. The regularity of formation of quasi-metallic defects in a number of alkali-halide crystals has been established.     

Lattice dynamics and superionic properties of Cd1−xPbxF2 crystals

Reflectivity in the infrared range, Raman scattering and ionic transport properties in Cd_1−xPb_xF₂ crystals have been investigated. A “one-mode” behaviour of these crystals has been found. The role of the PbF₂ cation sub-lattice in the anion disordering process has been discussed. A correlation has been found between the ionic transport properties and the fluorine-cation interaction coefficient for Cd_1−xPb_xF₂ crystals.     

Electro-optic effect in SrTiO3 and Sr1–x Ca x TiO3 (x = 0.014)

The results of the investigation of the quadratic electro-optic effect in Sr_1–x Ca _x TiO₃ with x = 0.014 (SCT) and in nominally pure SrTiO₃ (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.     

Studies of first-stage F-centre production in alkali halides doped with strontium

KBr crystals doped with strontium have been X-irradiated at 293 K. The reduction in single impurity-vacancy dipole concentration has been measured using I.T.C. techniques and is found to be proportional to and in quite close numerical agreement with F-centre growth in both first and second stage F-centre production, this being in agreement with previous results in KCl(Sr). The growth of the absorption band peaking at 263 nm was studied and was found to be proportional to F-centre growth during the earlier stages of colouration, indicating that the responsible centres are a major complement of the F-centres. Ionic conductivity experiments show little agreement between the decay of conductivity and the growth of F-centres during X-irradiation of KCl(Sr) crystals at 293 K.A model is developed in which interstitials are trapped at impurity-vacancy dipoles and in which the two stages of F-centre production is the result of dynamic processes involving modifications of the excitonic mechanism or of back reactions.     

Generation of color centers when annealing LiF crystals irradiated by Kr ions with an energy of 150 MeV

The effect thermal annealing has on F and F _n centers in LiF crystals irradiated by Kr ions with an energy of 150 MeV is studied with allowance for fluence and ionic currents. It is found that annealing at a temperature of 400 K using crystals irradiated at a fluence of ≥10¹³ iom/cm² reduces the concentration of F centers (due to annihilation with H centers) and raises the concentration of complex F _n centers.     

Mechanism of formation of cellular dislocation structures during propagation of intense shock waves in crystals

The mechanism of formation of a cellular dislocation structure in face-centered cubic (fcc) metal crystals subjected to shock compression at strain rates \(\dot \varepsilon \) > 10⁶ s^?1 has been considered theoretically within the dislocation kinetic approach based on the kinetic equation for the dislocation density (dislocation constitutive equation). A dislocation structure of the cellular type is formed in the case of a two-wave structure of the compression wave behind its shock front (elastic precursor). It has been found that, at pressures σ > 10 GPa, the dislocation cell size Λ _c depends on the pressure σ and the density ρ _G of geometrically necessary dislocations generated at the shock front according to the relationship Λ _c ～ ρ _G ^?n ～ σ^?m , where n = 1/4–1/2, m = 3/4–3/2, and m = 1, for different pressures and orientations of the crystal. It has been shown that, in copper and nickel crystals with the shock loading axis oriented along the [001] direction, the cellular structure is not formed after reaching the critical pressures σ _c equal to 31 and 45 GPa, respectively.     

F bands in KCl-KBr mixed crystals

Gamma ray-induced F bands in melt grown mixed crystals of KCl and KBr are studied at room temperature. The effect of dislocations on F-centre density in mixed crystals has been discussed. In addition, the influence of composition of mixed crystals on F band parameters has been studied. The behaviour of observed F bands is correlated to local strained regions and to the configuration of F centre in mixed crystals.     

Growth and comparison of physicochemical properties of pure,Ca and Sr doped NH4Sb3F10 single crystals for electro optic applications

Single crystals of pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ are grown by slow evaporation technique. The effect of dopants on the growth and physicochemical properties also have been investigated and reported for the first time. The grown crystals are characterized with the aid of single crystal X-ray diffractometry to confirm the crystal structure. EDAX studies are done to confirm the presence of dopants in the crystal lattice. The vibrational frequencies of various group ligands in the crystals have been derived from the Fourier transform infrared (FT-IR) spectrum. From the optical absorption spectrum the band gap energy was calculated and it was found to be 5.76, 6.29 and 6.35 eV for pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ crystals respectively. Thermal stability of the sample has been analysed using TG-DTA analysis. The activation energy of pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ crystals were calculated from the dc conductivity measurements and it is found to be 0.2728, 0.2816 and 0.3622 eV Experimental results shows improved physicochemical properties when the dopant is added to the pure material.     

Effect of Br− ions on the F-center formation in KCl crystals under u.v. light irradiation

The relative efficiency of the F-center formation, Y_F in KCl crystals has been studied as a function of photon energy of u.v. light between 5.7 and 10.0 eV. Y_F is maximum at the peak of the absorption band due to the localized excitons at Br^? ions, and increases with Br^? concentration. Results suggest an important role of the localized excitons in the F-center formation.     

Formation and nature of swirl defects in silicon

Point defect agglomerates in dislocation-free silicon crystals, usually called “swirls”, have been investigated by means of high-voltage electron microscopy. It was found that a single swirl defect consists of a dislocation loop or a cluster of dislocation loops. By contrast experiments it could be shown that these loops are formed by agglomeration of self-interstitial atoms. Generally the loops have a/2〈110〉 Burgers vectors, but in specimens with high concentrations of carbon (～10¹⁷ cm^?3) and oxygen (～10¹⁶ cm^?3) also dislocation loops including a stacking fault were observed. In crystals grown at growth rates higher thanv=4 mm/min no swirls are observed; lower growth rates do not markedly affect the size and shape of the dislocation loops. With decreasing impurity content (particulary of oxygen and carbon) the swirl density decreases, whereas the dislocation loop clusters become larger and more complex. A model is presented which describes the formation of swirls in terms of agglomeration of silicon self-interstitials and impurity atoms.     

Infuence of plastic deformation on IR photoconduction stimulated by <Emphasis Type="Italic">F</Emphasis>-center-exciting light in colored NaCl crystals

Plastic deformation of NaCl crystals colored by irradiation with gamma rays was found to lead to the occurrence of a band in the spectrum of IR photoconductivity stimulated by F-center-exciting light. This band corresponds to photon energies lying in the range from 0.65 to 1.11 eV and higher and has two distinct peaks, at 0.74 and 0.88 eV. The position of the former peak correlates well with the spectrum of IR quenching of the photoplastic effect, which suggests that this peak is associated with a dislocation electronic level.     

Enhanced photo-response of thin-film structures with nanocrystals

Thin-film multilayers with dielectric and semiconductor nanolayers of 200-10 nm thicknesses have been deposited by thermal evaporation onto irradiation-resistive substrates using pure crystals as evaporated targets. Some multilayers were γ-irradiated in air at room temperature with dose of 83 kGy. X-ray diffraction and microscopy studies reveal that the multilayers consist of nanometer-sized crystals with cubic structure and defined size. Film structures were oriented along the (1 1 1) plane. Absorption spectra of non-irradiated LiF nanocrystals of 100 nm size and those of initial crystals give evidence of metal colloids presence. Photoluminescence spectra of γ-irradiated nanostructures with various LiF content show the enhancement of F₃⁺-colour centres excitation in the region of metal colloids absorption and the increase is observed between emission intensities of F₃⁺ and F₂ centers with respect to initial crystals γ-coloured in identical conditions. Emission intensities of both centers under excitation in the M band correlate with LiF content. These effects, which are related to high-quality nanocrystals, but at the same time depend strongly on the defect content, especially as far as their 1-2 ps nonlinearities are concerned, could depend on nanocrystal purity and metal excess collection in their boundaries regions.     

F L V a + a and F L F color centers in lithium fluoride crystals

Using optical spectroscopy and X-ray diffraction analysis, the nature of defects in γ-irradiated lithium fluoride crystals is found, mechanisms of their formation are proposed, and the absorption bands at 385 (F _L F centers) and 506 nm (F _L V _a ⁺ centers) are interpreted.     

Lattice dynamics of PbxCd1-xF2 mixed superionic crystals

A modified rigid ion model has been used to investigate the one-mode behaviour in the Pb_xCd_1-xF₂ mixed crystals. It has been found that the said correlation between ionic transport properties and the flourine-cation interaction coefficient for Pb_xCd_1-xF₂ crystals is not reasonable. The agreement obtained for the zone centre frequencies with the experiment is satisfactory for all composition range.     

Softening processes in single-crystal tungsten ribbons

Recovery processes in rolling-deformed (001)[110] tungsten single crystals having various degrees of purity are investigated. It is shown that the dislocation structure formed in the plastic deformation of tungsten single crystals is transformed in subsequent high-temperature anneals to a system of dislocation subboundaries; only polygonization, which preserves the single-crystal structure, takes place in samples heated to a temperature close to the melting point. The formation of subboundaries proceeds in two stages with subsequent transformation of the unstable structure to a configuration having an energy minimum. The decisive factor affecting the polygonization rate is the stacking fault energy; the presence of impurities also has a significant influence. Fiz. Tverd. Tela (St. Petersburg) 40, 498–502 (March 1998)     

Some physical properties of ZnS crystals grown from the vapour phase

Wurtzite-type crystals with wide regions of high purity and good optical properties have been grown in a few hours by an improved flow method. Dislocations have been found to be mainly arranged inside isolated striation regions where a weak green fluorescence and phosphorescence has been observed. Under bending deformation the crystals show a plastic behaviour that is not directly correlated to the mean dislocation density of the samples. Rupture has been found at about 10³ kg/cm².The microwave method for the detection of crystal deflection has been suggested by Prof. A. Gozzini. His helpful advice and interest in this research has been deeply appreciated. Many thanks are also due to the Physical Institute of SAV in Prague, for the possibility to work on their crystals of ZnS. Finally we wish to thank Mr. Coli and Mr. Montanari for design and construction of the mechanical equipment and the furnace quartz assembly.     

UV and VUV spectroscopic study of Na0.4Y0.6F2.2 crystals doped with rare-earth ions

The short-wavelength transmission spectra of Na_0.4 R _0.6F_2.2 crystals with R = Y, Yb, or Lu have been investigated. For these crystals, the VUV transmission cutoffs are 78750, 58820, and 75200 cm^?1, respectively. The 4f ⁿ–4f ^n?15d absorption and excitation spectra of Na_0.4Y_0.6F_2.2 crystals activated with Ce³⁺, Pr³⁺, Nd³⁺, Er³⁺, Tm³⁺, and Yb³⁺ ions have been analyzed in the range 30000–80000 cm^?1. The energy positions of the lowest levels of the 4f ^n?15d configurations of these ions in the fluorite crystal matrix Na_0.4Y_0.6F_2.2 are determined. The absorption band in the spectral range 60600–70000 cm^?1 in Na_0.4(Y, Yb)_0.6F_2.2 crystals is due to the charge transfer from F^? to Yb³⁺. It is shown that the environmental symmetry of Ce³⁺ ions in Na_0.4R_0.6F_2.2 (R = Y, Yb, Lu) crystals is almost identical.     

Electric stimulation of magnetoplasticity and magnetic hardening in crystals

The effect of electric field E on the magnetoplastic effect (MPE) has been investigated in NaCl crystals with different impurities, which provide either the plasticization of the samples in the magnetic field (positive MPE) or their magnetic hardening (negative MPE). The mobility of individual dislocations under the joint action of the magnetic and electric fields and the mechanical load on the crystals has been studied. The sharp electric stimulation of the MPE of both signs has been revealed, i.e., an increase or a decrease in the mean free path of dislocations that is roughly proportional to exp(±E/E ₀) at E ? E ₀ ～ 1–10 kV/m. In particular, in the negative-MPE NaCl(Pb) crystals, the accompanying electric field enhances the magnetic suppression of plasticity. The results are attributed to the electrically induced transformation of the additional part of the pinning impurity ions Me⁺⁺ to the magnetically active state of Me⁺ on the dislocations. The subsequent magnetic transformation of the structure of these pinning centers should lead to a sharper variation of the dislocation pinning force (either an increase or a decrease, depending on the MPE sign).     

Thermal- and photo-induced transformations of luminescence centers in αd-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> anion-defective crystals

The thermal- and photo-induced transformations of luminescence centers in anion-defective crystals of α-Al₂O₃ have been investigated. It has been found that the exposure of crystals to ultraviolet light at temperatures in the range 50–900°C leads to substantial changes in their thermoluminescence and radioluminescence spectra. According to the optical absorption and photoluminescence data, the detected F-type centers have been identified and the temperature ranges of the F → F ⁺ → F ₂ transformations and their possible mechanisms have been determined. The special attention has been drawn to the detailed similarity in the formation of complex F ₂-type centers in the initially perfect α-Al₂O₃ crystals irradiated with fast neutrons and in the studied anion-defective crystals.     

Temperature behavior of the 6.05-eV band in optical absorption spectra of oxygen-deficient corundum

The effect of temperature on the 6.05-eV absorption band in α-Al₂O₃ has been studied in the 80–515 K region. The data obtained are analyzed in terms of a one-coordinate model with strong electron-phonon coupling. This band is shown to be formed by two peaks at 5.91 and 6.22 eV (T=293 K) originating from absorption at the F ⁺ and F centers, respectively. An analysis of the experimental temperature dependences has allowed us to calculate the energies of effective phonons responsible for the broadening and shift of the peaks. The energies calculated agree with the data obtained in other studies and lie in the region of corundum acoustic-vibration frequencies. The Huang-Rhys factors have been evaluated for both centers and found to be close to the estimates made by other authors. The results are discussed in detail and compared with independent data on optical absorption and luminescence of anion centers in colored and irradiated α-Al₂O₃ single crystals.