Influence of the degree of plastic deformation on the thermal photostability of F-centers in NaCl crystals

The effect of plastic deformations on the thermal stability and thermal photostability of F-centers in NaCl crystals was examined. The thermal photostability of F-centers and the thermal stability of some of these centers increases in proportion to the degree of plastic deformation. The concentration of the less stable F-centers increases due to plastic deformation. The effect was studied in crystals which were deformed before and after x-ray excitation. The results were interpreted from the viewpoint of an ionic activation mechanism in the thermal and photothermal decomposition of the color centers in alkalihalide crystals.The authors are indebted to Yu. L. Lukantsever for an evaluation of the results of this work.     

Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr → NaCl → KI → NaBr → CsBr → RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.     

Twinning in bismuth single crystals induced by thermal cycling

The role of twinning in bismuth single crystal plastic deformation under thermal cycling (cooling to 77 K and heating to 373 K) is studied. It is found that, under these conditions, the twinning of the crystals proceeds in several stages.     

Effect of the electric fields of immobile dislocations on photoluminescence and EPR in deformed ZnS crystals

The influence of the electric fields of immobile growth dislocations on the luminescence properties of deformed zinc sulfide crystals is considered. Variations in the photoluminescence and EPR spectra with plastic deformation of ZnS crystals are shown to be caused by an increase in the radius of Read cylinders during the breakaway of growth dislocations from Cottrell atmospheres.     

Crystal-Size Dependence of Replenishment Effect in BaFBr:Eu

Excitation spectra of photo-stimulated luminescence (PSL) in BaFBr:Eu and BaFBr:Eu ? Na crystals were studied. Replenishment of PSL at liquid helium temperature was investigated in crystals of various sizes including a commercially available X-ray storage phosphor plate (Image Plate). Although a weak replenishment effect was observed in the fine powder crystals, no replenishment effects were observed in Image Plate, while it showed strong PSL at liquid helium temperature. Results indicate that there are two types of PSL centers, one consists of spatially correlated electrons and holes and the other consists of electrons and holes trapped at isolated centers. Lack of the replenishment effect in Image Plate indicates that trapped electrons are stabilized by the surface and need thermal activation to recombine with holes which are possibly trapped at doped Eu^2?.     

NMR-NQR studies of structural phase transitions

Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.     

一种新型Ca3Mg3Si4O14:Eu2+绿色荧光粉的制备及其性能研究

采用CaCO3,MgO,SiO2,Eu2O3原料,通过高温固相法制备了Ca3Mg3Si4O14:Eu2+荧光粉.通过XRD图谱和PL光谱图,研究了Eu的掺杂浓度与助溶剂(NH4Cl,BaF2)对Ca3Mg3Si4O14:Eu2+荧光粉结构、发光性能和热稳定的影响.XRD图谱对比结果表明,制备的Ca3Mg3Si4O14:Eu2+荧光粉XRD图与理论计算得到的图谱几乎一致.Ca3Mg3Si4O14:Eu2+荧光粉在360～450 nm有很强的激发强度,并且在440 nm激发下发射峰值波长为530 nm的发射光.随着Eu2+离子浓度的增加,发射光谱出现了红移,且在Eu2+离子浓度约为6％时发生了浓度猝灭现象.当添加NH4Cl和BaF2作为助溶剂,Ca3Mg3Si4O14:Eu2+荧光粉的发光强度有一定提高.与未添加助溶剂的Ca3Mg3Si4O14:Eu2+荧光粉的发光强度相比,添加NH4Cl助溶剂后发光强度增加了70％.此外,当温度升高至150 ℃时,Ca3Mg3Si4O14:Eu2+荧光粉和商用绿色荧光粉的发光强度分别降低了7.6％和14％,表明Ca3Mg3Si4O14:Eu2+荧光粉具有良好的热稳定性.这些发光性能均表明Ca3Mg3Si4O14:Eu2+荧光粉是是一种可应用于固态照明的有前景的绿色荧光粉.     

Effect of a magnetic field on the flow stress of Co and Fe single crystals

In a range of plastic deformation, the flow stress drop and the stress relaxation of Co and Fe single crystals are observed under an a.c. magnetic field. The magnitude of these drops is discussed on the basis of different deformation modes due to the crystal structure.     

Luminescence of rare-earth ions in perovskite-type oxides: from basic research to applications

Luminescence of a rare-earth ion in SrTiO₃ or other perovskite-type oxides was used as a probe of physical properties of a host crystal, but not as a phosphor because observed luminescence efficiency was low. Recent investigations have shown that the luminescence efficiency of SrTiO₃:Pr³⁺ is intensified up to 200 times by addition of Al ions. This paper reviews experimental results on this phenomenon and discusses mechanism of the Al-addition effect. A survey was made to find other materials, the efficiency of which can be improved by the impurity addition. Host-to-activator energy transfer by a coupling of the band-edge recombination energy and an allowed transition of a rare-earth ion is essential to provide a high efficiency. A red phosphor, Al-added SrTiO₃:Pr³⁺, is promising for display applications.     

Kinetics of Relaxation of Stored Light Sums in Silver Chloride

Experimental investigation of the time and temperature dependences of the process of relaxation of excited silver chloride has been carried out by the method of a photostimulated burst of luminescence. It is found that the relaxation process can be divided into two components, the rates of which differ by an order of magnitude. The activation energies of these processes of 0.03 and 0.06 eV correspond to the thermal ionization energies of electrons from shallow traps. Thermally activated relocalization of charges from shallow to deep traps in the process of relaxation has been revealed. The depth of a corresponding trap appeared equal to 0.17 eV. A model of a crystal phosphor has been suggested that contains five levels in the forbidden band: a hole center of recombination (luminescence), a deep electron trap, two shallow electron traps located at a depth of 0.03 and 0.06 eV from the bottom of the conductivity band, and a hole trap located at a height of 0.17 eV from the valence band top. Within the framework of this model, we can qualitatively explain the time and temperature dependences of stored light sums. We have shown the possibility of applying the method of a photostimulated burst of luminescence to investigation of the processes of relaxation of electron excitations in crystals to establish the relaxation mechanisms.     

Remanenz und Ummagnetisierungsbeginn in Nickeleinkristallen

The hysteresis, especially the remanence, and the magnetostriction of nickel single crystal rods of 〈110〉, 〈111〉, 〈100〉 and medium orientation are measured at room temperature in a magnetic joke and their dependence on plastic deformation is investigated. In undeformed crystals the nucleation of reverse domains needs a negativeH-field several times the coercivity, thus causing an “overshooting” of the hysteresis. Very small plastic deformations initiate the formation of the reverse domains and destroy the overshooting. Similar to polycrystals, the remanence of the single crystals — except those with 〈100〉 as axis — falls steeply at the beginning of plastic deformation to about 40% of its initial value. The magnetization reversal and the role of nucleation processes are discussed for the different orientations.     

Behavior of alkali halide crystals under the influence of a low-energy flux of β particles

The effect of a low-energy flux of electrons on alkali halide crystals is investigated. The pulse luminescence of crystals in the visible and ultrashort wavelength ranges is found. The dependence of luminescence on the potential of the crystal and color centers is revealed.     

Heating of slip lines and bands as a quasi-athermic mechanism of plastic deformation of crystals at low temperatures

We discuss theoretically a mechanism of violation of the Arrhenius law for the rate of plastic deformation, on the one hand, and of the appearance of plateau-like segments in the temperature dependence of the thermal-activation parameters, on the other, during deformation of crystals at low (<10 K) temperatures, which is associated with heating of the crystal by slip lines and bands. Via a self-consistent solution of the heat-conduction equation with allowance for variation of its coefficients and the rate of plastic deformation with temperature it is found that both a stable and an unstable regime (in the thermal sense) of propagation of slip lines and expansion of slip bands are possible depending on the ratio between the heating level and the level of strain hardening of the strain localization sites. The first regime is associated with the appearance of quasi-athermic plateaus in the temperature dependences of the thermally-activation parameters, and the second one leads to an instability (stepped) in the plastic deformation that is characteristic at low temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1479–1485 (August 1998)     

Effect of the thermal heating of a crystal on the diffraction of pulses of a free-electron X-ray laser

Spatiotemporal dependences of the distribution of the crystal temperature under the effect of pulses of a free-electron X-ray laser are found using the solution of a thermal conductivity equation. The effect of temperature, its gradient, and the deformation of the crystal lattice on the diffraction reflection and the transmission of pulses in crystals of synthetic diamond is considered.     

Mechanical twinning of yttrium oxide single crystals

The deformation structure of an yttrium oxide single crystal under point load was studied by microhardness and electron microscopy. It was concluded that mechanical twinning is the main mechanism of plastic deformation of these single crystals. A twin was simulated by rotating the structure around the three-fold axis. It was shown that the average change of the distance between anions was about 5% and that between cations was 15%. The comparison with the structure of the regions of the fluorite single crystals and optical ceramics deformed in the analogous manner was performed.     

Accumulation of dislocations and thermal strengthening of alloys having an L12 superstructure

This paper discusses the dislocation-accumulation mechanism in alloys having an L1₂ superstructure, which is associated with the formation of Kira-Wilsdorf barriers and the retardation of superdislocations during plastic deformation. A model of the dislocation-accumulation kinetics during plastic deformation is constructed, on the basis of which a mathematical model is formulated for the thermal and deformation strengthening of single crystals of alloys having the L1₂ superstructure. The results of numerical calculations based on the model are compared with the experimentally observed regularities of the deformation and thermal strengthening of single crystals of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 41, 454–460 (March 1999)     

电子俘获材料SrS:Eu,Sm,Er的光谱特性和光存储特性

提出一种Er3+改进的电子俘获光存储材料SrS∶Eu0.002,Sm0.002,Erx,其中,0≤x≤0.006。通过水热反应,研究了不同退火温度对荧光粉晶相形成的影响,以及不同含量的Er3+对荧光粉的发光性质以及光存储特性的影响。结果表明,Er3+的引入导致荧光增强及光存储特性提高。当Er3+的摩尔分数x=0.003时,荧光强度、光激励发光强度及光存储量出现最大值,分别为不含Er3+时的1.9倍、2倍和3.5倍。同时,Er3+的掺入不改变样品的晶体结构和衰减特性。     

Mechanoluminescence induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates

Mechanoluminescence (ML) emission from coloured alkali halide crystals takes place during their elastic and plastic deformation. The ML emission during the elastic deformation occurs due to the mechanical interaction between dislocation segments and F-centres, and the ML emission during the plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. In the elastic region, the ML intensity increases linearly with the strain or deformation time, and in this case, the saturation region could not be observed because of the beginning of the plastic deformation before the start of the saturation in the ML intensity. In the plastic region, initially the ML intensity also increases linearly with the strain or deformation time, and later on, it attains a saturation value for large deformation. When the deformation is stopped, initially the ML intensity decreases at a fast rate; later on, it decreases at a slow rate. The decay time for the fast decrease of the ML intensity gives the relaxation time of dislocation segments or pinning time of the dislocations, and the decay time of the slow decrease of the ML intensity gives the diffusion time of holes in the crystals. The saturation value of the ML intensity increases linearly with the strain rate and also with the density of F-centres in the crystals. Initially, the saturation value of the ML intensity increases with increasing temperature, and for higher temperatures the ML intensity decreases with increasing temperature. Therefore, the ML intensity is optimum for a particular temperature of the crystals. From the ML measurements, the relaxation time of dislocation segments, pinning time of dislocations, diffusion time of holes and the energy gap between the bottom of the acceptor dislocation band and interacting F-centre level can be determined. Expressions derived for the ML induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates indicates that the ML intensity depends on the strain, strain rate, density of colour centres, size of crystals, temperature, luminescence efficiency, etc. A good agreement is found between the theoretical and experimental results.     

荧光粉中激活剂离子掺杂格位分析

荧光粉的发光特性由激活剂离子的电子构型决定,同时也受激活剂离子在基质中的配位环境的影响.配位环境对激活剂离子发光性能的影响主要体现在电子云膨胀效应和晶体场劈裂效应这两个方面.电子云膨胀效应的强弱取决于激活剂离子与配体离子的成键特性(离子键与共价键成分的比例)以及阴离子极化率的大小;而晶体场劈裂效应的大小取决于激活剂离子...