LiF Single Crystals Research Articles

Formation of Vk centers by mechanical treatment of LiF single crystals

Formation of Vk centers by mechanical treatment of LiF single crystals

The effect of γ radiation on the dislocation charge in LiF

Abstract A bent single crystal of LiF was mounted on a Marx composite resonator and exposed to π radiation at room temperature. The signal developed across the specimen faces by the vibrating dislocations was monitored together with the decrement and elastic modulus. It was found that the magnitude of the dislocation line charge decreased with radiation time, the rate being governed by the amount of deformation applied to the specimen prior to irradiation.

Stress dependence of elastic-wave attenuation in LiF

Shock-wave propagation studies along the 〈100〉 direction in LiF single crystals are presented for three elastic impact stresses, namely, 39, 29, and 15 kbar. Present results demonstrate a stress threshold for a large increase in plastic strain rate and establish stress relaxation as a bulk property. Analysis of the data reveals a one-to-one correspondence between plastic strain rate and stress at the elastic-shock front. This is in agreement with the constitutive relation generally used for stress-relaxing solids under plate-impact loading.

The early stages of irradiation hardening of LiF single crystals

By several spectral (absorption, thermoluminescence, thermoemission of exoelectrons, EPR) and mechanical (indentation, measuring of stress relaxation rate and velocity of individual dislocations) methods the nature of irradiation hardening in LiF: Mg on the early stages of irradiation is studied. It is stated that initial irradiation hardening (at doses < 104 R) is due to interaction of the irradiation with impurity complexes. Mit verschiedenen spektralen (Absorption, Thermolumineszenz, Thermoemission von Exoelektronen, EPR) und mechanischen (Eindruckhärte, Messung der Spannungsrelaxationsrate und Geschwindigkeit individueller Versetzungen) Methoden wird die Bestrahlungshärtung in LiF: Mg in frühen Stadien der Bestrahlung untersucht. Es wird gefunden, daß die anfängliche Bestrahlungshärtung (bei Dosen < 104 R) durch die Wechselwirkung der Strahlung mit Störstellenkomplexen hervorgerufen wird.

Strain Field around Cleavage Crack Stopped Halfway in LiF Single Crystal

The strain field developed by a spontaneously stopped cleavage crack in semi-brittle LiF single crystal is studied by the limited projection topography of X-rays. In front of the crack front a region of plastic deformation with midrib is observed over 0.4 mm in width and 560 µm in thickness. In the backside of the crack front are observed two kinds of interference fringes, Pendellösung fringes along a grown-in sub-grain boundary and moiré fringes resulting from X-ray interference between two crystal parts separated by the crack. From the analysis of the moire pattern local distributions of rotation and change in lattice spacing between two crystal parts facing each other across the crack are separately given.

The influence of pressurization-induced dislocations on the plastic deformation of LiF and NaCl monocrystals

Single crystals of LiF containing voids and of NaCl containing Na2SO4 precipitates were pressurized to introduce dislocations in the vicinities of the discontinuities and subsequently compressed along 〈100〉 at room temperature. The yield stress was raised in both materials; additionally, in LiF discontinuous yielding and easy glide were suppressed and work hardening rate increased by the pressurization-induced dislocations. Following pressurization at 0.85 GN m−2, for example, the 0.1 % shear flow stress of LiF was doubled to ∼ 4 MN m−2 and stage II work hardening rate quadrupled to ∼ 180 MN m−2. Pressurization of NaCl above 0.6 GN m−2 resulted in an increase in the 0.1 % flow stress from ∼ 1.2 to ∼ 2.0 MN m−2. If the slip bands in LiF were initiated by a precompression, pressurization prevented the broadening of these fresh slip bands during subsequent plastic flow. Deformation now took place at a higher stress both in LiF and NaCl. These effects resemble in some ways latent hardening in that oblique as well as conjugate dislocation intersections must take place to continue the deformation. In contrast to latent hardening data, the strain hardening rate was increased in LiF and was approximately proportional to the pressurization-induced dislocation density. This ratio, 5 to 6 dyne per dislocation, is in fair agreement with two sets of independent calculations reported by Gilman and Johnston. The results suggest, therefore, that in the present case also hardening may be due to defects left in the wakes of pressurization-induced moving dislocations.

Dislocation mechanisms for stress relaxation in shocked LiF

A study of shock wave propagation along the 〈100〉 direction in LiF single crystals is presented. Plate impact experiments were conducted to produce elastic impact stresses of approximately 29 kbar. Stress time profiles at the impact surface and rear surface for thicknesses up to 3.2 mm were observed. Experiments were done for two impurity concentrations and three different heat treatments. Material characterization to supplement the shock data was provided by quasistatic yield stress measurements, dielectric relaxation data, initial dislocation density counts, and spectrographic analysis. Elastic wave attenuation is strongly influenced by both Mg++ impurities and heat treatments. Impurity clustering generally reduces the rate of precursor decay. The plastic strain rate at the elastic shock front was computed from the data by a near−exact method which incorporates material nonlinearities. Beyond the first 1 or 2 mm of propagation, significant contributions to stress decay arise from overtaking by relief waves. Application of dislocation theory reveals dislocation densities to be approximately 3 orders of magnitude larger than grown−in dislocations, at least in the region of rapid stress decay. Present analysis contradicts the idea of regenerative multiplication of dislocations causing this large increase in density. A model for heterogeneous nucleation of dislocation based on an energy criterion is proposed which appears to be well suited for explaining large increases in dislocation densities. The present data suggest an applied shear stress of 3−5 kbar as the lower bound at which dislocations can nucleate at heterogeneities present in our crystals. Better material characterization concerning impurity clusters is needed to consider the quantitative aspects of rate and magnitude of heterogeneous nucleation. The mechanism for stress decay in the very soft LiF crystals is still not well understood.

On the interaction of cleavage cracks with second phase particles

The interaction of {100} cleavage cracks with hard and soft second phase particles is described in LiF and MgO single crystals. The principal effect of the second phase particles is to change the cleavage step density. The apparent fracture surface energy, γ i , of MgO is increased by one hundred per cent by the presence of cube shaped 10 μm on a side and spaced at 50 μm. The measured increase in γ i is consistent with the increased work done in forming cleavage steps by shearing at a stress equal to the theoretical shear strength.

Hydroxyl ions and the 200−nm absorption band in Mg− and Ti−doped thermoluminescent LiF single crystals

A study of the distribution of the optical absorption coefficient of the 200−nm band in Mg− and Ti−doped LiF single−crystal boules, grown from monocrystalline starting materials of varying purity, indicates that this absorption band is due to titanium−hydroxyl complexes. The measurements of ionic conductivity and infrared absorption on the starting materials and in doped crystals indicate that each titanium ion probably interacts with several hydroxyl ions to form these complexes, and that the association of hydroxyl ions is stronger with titanium ions than with magnesium ions. Since earlier work has shown that the 200−nm band is involved in the thermoluminescent recombination process, it is indicated that hydroxide impurities may be essential constituents in dosimetry grade LiF.

Generation of slip by pressurization of LiF single crystals containing cavities

Abstract Single crystals of LiF containing cubical and near-spherical cavities were subjected to hydrostatic pressures of up to 1·5 GNm−2 and the resultant microstructural changes were studied using the repeated etching technique. Nucleation of dislocations in the vicinities of the holes was observed to result from treatments above a threshold pressure characteristic of the cavity size. On raising the pressure, further dislocations were generated. These arrays were on the twelve 〈110〉{110} slip systems, each being associated with a cavity edge for the case of cubical cavities. An adequate representation of a first glissile half-loop is by the introduction of ’an extra (110) half plane ’connecting the distorted (010) cavity face with the (110) slip plane. This (110) plane is bounded by a glissile edge dislocation in (110) and sessile edge dislocations in (001) terminating at the cavity. By symmetry, twelve pairs of 〈110〉{110} half-loops can be produced and the process may continue to generate further slip....

Depths of the Regions Damaged by Protons, Deuterons and α-Particles in Lithium Fluoride Single Crystal

The depths of regions damaged by 0.6∼2.0 MeV protons, deuterons or α-particles penetrating the LiF single crystal were optically measured by utilizing the diffuse reflection of light from the metallic colloids formed by bombardment. The results were compared with the range-energy relations theoretically calculated on the Bethe formula. The depth data obtained agree well with the theoretical ranges with Z =10 or 12, depending on the bombarding ions, not with Z =6 which is usually taken for LiF. The rough approximation is given by the formulae with simple power of the energy value. As the sources of discrepancy between the depth of damaged region and the ion range, the following are possible: Channeling of the ions during penetration in the crystal and diffusion of the ion-induced defects.

Strain rate sensitivity of the flow stress of Mg++ doped LiF single crystals

Strain rate sensitivity of the flow stress of Mg++ doped LiF single crystals

Cluster arrangement of point defects in γ-irradiated LiF

The clastering of point defects in γ-irradiated LiF single crystals was investigated by measuring the small-angle X-ray scattering after various γ doses.

Behavior of individual dislocations in LiF under alternating stresses

The behavior of individual dislocations under alternating stresses has been investigated by the etch pit method in highly pure LiF single crystals. The applied stress level is varied from 1 4 to 1 2 of the macroscopic yield stress. Edge dislocations make jerky motion, and move backward when the stress is reversed, even at a low stress level. On the other hand, screw dislocations make no backward when the stress is reversed if the applied stress level is just above the minimum stress to move screw dislocations. When the stress is increased by 40 ∼ 60 g/mm 2, however, they can move back and forth under alternating stresses without any multiplication of dislocations Such as behaviour of screw dislocations is discussed on a basis of debris dipole formation.

Movement of gas-filled cavities in alkalihalide crystals produced by external force fields

The migration of gas-filled cavities in KBr, NaCl, LiF single-crystals is investigated experimentally in an electric field and a temperature gradient.It is shown, that migration in an electric field is strongly dependent on the surface purity, due to its influence on matter transport along cavity surface. It is found that the velocity of cavity (v), having a ‘dirty’ surface follows a V∼1/R dependence. (R—cavity size). In case of a ‘pure’ cavity surface. V is independent of R. Different types of V-vs-R dependence result in intermediate regions.Migration in a temperature gradient is also sensitive to surface purity. When the state of surface purity is determined, an appropriate critical size (R*) exists such that if R is smaller than R*, V∼R and if R is larger than R*, V is independent of R.Some constants of investigated process are determined from obtained experimental data.

The effect of plastic deformation on the thermoluminescence of LiF (TLD-100) single crystals

The thermoluminescent properties of LiF (TLD-100) have been investigated as a function of plastic deformation. Without exposure to ionizing radiation, plastic deformation up to fracture does not induce a thermoluminescent response. After subsequent exposure to ionizing radiation, the intensity of the thermoluminescent glow peaks decreases with the amount of plastic deformation. This is explained in terms of a relative decrease in the concentration of free dipoles and low-order dipole complexes which act as trapping centres for peaks 2 and 5. This decrease is a result of an interaction of these defect structures with the dislocations introduced during deformation.

DYNAMIC STRAIN AGEING IN LiF SINGLE CRYSTALS

Dislocation-point defect iiiterac~ions have been investigated in high purity LiF single crystals containing less than 5 ppni of Mg?-, utilising dynamic strain ageing techniques. Two mechanisms of strain ageing have been found. Below 200 the mechanism is one of weak pinning of dislocations by tetragonal strain fields around impurity-vacancy dipoles giving rise to a slight flow stress increase after ageing under stress. Above 200 C. the strain ageing is due to the strong pinning of dislocations by ordered atmospheres containing impurity-vacancy dipoles and othcr point defects. This results in the presence of serrated yielding, and a large flow stress increase after ageing under stress. Thcse erects may also be important in previo~~sly reported work hardening data in LiF single crystals.

Thermodepolarization currents and the electret effect in alkali halide single crystals

1. A stable electret state was observed in LiF single crystals. 2. The activation energy of the heterocharge, determined by the thermodepolarization method, is close to the energy of migration of cation vacancies. This is evidence of the fact that the formation of residual polarization (heterocharge) is connected with the accumulation of cation vacancies in the anode region.

Effect of a metaantimonate phase on the efficiency of calcium halophosphates

Evidently the increase in the luminescent yield beginning in the region of 5 eV is associated with the onset of characteristic absorption, while the maximum at 5.55 eV arises from the dependence of the luminescent efficiency on the absorption depth of the exciting light, in the presence of losses close to the surface. The range of excitation extending from 5 eV in the direction of lower exciting photon energies evidently corresponds to the absorption bands associated with the presence of manganese (for example, chargetransfer bands). These assumptions are supported by the spectral dependence of the reflection coefficient presented in Fig. 1. The reflection spectrum was measured with a relatively finely-ground, pure LiF single crystal. In the range hv > 5 eV the reflection coefficient falls on increasing the energy of the incident photons, and at hv = 5.6 eV reaches a value typical of the range of characteristic absorption of oxygencontaining compounds. In the range 4-5 eV the reflection coefficient is also relatively sinai1, which indicates a strong absorption of the exciting radiation. In order to estimate the luminescent efficiency of the separated phase, we compared its quantum yield with the quantum yield of the CHP on excitation with the mercury resonance line k = 254 nm, allowing for reflection. We found that the quantum yield of the separated phase amounted to 0.066 of the CHP luminescence.

Defect Formation by N++ Ion Bombardment in LiF and NaF

The F-light absorption was measured in LiF and NaF single crystals bombarded by 0.7 MeV N ++ ions from a Van de Graaff accelerator. The growth curves showed a maximum, from which the F-center concentration at the saturation was estimated to be 7.6 and 3.8×10 19 F-centers/cm 3 in LiF and NaF, respectively. The rates of production, 3020 and 1210 F-centers/N ++ ion, and the formation energies of an F-center, 232 and 578 eV, were determined from the initial portions of the growth curves in LiF and NaF, respectively. The formation of metallic film on the bombarded surface and other imperfections as the result of radiation damage are discussed, referring also to the micrographic observations. It is thus concluded that the damage by N ++ ions is fairly severe; the double charge is possibly responsible for this phenomenon.