LiF Single Crystals Research Articles

Deformation created by Berkovich and Vickers indenters and its influence on surface morphology of indentations for LiF and CaF2 single crystals

The study of the deformation zone around the indentations made by Berkovich and Vickers methods on LiF and CaF2 single crystals revealed several discrepancies in surface morphology. One of them is the presence of well-pronounced crack formation for Berkovich indentations and a total lack of that for the Vickers ones made at the same load on CaF2 crystals. The investigation of fine surface relief of indentations using atomic force microscopy revealed a faceted relief of the surface of Berkovich indentations as distinct from the smooth surface of the Vickers ones for both LiF and CaF2 crystals.Taking into account the peculiarities of each of the used methods, it was concluded that the geometry of indenters here has a major influence. The calculations showed that the trihedral Berkovich indenter with a smaller apical semi-angle against the tetrahedral Vickers indenter with a bigger semi-angle, on the one hand, and the smaller tip radius of the Berkovich indenter in comparison with the tip radius of the Vickers indenter, on the other hand, are the two factors which cause larger deformation of material under the Berkovich indenter as compared with the Vickers one. This fact leads to specific peculiarities of the deformation process and, as a result, to the appearance of specific surface morphology of indentations.

Volume expansion in heavily gamma irradiated NaCl and LiF single crystals

Linear expansion and stored energy measurements have been performed on NaCl and LiFsingle crystals gamma irradiated up to 150 MGy. The thermal recovery steps of theradiation-induced linear expansion upon heating coincide with the energy release peaks forboth materials. This, together with the amount of volume expansion measured afterirradiation, leads to the conclusion that this expansion is related to the formation ofdivacancies and divacancy clusters.

Electronic and ionic processes in LiF:Mg,Ti and LiF single crystals

Thermally stimulated luminescence (TSL, 5–750 K) and the annealing of induced optical absorption have been investigated in LiF:Mg,Ti and pure LiF single crystals irradiated at 5–295 K by electrons, X-rays, α particles, uranium swift ions and especially VUV radiation selectively forming anion excitons, electron–hole pairs and excitations localized near impurity centres. It is shown that besides electron–hole processes, the interstitial-vacancy (ionic) ones should be taken into account. The irradiation of LiF and LiF:Mg,Ti at high fluxes and fluences leads to the appearance of the TSL peaks at 570–630 K by a two-step process: thermal dissociation of two types of F 3 - trihalide molecules and sequent recombination of the products of this dissociation with intrinsic and impurity electron colour centres. The origin of some TSL peaks at 5–750 K is discussed.

Stages of surface erosion under ion irradiation

Initial stages of surface erosion have been studied for NaCl and LiF single crystals bombarded by Ar + ions with 20 keV. For irradiation with doses D=10 10–10 11 ions/cm 2, exoelectron emission has been used, whereas for higher doses, we have used electron microscopy. Two stages of initial surface erosion have been identified; for small doses, there is slow development of atomic-scale roughening, which reaches its peak when areas damaged by closest incident ions start to overlap, and then, beginning with D=10 16 ions/cm 2, there is rapid etching, deep into the crystal, followed by the emergence of secondary microscopic structure, i.e. caverns, concentric closed steps and terraces. Ion-induced surface structure of alkali halide crystals has been shown to depend strongly on the presence of foreign particles on the surface, as well as on segregation. Topography of thin carbon films and some metals have been discussed.

Color centers in neutron-irradiated Y 3Al 5O 12, CAF 2 and LiF single crystals

The thermal annealing behavior of the Y 3Al 5O 12, CaF 2 and LiF single crystals bombarded at Algiers with reactor neutrons has been monitored by optical absorption spectroscopy. The irradiation was performed at about 315 K. On heating samples after irradiation, the optical absorption bands decrease and disappear completely at 873 and 523 K in the case of Y 3Al 5O 12 and CaF 2, respectively. Activation energies of 1.2±0.02 and 0.9±0.2 eV are estimated for Y 3Al 5O 12 and CaF 2, respectively. On the other hand, the LiF crystal shows a complex annealing behavior. Here, the optical absorption spectrum presents different shapes after each annealing temperature. Four steps are distinguished and discussed on heating samples from 300 to 673 K. Above 673 K, the absorption drops by about 50%; it completely disappears at 773 K.

Neutron, ion and electron induced defects in activated LiF and NaF single crystals

AbstractThe neutron induced defects in activated LiF and NaF single crystals have been investigated by studying the absorption and luminescence spectra. For LiF and NaF neutron irradiated crystals the creation of F, F2 and F3+ aggregate color centers have been analysed in comparison with F, F2 and F3+‐center production in NaF crystals irradiated by ion and electron beams. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Toward control of point defects in lithium fluoride thin layers

AbstractPoint defect formation and stabilization properties, as well as their peculiar spectroscopic characteristics, locally modify the optical properties of insulating materials. Thin layers containing high concentrations of colour centres, hosted in a LiF single crystal and/or a polycrystalline matrix, offer the opportunity to develop innovative light‐emitting photonic devices. Control of all the critical parameters should be required on spatial dimension comparable with the optical wavelengths. Recent developments in laser technologies, electron and particles beam methods, and novel photon sources, have opened a wide range of opportunities. An overview of the most significant advances in this field is provided, with particular emphasis on colour‐centre LiF‐based innovative miniaturised light sources, optical amplifiers and lasers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal transformation of colour centres in LiF crystals with given content of oxygen, hydroxyl and metal ions

AbstractProcesses of radiation creation and annealing of Frenkel defects, as well as their transformation processes, have been studied in γ ‐irradiated LiF single crystals containing various impurities, by means of measurements of thermally stimulated changes in absorption and emission spectra. Important information about the density of colour centres in LiF, which depends on γ ‐irradiation doses, impurities and temperatures, has been obtained for small crystal contents of oxygen, hydroxyl and metal ions. A comparison of annealing parameters of the LiF crystals subjected to different doses of irradiation, but to the same thermal bleaching procedures, has been performed and the results are critically discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin Dynamics in Disordered Solids

Modern state of studies in spin dynamics of statically disordered media is presented. Next four fundamental problems are attended mainly: 1) delocalization of nuclear polarization in subsystem of impurity nuclei (it is exemplified in model nuclear spin system 8Li-6 Li in the LiF single crystal) 2) nuclear relaxation via paramagnetic impurities in crystals of arbitrary space dimension d 3) free induction decay and EPR line form function at d ≤ 3; and 4) form function of the hole, burned on the wing of the dipolar EPR line.

Optical transparency of z-cut LiF, Al2O3 and LiTaO3 single crystals under strong shock compression

In this paper, a comparative experimental study on optical transparency of z-cut LiF, sapphire and LiTaO3 under 1011 Pa shock compression at our two-stage light-gas gun facility is presented. In our experiments, intensity contrast variation of light emitted from a modified experimental setup of Mallory-type was monitored by a high-speed electronic camera. The results demonstrated that the LiF under the shock pressure of 102 GPa retains its initial transparency in the whole observation time interval, which is in good agreement with the common view of good sustained transparency of LiF. The LiTaO3 became basically opaque at 139 GPa. Sapphire at 131 GPa shock pressure showed a continuous transparency degradation till became completely darkened when the shock wave was arriving at the free-surface.

Accuracy limits and window corrections for photon Doppler velocimetry

Symmetric, plate-impact experiments were performed to validate photon Doppler velocimetry (PDV) with established shock wave diagnostics. Impact velocity measurements using shorting pins demonstrated that the velocity accuracy of PDV can be 0.1% or better. Shock velocities and refractive indices were also measured with PDV (at 1550 nm) and velocity interferometer system for any reflector (VISAR) (at 532 nm) to obtain window corrections for single crystal LiF (100), c-cut sapphire, and z-cut quartz. Time-dependent, free-surface velocity histories for shocked LiF(100) provide a direct comparison between PDV and VISAR diagnostics and illustrate the benefits and shortcomings of the new diagnostic. Further implications of these results are presented.

Experimental Investigation of Nuclear Polarization Transfer in Spatially Disordered Spin System by Means of β-NMR Spectroscopy

β-NMR studies of delocalization of polarization of 8Li β-active nuclei in spatially disordered 8Li–6Li nuclear system in LiF single crystals are presented. Unique peculiarity of the investigated 8Li–6Li nuclear spin system is that the g-factors of these isotopes are very close. So, the cross-relaxation between them dominates over all other depolarization mechanisms. Depolarization kinetics of 8Li nuclear probe in LiF crystals are measured in a wide range of 6Li concentration: 0.15, 3.28, 5.30, 8.81, and 10.06% and retraced up to (8–10) T 1/2(8Li). The obtained data show good agreement with theoretical descriptions.

X-ray diffraction measurements in shock compressed magnesium doped LiF crystals

X-ray diffraction measurements, utilizing multiple and single diffraction methods, were used to examine lattice compression of magnesium doped (approximately 100ppm) LiF single crystals shocked along the [100] direction. Unlike ultrapure LiF crystals, examined in previous x-ray diffraction studies, magnesium doped crystals display large elastic wave amplitudes under shock wave compression. Analysis of multiple diffraction data from the (200) and (202) planes and single diffraction data from the (200) planes showed that the crystal lattice for doped crystals was compressed isotropically in the peak state, similar to that observed for ultrapure crystals. This agreement demonstrates that the large elastic wave amplitude and subsequent stress relaxation observed in wave propagation measurements for the doped LiF crystals are a transient phenomenon and do not appear to affect the lattice compression in the final state. Implications of this finding are discussed.

Growth of CsI(Tl) scintillation film

CsI(Tl) is widely used as scintillation material for the high energy particles and ionizing radiation detectors. In the last decade imaging devices with high spatial resolution based on CsI(Tl) thin layers were developed due to the ability of CsI(Tl) film to grow in columnar morphology. This reduces the lateral light spread and promotes the directional light passing due to the total internal reflection on column boundaries. CsI(Tl) films of 5-200 μm thickness were obtained by physical vapour deposition technique onto the (100) cleavage planes of LiF single crystal used as orienting substrate and onto the amorphous glass substrates. In both cases columnarmorphology of films was observed. Vacuum deposition of CsI(Tl) onto the orienting LiF substrate at appropriate conditions leads to columnar morphology of the grown layer and single crystalline structure of blocks in two orientations, (110) and (112) (the halfwidth of rocking curves was from 8 to 20 arc min for (110) orientation and from 5 to 23 arc min for (112)), that are not coincident with (100) orientation of substrate. This may be the result of great lattice misfit and weak counteraction between the layer and substrate that provides theVolmer-Weber condensationmode and epitaxial growth according to coincident nodes mechanism. Columnar CsI(Tl) films demonstrate increased scintillation efficiency comparing with the standard CsI(Tl) single crystalline α detector at thicknesses more than 10 mkm. m10.p05

Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

Insufficient radiation resistance of construction materials is the Achilles heel for thermonuclear energetics. In wide-gap dielectrics, Frenkel defects are created not only because of the knock-out (impact) mechanism but also because of the decay of the electronic excitations formed during the irradiation (i.e. due to nonimpact mechanisms). The processes of the defect creation at the irradiation of highly pure LiF single crystals at 6–8K by 1–30-keV electrons, X-rays, or synchrotron radiation of 12–70eV have been investigated. The annealing processes of these defects in a temperature range up to 200K have been studied as well. In LiF, creation has been revealed for the following: (1) F–H pairs caused by the decay of anion excitons or by the recombination of electrons and holes, (2) F′–H–VK and F–I–VK defect groups at the decay of cation excitons (62eV), or (3) 20-keV electron irradiation. The mechanism of defect creation at the recombination of hot holes and hot electrons has been discussed for α-SiO2 crystals with an energy gap between the subbands of a valence band. One of the possible ways to suppress this mechanism (“luminescent defence”) is doping a material by luminescent impurities able to capture a part of the energy of hot carriers before their relaxation and recombination (e.g. in MgO:Cr).

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800nm, emission pulse duration 100fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F2 color centers of about 2×1018cm−3 and refractive index change of about 1% at 633nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F2 color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20mrad were achieved.

CaF2, MgF2, SiO2, Al2O3, SiC, LiF, BaF2, and ZrO2 optical single crystals used in studies in the VUV spectral region

Spectral characteristics of single crystals—fluorite, magnesium fluoride, quartz, sapphire, silicon carbide, lithium fluoride, barium fluoride, and fianite—used as materials for optical windows in studies in the VUV spectral region are presented. Spectral and photometric studies of single crystals in the spectral range 30–700 nm were performed on a BM-70 vacuum spectrophotometer. This instrument is equipped with a vacuum chamber for measuring the light transmission and reflection at radiation angles of incidence between 10° and 70°; this allows both characteristics to be measured sequentially in time without breaking vacuum. Replaceable radiation sources (with and without a window) were used to perform studies in the spectral range 30–700 nm, including measurements in flows of atmospheric and inert gases (Ne, He, Ar, N, and H).

Properties of the 4.45 eV optical absorption band in LiF:Mg,Ti

The optical absorption (OA) and thermoluminescence (TL) of dosimetric LiF:Mg,Ti (TLD-100) as well as nominally pure LiF single crystal have been studied as a function of irradiation dose, thermal and optical bleaching in order to investigate the role of the 4.45 eV OA band in low temperature TL. Computerised deconvolution was used to resolve the absorption spectrum into individual gaussian bands and the TL glow curve into glow peaks. Although the 4.45 eV OA band shows thermal decay characteristics similar to the 4.0 eV band its dose filling constant and optical bleaching properties suggest that it cannot be associated with the TL of composite peaks 4 or 5. Its presence in optical grade single crystal LiF further suggests that it is an intrinsic defect or possibly associated with chance impurities other than Mg, Ti.

Color center production by femtosecond-pulse laser irradiation in fluoride crystals

Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF4 single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm2 was determined in YLF and LiF, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F2, F +2 , and F +3 . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived F +2 centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.

Size Effects in LiF Micron-Scale Single Crystals of Low Dislocation Density

AbstractThis study examines the deformation response of 20, 5, and 1 micron diameter samples fabricated by FIB-milling from bulk ultrapure LiF single crystals. The bulk crystals have a very low initial dislocation density as revealed by an etch-pit technique. Two types of <001> microsamples were deformed preferentially by single slip when compressed inside a nanoindentation system. Similar to previously studied FCC-derivative metals, LiF microsamples demonstrate dramatic strengthening achieving the engineering flow stress σ ≈ 650 MPa in 1-μm samples. The stress-diameter dependence obeys a power law, σ ∼ D−m, where m ≈ 0.8. Stochastic variation of flow stress, fast intermittent deformation events (“avalanches”) and highly localized slip bands after avalanches - all characteristic of size effects in metals, are also observed in LiF. Possible dislocation mechanisms of the observed size effects are discussed.