LiF Crystals Research Articles

Isotopically Enriched Lithium Fluoride Crystals for Detection of Neutrons with the Fluorescent Track Technique.

In this work, the properties of LiF crystals grown using Li of different isotopic compositions are described from the standpoint of their application as fluorescent nuclear track detectors used in measurements in the neutron radiation fields. The crystals were grown using two techniques: the Czochralski method and the micro-pulling-down method. Three isotopic compositions of Li were studied: natural, highly enriched in 6Li, and highly enriched in 7Li. It was found that 6LiF detectors are about six times more sensitive to thermal (low energy) neutrons than natural LiF, which significantly decreases the lower detection limit. 7LiF detectors are insensitive to thermal neutrons, which makes it easier to detect tracks due to other radiation modalities, such as energetic ions or nuclei recoiled in collisions with high-energy neutrons. Besides the response to neutron radiation, no other significant differences in the crystal properties were identified, irrespective of the isotopic composition and crystal growth method employed.

Nonlinear Excitation of Color Centers in a LiF Crystal by Femtosecond Laser Radiation

Experiments were carried out in which the luminescence of color centers in lithium fluoride crystals was excited by two different femtosecond lasers with significantly different energies, durations and pulse repetition rates. It was established that in all experiments the main center, the luminescence of which was excited nonlinearly, was the F3+ color center. Unusual experimental data were obtained; a laser with low pulse energy (4 nJ) excited triplet luminescence of these centers (570 nm) but did not excite singlet luminescence (540 nm). Another laser with a higher pulse energy (0.3 mJ), on the other hand, excited singlet luminescence and did not excite triplet luminescence. A proposed diagram of energy levels and quantum transitions is presented, explaining the possible mechanisms of nonlinear excitation of luminescence in these experiments.

Thermal annealing of radiation damages produced by swift 14N and 16O ions in LiF crystals

Annealing of color centers was studied in lithium fluoride crystals, irradiated with 23-MeV nitrogen and 28-MeV oxygen ions. Basing on the optical absorption spectroscopy and reaction-rate modelling, a new interpretation of the annealing kinetics at the practically important temperatures below 500 K is suggested. Proposed model explains simultaneous decrease of the F and F2/F3 + peaks as a result of migration of the F centers and formation of larger aggregates, and does not include additional assumptions about impurities and cation vacancies. It specifies the migration energy of the F centers in the ground state to be about 1.3 eV, that corresponds to earlier studies.

Radiophotoluminescence response of LiF:Mg,Ti pellets irradiated with clinical proton beams in the 70–200 MeV energy range

Lithium fluoride doped with Mg and Ti (LiF:Mg,Ti) has been used for several decades as thermoluminescent dosimeter material, but recently also its radiophotoluminescence has been investigated for applications in dosimetry. In this work, LiF:Mg,Ti pellets (TLD-100) were irradiated in a water phantom at CNAO (Pavia, Italy) with therapeutic proton beams at five energies from 70 to 200 MeV in the dose range from 2 to 20 Gy. After irradiation, their visible radiophotoluminescence spectra were measured in controlled conditions and the spectrally-integrated red emission response of radiation-induced color centers has been investigated. The radiophotoluminescence signal, excited by a 445 nm continuous wave laser, was integrated within a 50 nm-wide band around the emission peak of the F2 color centers, located around 670 nm in LiF. The spectrally-integrated signal of the samples irradiated at the energy of 147.7 MeV exhibited a linear dependence with dose. Moreover, this radiophotoluminescence response appears independent from Linear Energy Transfer in the range from 0.8 to 1.6 keV/μm in all the samples irradiated at the dose of 5 Gy. Such independence was found up to 10.3 keV/μm in samples irradiated within two spread out Bragg peaks made of 36 energy components in the entire investigated proton energy range. The radiophotoluminescence response of the TLD-100 pellets was compared to that of nominally-pure LiF crystals irradiated in the same conditions, which show a similar behavior. The results are encouraging for the exploitation of TLD-100 pellets as passive solid-state radiophotoluminescent dosimeters for proton therapy.

Detection of proton tracks with LiF fluorescent nuclear track detectors

Fluorescent nuclear track detectors based on LiF crystals were successfully applied for detection of proton induced tracks. Irradiations were performed with protons with energy ranging from 1 MeV up to about 56 MeV and for all proton energies the fluorescent tracks were observed. The tracks are not continuous, but consist of a series of bright spots. The gaps between spots tend to narrow with decreasing proton energy (increasing ionization density). For the highest of the studied energies, the spots are scattered so sparsely, that it is not possible to link spots belonging to one track. The intensity (brightness) of the fluorescent tracks increases with the increasing LET and agrees well with the trend established earlier for various heavier ions.

Directing Fluorinated Solid Electrolyte Interphase by Solubilizing Crystal Lithium Fluoride in Aprotic Electrolyte for Lithium Metal Batteries

AbstractLithium fluoride (LiF) facilitates robust and fast‐ion‐transport solid electrolyte interphase (SEI) in lithium metal batteries. Fluorinated solvents/salts are ubiquitously employed to introduce LiF into SEI through electrochemical decomposition, but this approach is usually at the expense of their continuous consumption. A direct approach to fluorinate SEI that employs crystal LiF is limited by its poor solubility in the current battery electrolyte formulation. Dissolving crystal LiF in high‐dielectric‐constant solvents, like ethylene carbonate (EC) is nearly neglected. Herein, the feasibility of directly fluorinating SEI by the addition of crystal LiF in aprotic electrolyte with the assistance of EC is verified, and its mechanisms in fluorination of SEI and anti‐acidification of electrolyte are explored. The dissolved LiF is encapsulated by solvent‐/salt‐derived organic skins to promote the fluorinated SEI. Meanwhile, the presence of LiF in electrolyte alters hazardous thermodynamic equilibrium, suppressing the production of acid species to mitigate electrolyte acidification and SEI degradation. Such collective benefits yield a capacity retention ratio of ≈88% after 150 cycles at a high areal capacity (4.5 mAh cm−2) in Li||NCM622 cells. This facile and effective fluorination of SEI contributes to an in‐depth understanding of SEI formation and rational design of well‐performing lithium metal batteries.

Comparative analysis of copper X-radiation intensity with LiF and KBr crystals

In this research work, the physical, mechanical and performance characteristics of bituminous binder 90/130 before and after its modification with sulfur in the proportions of 10%, 30% and 50% of the binder weight were analyzed. Experimental results showed the effect of sulfur content on the mechanical characteristics of bituminous binder. By elemental analysis of the spectra of modified bitumen binder samples, it was found that there were no significant chemical changes with increasing sulfur content. The formation of new chemical compounds was not revealed, which is confirmed by the absence of new characteristic peaks in the spectra. Analysis and comparison of the experimental data of softening temperature showed an insignificant influence of the liquid heating rate on the concentration of the modifying agent. However, the increase of sulfur concentration more than 10% in the modified bitumen leads to an increase in softening temperature. This is due to the formation of denser structure and increase in viscosity of bitumen with increasing amount of sulfur in the system. Thus, the presence of sulfur as a dispersed phase in polymer bitumen has a natural effect on the mechanical characteristics of sulfur asphalt concretes, leading to an increase in their mechanical properties with increasing sulfur content.

Proton-beam energy diagnostics by color-center photoluminescence imaging in LiF crystals: Implementation of multiple Coulomb scattering into an analytical Bragg-curve model

Lithium fluoride crystals are used to assess proton-beam energy spectra. Proton irradiation induces laser-active color centers in the crystal, whose density correlates with the absorbed dose. The spatial distribution of photoluminescence emitted by these color centers is exploited to estimate the proton-beam energy spectrum using an analytical Bragg-curve model. This study integrates the effects of multiple Coulomb scattering (MCS) into the model. At high enough energies, MCS leads to proton leakage through the crystal faces with a reduction in absorbed dose along the crystal length. The model incorporates MCS using an empirical approach based on Monte Carlo simulations.

Effect of ion-induced secondary radiations on the formation of extended defects and radiolysis beyond the range of swift 12C, 50Ti, and 52Cr ions in LiF

The objective of this research is to study the role of ion-induced secondary radiations on structural damage in LiF crystals irradiated with ∼ 10 MeV/u energy 12C, 50Ti, and 52Cr ions using experimental methods of nanoindentation, Atomic Force Microscopy, optical absorption, depth-resolved photoluminescence and chemical etching. In LiF crystals irradiated at room temperature with 50Ti and 52Cr ions, the chemical etching method revealed formation of nano-sized colloids in a wide area (up to one mm) beyond the ion penetration depth. Annealing experiments on irradiated samples at temperatures above 620 K, show enhanced manifestations of radiolysis in ion-irradiated and beyond-range areas accompanied by the emission of radiolysis residues. The multicomponent secondary radiation following the ion-induced inelastic nuclear reactions is discussed as the plausible mechanism of the observed long-range damage.

Damage threshold of LiF crystal irradiated by femtosecond hard XFEL pulse sequence.

Here we demonstrate the results of investigating the damage threshold of a LiF crystal after irradiating it with a sequence of coherent femtosecond pulses using the European X-ray Free Electron Laser (EuXFEL). The laser fluxes on the crystal surface varied in the range ∼ 0.015-13 kJ/cm2 per pulse when irradiated with a sequence of 1-100 pulses (tpulse ∼ 20 fs, Eph = 9 keV). Analysis of the surface of the irradiated crystal using different reading systems allowed the damage areas and the topology of the craters formed to be accurately determined. It was found that the ablation threshold decreases with increasing number of X-ray pulses, while the depth of the formed craters increases non-linearly and reaches several hundred nanometers. The obtained results have been compared with data already available in the literature for nano- and picosecond pulses from lasers in the soft X-ray/VUV and optical ranges. A failure model of lithium fluoride is developed and verified with simulation of material damage under single-pulse irradiation. The obtained damage threshold is in reasonably good agreement with the experimentally measured one.

Photoluminescent Microbit Inscripion Inside Dielectric Crystals by Ultrashort Laser Pulses for Archival Applications.

Inscription of embedded photoluminescent microbits inside micromechanically positioned bulk natural diamond, LiF and CaF2 crystals was performed in sub-filamentation (geometrical focusing) regime by 525 nm 0.2 ps laser pulses focused by 0.65 NA micro-objective as a function of pulse energy, exposure and inter-layer separation. The resulting microbits were visualized by 3D-scanning confocal Raman/photoluminescence microscopy as conglomerates of photo-induced quasi-molecular color centers and tested regarding their spatial resolution and thermal stability via high-temperature annealing. Minimal lateral and longitudinal microbit separations, enabling their robust optical read-out through micromechanical positioning, were measured in the most promising crystalline material, LiF, as 1.5 and 13 microns, respectively, to be improved regarding information storage capacity by more elaborate focusing systems. These findings pave a way to novel optomechanical memory storage platforms, utilizing ultrashort-pulse laser inscription of photoluminescent microbits as carriers of archival memory.

Scalable conversion of residual lithium into conformal artificial cathode electrolyte interphase via a single-step wet-chemical process for high-voltage Ni-rich cathode operation

Ni-rich layered oxide has become an indispensable cathode for incumbent lithium-ion batteries due to their high energy density and low cost. Despite these advantages, the Ni-rich cathode suffers from surface residual lithium, which causes gas evolution and gelation issue during the electrode fabrication process. Here, we demonstrate a simple wet-chemical and mass-production compatible process that converts undesirable residual lithium into an artificial cathode electrolyte interface (CEI) layer. A scalable wet chemical route effective even in bulk scale is firstly developed, where LiOH and Li2CO3 residual lithium species convert into LiF crystal through NH4F dissolved in ethanol. Pre-existing residual lithium on LiNi0.8Mn0.1Co0.1O2 (NMC811) powder surface reacts with a fluorine source NH4F saturated in ethanol, reincarnating into an artificial LiF CEI layer. A conformally formed CEI layer improves high voltage cycling up to 4.6 V vs. Li/Li+ with ∼ 10 % increase of the initial capacity retention at the 500th cycle compared to that of the control electrode. Suppression of transition metal dissolution along with thermal oxygen evolution from the Ni-rich cathode by the artificial CEI are also demonstrated. Based on validation at 10 g level, a continuous reactor with high-throughput processibility was devised. This study provides a scalable, single-step, recyclable, and inexpensive process to achieve artificial CEI layer on Ni-rich cathode by replacing residual lithium without further post-processes.

Regulating Single-Crystal LiNiO2 Size and Surface Coating toward a High-Capacity Cathode for Lithium-Ion Batteries

Single crystals recently received a great deal of attention because the stabilities of cathode materials are improved. One of the major drawbacks of the single-crystal cathodes is that their achievable capacity is lower than that of the same composition polycrystalline cathodes. Although it is widely accepted that the large crystal size of single-crystal cathodes might be the main reason for their low capacity, a systematic study to verify all possible rationales is absent. In this work, we regulated the crystal size of a single-crystal LiNiO2 to investigate its relation to capacity for the first time. It was established that among the sizes studied, a 400 nm-sized single crystal LiNiO2 achieved high capacity, ∼240 mA h/g at 0.1 C, which is comparable to that of its polycrystalline counterpart. It is the first report that such a high capacity is obtained in a single crystal. Also, in our results, with increasing crystal size, a capacity decline was recorded as expected. Interestingly, it is first found that capacity loss occurs only in the high-lithium-composition region (x > 0.8 in LixNiO2), and polarization becomes high only in the same region upon increasing crystal size. This implies that kinetics of the region is significantly affected by the crystal size. Also, high capacity can be achieved in large single-crystal LixNiO2 once the region’s kinetics is optimized. In terms of capacity retention, large single-crystal LiNiO2 exhibits the highest stability. Accordingly, high capacity can be achieved when the crystal size is reduced by trading-off its cycling stability. In order to achieve both high capacity and stability, LiF surface coating was conducted on the small single-crystal LiNiO2. It was shown that the LiF coating can effectively protect against capacity degradation, and the capacity retention by such small single-crystal LiNiO2 can be made even better than that of large crystal LiNiO2. Therefore, both high capacity and cycle retention were achieved in single-crystal LiNiO2 by reducing its crystal size and LiF surface coating.

Simulation of the dynamics of single F3+ color centers in LiF crystals

Simulation of the dynamics of single F3+ color centers in LiF crystals

Radiation and Emission Phenomena in Combination

Radiation and Emission Phenomena in Combination

Low-energy protons shallow spread-out Bragg peak imaging with a lithium fluoride crystal

The 35 MeV proton beam produced by a modular linear accelerator was used to irradiate at grazing incidence a LiF crystal through a spinning Mylar multi-sector range modulation wheel and a Pyrex range shifter. Starting from a measured pristine depth dose curve, these energy-modulation devices were designed to plan a test shallow spread-out Bragg peak in water. The irradiation of the LiF crystal produced in its lattice a volume distribution of color centers, which could be visualized in a fluorescence microscope as a fluorescent image under blue-light illumination. Since the intensity of this fluorescent image was proportional point-by-point to the absorbed energy, it was elaborated to obtain an experimental depth dose curve in the LiF crystal, from which the proton beam energy distribution was estimated. This latter was finally used to evaluate the corresponding spread-out Bragg peak that would be obtained in water and compare it to the designed one.

Effect of ion-induced nuclear reactions on structure modification and radiolysis in LiF irradiated by 410 MeV 36S ions

The objective of this research is to study the structural damage in LiF crystals irradiated with 410 MeV 36S ions. We report on the appearance of a remarkable structural damage far (hundreds of μm) beyond the penetration depth (136 μm) of sulfur ions. Selective chemical etching on samples irradiated at room temperature revealed the formation of extended defects within the ion-irradiated zone and also beyond the range of sulfur ions. Based on AFM image analysis, the defects were identified as nano-size colloids (10 nm). In the irradiated zone, nanoindentation tests indicated a significant hardening effect. Annealing experiments showed manifestations of intense radiolysis both in the ion-irradiated and beyond the range areas with a maximum of around 620 K. Comparison of experimental results with those of performed model simulation using Monte-Carlo particle transport code FLUKA support the assumption that the cause of the observed long-range damage is irradiation by secondary particles created in inelastic nuclear collisions between incident sulfur ions and lithium and fluorine nuclei of the target atoms. Neutrons, γ-rays and protons are rated as the dominating components of the secondary particle flux. The results can be helpful for assessing the risks for damage delocalization and for reasonable selection of ions in the applications of ion technologies to optical materials like LiF.

Channels of Filaments of Axially Asymmetric Optical Vortices at a Wavelength of 1800 nm in a LiF Crystal

The formation of a set of filaments and plasma channels in a femtosecond optical vortex has been studied experimentally and numerically. A longitudinal distribution of color center tracks with a length of 1 cm written in a LiF crystal by an axially asymmetric beam in the single-pulse regime has been detected experimentally for the first time. It has been shown that, at a sufficient excess of the peak power over the critical value, two hot points on the annular profile of vortex beam separated by the phase dislocation region form sequences of color center tracks; each sequence in the cross section of the beam is localized near the initial hot point. Secondary filaments and the corresponding tracks appear with an increase in the pulse energy. The parameters of femtosecond filaments in LiF have been numerically estimated.

Influence of Neutron Irradiation Conditions on Maximal Frequency of IR Absorption Spectra of LiF:OH Single Crystal

For many solids operating under irradiation conditions, the study of their behavior at the neutron irradiation is of importance. Herein, the influence of neutron irradiation conditions on the infrared (IR) absorption spectra of LiF crystals containing different amounts of hydroxyl and metallic impurities is considered. The investigations are carried out for discovering new peculiarities under the extreme irradiation conditions of LiF—being a very important material for practical applications. In the crystals with OH− impurity after the action of neutron irradiation, quite clear bands are appeared in the region of 1900–2200 cm−1. The delivered doses vary from 1014 to 3 × 1017nnvt. LiF crystals are investigated by differing both in qualitative and quantitative contents of OH− ions. With the increase in irradiation dose, the maximum IR absorption of all investigated groups of crystals shifts to the region of low energies. The appearance of a new band in the IR spectrum and its shift as a function of dose is associated with the radiolysis of OH− molecule.

Influence of Elevated Temperature on Color Centers in LiF Crystals and Their Photoluminescence

The radiation-induced photoluminescence (PL) of LiF has found its way into many applications for the detection and imaging of ionizing radiation. In this work, the influence of thermal treatment at temperatures up to 400 °C on absorption and PL emission spectra as well as fluorescent nuclear tracks in irradiated LiF crystals was investigated. It was found that carrying out PL measurements with the crystals kept at the temperature of about 80 °C leads to a considerable increase in luminescence emission of F3+ color centers at 525 nm. This enhancement of PL intensity allows for the microscopic imaging of the fluorescent nuclear tracks using only F3+ emission, which is not possible at room temperature. It was also found that heating the irradiated crystals before measurement at temperatures from 100 °C to 200 °C increases the concentration of F3+ centers. However, the related enhancement of PL emission is insufficient in terms of enabling the observation of the fluorescent tracks in this part of the spectrum. In the case of the main PL emission at 670 nm related to F2 centers, the thermal treatment at around 290 °C substantially increases the intensity of fluorescent tracks. This effect, however, was found to occur only at low fluences of alpha particles (up to about 109 cm-2); therefore, it is barely visible in the emission spectrum and not noticeable in the absorption spectrum.