Defects In LiF Research Articles

Electronic structure investigation of intrinsic and extrinsic defects in LiF

A systematic study of geometries, formation energies, and electronic structures has been carried out for LiF considering monodoping with Mg and Ti at the interstitial, and lattice site positions, codoping in equal and different proportions, and vacancy defects in absence and presence of foreign elements using hybrid density functional. This study reveals that, both Mg and Ti, show preference for the Li lattice sites over interstitial sites. Li-poor condition is energetically more preferable over Li-rich condition, and codoping is found to be more stable over individual doping. Under this condition, formation of Li-vacancy in codoped system is found to be spontaneous. The presence of only Li vacancy results negligible changes on LiF electronic structure. Interestingly, in presence of Mg, and Ti, it plays an important role by modifying the band gap. Present results provide important insights to tune optical properties of LiF through the rational design of defect-controlled synthetic conditions.

(Invited) Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling

Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling A molecular scale insight into ion transport and decomposition is important for understanding deficiencies of the currently used aqueous and non-aqueous electrolytes. In this presentation I will summarize progress made towards improving molecular scale understanding of the structure and electrochemistry for a wide range of aqueous and non-aqueous electrolytes. Modeling of non-aqueous electrolytes will focus on the competitive solvent and salt reduction at the passivated electrochemical interfaces using Born Oppenheimer Molecular Dynamics (BOMD) simulations using DFT functionals. These BOMD simulations included critical factors needed to realistically represent electrolyte reactivity at electrodes such as explicit description of the substrate – electrolyte interactions; accurate representation of electrolyte structure, ion pairing and aggregation near an electrode; and collection of sufficient statistics from multiple unique simulations that were initiated with differing initial configurations. For example, 20 BOMD simulations starting from different initial conditions using 2.0M LiPF6 in THF tetrahydrofuran/2-methyl tetrahydrofuran showed no solvent decomposition nor HF formation while only LiF formation was observed as a result of LiPF6 salt decomposition.1 The most frequently observed reduction events included a PF6 − coordinated to Li+ cations from the electrolyte and LiF surface that lead to anion defluorination and formation of 3LiF and PF3 gas. The solvent separated LiPF6 and did not actively participate in reduction. When surface defects in LiF were present near a high population of PF6 − the anions there was a preference for the LiPF6 reduction and repair the SEI without ether solvent decomposition. Interestingly, a number of fast diffusion events for F- from the electrolyte | LiF interface to the LiF-lithium metal interface was observed that would be expected to occur during Li stripping indicating that F- re-arrangement in the thin LiF passivation films should be also considered. 1 Electrolyte reduction at the passivated interfaces from these simulations will be contrasted with other solvents ranging from ethers with mixed salts or carbonates and results from the representative quantum chemistry (QC) calculations performed on the small model electrolyte clusters to estimate oxidation and reduction.In the second part of the presentation the non-reactive molecular dynamics (MD) using APPLE&P polarizable force field to examine bulk and interfacial properties of the lithium and zinc aqueous electrolytes and electrochemical interfaces with the focus on the transport mechanism and relative contribution to charge flux from the anions and cations.2 Combined together information from these modeling scales suggests numerous strategies for stabilizing the electrolyte – electrode interfaces for numerous aggressive high energy density cathodes combined coupled with graphite and metal anodes. References Chen, J.; Li, Q.; Pollard, T. P.; Fan, X.; Borodin, O.; Wang, C., Electrolyte design for Li metal-free Li batteries. Materials Today 2020.Borodin, O.; Self, J.; Persson, K. A.; Wang, C.; Xu, K., Uncharted Waters: Super-Concentrated Electrolytes. Joule 2020, 4 (1), 69-100.

On the role of magnesium in a LiF:Mg,Ti thermoluminescent dosimeter

LiF doped with Mg and Ti is the most widely used thermoluminescent (TL) dosimeter for a large variety of applications. It has been argued that the Mg dopant is the most important defect in the TL process. Besides the common F-centre defects in LiF, optical absorption measurements have suggested the presence of Mg-related absorption bands at 380 nm (3.26 eV), and 310 nm (4.0 eV) when LiF:Mg is exposed to ionizing radiation, whose origin is not yet well understood. This work presents an investigation of the role of defects induced by Mg interstitials in LiF through electronic structure calculations. The calculations show that Mg interstitials induce a local lattice distortion characterized by the displacement of two opposite fluorine atoms, adjacent to the magnesium, away from their original sites by an average distance of 0.6 Å each, while the closest Li atoms are displaced by 0.1 Å. This defect introduces electronic states in the band-gap that can trap excess electrons produced during irradiation, thus enhancing the efficiency of the detector. Holes, on the other hand, are created and trapped in orbitals of mainly Mg-3s character. Additionally, the results suggest that irradiation can simultaneously remove a Li atom nearby a Mg interstitial; substitute a Li by a Mg atom or create a Li vacancy plus a Mg substitutional, giving rise to defects within the LiF gap that are more stable thermodynamically than the Mg interstitial itself. Interestingly, under irradiation the energy levels obtained for LiF:Mg–Lisub + e− (3.486 eV) and LiF:Mg + e− (4.224 eV) defects are very close to the experimental absorption bands.

Temperature and radiation effects at the fluorine K-edge in LiF

The fluorine K-edge of LiF is studied both experimentally and theoretically as a function of temperature. Instantaneous thermal fluctuations in atomic positions are shown in molecular dynamics simulations to increase in amplitude from 0.029 to 0.064nm in the temperature range from 40 to 298K. This is sufficient to cause instantaneous deviations from local octahedral atomic symmetry in this rock-salt crystal, resulting in altered electronic structure. The lowered symmetry of the lowest core-excited states of fluorine atoms is evident in X-ray absorption spectra at the F K-edge. In addition, sufficient radiation exposure produces a new X-ray absorption peak, below the F K-edge of LiF, which is assigned to defects in LiF based on both calculations and comparison to previous experiments.

Accumulation features of radiation-induced hydrogen defects in LiF

Infrared absorption and electron paramagnetic resonance spectra have been investigated for LiF:OH crystals irradiated in a nuclear reactor at low and normal reactor temperatures. The influence of irradiation temperature on accumulation of interstitial hydrogen ions Hi- (U1-centers) was studied. It was observed that these centers were predominantly accumulated under the low-temperature irradiation. The maximum frequency of local vibrations of Hi- ions in LiF depended on the irradiation dose. The shift of vibration frequency of U1-centers to low energy with the increase in the irradiation dose was studied and analyzed. Negative hydrogen ions [H−] in anionic sites (U-centers) were registered in irradiated LiF:OH crystals without their preliminary thermal treatment after the neutron irradiation both at reactor and low temperatures. Accumulation of hydrogen atoms (U2-centers) in LiF for different irradiation fluxes was studied by the thermal treatment. After the irradiation of LiF:OH at 20K the H0 atoms were observed without annealing of the crystal.

Broadband Optical Active Waveguides Written by Femtosecond Laser Pulses in Lithium Fluoride

Broadband waveguiding through light-emitting strips directly written in a blank lithium fluoride crystal with a femtosecond laser is reported. Light guiding was observed at several optical wavelengths, from blue, 458 nm, to near-infrared, at 1550 nm. Visible photoluminescence spectra of the optically active F2 and F3+ color centers produced by the fs laser writing process were measured. The wavelength-dependent refractive index increase was estimated to be in the order of 10−3−10−4 in the visible and near-infrared spectral intervals, which is consistent with the stable formation of point defects in LiF.

The efficiency of formation of primary radiation defects in LiF and MgF2 crystals

Processes of radiation formation of primary defects—F centers and self-trapped excitons—in lithium and magnesium fluorides, which have crystal lattices of different types and similar widths of the band gap and valence band, have been studied in a wide temperature range (11–500 K). It is shown that, along with qualitative similarity of the regularities of formation of the defects under study, LiF and MgF2 crystals are characterized at low temperatures (11–100 K) by different relationships between the energy dissipation channels for self-trapping electronic excitations and the types of self-trapped excitons arising.

Characterisation of defects in LiF implanted with Ar + using variable energy positron beam

Slow positron implantation spectroscopy has been used to determine the surface and near surface defect profile in LiF after implantation with 100 keV argon ions for a range of fluences from 10 13 to 10 16 cm −2. The measured S-parameter is used to characterise the radiation damage as a function of depth. The spatial extent of the defect distributions was estimated by employing the computer program VEPFIT. Maximum lattice damage is shown to occur at incident positron energy of 4.0 keV for low doses and 5.0 keV for the high dose. This energy corresponds to a sample depth of about 200 nm. The positron annihilation spectroscopy results are correlated with optical absorption measurements on the crystals.

A comparative study of defects in LiF implanted with 100 KeV Al+, Mg+ and Ar+ by slow Positron Annihilation Spectroscopy

The fluence-dependent evolution of defects in LiF implanted at room temperature with Al+, Mg+, Ar+ ions is measured using the slow positron beam technique. Samples were implanted with 100 KeV ions to doses of 1015 and 1016 cm−2. Doppler broadened spectra of positron annihilation as a function of positron incident energy are measured and are analyzed by the computer program VEPFIT. The depth profiles of the defect distribution are presented and discussed as a function of dose and nature of the implanted ions. The Positron Annihilation Spectroscopy (PAS) results are correlated with optical absorption measurements on the crystals, and are shown to provide important information about the spatial extent of defects in ion-implanted samples.

Infrared absorption and Raman scattering ofOH−andOD−stretching-mode vibrations in alkali fluorides: Isolated defects

The stretching-mode (SM) of ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ defects in LiF, NaF, and KF hosts has been studied with Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy, at temperatures from 4 K to 500 K. In all three hosts the $〈111〉$ orientation of the ${\mathrm{OH}}^{\ensuremath{-}}$ molecular axis is confirmed both by polarized Raman spectra and by stress-induced FTIR dichroism. The latter technique also yields the values of the disc-shaped elastic dipole tensor, aligning the ${\mathrm{OH}}^{\ensuremath{-}}$ parallel to an applied $〈111〉$ uniaxial stress with a Curie law. Accurate band-shape analysis of low-temperature FTIR absorption and ${T}_{2g}$ Raman spectra yields for low concentrations of ${\mathrm{OH}}^{\ensuremath{-}}$ in LiF and KF consistent values of their nearest-neighbor reorientational tunneling parameters $(\ensuremath{\Delta}=1.2$ and $0.6{\mathrm{cm}}^{\ensuremath{-}1},$ respectively). For ${\mathrm{OH}}^{\ensuremath{-}}$ in NaF and for ${\mathrm{OD}}^{\ensuremath{-}}$ in all three hosts spectral substructure from tunneling is undetectable in the narrow Lorentzian-shaped SM lines. The strong temperature broadening that occurs for the SM absorption and ${T}_{2g}$ Raman bands and the smaller broadening of the ${A}_{1g}$ line can be fitted with models for the dephasing of the transition by phonons, considering the presence of isotropic and anisotropic dephasing components. In spite of the very different amplitudes of the ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ SM vibrations, their relative infrared absorption and Raman intensities are found to be very similar; the only exception is an extremely weak oscillator strength of the ${\mathrm{OD}}^{\ensuremath{-}}$ absorption in KF.

Optical bands of F 2 and [formula omitted] centers in LiF

Optical absorption and emission spectra have been measured in LiF crystals colored with γ-rays. The optical bands of various samples, suitably treated in order to create different relative concentrations of F 2 and F 3 + defects in the crystal lattice, have been studied in detail at 77 K and at room temperature, and resolved into their components. An accurate determination of the spectroscopic parameters of the absorption and emission bands of the aggregated defects in LiF has been achieved, and a critical comparison with previous data has been performed.

Some peculiarities of the EPR spectra of radiation-induced defects in LiF and NaF crystals

To refine the structure of radiation-induced defects, a comparison is made of the EPR spectra of LiF and NaF crystals with various constituent impurities γ-irradiated with different doses, as well as the experimentally measured and calculated intensity distributions of HFS lines in the EPR spectra of these crystals. It is shown that the F-centers, whose model represents an ionized quasimolecule consisting of two metal atoms on each side of a halide vacancy, are responsible for the broad band observed independently of the impurity composition of the crystals and irradiation dose, while the HFS is caused by defects based on interstitial halide atoms.

Characterization of LiF and CaF 2 surfaces using MIES and UPS (HeI)

Metastable impact electron spectroscopy (MIES) and UPS (HeI) in combination with ab initio calculations (CRYSTAL-code) were applied to study surface and bulk defects in LiF and CaF 2. The investigated stoichiometric, defective and doped surfaces are LiF, LiF doped with Mg, and CaF 2. The experimental information obtained on the electronic structure of stoichiometric and defective surfaces of LiF (100), LiF on W (110) and CaF 2 (111) is discussed on the basis of the ab initio calculations. MIES spectra show features from Li agglomerates on the surface of electron bombarded LiF. The electronic structure of the LiF:Mg single crystal shows additional features above the valence band maximum caused by the Mg doping. These are identified as caused by the formation of (Mg O) surface bonds. Exposure of CaF 2 surfaces to X-rays (1486 eV) shows also formation of metallic regions on the surface.

Spatial distribution of defects in LiF induced by low-energy electron bombardment: Evidence for rapid hot-hole diffusion.

We report measurements of concentrations and spatial distributions of defects, defect clusters, and surface metal arising from low-energy electron bombardment of LiF crystals. The method employed to monitor defect concentration as a function of depth uses optical transmission spectroscopy in conjunction with depth-profile techniques. Investigations at -90 \ifmmode^\circ\else\textdegree\fi{}C indicate that the observed spatial distribution of defects can be explained best by long-range diffusion of hot holes.

Luminescence and defect creation in LiF and LiF-Na

The topic is the radiative and nonradiative decay of electronic excitations with the creation of radiation defects in LiF. A luminescence band with a peak at 3.45 eV is observed in the x-ray luminescence spectrum at 4.2 K; it may be assigned to the luminescence of a self-localized exciton. In LiF-Na, additional luminescence bands with peaks at 2.0 and 2.4 eV are observed; these may be attributed to the luminescence of electron excitations localized around the Na' ion. The formation of Frenkel defects at 4.2 K in LiF and LiF-Na is discussed.

An ab-initio Hartree-Fock perturbed-cluster study of neutral defects in LiF

An ab initio Hartree-Fock perturbed-cluster embedding scheme is applied to the problem of neutral defects in bulk LiF (specifically to Na substitutionals and the bound Schottky pair), using an extended basis set. The authors highlight the capabilities and drawbacks of the method, in particular regarding the convergence of the procedure and the description of polarization and relaxation effects. They also compare the results with simulations based on the Mott-Littleton theory. The results indicate that even in the case of neutral defects, there are important short-range polarization effects.

Ballistic-phonon propagation in LiF

The interaction between high-frequency ballistic phonons and mass defects in LiF of natural isotopic composition has been studied with the use of a heat-pulse technique. Studies were made as a function of input power for samples of various length oriented along the [111] direction. The results indicate that surface imperfections have a profound effect on the transmission of high-frequency phonons. The experiments on suitably prepared surfaces yield a phonon scattering rate which is in excellent agreement with theoretical predictions and thermal conductivity measurements.

Ti related defects in lif:ti and lif:Mg, ti crystals

Abstract The interaction of Ti, Mg, OH− and O2- ions in LiF have been studied. Ti usually enters the lattice as Ti4+, but in samples grown in inert atmosphere from a vitreous carbon tube another charge state (most probably Ti3+) has also been found. In “as-grown” LiF (inert) crystals Ti3+ ions appear to be in aggregated form giving rise to a broad absorption band at ∼660 nm. Quenching the crystal the broad band decreases simultaneously with the appearance of an ESR spectrum indicating the presence of some paramagnetic Ti3+ containing species. IR bands at 3662 and 3525 cm−1 in air grown LiF:Ti and LiF:Mg, Ti, respectively were attributed to Ti-OH complexes. Ionic conductivity measurements indicate that the extra charges of Ti ions in all investigated samples are compensated to some extent by cation vacancies. The evident differences in optical absorption and ESR results suggest that the coordinations of the Ti ions in LiF:Ti are different from those in double-doped crystals.

IMPURITIES AND THERMOLUMINESCENCETitanium-related defects in Lif : Mg, Ti

IMPURITIES AND THERMOLUMINESCENCETitanium-related defects in Lif : Mg, Ti

Positron Polarons and Positron Trapping by Defects in LiF

The momentum distribution of annihilation photon pairs in LiF observed at 293 K was found to be slightly broader than that at 89 K. Assuming that the positrons are thermalized and annihilate mainly in the polaron states, the broadening was analyzed in terms of their effective mass. The positron polarons are very likely to have a mass considerably larger than those of the electron polarons. This result is consistent with the consequence of a band-mass calculation made in the present work. Irradiation effects due to the positron source on the angular correlation, i.e. a gradual increase in the peak height at 293 K and a decrease at 89 K, were also detected and attributed to the irradiation-induced changes in the density of positron traps. Holes play an important role in the decrease at 89 K.