Optical properties of the surface transition-metal structures in fluoride ionic crystals and peculiarities of their formation during thermal diffusion

The possibility of forming surface films with an elevated concentration of an impurity metal during high-temperature diffusion has been analyzed for a wide series of ionic crystals: LiF with Co, Ni, Mg, Ca, Ba, and Sr impurities; NaF with Co, Mn, Mg, Ca, and Sr; MgF₂ with Co and Ni; and CaF₂ with Co. It is established that films are formed only on alkali halide crystals with impurities of transition metals and are not formed on alkaline earth fluorides with transition metals, as well as on alkali halide crystals activated with other divalent cationic impurities. The dynamics of the increase and decrease in the intensity of centers related to impurity-vacancy dipoles during thermal diffusion is shown. The mechanisms of film formation are explained in terms of the features of growth and structure of ionic crystals with cationic impurities and on the basis of isomorphism rules.     

First-principles calculations of the electronic structure of fluorite-type crystals (CaF2, BaF2, SrF2, and PbF2) containing Frenkel defects. Analysis of optical and transport properties

The electronic structure of the alkali-earth fluorides CaF₂, BaF₂, SrF₂, and PbF₂ with Frenkel defects is investigated in the tight-binding approximation by the LMTO method. The defect formation and migration energies are calculated. The electronic structure and optical excitations of a H center in a defective fluorite structure are examined. It is shown on the basis of calculations of the binding energies that CaF₂, BaF₂, and SrF₂ are ionic compounds, while the chemical bond in PbF₂ is partially covalent. Possible methods of displacement of interstitial fluorine atoms that lead to the observed optical spectra of an H center are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 2019–2025 (November 1998)     

Localised vibrations due to substitutional impurities in calcium fluoride

The localized modes in CaF₂ due to the introduction of substitutional impurities have been investigated using the greens functions method. The weakening of the nearest neighbour interaction in all the alkaline earth fluorides, is estimated from the infrared absorption spectra on localized modes in these crystals. The splitting of the localized modes in CaF₂, due to the presence of additional Sr²⁺ or Ba²⁺ impurities is estimated and satisfactory agreement exists between theory and experiment, within the limits of the simplifications in our calculations.     

Local structure of divalent silver complexes in fluorite crystals as seen by EPR method

The analytical expressions for spin-Hamiltonian parameters of Jahn-Teller paramagnetic centres [AgF₂F₆]^6? in fluorite crystals are deduced. Comparison with the experimental EPR data for CdF₂, CaF₂ and SrF₂ yields information about the local structure of centres [AgF₂F₆]^6?. It is found that two Ag²⁺-F^? bonds of the centre (along the 〈111〉 direction) are shorter than the rest six bonds by factor 1.1. Euler angles of six non-axial fluorines are almost the same as those in the undistorted fluorite structure.     

Laser-induced backside wet etching of fluoride and sapphire using picosecond laser pulses

Laser-induced backside wet etching (LIBWE) is a promising process for microstructuring of rigid chemical resistant and inert transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated in a number of studies. LIBWE in a time scale of femtosecond and picosecond pulse durations has been investigated only in a few studies and just on fused silica. In the present study LIBWE of fluorides (CaF₂, MgF₂) and sapphire with a mode-locked picosecond (t _p=10 ps) laser at a UV wavelength of λ=355 nm using toluene as absorbing liquid has been demonstrated. The influence of the laser fluence and the pulse number on the etching rate and the achieved surface morphology was investigated. The etching rate grows linearly with the laser fluence in the low and high-fluence ranges with different slopes. The achieved etching rates for CaF₂ and for sapphire were in the same range. Contrary to CaF₂ and sapphire the etching rates of MgF₂ were one magnitude less. For backside etching on sapphire at high fluences smooth surfaces and at low fluences ripples pattern were found, whereas fluoride surfaces showed a trend towards crack formation.     

A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF₂, SrF₂, and BaF₂ with Er³⁺ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er³⁺ ions proceeds differently for CaF₂, SrF₂, and BaF₂ host matrices. In the case of CaF₂, impurity centers are localized in a very thin near-surface layer of CaF₂ particles, in SrF₂, the impurity is distributed over the volume of particles, while, in BaF₂, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF₂, SrF₂, and BaF₂ considerably differ from each other.     

Fundamental absorption of calcium,magnesium, and lithium fluorides in the range of 8–12 μm

The fundamental absorption of calcium, magnesium, and lithium fluorides in the range of 8–12 μm has been studied experimentally by the spectrophotometric method. The known and experimentally obtained values of the absorption coefficients of these materials have been analyzed. It has been shown that the values experimentally measured in the range of 8–12 μm obey the Uhrbach rule and the multiphoton absorption determines the long-wavelength transmission cutoff of pure optical materials. The results presented make it possible to suggest that plates made of LiF and CaF₂ crystals and of optical ceramics based on MgF₂ can be used to fabricate calibrated radiation attenuators in the spectral range of 8–12 μm.     

UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass

The effect on transmittance and reflectance in the vacuum ultraviolet (VUV) region by doping alkali fluorides (LiF, NaF and KF) has been investigated in a ternary fluoroaluminate (18BaF₂-37CaF₂-45AlF₃) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali fluoride. The VUV reflection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10-20 times larger than that expected from the additive law on the magnitude of absorption coefficients of the glass and alkali fluorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the reflection peak upon doping supports that the excitonic state lies predominantly around the edge energies.     

Nonlinear processes in UV optical materials at 248 nm

Nonlinear processes in UV optical materials were investigated by using 280-fs, 248-nm pulses. Nonlinear absorption in CaF₂ was confirmed to be a two-photon process by using the luminescence of self-trapped excitons, which was also used for the single shot pulse width measurement. The absorption bands due to F centers were identified in CaF₂, MgF₂, and LiF after several hundred shots at 100 GW/cm². Absorption at 248 nm was considerable especially in MgF₂ and LiF. Self-focusing and self-phase modulation were observed in CaF₂.     

The role of defects in laser surface damage thresholds of fluoride crystals

The energy of the acoustic pulse generated by laser-surface interactions and measured by probe beam deflection was used to investigate laser surface damage thresholds of fluoride crystals with optical quality. It was found that damage thresholds decrease with increasing density of surface states. The defect density also controls the energy absorption mechanism: for surfaces with few defects, like polished MgF₂ and CaF₂, avalanche breakdown occurs at above 1 GW/cm₂, whereas for materials with lower damage thresholds, such as LiF, BaF₂, and roughened or incubated surfaces of CaF₂, multiphoton absorption across the band gap is observed.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Ion diffusion-controlled thermally stimulated processes in x-ray irradiated halide crystals

The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF₂, BaF₂, LiBaF₃ and KBr crystals were investigated above 290 K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF₂, BaF₂, LiBaF₃ and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal show that above 290 K the TSR processes are initiated and controlled by the ionic defect thermal detrapping, migration and interaction with the localised electronic and ionic charges and colour centres. The ion diffusion-controlled TSR processes take place in the above halide crystals.     

Contribution of the multipole polarization to the elastic constants in fluorite structure crystals

Numerical calculations are presented for the contribution of the multipole polarization to the elastic constants in fluorite structure crystals. The multipole polarizability is calculated by the self-consistent field treatment of the local density approximation and the spherical solid model. The elastic constants are significantly affected by the multipole polarization of the ions. The contributions of the multipole polarization of the ions to the elastic constant C₁₁, C₁₂, and C₄₄ are about 25%, 40% and 20% of the experimental values, respectively. The calculated values of the deviation from the Cauchy relation are in good agreement with the experimental values.     

Electrical conductivity of molten mixtures of potassium and sodium fluorides with calcium fluoride

The electrical conductivity of molten mixtures of calcium fluoride with sodium and potassium fluorides was investigated by impedance spectroscopy. The calcium fluoride additions to alkali metal fluoride melts decrease their electrical conductivity. Small additions of potassium and sodium fluorides to CaF₂ do not change its conductivity essentially.     

Equation of state and temperature variation of the bulk modulus of the alkaline earth fluorides

The isotherms for the alkaline earth fluorides (CaF₂, SrF₂ and BaF₂) have been computed using the expression for the total free energy of a crystal in the quasiharmonic approximation. The theoretical points for SrF₂ and BaF₂ have been compared with the points derived from Bridgman’s experimental relation. The temperature variation of the isothermal bulk modulus of the alkaline earth fluorides has been worked out on the basis of Axe’s shell model. The theoretica points are compared with the points obtained from the temperature variation of the elastic constants data. It is found that the vibrational contribution to the temperature variation of the elastic constants in these crystals is significant.     

Crystal fields of hexameric rare-earth clusters in fluorites

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er³⁺, Tm³⁺, Yb³⁺) in clusters of diamagnetic ions (Lu³⁺, Y³⁺) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g_∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF₂ crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er³⁺, Tm³⁺, and Yb³⁺ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.     

碱土氟化物中离子间相互作用势经验参数的确定

描述离子间非Coulomb短程互作用势的函数形式确定后，即可据此讨论离子晶体的一系列物理性质，势函数中出现的参数通过与实验数据的拟合而确定．基于电子壳模型对CaF₂，SrF₂，BaF₂离子晶体的讨论中，强调电子壳电量应一律取负值的基本前提，以经验参数化方法拟合势参数时注意引用近期实验数据．将F离子电量调整为0.97e之后，重新获得的模型参数集与上述碱土氟化物晶格形成能、弹性常数、长光学横波频率、Raman频率，以及极化率实验数据，有较满意的对应关系．     

Observations on a high temperature peak in the thermoluminescence of fluorites

A new peak at about 650°C has been observed in the thermoluminescence emission of gamma irradiated natural fluorites containing significant quantities of lanthanide rare-earth (RE) elements as impurities. Thermal activation energy of the corresponding trap has been evaluated to be 2·99 eV with a frequency factor of 10¹⁶ sec⁻¹. The optical activation energy, as deduced from photo-transfer-induced-TL from this trap, is 5·36 eV. Many types of natural fluorites as well as synthetically grown CaF₂ crystals doped with single and pairs of rare earth elements have been studied and the results indicate that the high temperature peak is associated with Sm impurity coexisting with Y, La or Ce in CaF₂. There are indications that this newly observed TL peak can be gainfully employed in ultraviolet dosimetry and geological dating of fluorite deposits.     

Molecular dynamic study of the melting temperature in MgF2 with the fluorite structure

The melting temperature of MgF₂ with cubic fluorite structure has been calculated by the constant temperature and pressure molecular dynamics (MD) simulation using the well-tested effective pair-wise potentials, which consist of the Coulomb, dispersion, and repulsion interaction by varying temperature from 300 to 2500 K. It is found that the potential parameters for MgF₂ derived from ab initio periodic Hartree-Fock calculations are very successful in reproducing accurately the DFT-GGA combined with quasi-harmonic Debye model calculated volumes of the cubic fluorite-type MgF₂ over a wide range of temperature at room pressure. Our simulated melting temperature of cubic fluorite-type MgF₂ is very close to the actual melting temperature 1539 K. Meanwhile, the radial distribution functions of Mg-Mg, F-F, and Mg-F ion pairs near the melting temperature are investigated from the isobaric and isothermal ensemble.     

Atomic structure of CaF2/MnF2-Si(1 1 1) superlattices from X-ray diffraction

X-ray reflectivity and non-specular crystal truncation rod scans have been used to determine the three-dimensional atomic structure of the buried CaF₂-Si(1 1 1) interface and ultrathin films of MnF₂ and CaF₂ within a superlattice. We show that ultrathin films of MnF₂, below a critical thickness of approximately four monolayers, are crystalline, pseudomorphic, and adopt the fluorite structure of CaF₂. High temperature deposition of the CaF₂ buffer layer produces a fully reacted, CaF₂-Si(1 1 1) type-B interface. The mature, “long” interface is shown to consist of a partially occupied layer of CaF bonded to the Si substrate, followed by a distorted CaF layer. Our atomistic, semi-kinematical scattering method extends the slab reflectivity method by providing in-plane structural information.