Alkali-earth Fluorides Research Articles

Charge Transfer and F-Type Centers in Alkaline-Earth Fluorides Activated by Cd2+ or Zn2+ Ions

Photoluminescence spectra of impurity centers containing anion vacancy Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ in alkali-earth fluorides have been studied. A comparison with luminescence spectra of photochromic centers (PCs), which can be considered as perturbed F centers, shows that luminescence of centers Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ is similar to that of F centers; therefore, it is caused by charge transfer from an impurity ion to an anion vacancy.

Ca2+/Sr2+/Ba2+ dependent phase separation, nanocrystallization and photoluminescence in fluoroaluminosilicate glass

AbstractFluoride phase separation is the initial stage of nanocrystallization in oxyfluoride glasses, and it is a key step in achieving transparent glass‐ceramics with good luminescence. In this work, we combine molecular dynamics (MD) simulations and experimental studies to investigate the phase separation, nanocrystallization and photoluminescence in fluoroaluminosilicate glass and glass‐ceramics containing alkali earth fluoride (MF2). The results reveal different phase separation behaviors due to the field strength difference of M2+. The composition and size similarity between the fluoride‐rich regions in the MD simulated glass and the fluoride nanocrystals in the experimental prepared glass‐ceramics are observed, suggesting that the separated fluoride phase is the structural origin of the observed MF2 nanocrystals. Besides, in order to understand the M2+ dependent glass structural features, the crystallization temperatures, the luminescent properties of Eu3+ and Eu2+ doped glass‐ceramics, and the lasing performance of Er3+ doped glass‐ceramics are discussed. Based on these comprehensions, some strategies are proposed to help to efficiently design oxyfluoride glass with desired luminescence performance.

Broadening of the lines due to the 4f n -4f n − 15d transitions of Ce3+, Pr3+, and Tb3+ ions in the absorption spectra of CdF2 crystals

The structure of the absorption spectra of Ce3+, Pr3+, and Tb3+ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce3+, Pr3+, and Tb3+ ions in CdF2 can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm−1. The broadening is related to the autoionization of electron from the local 5d(eg) level to the energy-degenerate states of the conduction band of CdF2 crystal.

Communication: Growing room temperature ice with graphene

Water becomes ordered in the form of hexagonal ice at room temperature under controlled humidity conditions upon confinement in the nanometer range between protective graphene sheets and crystalline (111) surfaces with hexagonal symmetry of the alkali earth fluoride BaF2. Interfacial water/substrate pseudoepitaxy turns out to be a critical parameter since ice is only formed when the lattice mismatch is small, an observation based on the absence of ice on (111) surfaces of isostructural CaF2.

Time-resolved spectroscopy of 5d-4f transitions in Pr3+ doped alkali-earth fluorides

We measured time-resolved spectra and emission decay times under pulsed X-ray and synchrotron excitation in alkali-earth fluorides doped with Pr3+ ions. Two fast decay components were found in the emission spectra of BaF2-Pr3+ and SrF2-Pr3+ . These were 4 ns and 21 ns in BaF2-Pr3+ and 8 and 24 ns in SrF2-Pr3+. The intensity of the faster components 4 ns and 8 ns depended on concentration of Pr3+. Thus, the presence of aggregates might be the cause of such faster components.

Formation of color centers during growth of alkali-earth fluoride crystals

New bands within the ranges of 1900–2200 and 700–1000 cm–1 have been discovered in the IR absorption spectra of crystalline alkali-earth fluorides. It is found that the defects responsible for the high frequency bands facilitate the formation of color centers during crystal growth. These bands are presumably caused by valence vibrations of CO molecules. The low frequency bands are attributed to molecular complexes which contain oxygen-vacancy dipoles.

MOCVD of Epitaxial Alkali-Earth Fluorides Thin Films

Thin films of alkali-earth fluorides and multilayer structures including such layers are known as perspective materials for optics, photonics and chemical sensors. They can also be considered as a material for buffer layers for 2G HTSC wires. Here we report on growing epitaxial CaF2 and SrF2 films on different substrates by MOCVD. The films' properties were examined using RBS, SEM, EDX, XRD, EBSD and AFM. The obtained films revealed perfect in- and out-of-plane orientation and had a smooth, crack-free surface. Depending on the substrate and deposition conditions, some films required the recrystallization annealing to form the epitaxial layer or that of good texture. Additionally, a number of multilayer films including MF2 layers were grown. The obtained films were successfully used as substrates for HTS deposition.

Structure and diffusion of intrinsic defects, adsorbed hydrogen, and water molecules at the surface of alkali-earth fluorides calculated using density functional theory

Using periodic density functional theory, we calculate the structure and migration energies of fluorine vacancies and interstitials in the bulk and at the stoichiometric bulk-truncated surface of three alkali-earth fluorides: ${\text{CaF}}_{2}$, ${\text{SrF}}_{2}$, and ${\text{BaF}}_{2}$. We then study the adsorption of water and hydrogen, in both molecular and dissociated form, at the ideal surface, and at neutral and charged vacancies in the surface and subsurface layers. The results demonstrate that in nearly all cases molecular adsorption is strongly favored. For the most probable configurations on the surfaces, we also studied the migration paths and barriers, and found that water is highly mobile on the surface, even when adsorbed at defects. In general, ${\text{CaF}}_{2}$ and ${\text{SrF}}_{2}$ show similar behavior with respect to water, while adsorption energies and migration barriers for ${\text{BaF}}_{2}$ are smaller. Finally, we discuss our results in the context of recent experimental Atomic Force Microscopy studies on ${\text{CaF}}_{2}$ and compare to calculations on other insulating surfaces.

Solid-phase epitaxy of alkali-earth fluorides with fluorite structure

Solid-phase epitaxy of alkali-earth fluorides with fluorite structure

Thermodynamic analysis of the system Na2O-K2O-CaO-Al2O3-SiO2-H2O-F2O−1: Stability of fluorine-bearing minerals in felsic igneous suites

Thermodynamic analysis of the system Na2O-K2O-CaO-Al2O3-SiO2-H2O-F2O−1 provides phase equilibria and solidus compatibilities of rock-forming silicates and fluorides in evolved granitic systems and associated hydrothermal processes. The interaction of fluorine with aluminosilicate melts and solids corresponds to progressive fluorination of their constituent oxides by the thermodynamic component F2O−1. The chemical potential μ(F2O−1) buffered by reaction of the type: MOn/2 (s)+n/2 [F2O−1]=MFn (s, g) where M=K, Na, Ca, Al, Si, explains the sequential formation of fluorides: carobbiite, villiaumite, fluorite, AlF3, SiF4 as well as the common coexistence of alkali- and alkali-earth fluorides with rock-forming aluminosilicates. Formation of fluorine-bearing minerals first starts in peralkaline silica-undersaturated, proceeds in peraluminous silica-oversaturated compositions and causes progressive destabilization of nepheline, albite and quartz, in favour of villiaumite, cryolite, topaz, chiolite. Additionally, it implies the increase of buffered fluorine solubilities in silicate melts or aqueous fluids from peralkaline silica-undersaturated to peraluminous silica-oversaturated environments. Subsolidus equilibria reveal several incompatibilities: (i) topaz is unstable with nepheline or villiaumite; (ii) chiolite is not compatible with albite because it only occurs only at very high F2O−1 levels. The stability of topaz, fluorite, cryolite and villiaumite in natural felsic systems is related to their peralkalinity (peraluminosity), calcia and silica activity, and linked by corresponding chemical potentials to rock-forming mineral buffers. Villiaumite is stable in strongly peralkaline and Ca-poor compositions (An 5) saturate with topaz or fluorite. Crystallization of topaz is restricted to peraluminous conditions, consistent with the presence of Li-micas or anhydrous aluminosilicates (cordierite, garnet, andalusite). Fluorite is predicted to be stable in peraluminous biotite granites, amphibole-, clinopyroxene- or titanite-bearing calc-alkaline suites as well as in peralkaline granitic and syenitic rocks. Fluorine concentrations in felsic melts buffered by the coexistence of F-bearing minerals and feldspars increase from peralkaline through metaluminous to mildly peraluminous compositions. At low-temperature conditions, the hydrothermal evolution of peraluminous granitic and greisen systems is controlled by white mica-feldspar-fluoride equilibria. With decreasing temperature, topaz gradually breaks down via: (i) (OH)F−1 substitution and fluorine transfer to fluorite by decalcification of plagioclase below 600 °C, (ii) formation of muscovite and additional fluorite at 475–315 °C, and (iii) formation of paragonite and cryolite, consuming F-rich topaz and albite below 315 °C. These equilibria explain the absence of magmatic fluorite in Ca-bearing topaz granitic rocks; its abundance in hydrothermal rocks is due to: (i) closed-system defluorination of topaz, (ii) open-system decalcification of plagioclase or (iii) hydrolytic alteration. These results provide a complete framework for the investigation of fluorine-bearing mineral stabilities in felsic igneous suites.

Photoelectron spectroscopy studies on the electronic structures of Al/RbF and Al/CaF2 cathodes for 8-hydroxyquinoline aluminium-based organic light-emitting devices

The electronic structures of Al/RbF/8-hydroxyquinoline aluminium (Alq3) and Al/CaF2/Alq3 interfaces were investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed that a significant valence band shift occurred by evaporating the thin fluoride layer on Alq3. However, the gap state formation and the evolution of N 1s core level spectra showed rather different trends, suggesting that the alkali fluoride and alkali–earth fluoride interlayer have different reaction mechanisms at the interface with the Al cathode. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF2. These results suggest that the complex charge transfer mechanism across the interface has considerably less effect on the enhancement of organic light-emitting device performance than the significant valence band shift as a result of deposition of interlayer materials.

Slow components of the emission decay in fluoride crystals doped with Ce3+

Spatially separated defects created by photons with energies 6–8 eV in alkali-earth fluoride crystals doped with cerium are investigated with the help of thermoluminescence. Measuring the spectra of creation of Vk and H peaks of thermostimulated luminescence inBaF2:Ce3+. we demonstrate that photons with energies higher than 6eV induce H centers (self-trapped holes captured by interstitialF ions), whereas the formation of self-trapped holes begins on exposure to photons with energies greater than 7 eV. The influence of photoionization on theCe3+ luminescence inBaF2, SrF2, CaF2, andCeF3 crystals is investigated in the range of photon energies 4–8 eV. An exponentialCe3+-emisson decay was observed for excitation energy lying in the range 4–6 eV. Slow and fast decay components were observed under excitation by photons with energies higher than 6 eV. We believe that the slow and fast components are due to the tunnel recombination of trapped electrons with hole centers.

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.

Band theory of linear and nonlinear susceptibilities of some binary ionic insulators.

The linear and nonlinear optical responses in a large number of cubic insulators are studied by means of first-principles local-density calculations. Complete results on band structures, frequency-dependent dielectric functions, and frequency-dependent third-order nonlinear susceptibilities ${\mathrm{\ensuremath{\chi}}}^{(3)}$(\ensuremath{\omega}) (in the simplest form as the third-harmonic generations) are presented for 27 alkali halides, alkali-earth fluorides, oxides, and sulfides. They are LiF, LiCl, LiBr, LiI, NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, RbF, RbCl, RbBr, RbI, ${\mathrm{CaF}}_{2}$, ${\mathrm{SrF}}_{2}$, ${\mathrm{CdF}}_{2}$, ${\mathrm{BaF}}_{2}$, MgO, CaO, SrO, BaO, MgS, CaS, and SrS. The results are compared with the existing experimental data and other calculations. The effectiveness of using a ``scissor operator'' to correct the gap underestimation in the local-density-approximation theory is assessed. It is shown that the full band-structure approach for the ${\mathrm{\ensuremath{\chi}}}^{(3)}$(0) calculation in these crystals gives results in very good agreement with experimental data, especially in the anisotropic coefficient of the nonvanishing tensor elements.

Correlations entre defauts ponctuels et proprietes de transport dans PbF 2 dope

The investigation of the transport properties of the Pb 1− x M 2+ n x F 2+ nx ( n = 1,2) solid solutions shows a maximum of the activation energy Δ E max associated to a minimum of the conductivity for a very small value of x : x min. ( x min. ≅ 0.01 for n = 1; x ,in ≅ 0.005 for n = 2). The value of Δ E max. depends closely on the nature of the substituting cation. Correlations have been established between the transport properties and the nature of the point defects which are supposed to occur in the considered composition range ( nn or innn pairs) by analogy with similar alkali-earth fluorides. The size difference Δ r between host and substitutional cations seems to play an essential role. The evolution of Δ E max. and the preferential stability of the nn or nnn pairs is discussed as a function of Δ r.

Light Induced Electron Transfer between La and Ce Ions in Alkali Earth Fluorides

Light Induced Electron Transfer between La and Ce Ions in Alkali Earth Fluorides

High-Resolution Measurements of Absorption, Fluorescence, and Crystal-Field Splittings of Solutions of Divalent, Trivalent, and Tetravalent Uranium Ions in Fluoride Crystals

Energy levels of dilute solutions of valence-pure divalent, trivalent, and tetravalent uranium in fluoride crystals are derived from absorption and fluorescence measurements. Fluorescence is observed which results from transitions between split ground-state and next higher split levels of trivalent uranium and divalent uranium in alkali-earth fluoride lattices. A new explanation is proposed for the fluorescence characteristics of trivalent uranium laser crystals, based upon the competition of energy absorptions of impurity valence states with the fluorescent emission of trivalent uranium ions.