Crystalline Fluoride Research Articles

Ba4Ni3F14·H2O: a ferrimagnetic compound with a staircase kagomé lattice.

A new crystalline fluoride Ba4Ni3F14·H2O was found to exhibit a rare S = 1 staircase kagomé lattice built by Ni2+ ions. Magnetic measurements indicate a ferrimagnetic transition at ∼28 K, while 1/3 plateau can be observed from the magnetization curve. We suggest that the hydrogen-bond interactions of O-H⋯F pathways may play an important role for magnetic properties of the system.

USING PERCOLATIVE CRYSTALLINE 0.3 CUO/PVDF NANOCOMPOSITE GATE DIELECTRIC FOR FABRICATING HIGH-EFFECT MOBILITY THIN FILM TRANSISTOR OPERATING AT LOW VOLTAGE

Thin-film transistor (TFT) with various layers of crystalline Poly-vinylidene fluoride (PVDF)/CuO percolative nanocomposites based on Anthracene as a gate dielectric insulator have been fabricated. A device with excellent electrical characteristics at low operating voltages (<1V) has been designed. Different layers (L) of the film were also prepared to achieve the best optimization of ideal gate insulator with various static dielectric constants (εr). Capacitance density, leakage current at 1V gate voltage and electrical characteristics of OFETs with a single and multi layer films have been investigated. This device was showed highest field effect mobility of 2.27 cm2/Vs, a threshold voltage of -1.6V, an exceptionally low sub threshold slope of 380 mV/decade and an on/off ratio of 106. Such a High-ε three layered (3L) PVDF/CuO gate dielectric appears to be highly promising candidates for organic non-volatile memory, sensor and field-effect transistors (FETs).

Glasses and glass-ceramics in InF3–BaF2–BiF3–BiPO4 systems

New glasses have been synthesized in the InF3–BaF2–BiF3–BiPO4 system. The introduction of the phosphate component into fluoroindate glass increases the resistance of the glass to crystallization, but the presence of phosphate polyhedra in the glass structure, even in small amounts, reduces the transmission region. It is shown that glasses in the InF3–BaF2–BiF3–BiPO4 system can be a convenient base for obtaining glass-crystalline materials consisting of an oxide glass matrix with crystalline fluoride phases. Luminescence was detected in the near IR region (1182 nm) in the glasses under discussion.

Optical properties of some crystalline fluorides in the terahertz region of the spectrum

Abstract The transmission in the terahertz spectral range of crystalline fluorides of Lithium (LiF), Calcium (CaF2), Barium (BaF2) and Magnesium (MgF2), which are widely used in ultraviolet and infrared spectral optics, was studied. The spectral dependences of the absorption coefficient of these materials were obtained, in the region of 200–3000 μm a transparency zone was registered. This is of interest for the possible use of fluorides in THz devices in the millimeter range.

Single-step approach to sensitized luminescence through bulk-embedded organics in crystalline fluorides

Luminescent materials enable warm white LEDs, molecular tagging, enhanced optoelectronics and can improve energy harvesting. With the recent development of multi-step processes like down- and upconversion and the difficulty in sensitizing these, it is clear that optimizing all properties simultaneously is not possible within a single material class. In this work, we have utilized the layer-by-layer approach of atomic layer deposition to combine broad absorption from an aromatic molecule with the high emission yields of crystalline multi-layer lanthanide fluorides in a single-step nanocomposite process. This approach results in complete energy transfer from the organic molecule while providing inorganic fluoride-like lanthanide luminescence. Sm3+ is easily quenched by organic sensitizers, but in our case we obtain strong fluoride-like Sm3+ emission sensitized by strong UV absorption of terephthalic acid. This design allows combinations of otherwise incompatible species, both with respect to normally incompatible synthesis requirements and in controlling energy transfer and quenching routes.

Phase Separation and Nanocrystallization in KF–ZnF2–SiO2 Glasses: Lessons from Solid-State NMR

In the present paper, we studied the structural evolution of the glass matrix and formation of crystalline fluoride phases for 15KF-15ZnF2-70SiO2 (glass A) and 25KF-25ZnF2-50SiO2 (glass B) glasses upon thermal treatment employing transmission electron microscopy, X-ray diffraction (XRD), and solid-state NMR. The first step marks a phase separation into an SiO2 rich phase and a phase enriched with Zn, K, and fluoride. As shown by XRD, depending on the nominal compositions, K2SiF6 (for 15KF-15ZnF2-70SiO2) and KZnF3 (for 25KF-25ZnF2-50SiO2) are found as the first crystalline phases being formed. Upon longer heat treatment, ZnF2 is additionally formed in both cases. Surprisingly, a significant amount of SiF62- units is detected employing 29Si-MAS and 29Si{19F}CPMAS-NMR spectroscopy even in the base glass, which is completely amorphous according to the X-ray results. The vast majority of Si, however, is found in an exclusive SiO4 environment as Q4, Q3, or Q2 units. The large fraction of nonbridging oxygens per SiO4 tetrahedron (ca. 0.7 for glass A and 0.65 for glass B) indicates quite large fluorine loss during glass synthesis (approx. 60-80%). Employing dipolar NMR techniques, i.e., 29Si-{19F}-REDOR (rotational echo double resonance) and 29Si{19F}CPMAS-NMR (cross polarization magic angle spinning), the presence of Si-F connectivity (in excess of the identified SiF62- units) could be ruled out. The observed differences in the 29Si-19F heteronuclear dipole couplings between the Si-Q4 and Si Q3 units-as determined by REDOR NMR-are compatible with the assumption of a core-shell structure with a mixed cation fluoride as the core and an SiO2-enriched shell.

Effect of Pre-Irradiation Annealing and Laser Modification on the Formation of Radiation-Induced Surface Color Centers in Lithium Fluoride

It is shown that surface color centers of the same type are formed in the surface layer and in regions with damaged crystal structure inside crystalline lithium fluoride after γ-irradiation. Results are presented from a study of the effect of pre-irradiation annealing on the efficiency with which surface centers are formed in lithium fluoride nanocrystals. Raising the temperature for pre-irradiation annealing from room temperature to 250°C leads to a substantial reduction in the efficiency with which these centers are created. Surface color centers are not detected after γ-irradiation for pre-irradiation annealing temperatures of 300°C and above. Adsorption of atmospheric gases on the crystal surface cannot be regarded as a necessary condition for the formation of radiation-induced surface centers.

Properties of fluoride microresonators for mid-IR applications.

We study crystalline fluoride microresonators for mid-infrared (mid-IR) applications. Whispering gallery mode resonators were fabricated with BaF2, CaF2, and MgF2 crystals. The quality factors were measured at wavelengths of 1.56 and 4.58μm. The impacts of post-fabrication processing, impurities, and surface water are highlighted. The mid-IR optical losses due to multiphoton absorption are measured. It is found that MgF2 resonators have a room temperature Q-factor of 8.3×106 at a wavelength of 4.58μm, limited by multiphoton absorption.

High yield synthesis and optical properties of MgF2nanowires with high aspect ratios

Ultrapure highly crystalline magnesium fluoride (MgF2) nanowires with high aspect ratios were successfully synthesized by utilizing ammonium boron trifluoride (NH3BF3) as a precursor material for the first time.

Evolution of microstructure of lithium fluoride crystals upon their vibrodispersion in ultraweak fields of thermalized neutrons

BET adsorption, electron-microscopy, and X-ray diffraction analysis have been employed to study the vibrodispergating kinetics of crystalline lithium fluoride particles under conditions of simultaneous irradiation of the material with a flux of thermalized neutrons with low intensity I = 100 neutron/(cm2 s). It has been found that, during vibratory treatment of the powder, the stationary state of the material microstructure is reached much earlier than the limiting (equilibrium) value of its external (specific) surface area is established. The reaction rate constants have been determined for the polygonization and recrystallization, which take place in the material during its vibratory grinding. The influence of the neutron irradiation on the numerical values of these constants has been revealed. The energy parameters of the processes occurring in the system have been analyzed taking into account the heat release caused by neutron absorption in the material.

Green, orange, and red Pr(3+)∶YLiF(4) epitaxial waveguide lasers.

Efficient guided laser oscillation in the visible spectral region is demonstrated with praseodymium-doped YLiF(4) layers grown by liquid phase epitaxy. By exciting Pr(3+) ions at 479.2 nm with an optically pumped semiconductor laser, maximum slope efficiencies of 40% at 639 nm and 32% at 604 nm are obtained in a cw regime. Green laser oscillation at 522.5 nm, in a quasi-cw regime, with an average output power of 66 mW, is demonstrated for the first time, to our knowledge, in a crystalline fluoride epitaxial waveguide. Furthermore, a record efficiency of 60% at 639 nm in the quasi-cw regime is reported.

NMR parameters in column 13 metal fluoride compounds (AlF3, GaF3, InF3 and TlF) from first principle calculations

The relationship between the experimental 19F isotropic chemical shift and the 19F isotropic shielding calculated using the gauge including projector augmented-wave (GIPAW) method with PBE functional is investigated in the case of GaF3, InF3, TlF and several AlF3 polymorphs. It is shown that the linear correlation between experimental and DFT-PBE calculated values previously established on alkali, alkaline earth and rare earth of column 3 basic fluorides (Sadoc et al., Phys. Chem. Chem. Phys. 13 (2011) 18539–18550) remains valid in the case of column 13 metal fluorides, indicating that it allows predicting 19F solid state NMR spectra of a broad range of crystalline fluorides with a relatively good accuracy. For the isostructural α-AlF3, GaF3 and InF3 phases, PBE-DFT geometry optimization leads to noticeably overbended M–F–M bond angles and underestimated 27Al, 71Ga and 115In calculated quadrupolar coupling constants. For the studied compounds, whose structures are built of corner shared MF6 octahedra, it is shown that the electric field gradient (EFG) tensor at the cationic sites is not related to distortions of the octahedral units, in contrast to what previously observed for isolated AlF6 octahedra in fluoroaluminates.

Enhanced upconversion in Ho3+-doped transparent glass ceramics containing BaYbF5 nanocrystals

Novel Ho3+-doped transparent oxyfluoride SiO2–Al2O3–Na2CO3–CaO–BaF2–YbF3 glass ceramics (GC) containing BaYbF5 nanocrystals were fabricated via melt-quenching technique with subsequent heat treatment. The formation of crystalline fluoride phase was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Compared to precursor glasses, the greatly enhanced green emission (40-fold), new emission band at ultraviolet-blue region and stark splittings of emission in GC, indicate that Ho3+ enters into BaYbF5 nanocrystals with low phonon energy. Besides, the origin of the previously unconfirmed emission band at 440–460nm is clearly identified by measuring spectra from thermally coupled luminescent levels at various temperatures. The outstanding upconversion properties of Ho3+ in GC may present potential application in all-solid-state upconversion lasers operating in the visible and ultraviolet range.

Nano-crystals of various lanthanide fluorides prepared using the ionic liquid bmimPF6

Lanthanide fluorides are important materials for a wide spectrum of applications. In this work, nano-sized crystalline fluorides of nearly all of the lanthanides were prepared via a one-pot reaction using the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) as both the solvent and as the fluorination agent and the appropriate lanthanide nitride as the source of the trivalent ion. The phase compositions of the samples (hexagonal, orthorhombic or a mixture of these) as well as the sizes of the nanocrystals were characterised by X-ray diffraction (XRD) and confirmed by high resolution transmission electron microscopy (HR-TEM). Fluorescence measurements were performed for all of the samples to gain insight into the optical properties of the samples.

Tm:LiYF_4 planar waveguide laser at 19 μm

Continuous wave laser operation at 1.87 μm of liquid-phase epitaxially (LPE) grown Tm(3+)-doped YLiF(4) (Tm:YLF) layers is demonstrated. The waveguide laser delivers 560 mW by pumping with a Ti:Sapphire laser at 780 nm leading to an efficiency of 76% with respect to the absorbed pump power. This constitutes the first Tm(3+)-doped crystalline fluoride waveguide laser ever demonstrated as well as a record in efficiency and output power for an LPE grown waveguide laser operating in the 2 μm spectral range.

An easy access to nanocrystalline alkaline earth metal fluorides – just by shaking

High energy ball milling as fast, direct and solvent free method allows an easy access to nanocrystalline alkaline earth metal fluorides MF2 (M: Mg, Ca, Sr, Ba). Comparable metal sources (acetates, carbonates, hydroxides, alkoxides) were used for the reaction with NH4F as fluorinating agent. Even very simple manual shaking experiments between NH4F and the corresponding hydroxides in the stoichiometric ratio (M:F = 1:2, M: Ca, Sr, Ba) give phase pure fluorides. Moreover, comparable classical thermal reactions in closed crucibles at higher temperatures provide phase pure crystalline fluorides in nearly all cases as well.

A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho 3+,Yb 3+):NaYF 4 crystallites

Glass-ceramics containing NaYF 4 nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho 3+ and Yb 3+ ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho 3+ and Yb 3+ ions is incorporated into the NaYF 4 ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the 5S 2 level in glass-ceramics and up-converted red emission (650 nm) originating in the 5F 5 level in as-melted glass were observed under optical pumping into Yb 3+ absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho 3+ and Yb 3+ excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed.

ChemInform Abstract: Structural Chemistry of Organically Templated Metal Fluorides

AbstractReview: 106 refs.

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.

The behavior of Er 3+ dopants during crystallization in oxyfluoride silicate glass ceramics

In this paper, the changes of local environment of Er 3+ during heat treatment process have been studied in 50SiO 2–45PbF 2–5PbO system. The samples were characterized by X-ray diffraction (XRD), absorption spectra, fluorescence decay curves and luminescence spectra. The experimental results indicate that PbF 2 crystals were precipitated in the precursor sample. However, a significant fraction of Er 3+ remains in the glassy phase. With increasing heat treatment time, the Er 3+ in the glass matrix may enter into fluoride nanocrystals gradually. When the heat treatment time reached 15 min, most of the Er 3+ ions were incorporated into the fluoride nanocrystals. Based on the experimental analysis, we considered that Er 3+ not only act as nucleating agents for the precipitation of crystalline fluoride phase during nucleation process but also enter into nanocrystals during crystals growth process.