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Single crystals of Ba1 –x–yYbxRyF2 +x+y (R = Tm, Ho) solid solutions have been grown by the Bridgman technique in vacuum using a CF4 fluorination atmosphere. We have measured their thermal conductivity in the range 50–300 K and their refractive index from the visible to IR spectral region. As the ytterbium content increases from 2 to 14 mol %, the room-temperature thermal conductivity of the solid solutions with R = Tm and Ho decreases from 3.39 to 1.18 and from 3.11 to 1.18 W/(m K), respectively. Raising the holmium and thulium content of the solid solutions leads to a gradual increase in their refractive index, and it gradually decreases with increasing wavelength. [ABSTRACT FROM AUTHOR]

Anodic processes were studied in chloride-fluoride, fluoride melts by voltammetry and gas chromatography mass spectrometry method. It was found that the reason for the occurrence of the anodic effect during the electrolysis of the KCl-KF-K2NbF7 melt is the formation of the compound CxFy on the electrode surface, which has electrical insulating properties. Influence of oxide ions on the anodic processes in the KCl-KF-K2NbF7 melt was investigated. It was determined that oxide ions and oxofluoride complexes of niobium oxidize at a glassy carbon electrode at more negative potentials than fluoride ions. The anodic effect in the chloridefluoride melt containing oxide ions and oxofluoride complexes of niobium can be reached if the anodic current density will be higher than the limiting current density of electrooxidation of oxide ions and niobium oxofluoride complexes. [ABSTRACT FROM AUTHOR]

Sodium yttrium fluoride codoped with ytterbium and erbium has been prepared by reacting sodium fluoride with rare-earth nitrates in molten sodium nitrate, using a sevenfold excess of the fluorinating agent. The synthesis temperature was 320 to 450°C and the synthesis time was varied from 15 to 500 min. After washing with double-distilled water, the synthesized powders were characterized by X-ray diffraction (to determine the crystallite size and lattice strain), scanning electron microscopy, upconversion luminescence measurements at an excitation wavelength of 974 nm, and the energy yield of their luminescence was determined. The synthesis products had the form of single-phase hexagonal β-NaYF4 samples. We observed the formation of distorted hexagonal prisms through nanoparticle aggregation. The luminescence energy yield was found to be a nonlinear function of the lattice strain e0 in the β-NaYF4 crystals, rising sharply with decreasing e0. Reducing the defect density in powder particles is an effective strategy for improving the efficiency of their upconversion luminescence. [ABSTRACT FROM AUTHOR]

Single crystals of Ca1 – ySryRхF2 +х (R = Ho, Pr) solid solutions with high optical quality have been grown by the Bridgman technique in vacuum using a fluorination atmosphere. The thermal conductivity of the Ca0.60Sr0.40 –хHoхF2 +х solid solutions decreases from 3.27 to 2.37 W/(m K) as the fraction of holmium rises from 0 to 3 mol %, respectively. The room-temperature thermal conductivity of the Ca0.50Sr0.50 –хPrхF2 +х solid solutions decreases from 3.38 to 2.18 W/(m K) as the fraction of praseodymium rises from 0 to 8 mol %, respectively. Increasing the holmium and praseodymium content of the ternary solid solutions leads to a gradual increase in their refractive index. With increasing wavelength, the refractive index gradually decreases. [ABSTRACT FROM AUTHOR]

The heat capacity Cp(T) of cadmium fluoride, strontium fluoride, and the Cd0.5Sr0.5F2 solid solution has been studied experimentally (2–300 K). We have detected deviations ΔCp(T) of the Cp(T) data from additivity. The low-temperature (2–50 K), positive deviations ΔCp(T) have been tentatively attributed to the effect of two-level systems associated with the glasslike character of the low-temperature thermal properties of the solution due to disorder in the Cd–Sr sublattice. The high-temperature (100–300 K), negative deviations ΔCp(T) are caused by the effect of anharmonicity of lattice vibrations, whose increasing role is also associated with the disordered arrangement of atoms in the crystal lattice upon formation of the solution. The Cp(T) data for the solution of the system studied have been analyzed in the Debye–Einstein model. We have determined model parameters: characteristic temperatures Θi and the fractions of different contributions, ai, to total thermal characteristics of the substances studied. [ABSTRACT FROM AUTHOR]

A concentration series of Ca(1 −X)SrXF2:4 mol % YbF3 crystals with Х varying from 0 to 1 has been grown. The absorption spectra of Yb3+ ions in these crystals are measured at temperatures of 300 and 77 K. It is shown that the intensity ratio of the absorption lines of clusters and tetragonal centers in the congruent crystals is higher than that in the other CaF2–SrF2 solid solution crystals. Analysis of the absorption profiles of Yb3+ optical centers in the concentration series of the solid solution crystals shows that it is energetically favorable for the Sr2+ ion to enter the local environment of both the tetragonal center and the cluster. [ABSTRACT FROM AUTHOR]

Abstract: Upconversion luminescence of fluoride phosphors SrF2:Er,Yb co-precipitated from aqueous solutions is studied. Under excitation of Er3+ ions to the 4I13/2 level, upconversion luminescence spectra are recorded in the visible and near-IR spectral regions, which correspond to the 4G11/2 → 4I15/2, 2H9/2 → 4I15/2, 4F5/2 → 4I15/2, 4F7/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4I15/2, 4I9/2 → 4I15/2, and 4I11/2 → 4I15/2, electronic transitions of Er3+ ions and to the 2F5/2 → 2F7/2 transition of Yb3+. The absolute quantum yields of visible upconversion luminescence under excitation in the region of 1.5 μm are determined for SrF2:Er,Yb phosphors with different concentrations of Er3+ and Yb3+ ions and analyzed comparatively with the corresponding values for SrF2:Er phosphor. A study of chromaticity coordinates showed that the yellow-green upconversion luminescence of SrF2:Er,Yb phosphors is characterized by color temperatures of 3037-3526 K at an incident power density of 850 W/cm2. [ABSTRACT FROM AUTHOR]

Using coprecipitation from aqueous solutions, we have synthesized transparent fluoride xerogels with both hexagonal (NdF, PrF, and CeF; tysonite structure) and cubic (SrYF, BaYF:Bi, and BaYF:Yb; fluorite structure) symmetries. As shown by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, the transparent xerogels have a hierarchical structural organization: primary nanoparticles 20-30 nm in size form agglomerates about 100 nm in size, which in turn form a 'skeleton' with many voids and channels up to hundreds of nanometers in size. [ABSTRACT FROM AUTHOR]

Standard charge transfer rate constants ( k) were defined for Nb(V)/Nb(IV) redox couples in NaF-KF (eutectic)-KNbF, KF-KNbF, and CsF-KNbF melts using the cyclic voltammetry technique. It was established that in fluoride melts, the k values change in the following order depending on the composition of the second coordination sphere: k (CsF) < k (KF) < k (NaF-KF). [ABSTRACT FROM AUTHOR]

The methods of optical, electron, and atomic force microscopy (AFM) are applied to the study of the real structure of optical lithium fluoride ceramic obtained by hot deformation of single crystals. A comparative analysis is carried out of the scattering mechanisms of weakly nonequilibrium thermal phonons at liquid helium temperatures in LiF single crystals and ceramics. It is demonstrated that the phonon scattering in the original single crystals is determined by the forced vibrations of dislocations in the stress field of an elastic plane wave (a phonon), i.e., by the flutter mechanism. As the degree of deformation of the original material increases, the ceramics exhibit a change in the plastic deformation mechanisms, which leads to a decrease in the average size of grains and to an ordered structure. In this case, the dominant scattering is that by intergrain boundaries. The thickness and the acoustic impedance of these boundaries are evaluated. [ABSTRACT FROM AUTHOR]

Standard rate constants ks of charge transfer for Nb(V)/Nb(IV) redox couple in NaCl-KCl (the equimolar mixture) -K2NbF7, KCl-K2NbF7, and CsCl-K2NbF7 melts are determined by using cyclic voltammetry. An unconventional series of the standard rate constants is found: ks (KCl) < ks (CsCl) < ks (NaCl-KCl). Ab initio calculations carried out by using a PC Gamess/Firefly quantum-chemical program showed that the charge transfer activation energy can change not monotonically in the Na-K-Cs series, in compliance with the reorganization energy relationship. This leads, in its turn, to nonmonotonic change in the charge transfer rate constants. [ABSTRACT FROM AUTHOR]

The article presents the study "Unusual Effect of the Second Coordination Sphere on the Standard Charge Transfer Rate Constants for the Nb(V)/Nb(IV) Redox Couple in Chloride-Fluoride Melts." The study aims to determine the standard constants of charge transfer in the Nb(V)/Nb(IV) redox couple in chloride-fluoride melts. It notes that the study is part of systematic studies of the effect of the composition of the first and second coordination spheres on standard rate constants of charge transfer

The thermal conductivity of single-crystal samples of SrLaNdF and SrLaNdF solid solutions, both with a cellular substructure and without it, have been investigated by the method of stationary longitudinal heat flow in the temperature range of 50-300 K. An anisotropy of thermal conductivity (related to the substructure) is established for a SrNdF single crystal. [ABSTRACT FROM AUTHOR]

Combination of electron microscopy and X-ray diffraction experimental data confirms the non-classical micro-block mechanism of crystal growth for fluoride nanopowders obtained by co-precipitation from aqueous solutions by aggregation and intergrowth of the corresponding nanoparticles. Major principles of these processes are discussed. [ABSTRACT FROM AUTHOR]