Phase formation in the BaF2-NdF3 and BaF2-PrF3 systems was studied by solid-phase annealing at 750 degrees & Scy; and syntheses in molten NaNO3 flux at 500 degrees & Scy;. We observed the formation of Ba1-& khcy;R & khcy;F2+& khcy; fluorite solid solutions in the aforementioned systems, and these solid solutions underwent ordering to form ordered Ba4Nd3F17 and Ba4Pr3F17 fluorite-like phases at 917 degrees & Scy; and 880 degrees & Scy;, respectively.

Single-phase powders of SrF2:Yb:Er solid solutions with different particle sizes were synthesized using different fluorinating agents, such as ammonium fluoride, sodium fluoride and potassium fluoride, under the same synthesis conditions by co-precipitation from the aqueous solution technique. The incorporation of K and Na into the crystal lattice of strontium fluoride was found to be about 1 and 4 mol%, respectively. The achieved values of potassium and sodium concentration were equal to their solubility limits in the crystal lattice of strontium fluoride. The particle size increased with rising alkali element content. The embedding of sodium and potassium led to a change in the up-conversion emission spectrum and manifested in a redistribution of the intensities of the red luminescence bands (F-4(9/2 )-> (4)I1(5/2) transition). The achieved quantum yield of up-conversion luminescence (6.80% at 300 W cm(-2)) for the powders synthesized using sodium fluoride as a fluorinating agent was comparable to the values for single crystals.

Методом соосаждения из водных растворов при различном порядке смешения реагентов синтезированы однофазные порошки твердого раствора KGd1,52Yb0,40Er0,08F7 со средним размером частиц 350 нм. Выявлены и исследованы процессы поверхностного вымывания калия. Определена оптимальная температура термообработки (525оС), позволившая достигнуть величины энергетического выхода ап-конверсионной люминесценции эрбия 1,30±0,02% при возбуждении на длине волны 974 нм с плотностью мощности накачки 1 Вт/см2.

The paper describes the study of the phase formation in the NaF-BaF2-YF3 system. It involved solid-phase sintering of the components in a fluorinating atmosphere at 750 °C for two weeks and quenching them in liquid nitrogen. The prepared samples were placed in nickel capillaries, which, together with barium hydrofluoride, BaF2·HF, were placed in copper containers. The containers were sealed by argon arc welding. The fluorinating atmosphere was created by pyrolysis of barium hydrofluoride, BaF2·HF. X-ray powder diffraction was carried out using a Bruker D8 Advanced diffractometer (CuKa‑radiation). TOPAS, DifWin, and Powder 2.0 software were used to process X-ray diffraction patterns. Sodium fluoride is a good sintering additive, its introduction in the amount of 5 mol % NaF was enough to synthesize sintered mass with clear X-ray diffraction patterns. The experiment revealed the formation of a solid solution based on the Ba4Y3F17 compound with a trigonally distorted fluorite structure (space group R-3) with a content of up to ~ 20 mol % of NaF. The parameters of the trigonal cell were related to the parameter а0 of the fluorite subcell by the ratios а ~ √7/2а0 and с ~2√3а0. The general formula for the resulting solid solution is Ba1-x-yYxNayF2+x-y. The introduction of sodium fluoride reduced the parameters of the trigonal lattice and was accompanied by the formation of anion vacancies. Structure stabilizationexpressed in the expansion of the homogeneity region of the phase based on Ba4Y3F17 seems to be associated with the disappearance of interstitial fluorine ions surrounded by anions in the Ba4Y3F17 structure, both in the cuboctahedral cavity of the Y6F36 clusters and in the centre of the F8 cubes. The corresponding solid solution can be used to create new photonics materials. The NaF-BaF2-YF3 system is similar to the previously studied NaF-BaF2-GdF3 system

Single-phase powders of a KGd1.52Yb0.40Er0.08F7 solid solution with an average particle size of 350 nm are synthesized by co-precipitation from aqueous solutions with a different order of mixing reagents. Surface leaching processes of potassium are revealed and studied. The optimal temperature of thermal treatment (525 °C) is determined. It enables the achievement of the energy yield of erbium up-conversion luminescence of 1.30±0.02

The possibility of doping the KGd2F7 matrix with ytterbium and erbium ions by introducing yttrium ions with a concentration of 25 mol.% was confirmed and the conditions were determined for the synthesis of anti-Stokes phosphors based on single-phase KGd2F7:Yb,Er solid solutions. The dependences were revealed of the sizes of coherent scattering regions, crystal lattice parameters, and energy yield of luminescence on the temperature and duration of heat treatment. Heat treatment conditions were determined to ensure the achievement of intense anti-Stokes luminescence. As a result, effective phosphors KGd2F7:Yb (20.0 mol.%),Er (4.0 mol.%) with an energy yield of up-conversion luminescence of 3.80 % were developed. Disordering of the crystal structure (transition from cubic to tetragonal modification) at a temperature of 600 degrees C was recorded, corresponding to the rule of Ostwald steps.

Optically transparent single-crystal blocks are prepared by the fusion of a barium fluoride charge with yttrium and erbium fluorides using sodium fluoride as a flux. The crystal structures were solved and composition of the following phases were determined: Na0.75Ba1.26Er1.99F9.24 (cubic crystal system, Fm3̅m space group, a = 11.4192(4) Å), Na0.25BaY2.75F10.5 (cubic crystal system, Fm3̅m space group, a = 11.4350(19) Å), Na0.05Ba0.9Y1.05F5 (orthorhombic crystal system, Cmmm space group, a = 5.7205(5) Å, b = 17.2348(11) Å, c = 5.7648 (4) Å).

The congruent melting compositions in the MF2–M'F2–RF3 systems (M ≠ M' = Ca, Sr, Cd, Ba, Pb; R are rare-earth elements (REEs)) have been determined based on thermodynamic topological analysis. Crystals of fluorite solid solution in the PbF2–CdF2–RF3 (R = Tb, Ho, Er, Tm, Yb, Lu) systems were grown by the vertical directional crystallization technique. Their phase composition and distribution of components over the crystalline boule length were investigated. Crystals of congruently melting solid solutions (Pb0.67Cd0.33) _1-x RxF _2 + x (R = Tb, Ho, Er, Tm, Yb, Lu) were grown for the first time. Traces of low-temperature transformation—a phase isostructural to the Pb2YF7 compound (sp. gr. I4/m), in which Y sites are occupied by the corresponding R cations and Pb sites can be partially occupied by Cd cations—were found in the crystals with R = Ho, Er, Tm, and Yb. The crystals with R = Tb and Lu have a high degree of homogeneity and are suitable for optical studies.

We synthesized powders of single-phase solid solutions Sr0.925–xBaxEu0.075F2.075 (x = 0.00, 0.20, 0.25, 0.30, 0.35 and 0.40) by a precipitation technique from nitrate aqueous solutions. The lattice parameters increase linearly as the barium content increases. We recorded a significant increase in the X-ray luminescence intensity of europium at increasing barium content. Upon increasing barium content, the intensity of the luminescence of strong 5D0 → 7F1 band increases exponentially, and we observed blue and red shifts in the position of the europium luminescence bands for 5D0 → 7F1 and 5D0 → 7F4, respectively

This article considers the features and fundamental difficulties of studying low-temperature phase equilibria associated with an exponential increase in the required duration of syntheses with a decrease in temperature. Methods for accelerating the achievement of equilibrium, including the use of salt solvents, are also considered. The results of phase equilibria studies in the SrF2–LaF3 system using sodium nitrate and in the ZrO2–Sc2O3 system using sodium sulfate as fluxes are presented. The methods of extrapolation of phase diagrams to absolute zero temperature in accordance with the third law of thermodynamics are considered. Phase diagrams of the Au–Cu, Cu–Pd, Ni–Pt, and ZrO2–Y2O3 systems are presented. Phase equilibria with plagioclase ordering are considered separately, and the phase diagram of the albite–anorthite (NaAlSi3O8–CaAl2Si2O8) system is presented. As the temperature approaches absolute zero, the homogeneity region of labradorite shrinks to the compound NaCaAl3Si5O16.

We propose a mathematical model to fit the temperature -dependent thermal conductivity of M1-xRxF2+x heterovalent solid solutions where M stands for alkaline -earth metals and R for rare-earth metals. These solid solutions experience composition -driven transition from the crystal -like to glass -like behavior of thermal conductivity. When tested on Ca1-xYbxF2+x solid solutions, the model showed a potential for use with an option for further improvements.

The thermal conductivity of natural monoliths of calcite, dolomite marble, and limestone from various deposits was measured using the absolute stationary method of longitudinal heat flow in the temperature range of 50–300 K and the dynamic method in the range of 323–573 K. A majority of calcite marbles were inferior in thermal conductivity to dolomite marbles. At room temperature, the thermal conductivity coefficients of all studied samples were lower k = 5 W/(m K). The obtained data were compared with the literature data. The diversity of experimental data from different authors on the thermal conductivity of carbonates is associated with qualitative differences in the samples studied

Efficient luminophores SrF2:Yb:Er were synthesized using different fluorinating agents, such as ammonium fluoride, sodium fluoride and potassium fluoride, under the same synthesis conditions by co-precipitation from the aqueous solution technique.

The thermal conductivity k of a series of single crystals of Na0.4(Y1 – xNdx)0.6F2.2 (x = 0–0.5) solid solutions has been measured in the range 50–300 K by an absolute steady-state axial heat flow technique. The temperature behavior of thermal conductivity for all of the crystals is characteristic of disordered materials. The room-temperature thermal conductivity of the solid solutions is low: k = 1.1–1.4 W/(m K).

The phase diagram of the SrF2–MgF2 system has been studied by differential thermal analysis (DTA) and X-ray powder diffraction (XRD). The stability area of the compound SrMgF4 is very small, and its extent is about 20°С between 870 and 890°С. The formation of compounds by magnesium fluoride and strontium fluoride with other metal fluorides is considered in the M versus X coordinates, where M is the cumulative moment of the cation and X is the cation electronegativity. Two areas of compound formation were identified for each of the fluorides. Magnesium and strontium fluorides are amphoteric compounds in terms of the Lewis acid and base theory.

— The thermal conductivity k of a series of single crystals of Na 0.4 (Y 1 – x Nd x ) 0.6 F 2.2 ( x = 0–0.5) solid solutions has been measured in the range 50–300 K by an absolute steady-state axial heat flow technique. The temperature behavior of thermal conductivity for all of the crystals is characteristic of disordered materials. The room-temperature thermal conductivity of the solid solutions is low: k = 1.1–1.4 W/(m K).

Novel tetragonal matrix Ba0.5−xLn0.5NaxF2.5−x with x = 0.08, doped by Yb3+, Ho3+, Er3+, was synthesized by molten salt synthesis (MSS) from nitrate flux. XRD data show that the tetragonal phase with a = 4.122(1) Å, c = 17.672(1) Å is stable in an argon atmosphere up to 960 °C. Luminescence spectra recorded in 500–900 nm and 1050–1700 nm upon 974 nm pumping demonstrated the characteristic luminescence at 1550 nm (4I13/2 → 4I15/2) for Er3+ and 1150 nm (5I6 → 5I8) for Ho3+. The relative thermal sensitivity (Sr) at 296–316 K were 0.3%×K−1 and 5.5%×K−1 in shortwave infrared (SWIR) and visible range, respectively. Synthesized luminophores can be used as dual-range optical temperature sensors, which simultaneously operate in visible and SWIR ranges.

— Barium lanthanum fluoride powders have been prepared by reacting barium nitrate and lanthanum nitrate in molten sodium nitrate at 350 and 450°C, using a sodium fluoride as a fluorinating agent. A fivefold excess of sodium fluoride has been shown to prevent pyrohydrolysis. We have identified a phase of variable composition with the fluorite structure, Ba 1– x La x F 2+ x (0.3 < x < 0.5), which has high ionic conductivity (2.3 × 10 –4 S/cm at 500 K) and an activation enthalpy for ionic transport of 0.50 ± 0.01 eV.