M. N. Mayakova

Coprecipitation from aqueous solutions to prepare binary fluorides

The synthesis of binary fluoride phases, specifically nanofluorides, by coprecipitation from aqueous solutions is considered in terms of phase equilibria. Phases have been precipitated in NaF-RF3 systems (where R stands for a rare-earth element), MF2-YF3 systems (where M = Ca, Sr, Ba), BaF2-ScF3, and CaF2-BaF2 systems. In the MF2-YF3 systems, unordered metastable congruently soluble phases are precipitated instead of ordered fluorite-like phases. In the BaF2-ScF3 system, the congruently soluble compound Ba3Sc2F12 is precipitated. In the NaF-RF3 systems, incongruently soluble metastable phases with a fluorite-like structure (for R = Er, Lu, Y) and the gagarinite structure (NaGdF4) are formed. In the CaF2-BaF2 system, intermediate phases have not been formed.

Coprecipitation of barium-bismuth fluorides from aqueous solutions: Nanochemical effects

The BaF2-BiF3 system was studied by the method of coprecipitation from aqueous solutions. The region of precipitation of single-phase nonoxygen powders was revealed (the atomic fraction of Bi in the initial solution was 0.35–0.43). The composition of the cubic fluorite-type phase is BaBiF5 according to an energy-dispersive X-ray spectroscopy analysis. Samples prepared from solutions with high Bi concentrations contain oxygen. This indicates that the hydrolysis temperature of BiF3 decreases when compared with that of the bulk samples and that this process takes place at room temperature. The coherent intergrowth of nanoparticles with the formation of single crystals with nonfaceted complex forms was shown by scanning electron microscopy and transmission electron microscopy.

Synthesis of ultrafine fluorite Sr1 − xNdxF2 + x powders

We have identified conditions that ensure the preparation of ultrafine Sr1 − xNdxF2 + x powders uniform in phase composition. The powders were characterized by X-ray diffraction and scanning electron microscopy. The powder particles have the form of faceted nano- and microcubes and range in size from 30–100 nm to 0.3–2.5 μm, depending on precipitation conditions.

Soft chemistry synthesis of powders in the BaF2-ScF3 system

The BaF2-ScF3 system is studied using coprecipitation from aqueous solutions. Compound Ba3Sc2F12 is found to form over a wide range (10–60 mol %) of scandium concentrations in the initial solution. At higher scandium concentrations in solution, a new phase of provisional composition BaSc2F8 · 2H2O (hexagonal system, a = 9.6346 Å, c = 4.0483 Å) was obtained. Scandium fluoride hydrolysis is not observed by energy-dispersive X-ray analysis.

Synthesis and Characterization of Fluoride Xerogels

Using coprecipitation from aqueous solutions, we have synthesized transparent fluoride xerogels with both hexagonal (NdF3, PrF3, and CeF3; tysonite structure) and cubic (Sr0.6Y0.4F2.4, Ba4Y3F17:Bi, and Ba4Y3F17: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.

Low-temperature phase formation in CaF 2 –HoF 3 system

Phase formation in CaF2–HoF3 system has been studied by coprecipitation followed by X-ray powder diffraction. Aqueous nitrate solutions have been used as initial substances, while hydrofluoric acid has been employed as fluorinating agent. Formation of hydrated nanophases: solid solution Ca1–xHoxF2 + x (х ≤ 0.1) and HoF3 has been revealed. Dehydration proceeds on heating to 600°C.

Phase diagram of the NaF–CaF2 system and the electrical conductivity of a CaF2-based solid solution

The phase diagram of the NaF–CaF2 system was studied by thermal analysis and X-ray powder diffraction analysis with the determination of the chemical composition. The system was found to be of the eutectic type. A narrow range of the existence of solid solution Ca1–xNaxF2–x was established. The NaF solubility reaches a maximal value of x = 0.035 at 1200 ± 50°C (the temperature at which there is a diffuse phase transition in fluorite). At 920 ± 25°C, the NaF solubility reaches a minimum (<0.4 mol %) and increases again to 2.2 ± 0.2 mol % at a eutectic temperature (818°C). The ionic conductivity increases by three orders of magnitude after adding NaF to CaF2.