— Magnesium double phosphates, MgHPO 4 ·3H 2 O and MgKPO 4 ·6H 2 O, isostructural with the minerals newberyite and struvite-K have been synthesized in powder form via coprecipitation. Ceramics with a relative density of ~76% have been prepared by cold pressing followed by firing. The highest sintering temperature was t max = 130°C. Using optical diffuse reflectance spectroscopy of polycrystalline samples, we have determined their band gap and absorption energy. The absorption energy of MgHPO 4 ·3H 2 O has been found to be 3.10 eV, and that of MgKPO 4 ·6H 2 O, 3.13 eV, which corresponds to absorption in the 400- and 396-nm ranges, respectively. The specific surface area of the powder samples studied has been determined by static vacuum volumetry. The results are 7.35 and 17.082 m 2 /g for MgHPO 4 ·3H 2 O and MgKPO 4 ·6H 2 O, respectively. To assess the photocatalytic activity of the synthesized phosphates for the oxidation of aromatic hydrocarbons in the vapor phase, using toluene as an example, we used purpose-designed test chambers. In our tests, UV radiation and light in the visible spectral region were used. The results obtained on photocatalytic decomposition of toluene demonstrate high catalytic activity of the synthesized polycrystalline phosphates. In a 2-h experiment with the use of visible light and UV radiation, the concentration of the aromatic hydrocarbon decreased by 12.2 and 1.9% in the case of MgHPO 4 ·3H 2 O and by 9.8 and 9.1% in the case of MgKPO 4 ·6H 2 O. To assess the chemical stability of the phosphates, we have carried out hydrolytic tests of the pressed ceramics in distilled water at room temperature.

— The Y 0.95 Gd 0.05 PO 4 phosphate with the xenotime structure has been synthesized in powder form and as ceramics. Ceramics with a relative density of ~99% have been produced by spark plasma sintering. The sintering temperature was 1140°C and the sintering time was ~18 min, without isothermal holding. We have assessed the radiation resistance of the ceramics under irradiation with 132 Xe 26+ ions and investigated the restoration of the obtained metamict phase to a crystalline one via high-temperature heat treatment. No complete amorphization of the samples has been reached at the fluences used in this study. The calculated critical fluence is (9.2 ± 0.1) × 10 14 cm –2 , and the calculated latent track radius is ~2.8 nm. Hydrolytic tests have shown that the phosphate under study is stable in water under dynamic conditions. The observed Y and Gd leaching rates were R i = 1.68 × 10 –6 and 1.5 × 10 –7 g/(cm 2 day), respectively.

— The use of magnetic fields in nonaqueous spent nuclear fuel treatment processes involving molten salts opens up new possibilities for selective extraction of radionuclides. In this research, this approach has been applied to the selective extraction of rare-earth elements and zirconium. We have studied strontium hexaferrite and a spinel ferrite as a magnetic carrier, examined the feasibility of utilizing it alkali chloride melts, and analyzed the effect of process conditions (temperature, holding time, and Zr content) on the effectiveness of Nd and Zr extraction by magnetic separation.

Fine-grained SrWO 4 and NaNd(WO 4 ) 2 ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.

Spark Plasma Sintering has been used to obtain first ever fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure and high density (95.8–99%) that are promising as materials for nuclear waste immobilization. It has been demonstrated that the densification intensity in tungstates at the initial sintering stage is determined by the agglomeration degree of input powders. The activation energy of fine-grained ceramics with the scheelite structure has been established to correlate at high temperatures with the activation energy of grain-boundary diffusion. Rapid sintering of fine-grained NaNd(WO4)2 ceramic results in internal tensile stress fields that lead to lowered hardness and fracture toughness of the ceramic.

Fine-grained ceramics based on solid solutions of ZrO2-x-Ln(2)O(3) (where Ln = Sm, Yb), which are used to simulate inert matrix fuel (IMF), were obtained by colloidal chemical synthesis of powders and their Spark Plasma Sintering (SPS). It has been established that the SPS activation energy (Q(s)) for fine-grain ceramics based on ZrO2 corresponds to the activation energy of oxygen diffusion on grain boundaries of zirconium oxide (zirconia). We have demonstrated that introducing oxides of samarium (Sm2O3) and ytterbium (Yb2O3) into zirconia decreases SPS activation energy and stabilizes the phase composition of zirconia. Ceramics with densities up to 100% were obtained by the following modes of SPS: sintering temperature 1000-1250 degrees C, sintering time 6-10 min. Hydrolysis testing in the static mode (T = 20 degrees C, P = 1 atm) in distilled water revealed no leaching of lanthanide ions at detection thresholds of 5 ppm for Sm ions and 0.1 ppm for Yb ions.

The Spark Plasma Sintering method was used to produce high-density ceramics from tungstates SrWO 4 and NaNd(WO 4 ) 2 with scheelite structure. These compounds are proposed as possible matrices for the consolidation of radwaste components. Powder samples were obtained by coprecipitation method and studied by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). After sintering, the samples retained their phase identity (scheelite structure). The total duration of sintering was ~ 13-15 min, the relative density was reached ~ 92, 99%.

Multicomponent oxides of pollucite structure, containing Cs and Ba, were synthesized as powders and ceramics. Their chemical compositions, Cs[MgAl0.5P1.5O6] and Cs0.875Ba0.125[Li0.125Zn0.875Al0.5P1.5O6], were modeled on the basis of the known structural features, taking into account the principles of iso- and heterovalent isomorphism of cations. From powdered samples synthesized using sol-gel process, a ceramic was prepared by spark plasma sintering (SPS). The sintering time was 3–4 min in the temperature interval 600–850°C. The relative densities were 97 and 99%. To evaluate the radiation resistance of the ceramics, the samples were irradiated with 132Xe26+ ions (E = 167 MeV) in the fluence interval from 6 × 1010 to 1 × 1013 cm−2 (ion flux density ∼109 s−1 cm−2). The amorphization took place at fluences of (1.2–1.3) × 1012 cm−2. This fact suggests the decisive role of the ion energy loss for ionization in the generation of radiation defects. Conditions were found for the transition of the metamict form into the crystalline form on heating.

The powder of phosphate Ca1/4Sr1/4Zr2(PO4)(3) was synthesized by sol-gel processes in the presence of citric acid and ethylene glycol. Ceramic samples were prepared from this powder by Spark Plasma Sintering (SPS), their relative densities were found to be 99.5 +/- 0.3% after the isothermal treatment at 860 degrees C for 3 min.Sintered disc-shaped ceramic samples (d = 10 mm, h = 4 mm) were bombarded at 300k by 167 MeV Xe26+ ions with fluences ranging from 6 10(10) to 1 . 10(13) ions/cm(2). It was found that exposure to the highest fluence (10(13) ion/cm(2)) led to a complete amorphization of the irradiated layer. The observed phase transition is ascribed to the formation of amorphous latent tracks via dense electronic excitations. Post-radiation heat treatment revealed that the transformation from metamict to crystalline form took place after annealing at T= 200, 300, 400, 500, 600 and 800 degrees C and t = 3, 13, 11, 5, 17 and 15 h, respectively. (C) 2013 Elsevier B.V. All rights reserved.

New pollucite-like phosphorus-containing compounds with cesium and uni-, bi-, and trivalent cations (Li; Mg, Mn, Co, Ni, Cu, Zn, Cd, Sr, Ba; Al, Cr, Fe), many of which can be present in various combinations in wastes from radiochemical processes, were synthesized. The individual phases obtained were characterized by X-ray phase analysis and IR spectroscopy. The symmetry of the unit cells of the phosphorus-containing compounds decreases relative to the silicon analog (pollucite natural mineral) from Ia3d to I4132. The behavior of the compounds on heating to 1000°C, under hydrothermal conditions at 90°C, in aqueous systems (distilled water and seawater) at 25°C, and in a CsCl melt at 680°C was studied. The chemical and phase composition of the samples remained unchanged under all these conditions. The Cs leaching rates varied from 7.1 × 10−6 to 1.46 × 10−5 g cm−2 day−1. The “crystal-chemical modeling” of the new possible compositions of the compounds with the expected pollucite structure was performed.