Crystalline silicophosphates M2Zr2SiP2O12 (M = K, Rb, Cs) of langbeinite structure were synthesized using the sol–gel method. These materials are of interest as candidate matrices for incorporating radioactive elements that are produced during the reprocessing of irradiated nuclear fuel. The composition, morphology, and structure of the samples as a function of the synthesis temperature were studied by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray microanalysis. Single-phase silicophosphates were obtained at 800°C. The sintering of M2Zr2SiP2O12 samples by hot pressing was described in terms of time and temperature. The thermal stability of silicophosphates up to 1200°C was confirmed by thermo-XRD and combined TG–DSC analyses. Silicophosphates are isotropic, moderately expanding materials; the coefficient of linear thermal expansion of Cs2Zr2SiP2O12 is 5.8 × 10–6 °C–1. The 28-day dynamic chemical durability test showed that the leach rate of Cs⁺ ions from the ceramics of langbeinite structure in distilled water at 90°C was approximately 10–5 g cm–2 day–1.

Silicophosphates A2Zr2SiO4(PO4)2 (A = K, Rb, and Cs) with langbeinite structure have been synthesized and studied for the first time. The samples have been characterized by XRD, IR spectroscopy, scanning electron microscopy, and energy-dispersive X-ray microanalysis. Single phase silicophosphates have been prepared at 800°C. Refining of crystal structure of Rb2Zr2SiO4(PO4)2 by Rietveld method have shown that the structure is based on 3D framework formed by ZrO6 octahedra and (Si/P)O4 tetrahedra, extra-framework alkali metal cations occupy separated cavities within the scaffold. Stretching and bending vibrations of SiO4 and PO4 tetrahedra have been determined for the compounds. Correlations between IR spectra and the nature of cation A+ have been revealed. Thermal stability of silicophosphates up to 1200°C has been confirmed by thermal X-ray diffraction and combined TG–DSC analysis. The linear thermal expansion coefficient of Cs2Zr2SiO4(PO4)2 has been found to be αa = 5.8 × 10–6 K–1. The obtained results allow one to conclude that the studied silicophosphate materials are highly persistent to extreme temperature conditions.

BiFe2(PO4)3 ceramic powder with controlled chemical and phase compositions has been prepared by evaporation of the salt solution, followed by heat treatment. The powder was consolidated by hot pressing and spark plasma sintering, which allowed high-density (92–98

Mineral-like single-phase NZP materials of variable composition M0.5Zr2(AsO4)x(PO4)3 − x (M − Sr, Ba, Cd) are of interest as matrices for isolation of high-level Sr, Ba and Cd wastes generated during reprocessing of irradiated nuclear fuel. Such polycrystalline phases were synthesized by sol-gel method with subsequent heat treatment and investigated by X-ray diffraction, IR spectroscopy and electron-probe microanalysis. Rietveld structure refinement showed that the framework is formed by ZrO6 octahedra and mixed AsO4/PO4 tetrahedra connected by vertices; Sr, Ba or Cd cations are located in extra-framework positions. Arsenate-phosphates can be classified as medium thermal expansion materials due to their average coefficient of linear thermal expansion of 2.8∙10−6 °C−1. Chemical durability test indicates that the leaching rate of Sr2+ ions from the NZP structure type solid solution is of the order of 10−5 g cm−2 d−1.

Phases of variable composition A2-(x)A'xMg0.5Zr1.5(PO4)(3) (AA' - KNa, KRb, KCs, 0 <= x <= 2) with langbeinite and kosnarite structures can serve as matrices immobilizing radioactive waste. They were synthesized by sol-gel method followed by heat treatment and investigated by X-ray diffraction at different temperatures, IR spectroscopy, scanning electron microscopy and electron probe microanalysis. A Rietveld refinement of the KCsMg0.5Zr1.5(PO4)(3) structure showed that the framework is formed by octahedra (Mg/Zr)O-6 and tetrahedra PO4 connected by vertices; potassium and cesium cations are located in cavities. When the temperature changes, phosphates with the langbeinite structure expand uniformly in all directions: alpha, = alpha b = alpha c = (4.2-6.6).10(-6) C- 1, without undergoing polymorphic transitions. Phosphate Na2Mg0.5Zr1.5(PO4)(3), which crystallizes in the kosnarite structure type, expands anisotropically: alpha a = -2.6.10(-6), alpha c = 8.2.10(-6), alpha(av) = 1.0.10(-6) C- 1. The chemical durability test shows that the leaching rate of Cs ions from the solid solution with langbeinite structure is of the order of 10-5 g cm (-2) d(-1).

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН

The manifestations of the Bi1- x Sb x Cr2(PO4)3 system with the α-CaMg2(SO4)3 structure were obtained and characterized by the evaporation of salt solutions with heat treatment. Refinement of the Rietveld method for the structure of BiCr2(PO4)3 ( x = 0) and SbCr2(PO4)3 ( x = 1) showed that the [Cr2(PO4)3]3∞ framework is formed by CrO6 octahedra doubled by faces, PO4 tetrahedra are between the dependences, attached to the octahedrons by oxygen vertices, the voids of the framework are populated by six-coordinated bismuth or antimony atoms. By varying the composition of the Bi1- x Sb x Cr2(PO4)3 solid solution everywhere, it is possible to obtain materials with low thermal expansion coefficients: 0.5×10-6 ≤ α av ≤ 1.9×10-6 °C-1.

Керамический порошок BiFe 2 (PO 4 ) 3 контролируемого химического и фазового состава получен упариванием раствора солей с последующей термообработкой. Консолидацию порошка проводили с использованием горячего прессования и электроимпульсного плазменного спекания, получена высокоплотная (92–98%) керамика BiFe 2 (PO 4 ) 3 со структурой α-CaMg 2 (SO 4 ) 3 . Методом лазерной вспышки в интервале 298−573 K исследована температуропроводность, определена теплопроводность высокоплотной (98%) керамической формы. Теплопроводность керамики убывает с повышением температуры. Температуропроводность и коэффициенты теплопроводности (0.9–1.4 Вт/(м К)) керамики BiFe 2 (PO 4 ) 3 характеризуют ее как теплоизолятор с высокой рабочей температурой.

Samples of the Bi 1– x Sb х Cr 2 (PO 4 ) 3 system with the structure of α-CaMg 2 (SO 4 ) 3 were obtained by evaporation of a solution of salts with subsequent heat treatment, and characterized. Refinement of the BiCr 2 (PO 4 ) 3 ( x = 0) and SbCr 2 (PO 4 ) 3 ( x = 1) structures by the Rietveld method showed that the [Cr 2 (PO 4 ) 3 ] 3∞ framework is formed by CrO 6 octahedra doubled by faces, between which PO 4 tetrahedra attached to the octahedra by oxygen vertices are located, the voids of the framework are populated by six coordinated bismuth or antimony atoms. By varying the composition of the unlimited solid solution Bi 1– x Sb х Cr 2 (PO 4 ) 3 , it is possible to obtain materials with low thermal expansion coefficients: 0.5×10 −6 ≤ α av ≤ 1.9×10 −6 °C −1 .

Phosphates Na 1 – x R 0.33 x Ti 2 (PO 4 ) 3 (R = Y or La; 0 ≤ х ≤ 1) were synthesized by the Pechini process and characterized by X-ray diffraction, electron microscopy with microprobe analysis, and IR spectroscopy. The systems exhibit isodimorphism to form a series of solid solutions belonging to the NaZr 2 (PO 4 ) 3 (NZP) structural type and crystallizing in space group R 3̅ c or R 3̅. The Rietveld structural studies confirmed the isomorphic miscibility of sodium and the rare-earth element in the interstices of the NZP structure. The unit cell parameter с in the phosphates studied tends to increase, and the parameter а tends to decrease, in response to rising temperature, which trends are typical of NZP phosphates.

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН

Arsenate phosphates(1.75 ≤ x ≤ 3.0) were prepared by coprecipitation. Their unit cell parameter was a linear function of composition: a (Å) = 0.0967x + 11.873. The Na3Cr2(AsO4)2PO4 crystal structure was refined by the Rietveld method; the coordinates of basal atoms and bond lengths were calculated. A garnet solid solution crystallizes at 600–650°C; transition to a high−temperature rhombohedral structure occurred at 994–1044°C. The thermal expansion of the low-temperature phase having a garnet structure was studied in the range from –130 to 200°C. The solid solution samples expanded isotropically and had small thermal expansion.

Na1 + 2xZnxZr2 – x(PO4)3 phosphates have been prepared by a sol–gel process followed by heat treatment. A limited series (0 ≤ x ≤ 0.4) of solid solutions with the NASICON structure has been obtained in the system studied. The crystal structure of Na1.8Zn0.4Zr1.6(PO4)3 has been refined by the Rietveld method. The results demonstrate that the structural basis of this phosphate has the form of a mixed framework made up of corner-sharing (Zr/Zn)O6 octahedra and PO4 tetrahedra. The Na+ ions partially occupy two types of structural voids. The sodium ion conductivity of the Na1 + 2xZnxZr2 – x(PO4)3 phosphates has been studied using impedance spectroscopy. Their conductivity has been shown to increase with increasing carrier concentration, reaching 2.7 × 10–4 S/cm at 723 K in the case of Na1.8Zn0.4Zr1.6(PO4)3. It has been shown that increasing the degree of zinc substitution for zirconium leads to a change in the mechanism of defect formation in the materials studied: from intrinsic disorder at x = 0 to impurity-related disorder for x > 0.2. We have estimated the enthalpy of intrinsic sodium ion disorder (72 kJ/mol) and the activation energy for sodium migration (61 kJ/mol).

A method has been developed for the synthesis of single-phase sulfate-phosphates, which provides keeping from sulfur losses by its binding to sulfate with a high decomposition temperature. The thermally stable sulfate-phosphate Pb2Mg2(PO4)2SO4 was studied using X-ray diffraction, IR spectroscopy, DTA, and electron-probe microanalysis. According to the results of the study by the Rietveld method, this compound belongs to the eulytite structural type (space group I4̅3d ). When the temperature changes, its crystals expand isotropically without polymorphic transitions.

M0.5+xMgxZr2–x(PO4)3 (M = Cd, Sr) phosphates have been synthesized via precipitation. In both systems, we have obtained limited solid solutions with the NaZr2(PO4)3 (NZP) structure (in the range 0 ≤ x ≤ 0.6 for M = Cd and 0 ≤ x ≤ 0.5 for M = Sr). Structure refinement of SrMg0.5Zr1.5(PO4)3 by the Rietveld method has shown that the Zr4+ and Mg2+ cations reside on framework sites, whereas Sr2+ ions occupy structural voids. The thermal expansion of the M0.5+xMgxZr2–x(PO4)3 (M = Cd, Sr) phosphates has been studied by X-ray diffraction in the temperature range 173–473 K. The results demonstrate that the incorporation of the larger (Sr2+) cations into voids in the NZP structure leads to a decrease in the magnitude of the axial linear thermal expansion coefficients. We have predicted and investigated a solid solution, Sr0.7Mg0.2Zr1.8(PO4)3, with a nearly zero thermal expansion anisotropy (0.55 × 10–6 K–1) and a small average thermal expansion coefficient (3.92 × 10–6 K–1).