Crystallographic evaluation of titanate ceramics as a host structure for immobilization of samarium

In order to simulate the mechanism of rare earth fixation in perovskite, Ca_1?x Sm_2/3x TiO₃ (x = 0.1–0.5) was synthesized via ceramic route at 1050°C. Up to ～4 mol % Sm could be loaded into calcium titanate formulations without significant changes of the three-dimensional framework structure. Their crystal structures were refined by Rietveld analysis of powder X-ray diffraction data. The structure refinement converged to satisfactory values of the Rietveld parameters (R _p and R _wp ) and goodness of fit. The lattice parameters, structure factors, bond lengths, and bond angles were calculated from General Structure Analysis System software (GSAS). The M-O stretching and bending vibrations in the IR region were assigned. The crystal morphology was examined by scanning electron microscopy. Energy-dispersive X-ray (EDX) analysis of the specimens shows that samarium enters into the structural framework of CaTiO₃.     

Diffusion of lanthanum into single-phase sodium zirconium phosphate matrix for nuclear waste immobilization

Sodium zirconium phosphate (NZP) is a potential material for immobilization of nuclear effluents. Up to ～2.17 mol % (10.71 wt %) lanthanum could be loaded into NZP formulations without significant changes in the three-dimensional framework structure. The crystal chemistry of Na_1+x Zr_2?x La _x P₃O₁₂ (x = 0.1?0.5) phases was investigated using General Structure Analysis System programming. The LaNZP phases crystallize in space group $R\bar 3c$ with Z = 6. Powder diffraction data were subjected to Rietveld refinement to arrive at a satisfactory structural convergence of R-factors. The PO₄ stretching and bending vibrations in the IR region were assigned. SEM and EDAX analysis provide evidence of La in the matrix.     

Immobilization of cesium by crystalline zirconium phosphate

HZr₂(PO₄)₃ was prepared by the thermal decomposition of NH₄Zr₂(PO₄)₃ which was synthesized in advance by a hydrothermal reaction from a mixed solution of ZrOCl₂, H₃PO₄ and H₂C₂O₄. Mixtures of HZr₂(PO₄)₃ with various amounts of CsNO₃ were treated at 700–1200°C, in order to investigate the immobilization of Cs ion. When a mixture of CsNO₃/HZr₂(PO₄)₃ in a molar ratio of 0.36 was treated at 700°C, the main product was suggested to be CsZr₂(PO₄)₃ from XRD measurements. The leaching rate of Cs ion from this product was less than 10^–10 g · cm^–2 · day^–1 in 0.1 mol · 1^–1 HCl solution at 100°C, indicating that HZr₂(PO₄)₃ reacts with CsNO₃ to give a stable Cs-immobilized product.     

Iron pyrophosphate matrix as a form for immobilization of cesium radionuclides

Several modifications of CsFeP₂O₇ were prepared by fine dispersion procedure: glass, glass ceramic, and finely crystalline composite. The cesium leach rates from these CsFe pyrophosphate forms at 90°C were determined. The pyrophosphate glass shows promise as a form for isolation of fission products in a deep underground repository.     

Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates containing tin

Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM³⁺SnP₃O₁₂ (A=Ca, Sr and Ba; M³⁺=Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073 K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the ‘sodium zirconium phosphate behaviour’. However, the strontium- and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds.     

Silicates of leucite structure as environmentally safe forms of cesium and strontium immobilization

Silicates containing binary combinations of Cs cations with Mg, Sr, Cu, or Fe cations or a ternary combination of Cs, Cu, and Fe cations were prepared by the sol-gel procedure followed by heat treatment of the dried gels. Silicates of the composition Cs₂MgSi₅O₁₂, Cs₂SrSi₅O₁₂, Cs₂CuSi₅O₁₂, CsFeSi₂O₆, and Cs₂CuSi₅O₁₂·CsFeSi₂O₆ (Cs₃CuFeSi₇O₁₈), according to the results of X-ray phase analysis, belong to the leucite family, space group Ia3d. The temperature range of their formation is 370–450°C (data of differential scanning calorimetry and X-ray phase analysis). The silicates underwent no chemical or phase transformations on heating to 900–1000°C. No egress of Cs from the silicates to the gas phase was observed on heating for 4–6 h at 800°C. The leaching rates of Cs and Sr at 25°C on the seventh day reached 10⁻⁷ g cm⁻² day⁻¹ (powdered samples, steady-state mode). The X-ray diffraction patterns of the silicates before and after hydrolytic tests were essentially similar.     

Formation of zirconium diboride nanoparticles as a result of reaction between zirconium tetrachloride and sodium borohydride

We have studied reaction between ZrCl₄ and NaBH₄ at temperatures between 300 and 725°C. The results demonstrate that single-phase zirconium diboride nanoparticles are formed starting at 575°C. According to electron microscopy data, the ZrB₂ powder obtained at 575 and 725°C consists of variously shaped particles, some of which are almost spherical, ranging in diameter from ~10 to 20 and from 25 to 35 nm, respectively. These values agree with the equivalent particle diameters evaluated from the measured specific surface area of ZrB₂, ~14 and ~32 nm, respectively, and with the crystallite size extracted from X-ray diffraction data: D _hkl ~ 13 and 28 nm.     

A new sodium iron phosphate as a stable high-rate cathode material for sodium ion batteries

Low-cost room-temperature sodium-ion batteries (SIBs) are expected to promote the development of stationary energy storage applications. However, due to the large size of Na⁺, most Na⁺ host structures resembling their Li+ counterparts show sluggish ion mobility and destructive volume changes during Na ion (de)intercalation, resulting in unsatisfactory rate and cycling performances. Herein, we report a new type of sodium iron phosphate (Na_0.71Fe_1.07PO₄), which exhibits an extremely small volume change (~ 1%) during desodiation. When applied as a cathode material for SIBs, this new phosphate delivers a capacity of 78 mA·h·g^?1 even at a high rate of 50 C and maintains its capacity over 5,000 cycles at 20 C. In situ synchrotron characterization disclosed a highly reversible solid-solution mechanism during charging/discharging. The findings are believed to contribute to the development of high-performance batteries based on Earth-abundant elements.

Open image in new window

     

Study on the structure and antibacterial activity of silver-carried zirconium phosphate

The structure and antibacterial activity of silver-carried zirconium phosphate (AgZrP) were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and viable cell counting method. There existed the exchange of sodium ions with silver ions in carrier, and the silver in AgZrP was in ionic state. Moreover, no significant changes in crystal structure of carrier were found by the exchange of silver ions except for its crystal space distance and crystallinity. The antibacterial tests showed that 100 mg l^− 1 of AgZrP possessed high antibacterial activity and was capable of killing all the Escherichia coli (E. coli) and more than 99.9% of the Staphylococci aureus (S. aureus) within 8.0 h of contact.     

Immobilization of lanthanum,cerium, and selenium into ceramic matrix of sodium zirconium phosphate

The crystallographic nature of NaCe_0.2Zr_1.8P₃O₁₂, NaSe_0.2Zr_1.8P₃O₁₂, and NaLa_0.13Ce_0.14Se_0.15·Zr_1.58P₃O₁₂ phases has been investigated with the aim of developing methods for radionuclide immobilization into sodium zirconium phosphate (NZP) phase. The phases have the NZP structure, space group \(R\bar 3c\) , Z = 6. Powder diffraction data have been subjected to Rietveld refinement, and satisfactory structural convergence of R-factors was achieved. The PO₄ stretching and bending vibration bands in the IR region have been assigned.     

Synthesis of castable sodium zirconium phosphate monoliths employing reactions between zirconyl nitrate hydrate and condensed phosphates

Reactions between zirconyl nitrate hydrate and condensed phosphates can be used to produce castable low CTE sodium zirconium phosphate (NZP) monoliths. Reaction between sodium nitrate, zirconyl nitrate hydrate and condensed phosphoric acid at room temperature (alkali nitrate method) produces monoliths having a heterogeneous microstructure, which are multiphasic in appearance. Except for the presence of crystalline sodium nitrate, they are X-ray amorphous. Differential thermal analysis revealed two distinct exothermic crystallization events when these materials are heated. The first event, with an onset temperature of 650°C, is the result of NZP and ZrO₂crystallization. The second is the result of ZrP₂O₇ crystallization. Reaction between zirconyl nitrate hydrate and condensed sodium phosphate (condensed alkali phosphate method) results in a more homogeneous microstructure in which crystalline zirconium hydrogen phosphate hydrate and sodium nitrate are present. Two exothermic peaks, with onset temperatures of approximately 570 and 860°C, are observed. The first exotherm is the result of NZP, ZrO₂ and ZrP₂O₇ crystallization; the second exotherm is the result of a further NZP formation. After heating materials made by these two methods at 940°C for 24 h, the condensed-alkali-phosphate-method-derived material converted to phase-pure NZP, while the alkali-nitrate-method-derived material contained ZrP₂O₇. The differences in phase evolution between the materials prepared by these two methods are attributable to the differences in chemical and microstructural homogeneity that result from the reactants used. This revised version was published online in September 2006 with corrections to the Cover Date.     

对两种不同晶型磷酸锆载银工艺探索

磷酸锆是一种具有离子交换性能的无机载体,被广泛应用于离子交换体。研究不同晶型磷酸锆的载银工艺,探索银离子交换的机理并制备出高载银量的磷酸锆载银抗菌粉。以锆磷比为1∶2的α-Zr(HPO4)2.H2O和锆磷比为2∶3的六面体磷酸锆为对象,研究在不同离子交换环境下对银离子的交换率。发现α-Zr(HPO4)2.H2O在层间隙扩大的情况下,银离子交换量大于8.47%,而六面体磷酸锆在强酸高温环境中,银离子交换量大于4.45%。具有层状间隙的α-Zr(HPO4)2.H2O比六面体磷酸锆对银离子具有更高的交换容量。为研制具有高效长效抗菌要求的制品,如抗菌医用导管做好准备。     

CsAlSi5O12: a possible host for 137Cs immobilization

CsAlSi₅O₁₂ exhibits more acid resistance than pollucite (CsAlSi₂O₆). At pH values of 1.02 and 1.40, the extraction of Cs from CsAlSi₅O₁₂ at 25° C was approximately proportional to the square root of leach time. The Cs extraction at 25° C varied as [H⁺]^0.36 over the pH range of 1 to 6. Also, the Cs extraction in various brines at 300° C and 30 MPa was comparable with that for pollucite. CsAlSi₅O₁₂ can be crystallized at about 1000° C from calcines if a small amount of CaO is present, but, in the absence of such sintering aids, crystallization temperatures of about 1400° C are necessary. Compatibility data were also obtained with respect to several other phases with which CsAlSi₅O₁₂ might be expected to co-exist in tailored ceramics designed for high-level defence waste.     

Effect of titanium and zirconium substitutions for calcium on the formation and structure of tricalcium phosphate and hydroxyapatite

The effect of Ti and Zr substitutions for Ca cations on the formation of tricalcium phosphate and hydroxyapatite has been studied in a wide concentration range: from 0.1 to 20 mol %. Upon the incorporation of Ti and Zr cations into tricalcium phosphate, the major forming phase is β-tricalcium phosphate. On the addition of low substituent concentrations to hydroxyapatite, we observe the formation of a single-phase material with the apatite structure. Increasing the substituent concentration to 10–20 mol % Ti or 20 mol % Zr leads to the formation of tricalcium phosphate. The unit-cell volume of the cation-substituted tricalcium phosphates has been shown to decrease with increasing substituent concentration. In the zirconium-containing hydroxyapatites, the unit-cell volume decreases with increasing zirconium concentration, whereas the titanium-containing hydroxyapatites exhibit an opposite tendency.     

Magnesium potassium phosphate matrices for immobilization of high-level liquid wastes

Physicochemical properties of magnesium potassium phosphate (MPP) matrices for conservation of highly saline high-level liquid wastes (HLW) after their solidification at room temperature were studied comprehensively. The matrices showed high chemical stability to leaching of radionuclides and other components at various temperatures. The leaching indices of ²³⁹Pu, ²³⁷Np, ²⁴¹Am, Sr, and Cs are in the range 12–14, and those of Tc, I, and Se, in the range 10–11. The mechanical strength (>20 MPa) and radiation resistance of the matrices and the chemical yield of radiolytically generated hydrogen (0.004 molecules/100 eV) were determined. The phase composition of the matrices and the character of radionuclide distribution in their bulk were determined. An enlarged installation was developed and fabricated at the Mayak Production Association, and solidification of simulated liquid radioactive wastes from this plant was performed in volumes of up to 2001.     

Framework flexibility of sodium zirconium phosphate: role of disorder, and polyhedral distortions from Monte Carlo investigation

A detailed Monte Carlo investigation of the structural changes of the framework of sodium zirconium phosphate, [Zr₂P₃O₁₂]⁻,—NASICON (acronym for Na-SuperIonic CONductor)—accommodating alkali ions of varying sizes (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) is carried out over a range of temperatures. Simulation results are critically compared with the structural models proposed earlier and available experimental results. Anisotropic changes of the rhombohedral cell parameters—a contracts while c expands with the size of the alkali ion substituted—is observed in good agreement with previous experimental results. The mechanism of anisotropic variation of lattice parameters involves dominantly, coupled rotations of the polyhedra as proposed by Alamo and co-workers. It is, however, observed that the distortions of the PO₄ tetrahedra and ZrO₆ octahedra are significant, and accounts for nearly one-third of the total change in a and c—parameters as the size of the alkali ion increases. This suggests that ‘rigid’ polyhedral models, permitting only angular distortions of the polyhedra, are of limited quantitative applicability in these solids. The same mechanism is found to be responsible for the low/anisotropic thermal expansion of these solids. Evidence that the polyhedral rotations are dynamic, opposed to a static-frozen-in disorder, is provided.     

Electrical conductivity of cesium phosphate doped with tetravalent cations

We have synthesized and characterized new cesium-phosphate-based solid electrolytes with the general formula Cs_3?4x M _x ^IV PO₄ (M = Zr, Hf, Ce) and high cesium ion conductivity. The increase in the conductivity of Cs₃PO₄ upon the introduction of the dopants studied is primarily due to the formation of cesium vacancies and the broadening of the stability range of the high-temperature phase of cesium phosphate, which has good transport properties.