Orthophosphates of Tetravalent Ce, Th, U, Np, and Pu with the Monazite Structure

The problem of development of a ceramic phosphate matrix for solidification of the lanthanideactinide fraction of radioactive wastes is tackled from the crystal-chemical viewpoint. New double orthophosphates of tetravalent f elements (Ce, Th, U, Np, Pu) with alkaline-earth elements and cadmium of the formulas B0.5M₂(PO₄)₃ and B1.5M1.5(PO₄)₃ (B = Mg, Ca, Sr, Ba, Cd; M = Ce, Th, U, Np, Pu) were prepared. The compounds were characterized by X-ray diffraction (including high-temperature measurements) and IR spectroscopy. They crystallize in the monazite structural type (space group P2₁/n). The thermal behavior of the compounds was studied. The unit cell parameters of the phases synthesized were determined. The influence of the cationic composition on the structure formation is analyzed.     

Double Phosphates of Ce(IV) and Some Mono- and Bivalent Elements

A series of double Ce(IV) orthophosphates NaCe₂(PO₄)₃ and B_0.5Ce₂(PO₄)₃ (B = Mg, Ca, Sr, Cd) was prepared by heat treatment of gels with the stoichiometric content of the components. These phosphates were characterized by powder X-ray diffraction, IR spectroscopy, and electron spin resonance. The com- pounds have the monazite structure. The unit cell parameters of the phosphates were determined. The crystal structure of the phosphates is stable at temperatures from 20 to 1600°C. The crystallographic parameters of the double Ce(IV) orthophosphates were compared with those of simple Ce(III) phosphate CePO₄ which is artificial analog of monazite. Crystal chemical features of formation of double phosphates of tetravalent f elements and mono- and bivalent elements were discussed.     

Synthesis and Crystal-Chemical Properties of Phosphates BI IRI I IMI V(PO4)3 Containing f, d, and Alkaline-Earth Elements

New triple phosphates containing f, d, and alkaline-earth elements or cadmium of the general formula BIIRIIIMIV(PO₄)₃ (B = Mg, Ca, Sr, Ba, Cd; R = Nd, Sm, Eu, Gd; M = Zr, Ce, Th, U) were prepared. The compounds were characterized by X-ray phase analysis, IR spectroscopy, and high-temperature X-ray diffraction. The compounds crystallize in the monazite and kosnarite structures (space groups P2₁/n and R-3c or R-3, respectively). The X-ray diffraction characteristics of the synthesized phases were determined. The influence of the cationic composition on the structure formation was analyzed.     

Sr-containing hydroxyapatite: morphologies of HA crystals and bioactivity on osteoblast cells

A series of Sr-substituted hydroxyapatites (HA), of general formula Ca_(10 ? x)Sr_x(PO₄)₆(OH)₂, where x = 2 and 4, were synthesized by solid state methods and characterized extensively. The reactivity of these materials in cell culture medium was evaluated, and the behavior towards MG-63 osteoblast cells (in terms of cytotoxicity and proliferation assays) was studied. Future in vivo studies will give further insights into the behavior of the materials.A paper by Lagergren et al. (1975), concerning Sr-substituted HA prepared by a solid state method, reports that the presence of Sr in the apatite composition strongly influences the apatite diffraction patterns. Zeglinsky et al. (2012) investigated Sr-substituted HA by ab initio methods and Rietveld analyses and reported changes in the HA unit cell volume and shape due to the Sr addition.To further clarify the role played by the addition of Sr on the physico-chemical properties of these materials we prepared Sr-substituted HA compositions by a solid state method, using different reagents, thermal treatments and a multi-technique approach. Our results indicated that the introduction of Sr at the levels considered here does influence the structure of HA. There is also evidence of a decrease in the crystallinity degree of the materials upon Sr addition. The introduction of increasing amounts of Sr into the HA composition causes a decrease in the specific surface area and an enrichment of Sr-apatite phase at the surface of the samples. Bioactivity tests show that the presence of Sr causes changes in particle size and/or morphology during soaking in MEM solution; on the contrary the morphology of pure HA does not change after 14 days of reaction. The presence of Sr, as Sr-substituted HA and SrCl_2, in cultures of human MG-63 osteoblasts did not produce any cytotoxic effect. In fact, Sr-substituted HA increased the proliferation of osteoblast cells and enhanced cell differentiation: Sr in HA has a positive effect on MG-63 cells. In contrast, Sr ions alone, at the concentrations released by Sr-HA (1.21–3.24 ppm), influenced neither cell proliferation nor differentiation. Thus the positive effects of Sr in Sr-HA materials are probably due to the co-action of other ions such as Ca and P.     

Hydrothermal Synthesis of nanocrystalline powders of alkaline-earth hydroxyapatites, A10(PO4)6(OH)2 (A = Ca, Sr and Ba)

In the present work, we report a direct precipitation of nanocrystalline powders of alkaline-earth hydroxyapatites of the compositions, A₁₀(PO₄)₆(OH)₂ (A = Ca, Sr or Ba) from aqueous solutions containing Na₃PO₄ and MCl₂ (M = Ca, Sr or Ba) at 150°C for 2 days and autogeneous pressure under hydrothermal conditions. The products were characterized by X-ray powder diffraction, transmission electron microscopy and scanning electron microscopy. The paper also discusses a convenient and economical hydrothermal route for the extraction of nanocrystalline calcium hydroxyapatite, from fish bone waste.     

On the possibility of realization of monazite and langbeinite structural types in complex americium and plutonium phosphates. Synthesis and X-ray diffraction studies

The existence of phosphates containing Am + Pu with monazite-type structure and of those containing Am + Zr with langbeinite-type structure was substantiated on the basis of the main principles of isomorphism. The compounds were synthesized and examined by X-ray diffraction. Samples of the composition CdAmPu(PO₄)₃ crystallized in the monazite structural type. In the sample containing Am + Zr, in which the formation of a langbeinite-type compound was expected by analogy with lanthanides and Zr [isoformula phosphates K₂LnZr(PO₄)₃, where Ln = Ce-Yb, are known], a mixture of monazite and kosnarite phases was obtained from the chosen reactants under the chosen conditions.     

Ultradisperse Composite Vitrified Host Materials of Monazite–Iron (Pyro)phosphate Type for Conservation of Nuclear Waste Concentrates

A new type of a promising glassy-crystalline host material was prepared by ultradisperse synthesis in model experiments. The starting product is the nitric acid fraction of fission products (Cs, Sr, Ba), obtained radiochemically from irradiated nuclear fuel of a WWER-1000 reactor (1000-MW_el water-cooled water-moderated energy reactor). Equivalent amounts of La(NO₃)₃, H₃PO₄, and Fe₂O₃ are added, and water is removed on heating. After complete removal of moisture, the residue is calcined at 600°С. The phosphated calcinate is dispersed in acetone to obtain slurry and, after removing acetone, converted to the glassy form at 950°С by self-igniting high-temperature synthesis. The whole process of preparing the composite is performed in one container crucible, which is then sealed. The major constituent of the host material is iron (pyro)phosphate glass (~55%); the finely grained crystalline phase, according to X-ray diffraction data, consists mainly of monazite of the composition La₂(Sr,Ba)₃(PO₄)₄. The monazite content is about 40%. Monazite forms with the glass via covalent bonding a composite exhibiting high corrosion resistance. The steady-state rate of Cs leaching form the monazite glass is 0.1 μg cm^–2 day^–1 (90°С, 100 days).     

Growth of cerium-containing films on titanium and aluminum

Cerium-containing films have been grown via plasma electrolytic oxidation in aqueous electrolytes containing Ce(III) polyphosphate complexes and have been characterized by x-ray microanalysis and x-ray diffraction. The films are shown to contain crystalline monazite, CePO₄. Under certain conditions, multilayer surface structures grow on titanium. Air annealing of titanium/film structures at 750°C leads to crystallization of NaTi₂(PO₄)₃ and CeP₅O₁₄.     

Structural variations in layered alkaline earth metal cyclohexyl phosphonates

Two series of alkaline earth metal cyclohexyl phosphonates, M(C₆H₁₁PO₃H)₂(H₂O) (M = Ca, Sr and Ba) (1–3) and M(C₆H₁₁PO₃)(H₂O) (M = Mg, Ca, Sr, and Ba) (4–7) have been synthesized under mild reaction conditions. All new compounds have been characterized using elemental analysis, IR, TGA and powder X-ray diffraction techniques. The molecular structure of compound 2 determined using single crystal X-ray diffraction technique reveals a layered polymeric structure.     

Radiation- and thermally-induced transformations of crystalline cerium(IV) phosphate CePE

The transformations caused by irradiation and thermal treatments of crystalline cerium(IV) phosphate CeP-E [Ce(NH₄PO₄)_0.44(HPO₄)_1.56], which has a high ion exchange capacity, have been studied by electron microscopy, infrared spectroscopy, and X-ray powder diffraction. Electron bombardment in the electron microscope transformed CeP-E into either hexagonal CePO₄ or a vitreous glass (dependent on sample thickness), while irradiation of CeP-E with X-rays produced an amorphous material. All of these materials were transformed into highly stable crystalline monoclinic CePO₄ (monazite) by thermal treatments.     

Hydrothermal synthesis and indication of room temperature ferromagnetism in CeO2 nanowires

Bundle of CeO₂ nanowires have been successfully synthesized by a simple hydrothermal process using Ce(NO₃)₃·6H₂O as cerium source and NaH₂PO₄·2H₂O as mineralizer, into which no surfactant or template was introduced. The synthesized nanowires were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and magnetization measurements. The XRD results indicated that CeO₂ nanowires have fluorite structure. Magnetization measurements indicate that CeO₂ nanowires exhibit room temperature ferromagnetism with remanent magnetization (M_r) and coercivity (H_C) of about 7.44 × 10^? 4 (emu/g) and 27.19 Oe, respectively, which may results due to the presence of defects in the CeO₂ nanowires.     

Hydrothermal conversion of calcite crystals to hydroxyapatite

Hydroxyapatite (HAp) has been successfully synthesized by hydrothermal treatment of calcite crystals in H₃PO₄ suspensions at 120 or 180 °C. Synthesis of HAp crystals under different H₃PO₄ concentrations and time conditions were studied. The results showed that HAp can be produced by the hydrothermal conversion of calcite crystals in the presence of a neutral suspension consisted of Ca 50 mM and PO₄ 60 mM, at 120 and 180 °C for 6 h. Unreacted CaCO₃ remained when PO₄ concentration was less than this at lower temperature. The major formation mechanism of HAp seems to be dissolution/precipitation.     

Hydrothermal synthesis, structural characterization and thermal, spectroscopic and magnetic properties of the inorganic-organic hybrid phosphate, [(C2N2H9)VF(PO4)]

The [(C₂N₂H₉)VF(PO₄)] compound has been synthesized by hydrothermal techniques under autogeneous pressure at 170°C. The compound was characterized from X-ray powder diffraction data with the Rietveld method. Its crystal structure consists of sheets linked by the ethylenediammonium cations and constructed by chains of VO₃F₂N octahedra. The thermal study indicates that the compound is stable up to 290°C. In the IR spectrum the bands of both the ethylenediammonium and phosphate ions are observed. From the reflectance diffuse spectrum the Dq and Racah parameters have been calculated. The magnetic measurements indicate the presence of antiferromagnetic interactions.     

Doped calcium–aluminium–phosphate cements for biomedical applications

Calcium–aluminium–phosphate cements (CAPCs) for biomedical applications, mainly intended for applications in the dental field as non-resorbable fillers, were obtained by reacting Ca-aluminates compounds, i.e. CaO·Al₂O₃ (CA) and CaO·2 Al₂O₃ (CA₂), with Al(H₂PO₄)₃ aqueous solution. Hydroxyapatite was also introduced as a bioactive dispersed phase. Suitable elements like Sr and La were used to increase the radiopacity of the set yielded pastes towards X-ray wavelength used in clinical diagnostic radiographic equipments. La and Sr doped Ca-aluminates powders have been synthesized by solid state reaction at 1,400°C from a mixture of CaCO₃, Al₂O₃, La₂O₃ and SrCO₃. The characteristics of the obtained powders were analyzed and related to the starting compositions and synthesis procedures. The microstructure, setting time, radiopacity and compressive strength of the CAPCs have been investigated and discussed.     

Facile preparation of Ag3PO4 rhombic dodecahedron microcrystals with enhanced catalytic activities under visible light irradiation

Rhombic dodecahedron shaped Ag₃PO₄ microcrystals were prepared by hydrothermal treatment of as-precipitated Ag₃PO₄ products. The phase formation and morphology of the synthesized products were characterized by powder X-ray diffraction and field emission scanning electron microscopy. Experimental results revealed that hydrothermal reaction time can effectively influence the formation of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals. The possible formation mechanism for rhombic dodecahedrons morphology was studied. Moreover, the rhombic dodecahedron-shaped Ag₃PO₄ microcrystals such as 48 h hydrothermally treated sample exhibited higher catalytic activity than as-precipitated and 12 h hydrothermally treated samples under visible light irradiation for the degradation of methylene blue. The enhanced photocatalytic activities of rhombic dodecahedron-shaped Ag₃PO₄ microcrystals is attributed to the existence of most exposed {110} facets.     

Combustion synthesis and photoluminescence of Ce-activated MHfO3 (M = Ba, Sr, or Ca)

A simple sol-combustion route involving EDTA was employed to prepare nanosized MHfO₃:Ce (M = Ba, Sr, or Ca) which were characterized by XRD, TEM and luminescent measurements. The as-prepared powders consist of particles with cube-like morphologies for Ce-doped BaHfO₃ and SrHfO₃ and uniform polyhedron-like morphology for Ce-doped CaHfO₃. The average sizes of crystallites of BaHfO₃:Ce, SrHfO₃:Ce and CaHfO₃:Ce are 48, 34 and 30.4 nm, respectively. Broad emissions for nanocrystals of MHfO₃:Ce (M = Ba, Sr, or Ca) resulting from Ce³⁺ 5d → 4f transitions were observed under UV excitation, and the Stokes shift for nanocrystals of Ce-doped BaHfO₃, SrHfO₃ and CaHfO₃ are around 9000, 7400 and 7020 cm⁻¹, respectively.     

[NZP], a new radiophase for ceramic nuclear waste forms

The NZP structure is a candidate for immobilization of certain types of nuclear waste. It will incorporate ¹³⁷Cs, ⁹⁰Sr, and a range of other nuclides. The leach resistance of CsZr₂(PO₄)₃ appears to be comparable with that of other phases under consideration for radio-Cs immobilization. This phase can be formed by sintering at ～850°C; it is reasonably refractory, and it is compatible with monazite, a favored immobilizing agent for waste actinides.     

Alpha-tricalcium phosphate (α-TCP): solid state synthesis from different calcium precursors and the hydraulic reactivity

The effects of solid state synthesis process parameters and primary calcium precursor on the cement-type hydration efficiency (at 37°C) of α-tricalcium phosphate (Ca₃(PO₄)₂ or α-TCP) into hydroxyapatite (Ca_10−xHPO₄(PO₄)_6−x(OH)_2−x x = 0–1, or HAp) have been investigated. α-TCP was synthesized by firing of stoichiometric amount of calcium carbonate (CaCO₃) and monetite (CaHPO₄) at 1150–1350°C for 2 h. Three commercial grade CaCO₃ powders of different purity were used as the starting material and the resultant α-TCP products for all synthesis routes were compared in terms of the material properties and the reactivity. The reactant CaHPO₄ was also custom synthesized from the respective CaCO₃ source. A low firing temperature in the range of 1150–1350°C promoted formation of β-polymorph as a second phase in the resultant TCP. Meanwhile, higher firing temperatures resulted in phase pure α-TCP with poor hydraulic reactivity. The extension of firing operation also led to a decrease in the reactivity. It was found that identical synthesis history, morphology, particle size and crystallinity match between the α-TCPs produced from different CaCO₃ sources do not essentially culminate in products exhibiting similar hydraulic reactivity. The changes in reactivity are arising from differences in the trace amount of impurities found in the CaCO₃ precursors. In this regard, a correlation between the observed hydraulic reactivities and the impurity content of the CaCO₃ powders—as determined by inductively coupled plasma mass spectrometry—has been established. A high level of magnesium impurity in the CaCO₃ almost completely hampers the hydration of α-TCP. This impurity also favors formation of β- instead of α-polymorph in the product of TCP upon firing.     

Structure and stability of monazite- and zircon-type LaVO4 under hydrostatic pressure

Pure monazite (m)- and zircon (t)-type LaVO₄ and LaVO₄:Eu³⁺ were successfully synthesized by a hydrothermal method. The high pressure behavior of m- and t-LaVO₄ nanoparticles has been investigated using Raman scattering techniques at room temperature. Raman measurements reveal a slight change for m-LaVO₄ at 11.2 GPa because of an isostructural phase transition. However, striking changes in Raman spectra indicate a pressure-induced irreversible phase transition from the zircon to monazite structure for t-LaVO₄ at around 5.9 GPa. The evolution of the luminescence spectra of t-LaVO₄:Eu³⁺ has also been studied during the pressure-induced phase transition. It is observed that pressure has a great influence on the fluorescence intensity and the energy levels, which allows a more in-depth understanding of the nature of the pressure-induced phase transition for t-LaVO₄. This result further confirms the conclusion that zircon-type RVO₄ compounds with larger rare-earth cations will experience zircon to monazite phase transition.     

The dependence of structure and conductivity in three-dimensional phosphate ionic conductors on composition

Two series of solid solutions, Na _x Ca_(1−x)/2Zr₂(PO₄)₃ (NCZP(x), 0⩽x⩽1) and Na _x Nb_1-x Zr_1+x (PO₄)₃(NNZP(x), 0⩽x⩽1), were synthesized. They were examined by powder X-ray diffraction, infra-red (i.r.) absorption and Raman scattering. Ionic conductivities of graphite coated samples were measured. A complete series of solid solutions was formed for NCZP(x), while a second phase was found forx<0.1 for NNZP(x). The i.r. and Raman spectra of their solid solutions consistently showed the formation of PO₄ tetrahedra with different geometries. The composition dependence of conductivity is interpreted on the basis of a structural change around Na⁺.