How temperature influences the stoichiometry of CeTi2O6

Of the many materials examined for the sequestration of nuclear waste, Ti oxides have received considerable attention. Brannerite (UTi₂O₆), in particular, has been studied extensively for this application. The Ce analogue of this material (CeTi₂O₆) has been widely investigated instead of the actinide versions owing to the reduced safety hazards and because Ce has similar crystal chemistry to U and Pu. In this study, examination of Ti K-, Ce L₃-, and Ce M_4,5-edge XANES spectra lead to the conclusion that CeTi₂O₆ was O-deficient when synthesized at high temperature and then quench cooled, and that the degree of O-deficiency was reduced upon post-annealing at lower temperatures. These observations can be ascribed to a temperature-dependant Ce³⁺/Ce⁴⁺ redox couple. This investigation suggests that Ce-containing materials may not properly simulate the actinide-bearing analogues; however, CeTi₂O₆ could be useful for other applications, such as catalysis.     

Heat capacities and absolute entropies of UTi<Subscript>2</Subscript>O<Subscript>6</Subscript> and CeTi<Subscript>2</Subscript>O<Subscript>6</Subscript>

Summary As part of a larger study of the physical properties of potential ceramic hosts for nuclear wastes, we report the molar heat capacity of brannerite (UTi₂O₆) and its cerium analog (CeTi₂O₆) from 10 to 400 K using an adiabatic calorimeter. At 298.15 K the standard molar heat capacities are (179.46±0.18) J K^-1 mol^-1 for UTi₂O₆ and (172.78±0.17) J K^-1 mol^-1 for CeTi₂O₆. Entropies were calculated from smooth fits of the experimental data and were found to be (175.56±0.35) J K^-1 mol^-1 and (171.63±0.34) J K^-1 mol^-1 for UTi₂O₆ and CeTi₂O₆, respectively. Using these entropies and enthalpy of formation data reported in the literature, Gibb’s free energies of formation from the elements and constituent oxides were calculated. Standard free energies of formation from the elements are (-2814.7±5.6) kJ mol^-1 for UTi₂O₆ and (-2786.3±5.6) kJ mol^-1 for CeTi₂O₆. The free energy of formation from the oxides at T=298.15 K are (-5.31±0.01) kJ mol^-1 and (15.88±0.03) kJ mol^-1 for UTi₂O₆ and CeTi₂O₆, respectively.     

Surface charge tuning of ceria particles by titanium doping: Towards significantly improved polishing performance

Titanium-doped ceria Ce_1 ? x Ti_xO₂ (x = 0–0.3) powders were prepared and their material removal rate (MRR) values for polishing the ZF₇ optical glass were evaluated with respect to their particle sizes, surface charges, crystallinity as well as the suspension stability. Significantly increased MRR values with a particle zeta potential dependence were observed for all the Ti-doped ceria powders, indicating that ceria abrasives with high MRR can be designed and synthesized by tuning particle surface charge using the titanium doping method. The XRD and Raman spectroscopic analyses revealed that the large increase in MRR and the surface negative zeta potentials were attributed to lattice defects due to the formation of CeO₂–TiO₂ solid solutions and the CeTi₂O₆ phase. A maximum MRR value of 544 nm min^?1 was obtained using Ce_0.9Ti_0.1O₂ solid solution as a polishing powder for the ZF7 glass. This value is ca. 2.2 times of that obtained from using pure ceria. With the x value further increasing to 0.2 and 0.3, the MRR value decreased slightly with the CeTi₂O₆ phase content increasing. This fact reveals that the contribution of CeTi₂O₆ to the MRR increase is less than that of CeO₂–TiO₂ solid solution.     

Formation of cerium titanate,CeTi<Subscript>2</Subscript>O<Subscript>6</Subscript>, in sol–gel films studied by XRD and FAR infrared spectroscopy

The process of formation of cerium titanate films as a function of annealing temperature and composition has been studied by combining X-ray diffraction analysis and far infrared spectroscopy. The films have been prepared by a sol–gel synthesis using metal chlorides as precursors; the synthesis allows obtaining cerium titanate films upon annealing in air. A brannerite type, CeTi₂O₆, phase has been identified by X-ray diffraction and Rietveld analysis on thin films. CeTi₂O₆ is formed upon annealing at 700 °C and in a limited range of ceria-titania mixed compositions. The far infrared spectra are useful to observe the formation of crystalline phases at the beginning of the crystallization process at lower firing temperatures, when the XRD analysis is not enough sensitive.     

Investigation of the stability of glass-ceramic composites containing CeTi2O6 and CaZrTi2O7 after ion implantation

Glass-ceramic composite materials have been investigated for nuclear waste sequestration applications due to their ability to incorporate large amounts of radioactive waste elements. A key property that needs to be understood when developing nuclear waste sequestration materials is how the structure of the material responds to radioactive decay of nuclear waste elements, which can be simulated by high energy ion implantation. Borosilicate glass-ceramic composites containing brannerite-type (CeTi₂O₆) or zirconolite-type (CaZrTi₂O₇) oxides were synthesized at different annealing temperatures and investigated after being implanted with high-energy Au ions to mimic radiation induced structural damage. Backscattered electron (BSE) images were collected to investigate the interaction of the brannerite crystallites with the glass matrix before and after implantation and showed that the morphology of the crystallites in the composite materials were not affected by radiation damage. Surface sensitive Ti K-edge glancing angle XANES spectra collected from the implanted composite materials showed that the structures of the CeTi₂O₆ and CaZrTi₂O₇ ceramics were damaged as a result of implantation; however, analysis of Si L_2,3-edge XANES spectra indicated that the glass matrix was not affected by ion implantation.     

Local structure in solid solutions of stabilised zirconia with actinide dioxides (UO2, NpO2)

The local structure of (Zr,Lu,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O_2−x and (Zr,Y)O_2−x precursor materials with the actinide oxide powders, respectively. Sintering at 1600 °C in Ar/H₂ yields a fluorite structure with U(IV) and Np(IV). As typical for stabilised zirconia the metal-oxygen and metal-metal distances are characteristic for the different metal ions. The bond lengths increase with actinide concentration, whereas highest adaptation to the bulk stabilised zirconia structure was observed for UO and NpO bonds. The ZrO bond shows only a slight increase from 2.14 Å at 6 mol% actinide to 2.18 Å at infinite dilution in UO₂ and NpO₂. The short interatomic distance between Zr and the surrounding oxygen and metal atoms indicate a low relaxation of Zr with respect to the bulk structure, i.e. a strong Pauling behaviour.     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemical solution deposited silver tantalate niobate, Ag x (Ta0.5Nb0.5)O3? y , thin films on (111)Pt/Ti/SiO2/(100)Si substrates

Silver tantalate niobate films are candidates for temperature stable microwave dielectrics. In this work, a chemical solution deposition synthesis method was developed for Ag _x (Ta_0.5Nb_0.5)O_3−y films on Pt-coated Si substrates. Stable solutions with a range of silver stoichiometries were prepared using 2-methoxyethanol and pyridine as solvents, from AgNO₃ and Nb and Ta ethoxide precursors. It was extremely difficult to prepare phase-pure perovskite films of Ag(Ta_0.5Nb_0.5)O₃ on Pt-coated Si subtrates; instead a mixture of perovskite and natrotantite phases was identified. Such mixed phase films had dielectric constant ɛ _r and dielectric loss tanδ values ranging from 200±20 to 270±25 and 0.006±0.002 to 0.002±0.001 at 100 kHz, respectively, depending on the firing temperature. For Ag₂(Ta_0.5Nb_0.5)₄O₁₁, Ag_0.8(Ta_0.5Nb_0.5)O_2.9, Ag_0.85(Ta_0.5Nb_0.5)O_2.925 and Ag_0.9(Ta_0.5Nb_0.5)O_2.95 films, mainly the natrotantite phase was observed. The ɛ _r values of these films were between 70±10 and 130±15 with tan δ values of 0.008±0.002 at 100 kHz.     

Self-irradiation induced structural changes in the transplutonium pyrochlores An2Zr2O7 (An=Am, Cf)

We have pursued the fundamental chemistry of actinide pyrochlore oxides, An₂Zr₂O₇ (An=Am, Cm, Bk, and Cf), using X-ray diffraction as well as optical spectroscopy. One recent facet of our studies has been to observe the structural changes of these materials under self-irradiation as a function of time. It has been reported that both titanate and silicate materials transform from a crystalline to an amorphous state under irradiation. With the Zr-based actinide pyrochlores studied here, we have observed a phase change from a pyrochlore structure to a fluorite-type structure with the retention of crystallinity. We focus here on the impact of α-radiation (²⁴³Am and ²⁴⁹Cf), rather than that from neutrons (²⁴⁸Cm) or β-radiation (²⁴⁹Bk), on the An₂Zr₂O₇ pyrochlore structures. As a result of this phase change, the local coordination environments of both the actinide and zirconium atoms are altered. We consider a defect/ion deficiency driven mechanism and also address the occurrence of oxidation of the trivalent actinides during the self-irradiation process as being potential mechanisms responsible for the observed phase change.     

Tetranuclear Uranyl Polyrotaxanes: Preferred Selectivity toward Uranyl Tetramer for Stabilizing a Flexible Polyrotaxane Chain Exhibiting Weakened Supramolecular Inclusion

Introduction of mechanically interlocked components into actinide‐based metal–organic materials such as polyrotaxanes will generate an entirely new type of inorganic–organic hybrid materials showing more supramolecular encapsulation‐based dynamics. In this work, tetranuclear uranyl‐directed polyrotaxanes (UO₂)₄O₂‐C5A3‐CB6 ( 1 ) and (UO₂)₄O₂‐C6A3‐CB6 ( 2 ), which are the first actinide pseudorotaxanes with high‐nuclearity uranium centers, were obtained through systematic extension of the string spacer in pseudorotaxane ligands from 1,4‐butylene (C4) to 1,5‐pentylene (C5) and 1,6‐hexylene (C6). Both of the as‐synthesized tetranuclear uranyl polyrotaxanes were structurally characterized and analyzed. Considering the structure of UO₂‐C4A3‐CB6 and the 1,4‐butylene string spacer, the preference for the uranyl tetramer may be related to the configurational inversion of the pseudorotaxane ligands from trans mode to cis mode on coordination to the uranyl center. Detailed structural analysis suggests that the length of the stretched string molecules for CB6 ‐encapsulated pseudorotaxanes has remarkable effect on the supramolecular inclusion interactions and the configurations of pseudorotaxanes, and should be responsible for the configurational inversion of pseudorotaxane spacers and subsequent distinct changes of the uranyl building units and geometric structures.     

High‐Pressure Synthesis and Characterization of New Actinide Borates,AnB4O8 (An=Th,U)

New actinide borates ThB₄O₈ and UB₄O₈ were synthesized under high‐pressure, high‐temperature conditions (5.5 GPa/1100 °C for thorium borate, 10.5 GPa/1100 °C for the isotypic uranium borate) in a Walker‐type multianvil apparatus from their corresponding actinide oxide and boron oxide. The crystal structure was determined on basis of single‐crystal X‐ray diffraction data that were collected at room temperature. Both compounds crystallized in the monoclinic space group C2/c (Z=4). Lattice parameters for ThB₄O₈: a=1611.3(3), b=419.86(8), c=730.6(2) pm; β=114.70(3)°; V=449.0(2) ų; R₁=0.0255, wR₂=0.0653 (all data). Lattice parameters for UB₄O₈: a=1589.7(3), b=422.14(8), c=723.4(2) pm; β=114.13(3)°; V=443.1(2) ų; R₁=0.0227, wR₂=0.0372 (all data). The new AnB₄O₈ (An=Th, U) structure type is constructed from corner‐sharing BO₄ tetrahedra, which form layers in the bc plane. One of the four independent oxygen atoms is threefold‐coordinated. The actinide cations are located between the boron–oxygen layers. In addition to Raman spectroscopic investigations, DFT calculations were performed to support the assignment of the vibrational bands.     

Phase transition of negative thermal expansion Zr1-xHfxW2O8 solid solutions

A powder X-ray diffraction experiment was performed on cubic Zr_1-xHf_xW₂O₈ (x=0.25, 0.50 and 0.75) solid solutions from 90 to 560 K. The lattice parameters of Zr_1-xHf_xW₂O₈ at 121 K decreased linearly with increasing Hf contents, due to smaller ionic radius of hafnium than that of zirconium. Transition temperatures due to α-β structural phase transition increased with increasing Hf contents, reflecting the decrease of lattice free volume related to the orientation of unshared vertex of WO₄. Anomaly in the heat capacity of Zr_0.5Hf_0.5W₂O₈ was observed around 450 K which was 9 K lower than that by X-ray diffraction method. Transition entropy of Zr_0.5Hf_0.5W₂O₈ was 2.1 J mol^-1 K^-1, consistent with those of ZrW₂O₈ and HfW₂O₈. This consistent entropy supports that Zr_1-xHf_xW₂O₈ (x=0-1.0) has the same order-disorder phase transition mechanism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural and electronic properties of PbZrxTi1‐xO3 (x = 0.5, 0.375): A quantum chemical study

The structural and electronic properties of the PZT materials PbZr_0.5Ti_0.5O₃ and PbZr_0.375Ti_0.625O₃ were studied by means of a Hartree–Fock quantum chemical semiempirical method that employs a periodic large unit cell (LUC) model. The atomic relaxation observed upon introduction of the Zr impurities resulted in outward oxygen atom displacements along the 〈100〉 direction for the cubic phases and varied oxygen and lead atom movements for the tetragonal structures. For these materials, the conduction bands (CB) were composed mainly of Pb 6p atomic orbitals with less important contributions of Zr 4d and Ti 3d states. The upper valence band (UVB) for the cubic phases was mostly Pb 6s in nature, with minor contribution of O 2p atomic orbitals. The tetragonal phase on the other hand was formed by Pb 6s with some contribution of admixed O 2p with Zr s atomic orbitals. The optical band gap (ΔSCF method) was found to decrease going from the cubic to the tetragonal phase in both titanates. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 37–43, 2003     

Sol–gel synthesis and structural characterization of niobium-silicon mixed-oxide nanocomposites

(Nb₂O₅) _x ·(SiO₂)_1−x gels of four different compositions with x = 0.025 (2.5Nb), 0.050 (5Nb), 0,10 (10Nb) and 0.20 (20Nb) were synthesized at room temperature from niobium penta-chloride and tetra-ethoxysilane and their structural evolution with the temperature was examined by X-ray diffraction, thermogravimetry/differential thermal analysis, Raman and IR spectroscopy (Fourier transform). The synthesis procedure tuned in this work allowed to obtain for each studied composition transparent chemical gels in which the niobium dispersion resulted to be strongly dependent on the Nb₂O₅ loading: it was on the atomic scale for the 2.5Nb and 5Nb gel samples whereas the gel structure of the 10Nb and 20Nb appears formed by phase separated niobia-silica nanodomains. All dried gels keep their amorphous nature up to 873 K, while at higher temperatures crystallization of T- and H-Nb₂O₅ polymorphs were observed according to the Nb₂O₅ loading: at low loading T-Nb₂O₅ was the main crystallising phase, whereas at higher one the H-Nb₂O₅ prevails. Particularly, T-Nb₂O₅ was the sole crystallising phase in the whole explored temperature range for the 2.5Nb, keeping its nanosize up to 1273 K for all samples except for the 20Nb.     

Synthesis, X-ray diffraction study, and ionic conductivity of new AB2O6 pyrochlores

The oxides A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl), A(Ti_0.5W_1.5)O₆ (A = Rb, Cs, Tl), and Cs(B_0.5W_1.5)O₆ (B = Zr, Hf) have been obtained as polycrystalline powders giving X-ray diffraction patterns characteristic of defect cubic pyrochlores, space group (No. 227), Z = 8. The best discrepancy R factors, from 0.0265 for Rb(Ti_0.5Te_1.5)O₆ to 0.0554 for Cs(Zr_0.5W_1.5)O₆, were obtained for the B cations randomly distributed at 16(d), A ions at one quarter of 32(e), and oxygen atoms at 48(f) positions. A linear relationship is observed between the a unit cell parameters and the ionic radii of the A cations, as well as the average ionic radii of the B atoms. The results of electrical resistivity measurements for A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl) are given.     

High‐pressure synthesis and crystal structure of the strontium tungstate Sr3W2O9

The strontium tungstate compound Sr₃W₂O₉ was prepared by a high‐pressure synthesis technique. The crystal structure was determined by single‐crystal X‐ray diffraction and transmission electron microscopy. The structure was found to be a hettotype structure of the high‐pressure phase of Ba₃W₂O₉, which has corner‐sharing octahedra with a trigonal symmetry. Sr₃W₂O₉ has a monoclinic unit cell of C2/c symmetry. One characteristic of the structure is the breaking of the threefold rotation symmetry existing in the high‐pressure phase of Ba₃W₂O₉. The substitution of Sr at the Ba site results in a significant shortening of the interlayer distances of the [AO₃] layers (A = Ba, Sr) and causes a distortion in the crystal structure. In Sr₃W₂O₉, there is an off‐centre displacement of W⁶⁺ ions in the WO₆ octahedra. Such a displacement is also observed in the high‐pressure phase of Ba₃W₂O₉.     

Separation of betacyanins from flowers of Amaranthus cruentus L. in a polar solvent system by high‐speed counter‐current chromatography

Betacyanin extract of Amaranthus cruentus L. flowers was fractionated by semi‐preparative high‐speed counter‐current chromatography in a highly polar solvent system: propan‐1‐ol/acetonitrile/(NH₄)₂SO_{4satd. soln}/H₂O (1.0:0.5:1.2:1.0, v/v/v/v) in tail‐to‐head mode with 76% retention of the stationary phase. The crude extract as well as the fractions containing betacyanins were analyzed by liquid chromatography with tandem mass spectrometry as well as by high‐resolution ion‐trap time‐of‐flight mass spectrometry detection technique for the molecular formulae and multi‐step fragmentation pattern elucidation. Four betacyanins; namely, amaranthin, betanin, 6′‐O‐formyl‐amaranthin, and 6′‐O‐malonyl‐amaranthin as well as their diastereomeric forms differing in the configuration of the C‐15 carbon atom were identified in the fractions. Amaranthin was the dominant pigment in the extract and was additionally analyzed by nuclear magnetic resonance correlation techniques after the counter‐current chromatographic and high‐performance liquid chromatographic isolation. Betacyanins were highly enriched during a single high‐speed counter‐current chromatographic step; therefore, the tentative identification of new compounds for the whole Amaranthaceae family, 6′‐O‐formyl‐amaranthin and 6′‐O‐malonyl‐amaranthin was possible. Different elution profiles of the pigments observed in the counter‐current chromatographic system in comparison to high‐performance liquid chromatography system confirm a complementarity of both the techniques especially in the separation of diastereomeric pairs of betacyanins.     

Lithium vanadate Li3+xV6O13 at low temperature

The structure of Li_3+xV₆O₁₃ [x = 0.24 (3)] at 95 K has been solved and refined using single‐crystal X‐ray diffraction. The refined lithium content corresponds to two fully occupied Li sites and one partially occupied Li site. A doubling of the c axis is observed upon cooling from room temperature, and this change is associated with shifts of the V atoms. The resulting space group is C2/c. The Li disorder present in the Li₃V₆O₁₃ phase at room temperature is also observed in the low‐temperature phase reported here.     

LixNi0.5Co0.5O2的态密度计算及电化学性能研究

>为获得综合性能更好的锂离子二次电池正极材料, 分析了Co掺杂对Li_xNiO₂电化学性能的影响. 采用密度泛函DFT理论对Li_xNiO₂和Li_xNi_0.5Co_0.5O₂的平均放电电压和态密度进行了计算. 同时, 用共沉淀法制备了Li_xNiO₂和Li_xNi_0.5Co_0.5O₂锂离子二次电池正极材料, 并对其进行了XRD结构分析和恒流充放电测试. 实验和计算结果表明: 随锂离子嵌入正极(电池放电), 电池的电压逐渐降低, 材料的态密度峰向低能量方向移动; 与Li_xNiO₂相比, Li_xNi_0.5Co_0.5O₂的电压平台相对较高(当0.25≤x≤0.5), 而且在Li^＋嵌/脱时, Li_xNi_0.5Co_0.5O₂的结构变化相对较小; Co离子的掺入, 减小了NiO₆八面体的畸变度, 使材料的电化学稳定性得以提高. 在钴掺杂镍酸锂体系中, NiO₆和CoO₆具有相互的稳定作用.