Cr-Mo-V-W: A new refractory and transition metal high-entropy alloy system

Cr-Mo-V-W high-entropy alloy (HEA) is studied, with 2553 K equilibrium solidus and high Cr content to promote protective oxide scale formation, suggesting potential applications in hot, oxidising environments. Alloy Search and Predict (ASAP) and phase diagram calculations found a single phase, body-centred cubic (BCC) solid solution at elevated temperatures, across the range of compositions present within the system - uncommon for a HEA of refractory and transition metals. Density functional theory identified solubility of 22 at.% Cr at solidus temperature, with composition-dependent drive for segregation during cooling. An as-cast, BCC single-phase with the composition 31.3Cr-23.6Mo-26.4 V-18.7 W exhibiting dendritic microsegregation was verified

Using in situ synchrotron x-ray diffraction to study lithium- and sodium-ion batteries: A case study with an unconventional battery electrode (Gd2TiO5)

Designing materials for application as electrodes in sodium-ion batteries may require the use of unconventional materials to realize acceptable reversible sodium insertion/extraction capabilities. To design new materials simple electrochemical methods need to be coupled with other techniques such as in situ x-ray diffraction (XRD) to correlate the influence of electrochemical performance on a parameter that can be modified, e.g., the crystal structure of the material. Here we use in situ synchrotron XRD data on Gd2TiO5-containing cells to show the minor changes in reflection positions during discharge/charge that illustrates minimal volume expansion and contraction due to insertion/extraction reactions. These small changes correlate to the Gd2TiO5 anode material in both lithium- and sodium-ion batteries showing reversible capacities of ∼45 and ∼23 mA h/g after 20 cycles, respectively. Analysis of sodium location in the crystal structure shows a preference for sodium in the smaller channels along the c axis direction during the first discharge before moving to the larger channels at the charged state. Therefore, in this work, in situ studies highlight minimal structural changes with respect to volume expansion during electrochemical cycling and illustrate where sodium ions locate within the Gd2TiO5 structure. © 2014 Materials Research Societ

Ion-beam irradiation of lanthanum compounds in the systems La2O3–Al2O3 and La2O3–TiO2.

Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation. © 2010, Elsevier Ltd

Ion beam irradiation of lanthanum compounds in the series La2O3-TiO2.

Thin crystals of La2O3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions in the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), at the Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1 × 1016 ions cm-2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for TiO2, are discussed with reference to the phase diagrams for La2O3-TiO2 systems and the different local environments within the crystal structures. Results suggest an observable inverse correlation between Tc and melting temperature (Tm) in the two systems.Australian Nuclear Science and Technology Organisation (ANSTO); Los Alamos National Laboratory; CEA Nuclear Energy Division; Materials Research Society (MRS

Soft chemical synthesis and structural characterization of Y2HfxTi2−xO7

A novel soft chemistry route was developed to synthesize Y2HfxTi2−xO7 (0≤x≤2.0) oxide solid solution. An aqueous solution containing reactants was produced to ensure the combustion reaction taking place at the molecular level. A suite of characterization techniques, including X-ray diffraction, Raman, transmission electron spectroscopy, as well as X-ray absorption near edge structure (XANES), is employed to investigate the structural and phase changes of the series. Both X-ray and electron diffraction patterns show that the Y2HfxTi2−xO7 system undergoes a clear composition-induced phase transition from ordered pyrochlore to disordered defect-fluorite at x~1.5. On the other hand, Raman and XANES spectra reveal a gradual evolution of the local structure with the substitution of Hf for Ti. © 2014 Elsevier Ltd and Techna Group S.r.l

Crystal chemistry of the orthorhombic Ln2TiO5 compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy.

The crystal structures of seven samples of orthorhombic (Pnma) Ln2TiO5 compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy were refined by Rietveld analysis of synchrotron X-ray powder diffraction (S-XRD) data. With increasing size of the lanthanide cation, the lattice parameters increase systematically: c by only ~1.5% whereas both a and b by ~6% from Dy2TiO5 to La2TiO5. The mean Ti–O bond length only increases by ~1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti–O3 bonds to opposite corners of the Ti–O5 square based pyramid polyhedra. For Dy2TiO5 and Tb2TiO5, a significant variation in Ti–O1 and Ti–O4 bond lengths results in an increased deformation of the Ti–O5 base. The particular configuration consists of large rhombic shaped tunnels and smaller triangular tunnels along the b axis, which have implications for defect formation and migration caused by radiation damage or the ionic conductivity. © 2017 Elsevier B.V

Radiation tolerance of M(n+1)AX(n) phases, Ti3AlC2 and Ti3SiC2.

During investigations of novel material types with uses in future nuclear technologies (ITER/DEMO and GenIV fission reactors), ternary carbides with compositions Ti3AlC2 and Ti3SiC2 have been irradiated with high Xe fluences, 6.25 × 1015 ions cm−2 (25–30 dpa), using the IVEM-TANDEM facility at Argonne National Laboratory. Both compositions show high tolerance to damage, and give indications that they are likely to remain crystalline to much higher fluences. There is a visible difference in tolerance between Ti3AlC2 and Ti3SiC2 that can be related to the changes in bonding within each material. These initial findings provide evidence for a novel class of materials (+200 compounds) with high radiation resistance, while, significantly, both of these materials are composed of low-Z elements and hence exhibit no long-term activation. © 2010, Elsevier Ltd

New pathway for the preparation of pyrochlore Nd2Zr2O7 nanoparticles

Pyrochlore Nd2Zr2O7 nanoparticles were prepared by complex-precipitation in aqueous media, followed by calcination in MgO matrix and subsequent dissolution processing. A suite of characterization techniques, including X-ray diffraction, Raman, TEM, SEM, dynamic light scattering, and nitrogen sorption, was employed to investigate the structure and particle size of the synthesized nano materials. Results show that calcination at 1200 °C for 20 h forms Nd2Zr2O7 with pyrochlore structure. The matrix phase (MgO) had no effect on the formation of pyrochlore phase. The MgO phase was readily removed by dissolution at 0.5 M HNO3 aqueous solution; and the remaining pyrochlore Nd2Zr2O7 nanoparticles had a diameter of approximately 200 nm estimated by TEM and approximately 550 nm determined by light scattering due to slight aggregation. The bulk density of the pelletized powder reached approximately 99% of theoretical value, after uniaxial pressing at 2.0–2.5 MPa and sintering at 1400 °C for 48 h. © 2015 Elsevier B.V

Uranium sorption on various forms fo titanium dioxide - influence of surface area, surface charge and impurities

Titanium dioxide (TiO(2)) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO(2) surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO(2) materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO(2) samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption. © 2011, American Chemical Societ

Crystal structures of orthorhombic, hexagonal, and cubic compounds of the Sm(x)Yb(2−x)TiO5 series

A series of single phase compounds with nominal stoichiometry Sm(x)Yb(2−x)TiO5 (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln2TiO5 series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. © 2014, Elsevier Inc