Anode free Na metal batteries are promising for future energy storage because they not only provide the highest energy densities but also eliminate the need of handling hazardous Na metals during battery manufacturing. However, they suffer from much faster degradation due to strong sensitivities even to trace levels of side reactions. In view of the crucial roles of surface chemistry on modulating electrochemical plating, this work systematically investigated a series of Cu surfaces for Na plating and stripping in the 1.0 M NaPF6 diglyme electrolyte. Our results suggest that Na plating and stripping on pure Cu surface without Cu oxide species exhibits much better reversibility and smaller overpotentials across a wide range of current densities, especially for the first plating/stripping cycle. The high performance includes consistently higher than 99.8% Faradaic efficiencies, much more stable interfacial resistance, and negligible formation of mossy Na after 500 cycles. This improved performance can be explained based on the stronger Na-Cu affinity compared with the Na-CuO affinity. Anode-free Na metal batteries equipped with high-capacity sodium vanadium phosphate cathodes and pure Cu current collector exhibited at least 70% capacity retention for 100 cycles.

Enhanced negative thermal expansion (NTE) properties are achieved by introducing a little amount of boron in the Fe43Mn28Ga28.97B0.03 alloy. As a result, this alloy shows a giant NTE coefficient of alpha(1) = -79.7 x 10(-6) K-1 in a wide temperature range from 277 K to 136 K. Compared to the NTE characteristics in Fe43Mn28Ga29, the NTE operation temperature window has expanded by 74% with the corresponding coefficient of thermal expansion increased by 57% within the NTE temperature window for the boron-doped Fe43Mn28Ga28.97B0.03. In-situ synchrotron high-energy X-ray diffraction results suggest that by boron substitution, the large unit cell volume change across martensitic transformation and the wide phase transition temperature interval are responsible for the pronounced NTE behavior in Fe43Mn28Ga28.97B0.03. Moreover, for the Fe43Mn28Ga28.97B0.03 NTE material, the compressive strength and strain are significantly improved compared with that of Fe43Mn28Ga29. The present study indicates that Fe43Mn28Ga28.97B0.03 with enhanced NTE across martensitic transformation may be used for practical application as thermal-expansion compensators. (C) 2020 Elsevier B.V. All rights reserved.

Polymorphism and polymorphic transitions are attracting considerable interest because of their significant effect on the phase stability, switchable properties, and the atomic rearrangement mechanism in materials. Here, by employing in situ high-pressure synchrotron radiation X-ray diffraction, we reveal surprisingly abundant polymorphic transitions in an Al0.6CoCrFeNi high-entropy alloy (HEA). The original body-centered cubic (bcc) phase transfers to an orthorhombic phase at similar to 10.6 GPa during compression at room temperature, and the orthorhombic phase remains stable up to similar to 40 GPa. When the lattice stress is released entirely, the orthorhombic phase can further transform into a body-centered tetragonal structure. On the other hand, when the original bcc phase is heated to high temperatures, a face-centered cubic phase forms, and it can further transform to a hexagonal close packing phase by applying pressure. In total, we observed five polymorphs in the Al06 HEA, and they all can exist at ambient conditions. The complex chemistry and the pressure/temperature tuned lattice distortion may play a key role in introducing and stabilizing the rich polymorphs in the Al06 HEA. Our findings may bring new insights into the correlation between lattice distortion and phase stability/transitions of HEAs and help to explore HEAs with novel structures and properties. (C) 2018 Elsevier Ltd. All rights reserved.

Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (αl) have been achieved in Fe44−xMn28Ga28+x (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and αl = −50.2 × 10−6 K−1 were realized in the Fe43Mn28Ga29 alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects for use as advanced NTE materials.

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high-temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of certain hole-doped compounds, revealing the existence of a new magnetic phase at compositions close to the onset of superconductivity. Here, we present Mössbauer data that show that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. Instead, this state is naturally explained as the interference between two commensurate spin-density waves, a rare example of collinear double-Q magnetic order. Our results demonstrate the itinerant character of the magnetism of the iron pnictides, and the primary role played by magnetic degrees of freedom in determining their phase diagram. A combination of neutron scattering, X-ray scattering and Mössbauer spectroscopy experiments reveal the existence of a collinear double-Q magnetic ordering in an iron arsenide superconductor.

The crystal structure of the CoMnSi compound during zero-field cooling and field cooling from room temperature down to 200 K was studied using the synchrotron radiation X-ray diffraction technique. The results show that the lattice parameters and thermal expansion behavior of the sample are changed by the applied magnetic fields. The lattice contracts along the a axis, but expands along the b and c axes. Due to enlarged and anisotropic changes under a magnetic field of 6 T, the lattice shows an invar-like behavior along all three axes. Critical interatomic distances and bond angles also show large changes under the influence of such a high magnetic field. These magnetic field-induced changes of the lattice are discussed with respect to their contributions to the large magnetocaloric effect of the CoMnSi compound.

Single-phase polycrystalline samples of stoichiometric RMnO3+delta (R = Er, Y, and Ho) were achieved in the hexagonal P6(3)cm structure through solid state reaction at, similar to 1300 degrees C. Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger Ho and Y show rapid and reversible incorporation of large amounts of excess oxygen (0.3 > delta> 0) at an unusually low temperature range of similar to 190-325 degrees C, indicating the industrial usefulness of RMnO3+delta materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to delta similar to 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with 6 = 0, 0.28, and 0.38 was confirmed by thermogravimetric measurements, synchrotron X-rays, and neutron diffraction. In situ synchrotron diffraction proved the thermal stability of these single phases and the regions of their creation and coexistence, and demonstrated that the stability of the delta = 0.28 phase increases with the ionic size of the R ion. Structural modeling using neutron powder diffraction for oxygen excess phases describes the formation and details of a large R3c superstructure observed for HoMnO3.28 by tripling the c-axis of the original parent unit cell. Modeling of the RMnO3.38 (R = Y and Er) oxygen-loaded phase converged on a structural model consistent with the symmetry of Pca2(1).

A fluorozirconate glass (ZBLAN) containing BaCl2 nanocrystals doped with divalent Eu is a promising material for x-ray image plates for medical diagnosis. Since it is known that Eu2+ readily oxidizes to Eu3+, which reduces fluorescence efficiency of the image plates, 151Eu Mössbauer spectroscopy was used in this work to monitor the Eu oxidation state of the samples during degradation over time in the presence of ambient humidity. In addition, Mössbauer spectroscopic experiments show that the oxidation state has already changed during the glass melt: The sample made from 5 mol% EuCl2 contained 78% EuCl2 + 22% EuCl3 deduced from the relative areas of the absorption lines. The sample made from 2.5 mol% EuCl2 + 2.5 mol% EuCl2 contained 37% EuCl2 + 63% EuCl3, i.e. 26% of the original EuCl2 was oxidized to EuCl3.

Mossbauer spectra of Eu-151 in 20 nm nanoparticles of EuS were measured at several temperatures. The samples were found to contain up to 30% of Eu3+, which suggests that surface oxidation of the nanostructures penetrated farther than the similar to 12% of atomic sites on the outermost surface of the nanoparticles. Below the Curie temperature, T-c = 16.6K, the spectra showed line broadening due to superparamagnetic fluctuations with an average Mossbauer blocking temperature T-B(M) of about 16 K. Below 10 K, the spectrum became a fully split magnetic hyperfine pattern with an effective field of 29.9 T at 5 K. The spectra were compared with those of bulk ferromagnetic EuS. This approach provides considerable information about the composition of oxygen-sensitive nanoscale materials.

Fe-doping (up to 11 mole%) into SrRuO 3 (SRO) thin films on SrTiO 3 substrates decreased correlation lengths of both surface and interface. It turned out that Fe was doped in the valence state of 3+ without formation of the Fe 2 O 3 phase, which caused orthorhombic distortion. T C values decreased from 145 K to 97 K with increasing Fe concentration ( C Fe ). High magnetic switching fields were observed for all Fe-doped SRO thin films and their strengths showed a linear relationship with C Fe . Detail structural characterization using synchrotron X-ray diffraction and X-ray photoemission spectroscopy were used to understand its unique magnetic switching field properties.

The luminescent properties of divalent europium ions can be exploited to produce storage phosphors for x-ray imaging applications. The relatively high cost and limited availability of divalent europium halides makes it desirable to synthesize them from the readily available trivalent salts. In this work, samples of pure EuCl(3) and fluoride glass melts doped with EuCl(3) were processed at 700-800 °C in an inert atmosphere furnace. The Eu oxidation state in the resulting materials was determined using fluorescence and Mössbauer spectroscopy. Heat treatment of pure EuCl(3) for 10 min at 710 °C resulted in a material comprising approximately equal amounts of Eu(2+) and Eu(3+). Glasses made using mixtures of EuCl(2) and EuCl(3) in the starting material contained both oxidation states. This paper describes the sample preparation and analysis and discusses the results in the context of chemical equilibria in the melts.

Nanocrystalline EuCrO3 particles (∼25 nm) have been prepared by pre-milling a 1 : 1 molar mixture of Eu2O3 and Cr2O3 for 60 h followed by sintering at 700 °C (12 h). This temperature is ∼500–600 °C lower than those at which the material, in bulk form, is conventionally prepared. Rietveld analysis of the x-ray powder diffraction pattern of the EuCrO3 nanoparticles favours a structural model involving a slight degree of cationic exchange where ∼11% of the Eu3+ and Cr3+ ions exchange their normal dodecahedral A- and octahedral B-sites, respectively, in the perovskite-related structure. This cationic site exchange, which is unusual in a perovskite structure, has been well supported by the corresponding room-temperature 151Eu Mössbauer spectrum of the nanoparticles that in addition to displaying a distribution in the principal component of the EFG tensor (V zz ) at the usual A-sites of the 151Eu nuclei, also revealed the presence of a subcomponent with ∼11% area fraction and a considerably increased |V zz | value that was associated with Eu3+ ions at octahedral B-sites. X-ray photoelectron and Auger electron spectroscopic techniques reveal a complex surface structure where extremely thin layers of un-reacted Eu2O3 and Cr2O3 cover most of the EuCrO3 nanoparticles' surfaces together with some traces of elemental Cr. The binding energies associated with Eu3+ 3d5/2, Eu3+ 4d3/2, Cr3+ 2p3/2 and O2− 1s core-level electrons in EuCrO3 are estimated from the x-ray photoelectron data for the first time.

A nanocomposite composed of NbTi nanowires uniformly embedded in NiTi matrices was fabricated, which exhibits an ultrahigh mechanical-damping capability. The absorption energy measured under an applied 8% strain is up to 54 MJ/m3, which is over three times higher than that (∼16 MJ/m3) found in the well-known Ni-Ti alloys. In-situ synchrotron x-ray diffraction reveals that a redistribution of stress between the nanowires and matrices was evidenced from an abrupt change in residual lattice strains. The ultrahigh mechanical-damping property is attributed to a combination of the strong interaction of nanowires and matrices and the plastic deformation occurring in NbTi nanowires during deformation causing large energy dissipation.

The high-energy ball-milling method was used for fabricating Ni50Mn36.7In13.3 fine-sized particles. The as-melt polycrystalline Ni50Mn36.7In13.3 alloy exhibits a 14 M modulated martensite structure at room temperature (RT). The atomic pair distribution function analysis together with the differential scanning calorimetry technique proved that the 14 M modulated martensite transformed to a metastable amorphous-like structure after ball milling for 8 hours. Annealing of the ball-milled particles with the amorphous-like phase first led to the crystallization to form a B2 structure at 523 K (250 °C), and then an ordered Heusler L21 structure (with a small tetragonal distortion) at 684 K (411 °C). The annealed particles undergo different structural transitions during cooling, tailored by the atomic arrangements of the high-temperature phase. Low-field thermomagnetization measurements show that the ball-milled particles with the amorphous-like structure or the atomically disordered crystalline structure exhibit a magnetic transition from the paramagnetic-like to the spin-glass state with decreasing temperature, whereas the crystalline particles with the ordered Heusler L21 structure present a ferromagnetic behavior with the Curie temperature T c ≈ 310 K (37 °C).

The phase-stress partition and stress-induced martensitic transformation in a NbTi/NiTi nanocomposite were investigated by employing in situ synchrotron x-ray diffraction during tensile cycling. The phase-stress partition behavior in the nanocomposite is significantly different from that previously reported in the metal-matrix composites. Beyond the initial elastic deformation, the stress carried by the NbTi nanowires increased significantly with increasing macroscopic strain, while the stress taken by the NiTi matrix decreased gradually. We also found that the stress-induced martensitic transformation of the NiTi matrix still proceeded even though the matrix carried decreasing stress rather than constant or increasing stress well known in binary NiTi alloys.

Using neutron and x-ray diffraction, we report the discovery of competing ground states near a multicritical point in A-site layer-ordered La1-xBa1+xMn2O6 materials. We demonstrate the dual effects of deliberate disorder on the system's stability, the freezing of the competing states, and the drastic reduction in magnetic fields required for the suppression of charge-and orbital-ordered phases. Our work suggests that quenched disorder is not the primary reason for phase separation and magnetoresistance and that increased doping leads to electronic phase separation.

The in situ high-energy x-ray diffraction was used for revealing an atomic mechanism on the two-step pseudoelastic behavior found in the premartensitic phase of Ni2MnGa magnetic shape memory alloy. The applied stress first suppresses the three-dimensional modulated structure of the premartensitic phase to a two-dimensional modulated one, which is accompanied by a change in the modulation wave vector and accommodates a large lattice strain reaching similar to 1%. With further increasing stress, the two-dimensional modulated premartensite transforms to the five-layered modulated martensite. The observation of the stress-induced dimensionality crossover of atomic modulation has broad impacts in understanding not only the mechanical properties of advanced shape memory alloys but also the physical properties of condensed matter with heterogeneous structures. (C) 2010 American Institute of Physics. [doi:10.1063/1.3506508]

A polymer-bonded Ni45Co5Mn36.6In13.4 ferromagnetic shape–memory composite was fabricated, having magnetic-field-driven shape recovery properties. The thermo-magnetization curves of the composite suggested that the magnetic-field-induced reverse martensitic transformation occurs in the composite. The effects of temperature, stress, and magnetic-field on the phase transformation properties were systematically investigated using an in-situ high-energy X-ray diffraction technique. A temperature-induced reversible martensitic phase transformation was confirmed within the composite, showing a broad phase transformation interval. Stress-induced highly textured martensite was observed in the composite during uniaxial compressive loading, with a residual strain after unloading. The origin of the textured martensite can be explained by the grain-orientation-dependent Bain distortion energy. A recovery strain of ∼1.76% along the compression direction was evidenced in the pre-strained composite with an applied magnetic-field of 5T. This recovery was caused by the magnetic-field-induced reverse martensitic phase transformation. The phase transformation properties of the ferromagnetic shape–memory composite, different from its bulk alloys, can be well explained by the Clausius–Clapeyron relation. The large magnetic-field-induced strain, together with good ductility and low cost, make the polymer-bonded Ni–Co–Mn–In composites potential candidates for magnetic-field-driven actuators.