The microstructure and hardness of Ni-Co-Al-Ti-Cr quinary alloys

The effects of Ni:Co and Al:Ti ratios on the room temperature microstructure, hardness and lattice parameter of twenty-seven quinary Ni-Co-Al-Ti-Cr alloys have been evaluated. All of the alloys exhibited a uniform $\gamma$-$\gamma ^\prime$ microstructure. Differential scanning calorimetry (DSC) showed that the liquidus and solidus temperatures of the alloys increased with greater Al:Ti ratios, decreased with Cr concentration and remained largely unchanged with respect to the Ni:Co ratio. Neutron diffraction measurements of the $\gamma$ and $\gamma ^\prime$ lattice parameters revealed that the lattice misfit in all of the alloys was positive and increased with Ti concentration (i.e. lower Al:Ti ratio) regardless of the concentration of Cr, or the ratio of Ni:Co. Importantly, alloys with a Ni:Co ratio of 1:1, were found to have consistently greater lattice misfits than alloys with Ni:Co ratios of either 1:3 or 3:1. The measured lattice misfits were found to be strongly correlated with the Vickers hardness of the alloys, suggesting that lattice misfit plays a key role in the strengthening of $\gamma$-$\gamma ^\prime$ alloys of this type.Rolls-Royce/EPSRC Strategic Partnership (Grant IDs: EP/H022309/1, EP/H500375/1 and EP/ M005607/1)This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jallcom.2016.07.15

Nematic Fluctuations in Iron-Oxychalcogenide Mott Insulators

Nematic fluctuations occur in a wide range of physical systems from liquid crystals to biological molecules to solids such as exotic magnets, cuprates and iron-based high-$T_c$ superconductors. Nematic fluctuations are thought to be closely linked to the formation of Cooper-pairs in iron-based superconductors. It is unclear whether the anisotropy inherent in this nematicity arises from electronic spin or orbital degrees of freedom. We have studied the iron-based Mott insulators La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ $M$ = (S, Se) which are structurally similar to the iron pnictide superconductors. They are also in close electronic phase diagram proximity to the iron pnictides. Nuclear magnetic resonance (NMR) revealed a critical slowing down of nematic fluctuations as observed by the spin-lattice relaxation rate ($1/T_1$). This is complemented by the observation of a change of electrical field gradient over a similar temperature range using M\"ossbauer spectroscopy. The neutron pair distribution function technique applied to the nuclear structure reveals the presence of local nematic $C_2$ fluctuations over a wide temperature range while neutron diffraction indicates that global $C_{4}$ symmetry is preserved. Theoretical modeling of a geometrically frustrated spin-$1$ Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of magnetic fluctuations in terms of hidden quadrupolar spin fluctuations. Nematicity is closely linked to geometrically frustrated magnetism, which emerges from orbital selectivity. The results highlight orbital order and spin fluctuations in the emergence of nematicity in Fe-based oxychalcogenides. The detection of nematic fluctuation within these Mott insulator expands the group of iron-based materials that show short-range symmetry-breaking

Nematic fluctuations in iron-oxychalcogenide Mott insulators

Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-Tc superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La2O2Fe2OM2M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C2 fluctuations while neutron diffraction indicates that global C4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity

Performance of Cu-coated vanadium cans for in situ neutron powder diffraction experiments on hydrogen storage materials

In situ neutron powder diffraction (NPD) measurements of hydrogenation processes taking place at high temperatures pose difficulties related to the choice of sample can material. This article describes a simple design for a copper-coated vanadium can and its connection to the gas-handling system, tested up to 523 K. High-quality NPD patterns of TiV1.2Mn0.8 body-centred cubic alloy, as-cast and partially hydrogenated, were collected at 373 K and deuterium pressures up to 2 bar (200 kPa).Peer reviewed: YesNRC publication: Ye

Structural phase transitions induced by pressure in ammonium borohydride

A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH4BH4) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car\u2013Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH4BH4 transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.Peer reviewed: YesNRC publication: Ye

Response to reply on "Structural and magnetic behavior of the cubic oxyfluoride SrFeO2F studied by neutron diffraction"

Clemens et al. reported on the results published by us (Thompson et al. J. Solid State Chem. 219 (2014) 173-178) on the crystal structure of SrFeO2F, which they suggest to actually crystallize in the orthorhombic space group Imma rather than the cubic Pm-3m structure at lower temperatures (Clemens et al. J. Solid State Chem. (2015), http://dx.doi.org/10.1016/j.jssc.2015.02.022). In this report, we provide evidence to support their claim that at lower temperatures (<523K) the structure is evidently Imma. Furthermore, we will highlight the significance of our previous report and comment on the proposed explanations of the magnetic behavior of SrFeO2F reported by both groups.Peer reviewed: YesNRC publication: Ye

Structural and magnetic behavior of the cubic oxyfluoride SrFeO 2F studied by neutron diffraction

The oxyfluoride SrFeO2F has been prepared via a low temperature route involving the infinite-layer SrFeO2 and XeF2. SrFeO2F crystallizes in the cubic space group Pm-3m with disordered oxygen and fluorine atoms on the anion site. Recent reports demonstrated that SrFeO2F is antiferromagnetic at room temperature and the zero field cooled and field cooled curves diverge at ~150 K and ~60 K, suggesting that the material has a spin glassy magnetic state at low temperatures. In this article, variable-temperature neutron diffraction (4-723 K) was performed to clarify the magnetic behavior observed in this material. Neutron powder diffraction measurements confirmed the antiferromagnetic (AFM) ordering of the system at room temperature. Below 710(1) K, the magnetic structure is a G-type AFM structure characterized by a propagation vector k=(12, 12, 12). The ordered moments on Fe3+ are 4.35(6)\u3bcB at 4 K and 4.04(5)\u3bcB at 290 K. Our results indicate that the cubic structure is retained all the way to base temperature (4 K) in contrast to PbFeO 2F. These results are compared with those of Pb and Ba analogs which exhibit very similar magnetic behavior. Furthermore, the observation of magnetic reflections at 4 K in the diffraction pattern shows the absence of the previously proposed spin glassy behavior at low temperatures. Previous proposals to explain the ZFC/FC divergences are examined.Peer reviewed: YesNRC publication: Ye