X-ray and neutron scattering

Abstract:

We report crystal growth of several rare-earth stannates RE 2 Sn 2 O 7 (RE=Pr, Tb, Ho, Dy, Yb and Lu) using the flux technique. Different combinations of flux were tried, and a Na 2 B 4 O 7 -NaF (1.2:1) mixture was found to be suitable for crystal growth. X-ray diffraction and thermal characterisation data are presented, as well as some initial measurements of magnetic and thermodynamic properties of the crystals. Little effect was observed with changing oxygen content by Sc substitution for Sn.

Abstract:

The atomic and magnetic structures of CoO and NiO have been probed using reverse Monte Carlo refinements of neutron total scattering data. The results obtained show that the known magnetic structure for NiO can be recovered by the reverse Monte Carlo process starting from random spin configurations, but it is insensitive to the spin direction in the {111} ferromagnetic planes. Refinements of the magnetic structure of CoO starting from random spin configurations result in collinear or non-collinear magnetic structure, consistent with those reported by other techniques. Starting from an ordered collinear spin structure for CoO and NiO leads to different results than when starting from a random arrangement of spins, evidence for configurational bias that highlights the need to take care when selecting a starting model for reverse Monte Carlo refinements of magnetic structures.

Abstract:

We have performed inelastic neutron-scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li1−xFexODFe1−ySe (x ≈ 0.16, y ≈ 0.02, Tc = 41 K). The spectrum shows an enhanced intensity below Tc over an energy range of 0.64 × 2Δ < E < 2Δ, where Δ is the superconducting gap, and has maxima at the wave vectors Q1 ≈ 1.46 and Q2 ≈ 1.97 °A−1. The behavior of this feature is consistent with the spin-resonance mode found in other unconventional superconductors, and in particular strongly resembles the spin resonance observed in the spectrum of another molecular-intercalated iron selenide Li0.6(ND2)0.2(ND3)0.8Fe2Se2. The signal can be described with a characteristic two-dimensional wave-vector (π,0.67π) in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets.

Abstract:

We report growth of large single crystals of the triangular-lattice antiferromagnetic compound Ba3CoSb2O9 by the floating-zone technique in an image furnace. Evaporation of Sb due to its high volatility was controlled by high pressure and addition of excess Sb in the starting materials to compensate for the losses. The crystal quality was analysed using different X-ray techniques, and the magnetic transition temperature was confirmed by magnetization and heat capacity measurements.

Abstract:

Carbon capture and storage (CCS) offers a possible solution to curb the CO₂ emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO₂ capture, such as the CaO-CaCO₃ system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation-regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions.