27 research outputs found
Synchrotron X-Ray Scattering as a Tool for Characterising Catalysts on Multiple Length Scales
Optimising the properties of catalysts for industrial processes requires a detailed knowledge of their structure and properties on multiple length scales. Synchrotron light sources are ideal tools for characterising catalyts for industrial R&D, providing data with high temporal and spatial resolution, under realistic operating conditions, in a non-destructive way. Here, we describe the different synchrotron techniques that can be employed to gain a wealth of complementary information, and highlight recent developments that have allowed remarkable insight to be gained into working catalytic systems. These techniques have the potential to guide future industrial catalyst design
High-Pressure Annealing of a Prestructured Nanocrystalline Precursor to Obtain Tetragonal and Orthorhombic Polymorphs of Hf3N4
Transition metal nitrides containing metal ions in high oxidation states are a significant goal for the discovery of new families of semiconducting materials. Most metal nitride compounds prepared at high temperature and high pressure from the elements have metallic bonding. However amorphous or nanocrystalline compounds can be prepared via metal-organic chemistry routes giving rise to precursors with a high nitrogen:metal ratio. Using X-ray diffraction in parallel with high pressure laser heating in the diamond anvil cell this work highlights the possibility of retaining the composition and structure of a metastable nanocrystalline precursor under high pressure-temperature conditions. Specifically, a nanocrystalline Hf3N4 with a tetragonal defect-fluorite structure can be crystallized under high-P,T conditions. Increasing the pressure and temperature of crystallization leads to the formation of a fully recoverable orthorhombic (defect cottunite-structured) polymorph. This approach identifies a novel class of pathways to the synthesis of new crystalline nitrogen-rich transition metal nitrides
Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity
We use neutron scattering to study the structural and magnetic phase
transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a
semimetal to a high-transition-temperature (high-Tc) superconductor through
Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural
lattice distortion followed by a stripe like commensurate antiferromagnetic
order with decreasing temperature. With increasing Fluorine doping, the
structural phase transition decreases gradually while the antiferromagnetic
order is suppressed before the appearance of superconductivity, resulting an
electronic phase diagram remarkably similar to that of the high-Tc copper
oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other
Fe-based superconductors reveals that the effective electronic band width
decreases systematically for materials with higher Tc. The results suggest that
electron correlation effects are important for the mechanism of high-Tc
superconductivity in these Fe pnictides.Comment: 19 pages, 5 figure
Satellites and large doping- and temperature-dependence of electronic properties in hole-doped BaFe2As2
Over the last years, superconductivity has been discovered in several
families of iron-based compounds. Despite intense research, even basic
electronic properties of these materials, such as Fermi surfaces, effective
electron masses, or orbital characters are still subject to debate. Here, we
address an issue that has not been considered before, namely the consequences
of dynamical screening of the Coulomb interactions among Fe-d electrons. We
demonstrate its importance not only for correlation satellites seen in
photoemission spectroscopy, but also for the low-energy electronic structure.
From our analysis of the normal phase of BaFe2As2 emerges the picture of a
strongly correlated compound with strongly doping- and temperature-dependent
properties. In the hole overdoped regime, an incoherent metal is found, while
Fermi-liquid behavior is recovered in the undoped compound. At optimal doping,
the self-energy exhibits an unusual square-root energy dependence which leads
to strong band renormalizations near the Fermi level
Ultrafast transient generation of spin-densitywave order in the normal state of BaFe2As2 driven by coherent lattice vibrations
The interplay among charge, spin and lattice degrees of freedom in solids
gives rise to intriguing macroscopic quantum phenomena such as colossal
magnetoresistance, multiferroicity and high-temperature superconductivity.
Strong coupling or competition between various orders in these systems presents
the key to manipulate their functional properties by means of external
perturbations such as electric and magnetic fields or pressure. Ultrashort and
intense optical pulses have emerged as an interesting tool to investigate
elementary dynamics and control material properties by melting an existing
order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the
evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2
following excitation with a femtosecond optical pulse. When starting in the
low-temperature ground state, optical excitation results in a melting of the
SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced
when we excite the normal state above the transition temperature. Very
surprisingly, the transient ordering quasi-adiabatically follows a coherent
lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a
pronounced spin-phonon coupling in pnictides that supports rapid development of
a macroscopic order on small vibrational displacement even without breaking the
symmetry of the crystal
Spin-chain correlations in the frustrated triangular lattice material CuMnO2
The Ising triangular lattice remains the classic test-case for frustrated
magnetism. Here we report neutron scattering measurements of short range
magnetic order in CuMnO, which consists of a distorted lattice of Mn
spins with single-ion anisotropy. Physical property measurements on CuMnO
are consistent with 1D correlations caused by anisotropic orbital occupation.
However the diffuse magnetic neutron scattering seen in powder measurements has
previously been fitted by 2D Warren-type correlations. Using neutron
spectroscopy, we show that paramagnetic fluctuations persist up to 25 meV
above TN= 65 K. This is comparable to the incident energy of typical
diffractometers, and results in a smearing of the energy integrated signal,
which hence cannot be analysed in the quasi-static approximation. We use low
energy XYZ polarised neutron scattering to extract the purely magnetic
(quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which
reveals that two directions in the triangular layers are perfectly frustrated
in the classical spin-liquid phase at 75 K. Strong antiferromagnetic
correlations are only found along the b-axis, and our results hence unify the
pictures seen by neutron scattering and macroscopic physical property
measurements
Interlayer tuning of electronic and magnetic properties in honeycomb ordered Ag3LiRu2O6.
We report a switching of electronic, magnetic and lattice properties in honeycomb ruthenates by interlayer cation exchange. The new material Ag3LiRu2O6 was made by ion-exchange of the ordered Li/Ru honeycomb material Li2RuO3 in an AgNO3 melt at 200°C. Neutron powder diffraction and electron microscopy show that the Li/Ru order is preserved in the honeycomb layers, however, significant stacking disorder is found between layers. In contrast to Li2RuO3, which is insulating, dimerised and diamagnetic, Ag3LiRu2O6 has low electrical resistivity (0.01 ohm cm−1) and a large magnetic susceptibility at room temperature. This is attributed to the electronic influence of the highly polarisable interlayer Ag+ cation. The combination of two dimensionality, good conductivity and stacking disorder means this family of materials have potential for thermoelectric applications. © 2010, Royal Society of Chemistr
An eigenstrain-based finite element model and the evolution of shot peening residual stresses during fatigue of GW103 magnesium alloy
Magnesium alloy GW103 samples were heat treated to different ageing conditions and then shot peened using process parameters that deliver optimized high cycle fatigue (HCF) life. Significant HCF life improvements were observed in all samples, with a peak-aged sample showing the biggest increase. In order to simulate the effect and evolution of residual stresses during low cycle fatigue (LCF), a Finite Element (FE) model was employed, taking into account both the shot-peening-induced plastic strains and the influence of hardening on subsequent deformation. Experimental and modelling results offer a basis for explaining the observed fatigue performance improvement due to shot peening. © 2012 Elsevier Ltd. All rights reserved