ORCID Profile
0000-0002-9888-7468
Current Organisation
Oak Ridge National Laboratory
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Publisher: American Physical Society (APS)
Date: 25-10-2019
Publisher: Informa UK Limited
Date: 09-2009
Publisher: American Physical Society (APS)
Date: 25-04-2014
Publisher: AIP Publishing
Date: 10-2023
DOI: 10.1063/5.0152136
Publisher: Informa UK Limited
Date: 28-01-2015
Publisher: International Union of Crystallography (IUCr)
Date: 15-05-2013
DOI: 10.1107/S0021889813010509
Abstract: The pressure-induced phase transformations of a form of amorphous silicon (a-Si) with well characterized impurity levels and structure are examined at pressures up to 40 GPa using in situ synchrotron X-ray radiation. At ∼12 GPa crystallization commences, but it is not completed until ∼16 GPa. At higher pressures, not all the crystalline phases observed for crystalline silicon (c-Si) appear. On pressure release, none of the metastable crystalline phases observed for c-Si nucleate. Instead an amorphous phase is re-formed. This is in contrast to all previous diamond-anvil studies on a-Si. If full pressure-induced crystallization occurred, the material remained crystalline on unloading. The formation of a-Si upon unloading was only observed when a high-density amorphous phase was reported on loading. The fully characterized nature of the a-Si used in this current study allows for the interpretation of this significant ersity in terms of impurity content of the a-Si used. Namely, this suggests that `ideal' (pure, voidless, structurally relaxed) a-Si will follow the same transition pathway as observed for c-Si, while crystallization of a-Si forms with a high impurity content is retarded or even inhibited. The a-Si used here straddles both regimes and thus, although full crystallization occurs, the more complex crystalline structures fail to nucleate.
Publisher: AIP Publishing
Date: 02-12-2021
DOI: 10.1063/5.0069425
Abstract: High-pressure neutron diffraction is an extremely useful technique in the quest for making and understanding novel hydride superconductors. Neutron diffraction can be used to directly determine elemental stoichiometries and atomic positions of many light elements such as hydrogen or deuterium, even in the presence of heavy elements such as rare-earth metals. Here, we report on the current status and ongoing developments on high-pressure neutron diffraction for hydride superconductors and other metal hydrides with a special focus on current advancements at the Spallation Neutrons and Pressure (SNAP) beamline of the Spallation Neutron Source at Oak Ridge National Laboratory. For broader context, an overview of high-pressure neutron diffractometers and pressure cells is included together with insight into critical s le considerations. There, attention is given to the requirements for powdered hydride s les and the need for deuterium rather than hydrogen. Additionally, the advantages of angular access and data representation as possible at SNAP are described. We demonstrate the current capability for high-pressure neutron diffraction on two different s les created via hydrogen gas loading, specifically pure deuterium and nickel-deuteride. The deuterium ex le highlights the usefulness of adding s le materials that facilitate the formation of a good powder while the nickel-deuteride ex le demonstrates that atomic deuterium positions and stoichiometry can be directly determined. Both ex les highlight the importance of large scattering apertures. These enable investigation of the data resolved by scattering angle that is needed to identify parasitic peaks and background features. Finally, future directions beyond current high-pressure neutron powder diffraction are also discussed.
Publisher: American Physical Society (APS)
Date: 11-03-2019
Publisher: Royal Society of Chemistry (RSC)
Date: 2017
DOI: 10.1039/C6SC02830F
Abstract: Pressure induced polymerization of CaC 2 in the amorphous phase is evidenced for the first time, accompanied by a 10 7 fold enhancement of the electrical conductivity.
Publisher: Informa UK Limited
Date: 22-05-2020
Publisher: Springer Science and Business Media LLC
Date: 23-03-2023
Publisher: AIP Publishing
Date: 09-08-2010
DOI: 10.1063/1.3455889
Abstract: We have determined the full crystal structure of the high-pressure phase methane A. X-ray single-crystal diffraction data were used to determine the carbon-atom arrangement, and neutron powder diffraction data from a deuterated s le allowed the deuterium atoms to be located. It was then possible to refine all the hydrogen positions from the single-crystal x-ray data. The structure has 21 molecules in a rhombohedral unit cell, and is quite strongly distorted from the cubic close-packed structure of methane I, although some structural similarities remain. Full knowledge of this structure is important for modeling of methane at higher pressures, including in relation to the mineralogy of the outer solar system. We discuss interesting structural parallels with the carbon tetrahalides.
Publisher: American Physical Society (APS)
Date: 23-05-2019
Publisher: IOP Publishing
Date: 23-09-2005
Publisher: AIP Publishing
Date: 09-2018
DOI: 10.1063/1.5033906
Abstract: The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder s les over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of s le environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United States of America
No related grants have been discovered for Malcolm Guthrie.