R. Boehler

Carnegie Institute, Washington, Washington, D.C., United States

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Publications (180)729.82 Total impact

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    ABSTRACT: A novel rock salt Mg0.4Fe0.6N solid solution is obtained by hot pressing a 70:30 at.% mixture of Fe and Mg together with NaN3 (MgO capsule in multianvil cell, 15 GPa, 1800 K, 1 min).
    No preview · Article · Feb 2016 · ChemInform
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    ABSTRACT: Alloy and nitride solid solutions are prominent for structural, energy and information processing applications. There are frequently however barriers to making them. We remove barriers to reactivity here using pressure with a new synthetic approach. We target pressures where the reasons for cubic endmember nitride instability can become the driving force for cubic nitride solid solution stability. Using this approach we form a novel rocksalt Mg0.4Fe0.6N solid solution at between 15 and 23 GPa and up to 2500 K. This is a system where, neither an alloy nor a nitride solid solution form at ambient conditions and bulk MgN and FeN endmembers do not form, either at ambient or at high pressure. The new nitride is formed, by removing endmember lattice mismatch with pressure, allowing a stabilizing redistribution of valence electrons upon heating. This approach can be employed for a range of normally unreactive systems. Mg, Fe and enhanced nitrogen presence, may also indicate a richer reaction chemistry in our planets interior.
    No preview · Article · Dec 2015 · Angewandte Chemie International Edition
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    Full-text · Dataset · Dec 2015
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    ABSTRACT: Temperature, thermal history, and dynamics of Earth rely critically on the knowledge of the melting temperature of iron at the pressure conditions of the inner core boundary (ICB) where the geotherm crosses the melting curve. The literature on this subject is overwhelming, and no consensus has been reached, with a very large disagreement of the order of 2,000 K for the ICB temperature. Here we report new data on the melting temperature of iron in a laser-heated diamond anvil cell to 103 GPa obtained by X-ray absorption spectroscopy, a technique rarely used at such conditions. The modifications of the onset of the absorption spectra are used as a reliable melting criterion regardless of the solid phase from which the solid to liquid transition takes place. Our results show a melting temperature of iron in agreement with most previous studies up to 100 GPa, namely of 3,090 K at 103 GPa.
    Full-text · Article · Sep 2015 · Proceedings of the National Academy of Sciences
  • D. Errandonea · R. Boehler · M. Ross
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    ABSTRACT: In this Comment we discuss recent results presented by Nguyen et al. on shock compressed molybdenum up to 438 GPa [Phys. Rev. B 89, 174109 (2014)10.1103/PhysRevB.89.174109]. The aim of Nguyen's article is to show that there is no phase transition near 210 GPa. We propose instead an interpretation of their data that this material shows the onset of partial melting along the Hugoniot at 240(20)GPa, which is evident from abrupt changes in the pressure dependence of the shear modulus. This interpretation may solve the significant controversy in the melting slopes derived from shock and static experiments. © 2015 American Physical Society.
    No preview · Article · Jul 2015 · Physical Review B
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    ABSTRACT: Neutron diffraction provides many unique advantages for structural studies of materials under extremes of pressure. In addition to the famous sensitivity to light atom positions, neutrons are sensitive to long-range magnetic order and have an extremely high spatial resolution. However, a major downside of neutron techniques, that is keenly felt in high pressure studies, is the comparative weakness of available sources. Some of these limitations have been recently overcome at the Spallation Neutron Source, ORNL, using a newly developed supported diamond-anvil device. For the first time, this new capability allows the possibility of conducting neutron diffraction measurements at pressures approaching 100 GPa. These new developments will be discussed with a look towards the prospects for advances in neutron scattering at high pressure in the near future.
    No preview · Article · Aug 2014 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: The properties of solid and liquid phases of H2O at high pressure and temperature remain an active area of research. In this study, Brillouin spectroscopy has been used to determine the temperature dependence of sound velocities in H2O as a function of pressure up to 26 GPa through the phase field of ice VII and into the liquid to a maximum temperature of 1200 K. The Brillouin shift of the quasi-longitudinal acoustic mode moves to lower frequencies upon melting at each pressure. As a test of the method, measurements of the melting of Ar by Brillouin scattering at several pressures show a similar behavior for the acoustic mode, and measured melting points are consistent with previous results. The results of H2O melting are consistent with previously reported melting curves below 20 GPa. The data at higher pressure indicate that ice melts at a higher temperature than a number of previous studies have indicated.
    Full-text · Article · Jul 2014 · High Pressure Research
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    ABSTRACT: Quantitative high pressure neutron-diffraction measurements have traditionally required large sample volumes of at least ∼25 mm3 due to limited neutron flux. Therefore, pressures in these experiments have been limited to below 25 GPa. In comparison, for X-ray diffraction, sample volumes in conventional diamond cells for pressures up to 100 GPa have been less than 1×10−4 mm3. Here, we report a new design of strongly supported conical diamond anvils for neutron diffraction that has reached 94 GPa with a sample volume of ∼2×10−2 mm3, a 100-fold increase. This sample volume is sufficient to measure full neutron-diffraction patterns of D2O–ice to this pressure at the high flux Spallation Neutrons and Pressure beamline at the Oak Ridge National Laboratory. This provides an almost fourfold extension of the previous pressure regime for such measurements.
    No preview · Article · Aug 2013 · High Pressure Research
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    ABSTRACT: The motif of distinct H2O molecules in H-bonded networks is believed to persist up to the densest molecular phase of ice. At even higher pressures, where the molecule dissociates, it is generally assumed that the proton remains localized within these same networks. We report neutron-diffraction measurements on D2O that reveal the location of the D atoms directly up to 52 GPa, a pressure regime not previously accessible to this technique. The data show the onset of a structural change at ∼13 GPa and cannot be described by the conventional network structure of ice VII above ∼26 GPa. Our measurements are consistent with substantial deuteron density in the octahedral, interstitial voids of the oxygen lattice. The observation of this "interstitial" ice VII form provides a framework for understanding the evolution of hydrogen bonding in ice that contrasts with the conventional picture. It may also be a precursor for the superionic phase reported at even higher pressure with important consequences for our understanding of dense matter and planetary interiors.
    Preview · Article · Jun 2013 · Proceedings of the National Academy of Sciences
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    ABSTRACT: Phase IV of dense solid hydrogen has been identified by its infrared spectrum using high-pressure synchrotron radiation techniques. The spectrum exhibits a sharp vibron band at higher frequency and lower intensity than that for phase III, indicating the stability of molecular H_{2} with decreased intermolecular interactions and charge transfer between molecules. A low-frequency vibron having a strong negative pressure shift indicative of strongly interacting molecules is also observed. The character of the spectrum is consistent with an anisotropic, mixed layer structure related to those recently predicted theoretically. Phase IV was found to be stable from 220 GPa (300 K) to at least 340 GPa (near 200 K), with the I-III-IV triple point located. Infrared transmission observed to the lowest photon energies measured places constraints on the electronic properties of the phase.
    Full-text · Article · May 2013 · Physical Review Letters
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    Liuxiang Yang · Amol Karandikar · Reinhard Boehler
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    ABSTRACT: A new method for measuring melting temperatures in the laser-heated diamond cell is described. This method circumvents previous problems associated with the sample instability, thermal runaway, and chemical reactions. Samples were heated with a single, 20 milliseconds rectangular pulse from a fiber laser, monitoring their thermal response with a fast photomultiplier while measuring the steady state temperature with a CCD spectrometer. The samples were recovered and analyzed using scanning electron microscopy. Focused ion beam milling allowed to examine both the lateral and the vertical solid-liquid boundaries. Ambient pressure tests reproducibly yielded the known melting temperatures of rhenium and molybdenum. Melting of Re was measured to 50 GPa, a 5-fold extension of previous data. The refractory character of Re is drastically enhanced by pressure, in contrast to Mo.
    Full-text · Article · Jun 2012 · The Review of scientific instruments
  • R. Boehler · A. Karandikar · L. Yang
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    ABSTRACT: From the observed decreasing mobility of liquid iron at high pressure in the laser-heated diamond cell and the gradual decrease in the shear modulus in shock experiments, one may derive high viscosity in the liquid outer core of the Earth. A possible explanation could be the presence of local structures in the liquid as has been observed for several transition metals. In order to bridge the large gap in the timescales between static and dynamic melting experiments, we have developed new experimental techniques to solve the large discrepancies in the melting curves of transition metals (Fe, W, Ta, Mo) measured statically in the laser-heated diamond cell and in shock experiments. The new methods employ "single-shot" laser heating in order to reduce problems associated with mechanical instabilities and chemical reactions of the samples subjected to several thousand degrees at megabar pressures. For melt detection, both synchrotron X-ray diffraction and Scanning Electron Microscopy (SEM) on recovered samples are used. A third approach is the measurement of latent heat effects associated with melting or freezing. This method employs simultaneous CW and pulse laser heating and monitoring the temperature-time history with fast photomultipliers. Using the SEM recovery method, we measured first melting temperatures of rhenium, which at high pressure may be one of the most refractory materials. From the melt textures of Re, we did not observe a significant pressure dependence of viscosity.
    No preview · Article · Dec 2011
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    Preview · Article · Aug 2011 · Acta Crystallographica Section A Foundations of Crystallography
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    ABSTRACT: A new phase of magnesium silicon nitride, beta-MgSiN2, is synthesized in a laser-heated diamond anvil cell at pressure of 27 GPa and temperatures exceeding 2000 K. It crystallises in a rhombohedral caswellsilverite-type structure (space group R (3) over barm) with both Mg and Si atoms in octahedral coordinations - in agreement with earlier DFT calculations. In contrast to recent shock compression investigation, the beta-phase is found to metastably persist at ambient conditions. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    No preview · Article · Jun 2011 · physica status solidi (RRL) - Rapid Research Letters
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    ABSTRACT: Pressure dependent studies on technologically important ferroelectric material Pb0.70Ca0.30TiO3 show the occurrence of a new hitherto unreported pressure dependent phase transition around 4GPa. In the pressure range 4–14GPa, the parent tetragonal (P4mm) phase of Pb0.70Ca0.30TiO3 transforms in to a monoclinic (Cm) phase before attaining its paraelectric cubic (Pm3m) phase around 15GPa. High pressure Raman studies reveal the presence of a critical pressure above which the ferroelectric phase starts to reappear in the paraelectric phase. This critical pressure is found to be much lower than the critical pressure observed in pure PbTiO3. Possible reasons for this lowering of the critical pressure are presented. KeywordsXRD–Phase transition–Raman–Ferroelectric
    No preview · Article · Jun 2011 · Journal of Electroceramics
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    ABSTRACT: The sound velocity in polycrystalline ice was measured as a function of pressure at room temperature to 100 GPa, through the phase field of ice VII and crossing the ice X transition, by Brillouin scattering in order to examine the elasticity, compression mechanism, and structural transitions in this pressure range. In particular, we focused on previously proposed phase transitions below 60 GPa. Throughout this pressure range, we find no evidence for anomalous changes in compressibility, and the sound velocities and elastic moduli do not exhibit measurable discontinuous shifts with pressure. Subtle changes in the pressure dependence of the bulk modulus at intermediate pressures can be attributed to high shear stresses at these compressions. The C(11) and C(12) moduli are consistent with previously reported results to 40 GPa and increase monotonically at higher pressures.
    Full-text · Article · Mar 2011 · The Journal of Chemical Physics
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Sep 2010 · ChemInform
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    Beate Schwager · Marvin Ross · Stefanie Japel · Reinhard Boehler
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    ABSTRACT: Measurements for Sn, made using the laser-heated diamond cell, are reported that extend the melting curve to 68 GPa and 2300 K. Initially the melting temperature of Sn increases linearly with increasing pressure (dT/dP approximately 40 K/GPa) and near 38 GPa (2200 K) the melting curve flattens (dT/dP approximately 0), indicating a zero volume phase change at melting. The results are in good agreement with previously reported shock melting studies. In comparison to Sn the melting curve of Pb is relatively linear to 100 GPa, the highest pressure at which measurements have been made.
    Full-text · Article · Aug 2010 · The Journal of Chemical Physics
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    ABSTRACT: New data on the high-pressure melting curve of Ta up to 48GPa are reported. Evidence of melting from changes in sample texture was found in five different experiments using scanning electron microscopy. The obtained melting temperatures are in excellent agreement with earlier measurements using X-ray diffraction or the laser-speckled method but are in contrast with several theoretical calculations. The results are also compared with shock-wave data. These findings are of geophysical relevance because they confirm the validity of earlier experimental techniques that resulted in low melting slopes of the transition metals measured in the diamond-anvil cell, including iron.
    Full-text · Article · Jul 2010 · Physics of The Earth and Planetary Interiors
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    ABSTRACT: The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi(2)Ga(4)O(9), was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi(3+) is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.
    Full-text · Article · Jun 2010 · Acta crystallographica. Section B, Structural science

Publication Stats

6k Citations
729.82 Total Impact Points

Institutions

  • 2013-2015
    • Carnegie Institute
      Washington, Washington, D.C., United States
  • 2010-2013
    • Carnegie Institution for Science
      • Geophysical Laboratory
      Washington, West Virginia, United States
  • 1989-2011
    • Max Planck Institute for Chemistry
      • Department of Particle Chemistry
      Mayence, Rheinland-Pfalz, Germany
  • 2008
    • Universidad de La Laguna
      • Department of Basic Physics II
      San Cristóbal de La Laguna, Canary Islands, Spain
  • 2007
    • University of Valencia
      Valenza, Valencia, Spain
  • 1994
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 1977-1987
    • University of California, Los Angeles
      • • Department of Chemistry and Biochemistry
      • • Institute of Geophysics and Planetary Physics
      Los Angeles, CA, United States