R. Boehler

Carnegie Institution for Science, Washington, WV, United States

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Publications (161)581.83 Total impact

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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.
    High Pressure Research 08/2013; 33(3):546-554. · 0.90 Impact Factor
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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.
    Proceedings of the National Academy of Sciences 06/2013; · 9.74 Impact Factor
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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.
    Physical Review Letters 05/2013; 110(21):217402. · 7.73 Impact Factor
  • 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.
    The Review of scientific instruments 06/2012; 83(6):063905. · 1.52 Impact Factor
  • 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.
    AGU Fall Meeting Abstracts. 12/2011;
  • physica status solidi (RRL) - Rapid Research Letters 05/2011; 5(5‐6):196 - 198. · 2.39 Impact Factor
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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.
    The Journal of Chemical Physics 03/2011; 134(12):124517. · 3.12 Impact Factor
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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
    Journal of Electroceramics 01/2011; 26(1):191-199. · 1.14 Impact Factor
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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.
    The Journal of Chemical Physics 08/2010; 133(8):084501. · 3.12 Impact Factor
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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.
    Acta crystallographica. Section B, Structural science 06/2010; 66(Pt 3):323-37. · 1.80 Impact Factor
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    ABSTRACT: The melting curves of He and Ne were measured up to 80 and 70 GPa, respectively, significantly extending the pressure range of previous measurements. Melting was detected in situ by the laser speckle method using the laser-heated diamond-anvil cell. Temperatures were measured in the visible as well as infrared range. Our He melting curve differs considerably from earlier experimental data above 30 GPa. The present Ne melting curve does not agree with the predictions from corresponding states theory in the range from 15 to 70 GPa.
    Physical Review B 03/2010; 81. · 3.66 Impact Factor
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    ABSTRACT: Contribution to a conf. proceeding (journal)
    Journal of Physics Conference Series 03/2010;
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    Acta Crystallographica Section B-structural Science - ACTA CRYSTALLOGR B-STRUCT SCI. 01/2010; 66(3):323-337.
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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.
    Physics of The Earth and Planetary Interiors 01/2010; · 2.38 Impact Factor
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    ABSTRACT: The pressure-induced B4⇆B1 structural phase transition of ZnO has been studied with the second harmonic generation (SHG) technique. Measurements in nonhydrostatic and hydrostatic pressure transmitting media show slightly different transition pressures (9–11 GPa) and a different pressure dependence of the SHG intensities. These observations are consistent with the presence of a tetragonal and hexagonal intermediate phase as a result of hydrostatic and axial compression, respectively. In contrast to earlier work, it is shown that it is not necessary to use nanocrystalline starting material to be able to recover the B1 phase at ambient conditions.
    Applied Physics Letters 09/2009; · 3.79 Impact Factor
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    ABSTRACT: A compact, double-sided laser-heating system for diamond-cell synchrotron applications is described. The optical table, containing laser, spectrometer, and all optics for visual observation and measuring temperatures and pressures has an area of less than 1/2 m(2) and weighs less than 20 kg. All components can be remotely controlled at micron levels with simple dc motors and pneumatic drives. The design allows quick alignment of the laser-heated hot spot with the x-ray beam and the spectrometer. The prealigned system can be set up at most synchrotron beamlines within about 1 h. We carried out measurements on a variety of materials above one megabar and up to over 4000 K at both the x-ray diffraction beamline ID 27 and the x-ray absorption beamline ID 24 at the European Synchrotron Facility. A new measurement of the melting temperature of iron by x-ray absorption spectroscopy is presented.
    The Review of scientific instruments 05/2009; 80(4):045103. · 1.52 Impact Factor
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    ABSTRACT: In this paper, we report angle-dispersive X-ray diffraction data of molybdenum melting, measured in a double-sided laser-heated diamond-anvil cell up to a pressure of 119 GPa and temperatures up to 3400 K. The new melting temperatures are in excellent agreement with earlier measurements up to 90 GPa that relied on optical observations of melting and in strong contrast to most theoretical estimates. The X-ray measurements show that the solid melts from the bcc structure throughout the reported pressure range and provide no evidence for a high temperature transition from bcc to a close-packed structure, or to any other crystalline structure. This observation contradicts earlier interpretations of shock data arguing for such a transition. Instead, the values for the Poisson ratios of shock compressed Mo, obtained from the sound speed measurements, and the present X-ray evidence of loss of long-range order suggest that the 210 GPa (approximately 4100 K) transition in the shock experiment is from the bcc structure to a new, highly viscous, structured melt.
    The Journal of Chemical Physics 04/2009; 130(12):124509. · 3.12 Impact Factor
  • Goutam Dev Mukherjee, Reinhard Boehler
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    ABSTRACT: A Reply to the Comment by E. Gregoryanz and A. F. Goncharov.
    Physical Review Letters 01/2009; 102(4). · 7.73 Impact Factor
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    ABSTRACT: We performed high-pressure ADXRD studies on Fe5Si3 and Ni2Si up to 75 GPa. No evidence of the occurrence of a phase transition was observed in them. Fe5Si3 was found to compress isotropically, but an anisotropic compression was observed in Ni2Si. These results are supported by ab initio total-energy calculations, which for Fe5Si3 also predicted a transition at 283 GPa from the hexagonal P63/mcm phase to a cubic phase. High-pressure melting studies were conducted on FeSi up to 70 GPa. We found a change in the melting slope at 12 GPa, which is attributed to the intersection of the melting curve with the phase boundary between epsilon-FeSi and CsCl-type FeSi. Finally, an equation of state for Fe5Si3 and Ni2Si is reported.
    Journal of Physics Conference Series 03/2008; 121.
  • D. Santamaria-Perez, R. Boehler
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    ABSTRACT: The melting curve of iron monosilicide, FeSi, has been determined in a laser-heated diamond anvil cell from 6 up to 70 GPa by direct visual observation of the continuous laser-speckle motion in the liquid state. At 12 GPa and 1700 K, a discontinuous change in the slope of the melting curve indicates the first-order phase transition between the ɛ-FeSi (B20) and the CsCl-type FeSi structures (B2). During the phase transition the coordination number of both, Fe and Si atoms, increases from 7 to 8. Above this pressure, the melting curve rises steeply but shows significant flattening at higher pressures. A comparison with the melting curve of Fe shows that both curves cross at 32±3 GPa, FeSi having higher melting temperatures (about 100 K) at high pressures.
    Earth and Planetary Science Letters 03/2008; 265:743. · 4.72 Impact Factor

Publication Stats

3k Citations
581.83 Total Impact Points

Institutions

  • 2010–2013
    • Carnegie Institution for Science
      • Geophysical Laboratory
      Washington, WV, United States
  • 1989–2011
    • Max Planck Institute for Chemistry
      Mayence, Rheinland-Pfalz, Germany
  • 2009
    • Goethe-Universität Frankfurt am Main
      • Institut für Geowissenschaften
      Frankfurt am Main, Hesse, Germany
  • 2004–2009
    • Max Planck Institute for Empirical Aesthetics
      Frankfurt, Hesse, Germany
  • 2007
    • Technical University Darmstadt
      • Research Area of Materials Science
      Darmstadt, Hesse, Germany
  • 1997
    • CSU Mentor
      Long Beach, California, 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