The general availability of third generation synchrotron sources has ushered in a new era of high... more The general availability of third generation synchrotron sources has ushered in a new era of high pressure research. The crystal structure of materials under compression can now be determined by X-ray diffraction using powder samples and, more recently, from multi-nano single crystal diffraction. Concurrently, these experimental advancements are accompanied by a rapid increase in computational capacity and capability, enabling the application of sophisticated quantum calculations to explore a variety of material properties. One of the early surprises is the finding that simple metallic elements do not conform to the general expectation of adopting 3D close-pack structures at high pressure. Instead, many novel open structures have been identified with no known analogues at ambient pressure. The occurrence of these structural types appears to be random with no rules governing their formation. The adoption of an open structure at high pressure suggested the presence of directional bond...
We report the results of in situ structural characterization of the amorphization of crystalline ... more We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ∼1 Pa, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an...
The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigate... more The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure (119)Sn nuclear resonant inelastic x-ray scattering measurements. Above ∼8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ∼64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms.
The pressure-induced transformation of diatomic nitrogen into nonmolecular polymeric phases may p... more The pressure-induced transformation of diatomic nitrogen into nonmolecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B_{3}N_{5} that are stable or metastable relative to solid N_{2} and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B_{3}N_{5}) containing hexagonal B_{3}N_{3} layers sandwiched with intercalated freely rotating N_{2} molecules. At 15 GPa, a three-dimensional C222_{1} structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C222_{1}-B_{3}N_{5} is metastable, and can be recovered under ambient conditions. Its energy density of ∼3.44 kJ/g makes it a potential high-energy-density material. In addition, stre...
Physical chemistry chemical physics : PCCP, Jan 6, 2015
It is now known that the structure and properties of a material can be significantly altered unde... more It is now known that the structure and properties of a material can be significantly altered under extreme compression. In this work, a structural search was performed to investigate the phase stabilities and structures of SrH2n (n = 1-5) in the pressure range of 50-300 GPa. The high-pressure polymorphs reveal a variety of hydrogen structural units ranging from monatomic hydride to linear and bent H3 and spiral polymer chains. A novel graphene like H-layer structure was found to exist in SrH10 at 300 GPa. The structural diversity in the predicted high pressure structures provides an opportunity for an in-depth analysis of the chemical bonding in the high pressure polyhydrides. It is shown from theoretical calculations that the electronegativity of molecular hydrogen is similar to that of group 13 and 14 elements. This resulted in electrons being transferred from Sr to the hydrogen molecules. Thus, a consideration of the number of valence electrons available from Sr that can be share...
The structures of the high and low-density amorphous phases of ice are studied using several tech... more The structures of the high and low-density amorphous phases of ice are studied using several techniques. The diffraction patterns of high and low density amorphous ice are analyzed using reverse Monte Carlo methods and compared with molecular dynamics simulations of these phases. The spectra of crystalline and amorphous phases of ice obtained by Raman and incoherent inelastic neutron scattering are analyzed to yield structural features for comparison with the results of molecular dynamics and Reverse Monte Carlo analysis. The structural details obtained indicate that there are significant differences between the structure of liquid water and the amorphous phases of ice.
Proceedings of the National Academy of Sciences, 2008
The detailing of the intermolecular interactions in dense solid oxygen is essential for an unders... more The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O 2 molecules interact predominantly through the half-filled 1π g * orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1π g * orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O 2 molecules at the transition to the ε-phase. The ε-phase, which consists of (O 2 ) 4 clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O 2 ) 4 clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bo...
The general availability of third generation synchrotron sources has ushered in a new era of high... more The general availability of third generation synchrotron sources has ushered in a new era of high pressure research. The crystal structure of materials under compression can now be determined by X-ray diffraction using powder samples and, more recently, from multi-nano single crystal diffraction. Concurrently, these experimental advancements are accompanied by a rapid increase in computational capacity and capability, enabling the application of sophisticated quantum calculations to explore a variety of material properties. One of the early surprises is the finding that simple metallic elements do not conform to the general expectation of adopting 3D close-pack structures at high pressure. Instead, many novel open structures have been identified with no known analogues at ambient pressure. The occurrence of these structural types appears to be random with no rules governing their formation. The adoption of an open structure at high pressure suggested the presence of directional bond...
We report the results of in situ structural characterization of the amorphization of crystalline ... more We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ∼1 Pa, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an...
The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigate... more The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure (119)Sn nuclear resonant inelastic x-ray scattering measurements. Above ∼8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ∼64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms.
The pressure-induced transformation of diatomic nitrogen into nonmolecular polymeric phases may p... more The pressure-induced transformation of diatomic nitrogen into nonmolecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B_{3}N_{5} that are stable or metastable relative to solid N_{2} and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B_{3}N_{5}) containing hexagonal B_{3}N_{3} layers sandwiched with intercalated freely rotating N_{2} molecules. At 15 GPa, a three-dimensional C222_{1} structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C222_{1}-B_{3}N_{5} is metastable, and can be recovered under ambient conditions. Its energy density of ∼3.44 kJ/g makes it a potential high-energy-density material. In addition, stre...
Physical chemistry chemical physics : PCCP, Jan 6, 2015
It is now known that the structure and properties of a material can be significantly altered unde... more It is now known that the structure and properties of a material can be significantly altered under extreme compression. In this work, a structural search was performed to investigate the phase stabilities and structures of SrH2n (n = 1-5) in the pressure range of 50-300 GPa. The high-pressure polymorphs reveal a variety of hydrogen structural units ranging from monatomic hydride to linear and bent H3 and spiral polymer chains. A novel graphene like H-layer structure was found to exist in SrH10 at 300 GPa. The structural diversity in the predicted high pressure structures provides an opportunity for an in-depth analysis of the chemical bonding in the high pressure polyhydrides. It is shown from theoretical calculations that the electronegativity of molecular hydrogen is similar to that of group 13 and 14 elements. This resulted in electrons being transferred from Sr to the hydrogen molecules. Thus, a consideration of the number of valence electrons available from Sr that can be share...
The structures of the high and low-density amorphous phases of ice are studied using several tech... more The structures of the high and low-density amorphous phases of ice are studied using several techniques. The diffraction patterns of high and low density amorphous ice are analyzed using reverse Monte Carlo methods and compared with molecular dynamics simulations of these phases. The spectra of crystalline and amorphous phases of ice obtained by Raman and incoherent inelastic neutron scattering are analyzed to yield structural features for comparison with the results of molecular dynamics and Reverse Monte Carlo analysis. The structural details obtained indicate that there are significant differences between the structure of liquid water and the amorphous phases of ice.
Proceedings of the National Academy of Sciences, 2008
The detailing of the intermolecular interactions in dense solid oxygen is essential for an unders... more The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O 2 molecules interact predominantly through the half-filled 1π g * orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1π g * orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O 2 molecules at the transition to the ε-phase. The ε-phase, which consists of (O 2 ) 4 clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O 2 ) 4 clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bo...
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