Structure Of High-pressure Phase Research Articles

Structural and vibrational properties of methane up to 71 GPa

Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The structure of HP phase is trigonal R3, which can be represented as a distortion of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The positions of hydrogen atoms in HP phase have been 19 obtained from first principles calculations. The molecules occupy four different crystallographic sites in phases B and eleven sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on 22 a good agreement with the Raman spectra calculated from the first principles.

Copper Delafossites under High Pressure—A Brief Review of XRD and Raman Spectroscopic Studies

Delafossites, with a unique combination of electrical conductivity and optical transparency constitute an important class of materials with their wide range of applications in different fields. In this article, we review the high pressure studies on copper based semiconducting delafossites with special emphasis on their structural and vibrational properties by synchrotron based powder X-ray diffraction and Raman spectroscopic measurements. Though all the investigated compounds undergo pressure induced structural phase transition, the structure of high pressure phase has been reported only for CuFeO2. Based on X-ray diffraction data, one of the common features observed in all the studied compounds is the anisotropic compression of cell parameters in ambient rhombohedral structure. Ambient pressure bulk modulus obtained by fitting the pressure volume data lies between 135 to 200 GPa. Two allowed Raman mode frequencies Eg and A1g are observed in all the compounds in ambient phase with splitting of Eg mode at the transition except for CuCrO2 where along with splitting of Eg mode, A1g mode disappears and a strong mode appears which softens with pressure. Observed transition pressure scales exponentially with radii of trivalent cation being lowest for CuLaO2 and highest for CuAlO2. The present review will help materials researchers to have an overview of the subject and reviewed results are relevant for fundamental science as well as possessing potential technological applications in synthesis of new materials with tailored physical properties.

Electrical resistance of single-crystal magnetite (Fe3O4) under quasi-hydrostatic pressures up to 100 GPa

The pressure dependence of electrical resistance of single-crystal magnetite (Fe3O4) was measured under quasi-hydrostatic conditions to 100 GPa using low-temperature, megabar diamond-anvil cell techniques in order to gain insight into the anomalous behavior of this material that has been reported over the years in different high-pressure experiments. The measurements under nearly hydrostatic pressure conditions allowed us to detect the clear Verwey transition and the high-pressure structural phase. The appearance of a metallic ground state after the suppression of the Verwey transition around 20 GPa and the concomitant enhancement of the electrical resistance caused by the structural transformation to the high-pressure phase form reentrant semiconducting-metallic-semiconducting behavior, although the appearance of the metallic phase is highly sensitive to stress conditions and details of the measurement technique.

Germanium nanoparticles with non-diamond core structures for solar energy conversion

Multiple Exciton Generation (MEG) in nanoparticle-based solar cells promises to increase the cell-efficiency above the Shockley–Queisser limit. However, utilizing MEG is hampered by the Quantum Confinement Dilemma (QCD): quantum confinement advantageously increases the effective Coulomb interaction, but at the same time disadvantageously increases the electronic gap. Using ab initio calculations we showed that germanium nanoparticles with core structures of high pressure phases of bulk Ge can transcend the QCD, by simultaneously lowering gaps and increasing the MEG rates above those of NPs with a cubic diamond core. Synthesis routes to obtain Ge colloidal ST12 core structures are available and hence we propose that exploring ST12 Ge NPs for MEG solar cells is a promising research effort.

Determining complex crystal structures from high pressure single-crystal diffraction data collected on synchrotron sources

As part of a Long Term Project, single-crystal diffraction techniques have been developed for use at the high pressure beamlines ID09 and ID27 at the European Synchrotron Radiation Facility, and have been utilised to determine the crystal structures of various high pressure phases, including those with incommensurate structures, at both high and low temperatures. The same techniques have also been used to determine the structures of high pressure phases at the SRS, Diamond and Petra-III synchrotron sources. In this paper, we describe technical details of the methods developed, and describe some of the considerations necessary for planning experiments and collecting and processing the data. We then illustrate the quality of data that can be obtained, and the complexity of the structures that can be refined, using recent results obtained from complex high pressure phases of N2 and Ba.

Crystal Structure of High-Pressure Phases V and VI of Potassium Dihydrogen Phosphate

Potassium dihydrogen phosphate KH 2 PO 4 is the most typical hydrogen-bonded ferroelectric. The P – T phase diagram and the existence of high-pressure phases V and VI have already been reported. However, their crystal structures remain unknown. We performed a powder x-ray diffraction experiment under high pressure using synchrotron radiation and analyzed the structures from the obtained data. The structures of phases V and VI were determined to be orthorhombic C 222 1 and triclinic P \bar1. Their hydrogen positions were predicted by a density functional theory calculation.

The properties of ions with intermediate valence and the theory of high-pressure phase structure

Two models of phase transitions under pressure, accompanied by electronic structure rearrangement, are proposed. A distinctive feature of the proposed models is the consideration of pressure-induced isotropic deformations and phonons breaking the symmetry. A mutual solid solution of ions of the same element in different ionization states is assumed to be regular. The second model supplements the first one by taking into account the physical nature of deviation of the mutual solution from the ideal one. The violation of the Curie principle during phase transitions under isotropic pressure is explained within these models.

Structural Phase Transitions of Mg(BH4)2 under Pressure

The structural stability of Mg(BH4)2, a promising hydrogen storage material, under pressure has been investigated in a diamond anvil cell up to 22 GPa with combined synchrotron X-ray diffraction and Raman spectroscopy. The analyses show a structural phase transition around 2.5 GPa and again around 14.4 GPa. An ambient-pressure phase of Mg(BH4)2 has a hexagonal structure (space group P61, a = 10.047(3) A, c = 36.34(1) A, and V = 3176(1) A3 at 0.2 GPa), which agrees well with early reports. The structure of high-pressure phase is found to be different from reported theoretical predictions; it also does not match the high-temperature phase. The high-pressure polymorph of Mg(BH4)2 is found to be stable on decompression, similar to the case of the high-temperature phase. Raman spectroscopic study shows a similarity in high-pressure behavior of as-prepared Mg(BH4)2 and its high-temperature phase.

Spiral chain structure of high pressureselenium−II′andsulfur−IIfrom powder x-ray diffraction

The structure of high pressure phases, selenium-II{sup '} (Se-II{sup '}) and sulfur-II (S-II), for {alpha}-Se{sub 8} (monoclinic Se-I) and {alpha}-S{sub 8} (orthorhombic S-I) was studied by powder x-ray diffraction experiments. Se-II{sup '} and S-II were found to be isostructural and to belong to the tetragonal space group I4{sub 1}/acd, which is made up of 16 atoms in the unit cell. The structure consisted of unique spiral chains with both 4{sub 1} and 4{sub 3} screws. The results confirmed that the structure sequence of the pressure-induced phase transitions for the group VIb elements depended on the initial molecular form. The chemical bonds of the phases are also discussed from the interatomic distances that were obtained.

High-Pressure Structural Phase Transitions

The study of structural phase transitions that occur as the result of the application of pressure is still in its long-drawn-out infancy. This is a direct result of the non-quenchability of structural phase transitions; their characterisation requires measurements of the material to be made in situ at high pressures. Although structural phase transitions could be detected by simple macroscopic compression measurements in piston-cylinder apparatus when the volume change arising from the transition was sufficiently large (e.g. calcite by Bridgman 1939, and spodumene by Vaidya et al. 1973) the limitations on sample access precluded their proper microscopic characterisation. The development in the 1970s of in situ diffraction methods, specifically the diamond-anvil cell (DAC) for X-ray diffraction and various clamp and gas-pressure cells for neutron diffraction, allowed both the structure of high-pressure phases and the evolution of the unit-cell parameters of both the high- and low-symmetry phases involved in a phase transition to be determined. Several classic studies of phase transitions at relatively low pressures were performed by high-pressure neutron diffraction methods in the 1970s and 1980s (e.g. Yelon et al. 1974, Decker et al. 1979), but the instrument time required for high-pressure studies (a result of limited sample access through, and attenuation by, the high-pressure cells as well as small sample sizes) limited the number of studies performed. In addition, until the advent of the Paris-Edinburgh pressure cell which can achieve pressures of up to 25 GPa (Besson et al. 1992, Klotz et al. 1998), pressure cells for neutron diffraction were limited to maximum pressures of the order of 2 to 4 GPa. In the past two decades the precision and accuracy of high-pressure X-ray diffraction methods has advanced considerably (see Hazen 2000). Single-crystal X-ray diffraction, which can be performed in the laboratory, is routine to pressures of …

Hydrous Phases and Hydrogen Bonding at High Pressure

Hydrogen is the simplest and most abundant element in the universe. It is the third most abundant element on the surface of the planet. Studies and interpretation of the spectroscopic properties of hydrogen, and those of its isotopes, have been intertwined with the development of experimental and theoretical physics and chemistry. It is now realized that hydrogen forms more chemical compounds than any other element, including carbon, and a survey of its chemistry would encompass the whole periodic table. In mineralogy hydrogen is also ubiquitous. The range of minerals capable of hosting hydrogen range from ice to nominally anhydrous silicate phases implicated in water storage in the mantle. The later observation has led to proposals of hydrogen content equivalent to several oceans in Earth’s deep interior and has provided new impetus for the study of hydrogen and its role in stabilizing high-pressure hydrous phases. This chapter will largely deal with this issue and will concentrate on the structures of high-pressure phases that are potential hosts for water, the properties of these materials, and the hydrogen bond in these and model systems. The properties of many substances suggest that, in addition to the normal “strong” interactions attributed to chemical bonding between atoms and ions, there exists some further interaction (10–60 kJ per mole) involving a hydrogen atom placed between two or more other groups of atoms. The definition of this interaction, the hydrogen bond, and its history is well described in chapter one of Jeffrey’s (1997) book. The concept is introduced in Pauling (1939; Nature of the Chemical Bond ): “Under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them. This is called a hydrogen bond.” He …

High Pressure Lattice Instabilities and Structural Phase Transformations in Solids from Ab-Initio Lattice Dynamics

ABSTRACTThis paper is devoted to some recent applications of density-functional perturbation theory to the prediction of lattice instabilities occurring in ionic solids at high pressure. We first briefly review some work aimed at understanding the interplay between shear instabilities and phonon softening in the pressure-induced phase transformations of Cesium halides and hydride. We then report on preliminary results of an attempt of ours to apply similar techniques to the long-standing problem of the pressure-induced amorphization of quartz.

Determination of the structure of high pressure phases combining X-ray absorption and diffraction studies

X-ray absorption spectroscopy in the dispersive mode has been combined with energy dispersive diffraction to determine in situ the structure of the high pressure phases of semiconductor materials in a diamond anvil cell. The difficulties involved in the determination of the structure of solid high pressure phases will be first assessed. Examples on GaSb, HgTe and ZnTe will illustrate how these two techniques combine to give the structure of high pressure phase.

Extension of crystal coordination formulae: Application to a prediction of high pressure phases

The intermediate notation for describing the connectivity of coordination polyhedra is extended from the conventional local environment notations. This notation is appropriate for describing the structures of coordination polyhedra. The merit of using this notation is illustrated by classification of structures for MX 3 compounds. This notation also helps us to notice the structure-density relation. Consequently the prediction of the structures of high pressure phases for ABO 3 perovskite compounds is tried and confirmed.

Theoretical and Experimental Investigation of Electronic Structure of High Pressure Phases of Li Intercalated Graphite

Abstract Electronic structure of a high pressure phase of lithium intercalated graphite has been investigated by theoretical and experimental means. Total energy calculation has shown the existence of a stable phase with LiC2 composition at c=3.38°. A Compton measurement indicates a possible existence of excess Li in the sample.

Electronic structure of high pressure phase of AlN

Results of ab initio Hartree–Fock calculations for the electronic structure of aluminum nitride in the (high-pressure) rocksalt phase are reported. In the rocksalt phase, the calculated lattice constant is 3.982 Å with the bulk modulus of 329 GPa. The band structure is predicted to be indirect at the X point with a gap of 8.9 eV. In this phase, the bonding is shown to be essentially ionic between Al and N. The direct gap shows a stronger linear dependence on pressure with a pressure derivative of 68 meV/GPa compared to that of the indirect gap with a pressure derivative of 31.7 meV/GPa.

Band structures and cohesive properties of high pressure phases of solid iodine

The band structures of high pressure phases of iodine are calculated on the basis of the local-density-functional formalism and a norm-conserving pseudopotential. The density of states (DOS) of the body-centered-tetragonal phase (BCT phase) at 49GPa is compared with that of a hypothetical face- centered-cubic phase (FCC phase) with the same atomic volume. It is found that the Fermi-level of the BCT phase is located off a peak of the DOS but that of the FCC phase is on a plateau, which originates from a saddle point singularity due to degenerate p-bands. This is in agreement with the experiment that the BCT structure is a stable phase at this atomic volume over the FCC structure. We argue from the band structures the reason why the FCC structure becomes a stable phase over the BCT structure at high pressures. We have calculated also the total energy of the FCC phase as a function of the atomic volume. The lattice constant at 64GPa is reproduced within 1% by the calculation.

In situ determination of crystal structure for high pressure phase of ZrO2 using a diamond anvil and single crystal X-ray diffraction method

Combining a miniature diamond-anvil pressure cell with a single crystal four-circle diffractometer, the crystal structure of a synthetic ZrO2 has been studied in situ up to 51 kbar at room temperature. The space group of the unquenchable orthorhombic high pressure phase is Pbcm. The directions of the b and c axes are preserved through the transition and the transformation is displacive. The coordination configurations of the Zr atoms and oxygen atoms are the same in the high pressure and low pressure phases. The orthorhombic high pressure phase has a higher entropy than that of low pressure monoclinic phase.

Crystal Structure of High Pressure Phase of Polytetrafluoroethylene

The crystal structure of high pressure phase of polytetrafluoroethylene (PTFE) has been determined by X-ray methods. A high pressure X-ray diffraction apparatus which enables to get the fiber pattern of crystalline sample under purely hydrostatic high pressure at high temperature has been constructed. A small specimen of PTFE fiber was held in the bore of a beryllium window for X-ray. The fiber pattern of high pressure phase of this polymer has been recorded on a cylindrical film at 5500km/cm2, 78°C. It is found that in this phase the molecules have a planar zigzag arrangement different from that of a helix at atmospheric pressure. The unit cell is orthorhombic, with a=8.73 A, b=5.69 A, c=2.62 A. The result of the X-ray diffraction studies agrees with that of the other physical properties of PTFE in high pressure phase.

Chemistry and structure of high pressure phase of garnets rich in almandite

I HAVE previously reported1,2 high pressure phase transformations for garnets of the almandite–pyrope (Fe3Al2Si3O12–Mg3Al2Si3O12). Here the chemical formula and the crystal structure for the high pressure phase of almandite-rich garnets quenched from a loading pressure of approximately 120×105 kPa and temperatures of about 1,400–1,800 °C are given.