High-pressure synthesis of a novel form of endohedral Li diamond from Li graphite intercalation compound

A novel form of hexagonal diamond containing Li atoms in the open rooms surrounded by sp³-bonded carbon atoms was successfully synthesized from a Li graphite intercalation compound under high pressure, as had been predicted by theoretical studies. High-pressure experiments with LiC₆ were performed in the pressure range from 0.1 MPa to 43 GPa using a diamond-anvil cell. In situ X-ray diffractometry and optical microscopy revealed that LiC₆ was transformed to a hexagonal-diamond form without losing Li atoms. The c-axis of the hexagonal-diamond form was considerably longer than that of the hexagonal diamond transformed from pure graphite, which was consistent with the predicted structure of the endohedral Li diamond. The observed high-pressure form exhibited a golden metallic gloss, which was also consistent with the calculated metallic band structure.     

High-pressure X-ray diffraction study of tungsten diselenide

Synchrotron X-ray diffraction was used in conjunction with a diamond anvil cell to investigate the properties of a tungsten diselenide (WSe₂) sample to 35.8 GPa at room temperature. By fitting the pressure-volume data to the third-order Birch-Murnaghan equation of state, the bulk modulus, K_0T, of WSe₂ was determined to be 72±1 GPa with its pressure derivative, , being 4.1±0.1. It was also found that the c-direction of the hexagonal structure is significantly more compressible than the a-direction. No phase transformation was clearly observed in the pressure range of our measurements.     

High-pressure X-ray diffraction study of dimedone

Abstract

Dimedone (i.e. 5.5-dimethyl-I,3-cyclohexanedione) crystals, C₈HI₁₂O₂, have been studied at high pressures by X-ray diffraction using a Merrill-Bassett diamond-anvil cell. The unit-cell dimensions have been measured to 1.20(5) GPa and the structure has been determined at 0.95(5) GPa. The crystal compressibility is strongly anisotropic and non-linear, relatively strong compressibility of the crystals is observed along the helices of the hydrogen-bonded molecules. Small anomalous changes of the unit-cell dimensions are observed between 0.1 and 50 MPa. The main structural changes are compression of intermolecular contacts, but also an alongation of the O=C bond—accompanied with the compression of the hydrogen bond involving the carbonyl oxygen atom—has been observed. This elongation is consistent with similar effects reported on compression of the hydrogen bonds in 1,3-cyclohexanedione and 2-methyl-1,3-cyclopentanedione. Crystal data for the dimedone structure at 0.95 GPa: monoclinic, P2₁/c, a=9.909(6), b= 6.505(3), c=12.313(6) Å, β=14.51°, V=722.1(5) Å, Z=4, R=0.139 for 336 independent reflections.     

Solvothermal synthesis of AlN

Abstract

Aluminum nitride is an important material due to its physical properties (electronic, thermal conductivity…). Different processes have been used for preparing such a material. The solvothermal synthesis is characterised by the use of a solvent in supercritical conditions in order to improve the reactivity of the precursors and to control the size of the crystalhtes. Using finely divided aluminum particles as precursor, a new process for the preparation of AlN has been optimised versus the thermodynamical parameters: pressure and temperature and the nature of the additive used for improving the kinetics of nitridation.     

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract

We have synthesized the rhombohedrally distorted perovskite phase of LaCuO₃ by reacting mixtures of La₂O₃ and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.     

High-pressure phase transition of carbon disulfide

The high-pressure phase transition of CS₂ was studied by combing ab initio molecular dynamics with total energy calculations. At 300 K the pieces of polymer structure were found to appear at 10 GPa in the molecular dynamics run, and further the CS₄ tetrahedral structure to appear at about 20 GPa. The phase transition was then studied in the structure of Cmca, α-quartz and β-quartz by using the first-principle total energy calculation method. A phase transition from Cmca to β-quartz was found at 10.6 GPa. The calculated lattice constants of β-quartz at atmospheric pressure are a=5.44 and c/a=1.138 with B₀=95 GPa. The calculation has also indicated that CS₂ decomposes at 20 GPa and below 1000 K.     

High-pressure far-infrared measurements at SINBAD

First results from the SINBAD beam line at DAΦNE are reported. The SR brilliance, as measured in the far infrared at the sample position, has been compared with the one of the mercury lamp. For λ=100 μm and through an aperture of 1 mm, the gain in intensity is larger than 20. The high brilliance of the synchrotron radiation in the far-infrared is fully exploited by high-pressure experiments. Preliminary experiments with a diamond-anvil cell in the 10 GPa range are reported.     

High-pressure powder diffraction study of TaO2F

TaO₂F, with a ReO₃-type structure, has been studied at up to 12.8 GPa using monochromatic synchrotron powder diffraction and diamond anvil cells. Two-phase transitions at ∼0.7 and 4 GPa were observed on compression. Below ∼0.7 GPa the cubic material was found to have a bulk modulus (K₀) of 36(3) GPa (K_p fixed at 4.0), similar to that reported for NbO₂F but much smaller than that of ReO₃. Immediately above 0.7 GPa on compression, the diffraction data were not fully consistent with a VF₃-type structure as previously proposed for NbO₂F. On decompression, the data between 8 and 4 GPa could be satisfactorily attributed to a single R-3c phase with a VF₃-type structure and an average bulk modulus of 60(2) GPa.     

High-pressure Raman and X-ray studies of barite,BaSO4

High-pressure Raman spectroscopic and X-ray diffraction experiments of barite, BaSO₄, were carried out in a diamond anvil cell up to 25?GPa at room temperature. On the basis of the changes in the diffraction patterns and the variation of lattice parameters with pressure, it is inferred that barite undergoes a phase transformation at 10?GPa. The phase transition accompanies the change in the force constant of vibrational modes in barite. Further compression beyond the phase transition causes the distortion of SO₄ tetrahedron as indicated by the splitting in the SO₄ stretching modes. Both X-ray and Raman data support that the phase transition in BaSO₄ is reversible. The compression data yield a bulk modulus of 63?±?2?GPa for barite. Barite shows anisotropic compressibility along three crystallographic axes with c being the most compressible axis.     

High-pressure and high-temperature bulk modulus of cubic fluorite-type MgF2 from quasi-harmonic Debye model

A detailed theoretical study of the isothermal and adiabatic bulk moduli of MgF₂ with a fluorite structure under high pressure and temperature has been carried out by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the prediction of the isothermal bulk modulus and its first and second pressure derivatives for the first time. The calculated ground state properties agree well with other theoretical values. At extended pressure and temperature ranges, the variation of the bulk modulus which plays a central role in the formulation of approximate equations of state has also been predicted. The properties of MgF₂ with a fluorite structure are summarized in the pressure range of 0–135 GPa and the temperature up to melting temperature 1500 K.     

High-pressure disproportionation phases of CO2 and CO

First-principles enthalpy minimization simulations with target pressures suggest that dense-packed disproportionation phases of CO₂ and CO can be transformed from selected molecular precursors under hydrostatic compression. In transformations, electrons are pushed from oxygen to carbon atoms to form electronic disproportionation phases with their chemical formulas of C^?2O${}_2^{+}$ and C^?O⁺. Simulations indicate that the space packing properties of reactive groups in a molecular crystal precursor, including the orientation and the number of bonding partners of each reactive group, mainly control transformation bonding pathways. Phonon dispersion spectra and molecular dynamic simulations confirm the metastability of both electronic disproportionation phases under high pressures.     

High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of

Two-dimensional polymerisation of a C₆₀ single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C₆₀-C₆₀ dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P4₂/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C₆₀ is discussed. Received 8 October 1999     

Modified Bridgman anvils for high pressure synthesis and neutron scattering

ABSTRACT

A simple modified Bridgman design for large volume pressure anvils usable in the Paris-Edinburgh (PE) press has been demonstrated at Oak Ridge National Laboratory Spallation Neutron Source. The design shows advantages over the toroidal anvils typically used in the PE press, mainly rapid compression/decompression rates, complete absence of blow-outs upon drastic phase transitions, simplified cooling, high reliability, and relative low loads (～40 tons) corresponding to relatively high pressures (～20?GPa). It also shows advantages over existing large-volume diamond cells as sample volumes of ～2–3?mm³ can be easily and rapidly synthesized. The anvils thus allow sample sizes sufficient for in situ neutron diffraction as well as rapid synthesis of adequate amounts of new materials for ex situ analysis via total neutron scattering and neutron spectroscopy.     

Strong tolerance of blue-green alga Microcystis flos-aquae to very high pressure

It was shown in our previous reports that a few spores of moss Venturiella could tolerate the very high pressure of 20 GPa for 30 min and germinated a protonema to the length of 30 μm. However, these spores did not grow any further, and disappeared at around 30 days of incubation after seeded. On the other hand, colonies of blue-green alga Microcystis flos-aquae came to appear about 76 days after the moss spores were seeded. Many of these colonies appeared at the places where the moss spores had disappeared. These colonies were formed by the algae that had adhered to the spore cases of the moss and survived after exposure to the very high pressure of 20 GPa. Though the appearance of the colonies of high pressure exposed algae was delayed by about 50 days compared with that of the control group which was not exposed to high pressure, there seems no difference in their shape and color from those of the control group. The pressure tolerance of blue-green alga is found to be enormously strong, and it can survive after exposure to the high pressure which corresponds to the depth of about 550–600 km from the surface of the Earth, just above the lower mantle.     

High-pressure properties of group IV clathrates

We review the investigations on clathrate materials having C, Si, Ge or Sn framework atoms in which the pressure parameter plays an important role. In this article, we studied the synthesis of clathrates, superconducting properties and those studies relative to the structural cohesivity and phase diagrams. We have tried to extensively review these subjects. Key references on other important properties of the group IV clathrates are provided in the introductory sections. As a main result of this review, we note that pressure appears as a key parameter for the elaboration of present and future clathrates in particular those exhibiting superconducting properties. We show how high-pressure research has also played an important role in the understanding of the parameters governing clathrate superconductivity. In contrast, the study of the structural evolution of group IV clathrates (bulk modulus and stability) gives abundant clues for tailoring new materials with improved mechanical properties. Finally, practically all investigations converge to point out that today hypothetical carbon clathrates are candidates for extraordinary superconducting and mechanical properties.     

High-pressure structural studies of group-15 elements

Recent advances in high-pressure experimental techniques have yielded high-quality x-ray diffraction data for the high-pressure phases of the group-15 elements Bi, Sb and As, and have made it possible to solve several long-standing problems in their structures. In particular, several complex incommensurate host–guest structures have been identified. This paper reviews the present state of knowledge of the structural transition sequences for these elements at high pressure and room temperature, including a summary of previous work, a detailed presentation of the new structures, and revised equations of state.     

High-pressure [2 + 2]cyloaddition of acyl isocyanates to glycals

Abstract

Isocyanates derived from uretanes 4, s and benzamides 6, 7 react with di-O-benzyl-D-arabinal 3 under 12 kbar pressure to afford the corresponding cycloadducts with high stereoselectivity. Alkoxycarbonyl isocyanates 4 and 5 from [2 + 2]cycloadducts almost exclusively, where benzoyl iSocyanates produce 1:1 mixtures of [2 + 21 and [4 + 2]cycloadducts.     

High-pressure Raman spectra of L-isoleucine crystals

Raman spectroscopy investigations of L-isoleucine crystals under high pressures have been carried out up to 7.3 GPa. From this study it was possible to observe modifications on bands associated to both rocking vibrations of r(NH₃⁺) and r(CO₂⁻) as well as to lattice modes at about 2.3 and 5.0 GPa. These modifications were correlated to either conformational change of molecules or to a solid–solid phase transition undergone by the crystals involving the hydrogen bonds that maintain the molecules held in the unit cell. A comparison with a few results on other amino acid crystals is also given.     

新型带宽调制型Mott化合物的高压合成及其金属化相变

利用高温高压条件,制备了(SrCa)CrO3系列带宽可调型的钙钛矿Mott化合物。在10 GPa的外加压力下,观察到SrCrO3的绝缘体-金属化相变。原位高压X射线衍射实验表明,Sr/CaCrO3晶体结构在0-9 GPa压力范围内保持稳定。但SrCrO3在4 GPa时存在着电子结构变化所引起的等结构相变,表现为体弹性模量的反常软化。此外,由于电子关联效应,磁性和热输运性质的测试结果表明了材料的奇异电子态特征。     

新型带宽调制型Mott化合物的高压合成及其金属化相变

利用高温高压条件，制备了（SrCa）CrO3系列带宽可调型的钙钛矿Mort化合物。在10 GPa的外加压力下，观察到SrCrO3的绝缘体-金属化相变。原位高压x射线衍射实验表明，Sr／CaCrO3晶体结构在0-9 GPa压力范围内保持稳定。但SrCrO3在4 GPa时存在着电子结构变化所引起的等结构相变，表现为体弹性模量的反常软化。此外，由于电子关联效应，磁性和热输运性质的测试结果表明了材料的奇异电子态特征。