Static compression of tetramethylammonium borohydride

Raman spectroscopy and synchrotron X-ray diffraction are used to examine the high-pressure behavior of tetramethylammonium borohydride (TMAB) to 40 GPa at room temperature. The measurements reveal weak pressure-induced structural transitions around 5 and 20 GPa. Rietveld analysis and Le Bail fits of the powder diffraction data based on known structures of tetramethylammonium salts indicate that the transitions are mediated by orientational ordering of the BH(4)(-) tetrahedra followed by tilting of the (CH(3))(4)N(+) groups. X-ray diffraction patterns obtained during pressure release suggest reversibility with a degree of hysteresis. Changes in the Raman spectrum confirm that these transitions are not accompanied by bonding changes between the two ionic species. At ambient conditions, TMAB does not possess dihydrogen bonding, and Raman data confirms that this feature is not activated upon compression. The pressure-volume equation of state obtained from the diffraction data gives a bulk modulus [K(0) = 5.9(6) GPa, K(0)' = 9.6(4)] slightly lower than that observed for ammonia borane. Raman spectra obtained over the entire pressure range (spanning over 40% densification) indicate that the intramolecular vibrational modes are largely coupled.     

Microscopic observation and in-situ Raman scattering studies on high-pressure phase transformations of Kr hydrate

Direct observations through a microscope and in-situ Raman scattering measurements of synthesized single-crystalline Kr hydrate have been performed at pressures up to 5.2 GPa and 296 K. We have observed that the initial cubic structure II (sII) of Kr hydrate successively transforms to a cubic structure I (sI), a hexagonal structure, and an orthorhombic structure (sO) called "filled ice" at 0.45, 0.75, and 1.8 GPa, respectively. The sO phase exists at least up to 5.2 GPa. In addition to these transformations, we have also found the new phase behavior at 1.0 GPa, which is most likely caused by the change of cage occupancy of host water cages by guest Kr atoms without structural change. Raman scattering measurements for observed phases have shown that the lattice vibrational peak at around 130 cm(-1) disappears in the pressure region of sI, which enables us to distinguish the sI phase from sII and sH phases.     

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.     

Pressure response of three-dimensional cyanide-bridged bimetallic magnets

Effects of pressure on the structures and magnetic properties of three types of 3-D cyanide-bridged bimetallic coordination polymer magnets, MnIICrIII ferrimagnet [Mn(en)]3[Cr(CN)6]2.4H2O (1; en = ethylenediamine), NiIICrIII ferromagnet [Ni(dipn)]3[Cr(CN)6]2.3H2O (2; dipn = N,N-di(3-aminopropyl)amine), and NiIIFeIII ferromagnet [Ni(dipn)]2[Ni(dipn)(H2O)][Fe(CN)6]2.11H2O (3), were systematically examined under hydrostatic pressure up to 19.8 GPa using a piston-cylinder-type pressure cell and a diamond anvil cell. The ferrimagnet 1 showed the reversible crystalline-to-amorphous-like phase change, and the magnetic phase transition temperature (TC) was reversibly changed from 69 K at 0 GPa to 126 K at 4.7 GPa. At higher pressure, the net magnetization was suppressed with increasing pressure, and the magnetic state at 19.8 GPa was assumed to be paramagnetic. The initial ferrimagnetic phase of 1 was not recovered after releasing the pressure from 19.8 GPa. The magnetic phase of 2 was reversibly converted between ferromagnetic and paramagnetic-like phase in the range 0 相似文献   

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO₂F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO₂F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4 GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4 GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO₂F. Disorder-induced first order Raman modes at 176, 212, 381 and 485 cm⁻¹ soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K⁻¹) is in fair agreement with the reported values (−1 to +1 ppm K⁻¹). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.     

Optical microscopy and in situ Raman scattering of single crystalline ethylene hydrate and binary methane-ethylene hydrate at high pressures

Visual observations through a microscope and in situ Raman measurements have been made for single crystalline ethylene hydrate (EH) and binary methane-ethylene hydrate (MEH) at pressures up to 3.7 GPa and room temperature. Both hydrates showed pressure-induced phase transitions at 1.6, 2.0, and 3.0 GPa for EH and at 1.7, 2.1, and 3.3 GPa for MEH. The cubic sI phase of EH and MEH remains stable up to 1.6 and 1.7 GPa, respectively, which are more widely ranging values than the values for the methane hydrate sI phase. In this sI phase of binary MEH, the cage occupancies by methane and ethylene molecules are investigated from Raman spectra. Above P = 3.0 GPa for EH and 3.3 GPa for MEH, they decomposed by associating with the formation of the polyethylene.     

Vibrational dynamics, intermolecular interactions, and compound formation in GeH4-H2 under pressure

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH(4)) + hydrogen (H(2)). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH(4)(H(2))(2), when the constituents are compressed above 7.5 GPa. Raman and infrared spectroscopic measurements show multiple H(2) vibrons substantially softened from bulk solid hydrogen. With increasing pressure, the frequencies of several Raman and infrared H(2) vibrons decrease, indicating anomalous attractive interaction for closed-shell, nonpolar molecules. Synchrotron powder x-ray diffraction measurements show that the compound has a structure based on face-centered cubic (fcc) with GeH(4) molecules occupying fcc sites and H(2) molecules likely distributed between O(h) and T(d) sites. Above ca. 17 GPa, GeH(4) molecules in the compound become unstable with respect to decomposition products (Ge + H(2)), however, the compound can be preserved metastably to ca. 27 GPa for time-scales of the order of several hours.     

Phonon behavior of CaSnO3 perovskite under pressure

Raman scattering and x-ray diffraction studies of CaSnO(3) perovskite were performed under high-pressure conditions. This high-pressure study was motivated by a recent theoretical study predicting a phase transition in CaSnO(3) from GdFeO(3)-type perovskite to CaIrO(3)-type structure occurred at 12 GPa. Despite no obvious structure change up to a pressure of 26 GPa based on the x-ray diffraction data, high pressure Raman measurements revealed that some Raman modes disappeared upon compression; either merging into neighboring bands or vanishing. The signals for these Raman peaks were recovered during decompression. The measured pressure derivative of Raman shift (?ν∕?P) of CaSnO(3) ranged from ~1.29 to ~4.35, up to 20 GPa. Due to the lack of lattice dynamic study for CaSnO(3) perovskite, the mode symmetry for CaSnO(3) was tentatively assigned based on the empirical relation among Ca-bearing perovskites. The pressure derivative of the Raman shifts was found to be related to their mode vibrations: modes related to Ca and O shifts had a strong pressure dependence compared with those associated with oxygen octahedral rotation.     

Pressure-induced structural changes of the tetragonal Bi2CuO4

The tetragonal compound Bi₂CuO₄ was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.     

Extending reversed-flow chromatographic methods for the measurement of diffusion coefficients to higher temperatures

A reversed-flow gas-chromatography (RF-GC) apparatus for the measurement of binary diffusion coefficients is described and utilized to measure the binary diffusion coefficients for several systems at temperatures from (300 to 723)K. Hydrocarbons are detected using flame ionization detection, and inert species can be detected by thermal conductivity. The present apparatus has been utilized to measure diffusion coefficients at substantially higher temperatures than previous RF-GC work. Characterization of the new apparatus was accomplished by comparing measured binary diffusion coefficients of dilute argon in helium to established reference values. Further diffusion coefficient measurements for dilute helium in argon and dilute nitrogen in helium (using thermal conductivity detection) and dilute methane in helium (using flame ionization detection) were performed and found to be in excellent agreement with literature values. The measurement of these well-established diffusion coefficients has shown that specific experimental conditions are required for accurate diffusion measurements using this technique, particularly at higher temperatures. Numerical simulations of the diffusion experiments are presented to demonstrate that artifacts of the analysis procedure must be specifically identified to ensure accuracy, particularly at higher temperatures.     

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.     

Pressure-induced phase transitions in Al2(WO4)3

The high pressure behavior of aluminum tungstate [Al₂(WO₄)₃] has been investigated up to ∼18 GPa with the help of Raman scattering studies. Our results confirm the recent observations of two reversible phase transitions below 3 GPa. In addition, we find that this compound undergoes two more phase transitions at ∼5.3 and ∼6 GPa before transforming irreversibly to an amorphous phase at ∼14 GPa.     

Application of principal component analysis and Raman spectroscopy in the analysis of polycrystalline BaTiO3 at high pressure

The principal component analysis (PCA) was applied to Raman spectra of polycrystalline BaTiO(3) under pressure from atmospheric pressure to approximately 6.72 GPa. For the system utilized, PCA was able to distinguish spectral features and to determine the phase transition pressure: tetragonal to cubic at approximately 2.0 GPa. The present study demonstrates the potentialities of the application of PCA to the investigation on phase transitions at high pressure by Raman spectroscopy.     

Structural change of layered perovskite La2Ti2O7 at high pressures

The perovskite-related layered structure of La₂Ti₂O₇ has been studied at pressures up to 30 GPa using synchrotron radiation powder X-ray diffraction (XRD) and Raman scattering. The XRD results indicate a pronounced anisotropy for the compressibility of the monoclinic unit cell. The ratio of the relative compressibilities along the [100], [010] and [001] directions is ∼1:3:5. The greatest compressibility is along the [001] direction, perpendicular to the interlayer. A pressure-induced phase transition occurs at 16.7 GPa. Both Raman and XRD measurements reveal that the pressure-induced phase transition is reversible. The high-pressure phase has a close structural relation to the low-pressure monoclinic phase and the phase transition may be due to the tilting of TiO₆ octahedra at high pressures.     

The direct determination of partial molar volumes and reaction volumes in ultra-dilute non-reactive and reactive multi-component systems using a combined spectroscopic and modified response surface model approach

Two experimental multi-component organometallic systems were studied, namely, (1) a non-reactive system consisting of [Mo(CO)(6)], [Mn(2)(CO)(10)], and [Re(2)(CO)(10)] in toluene under argon at 298.15 K and 0.1 MPa and (2) a reactive system consisting of [Rh(4)(CO)(12)] + PPh(3)--> [Rh(4)(CO)(11)PPh(3)] + CO in n-hexane under argon at 298.15 K and 0.1 MPa. The mole fractions of all solutes were less than 140 x 10(-6) in system (1) and less than 65 x 10(-6) in system (2). Simultaneous in-situ FTIR spectroscopic measurements and on-line oscillatory U-tube density measurements were performed on the multi-component solutions. A newly developed response surface methodology was applied to the data sets to determine the individual limiting partial molar volumes of all constituents present as well as the reaction volume. The limiting partial molar volumes obtained for system (1) were 176.4 +/- 2.5, 265.1 +/- 2.4, and 276.8 +/- 2.4 cm(3) mol(-1) for [Mo(CO)(6)], [Mn(2)(CO)(10)], and [Re(2)(CO)(10)], respectively and are consistent with independent binary experiments. The limiting partial molar volumes obtained for system (2) were 310.7 +/- 2.7, 219.8 +/- 2.2 and 461.5 +/- 4.5 cm(3) mol(-1) for [Rh(4)(CO)(12)], PPh(3) and [Rh(4)(CO)(11)PPh(3)], respectively. In addition, a reaction volume Delta(r)V equal to -17.0 +/- 5.7 cm(3) mol(-1) was obtained. The present results demonstrate that both partial molar volumes and reaction volumes can be obtained directly from multi-component organometallic solutions. This development provides a new tool for physico-chemical determinations relevant to a variety of solutes and their reactions.     

High-pressure vibrational spectroscopy of sulfur dioxide

Solid sulfur dioxide was investigated by vibrational spectroscopy over a broad pressure and temperature range, extending to 32.5 GPa at 75-300 K in diamond anvil cells. Synchrotron infrared spectra provided the first measurements of the pressure dependence of the lattice modes in the far-IR region. Below 17.5 GPa, two fundamentals exhibit splittings enhanced by pressure. The asymmetric stretching mode of SO(2) exhibits a remarkable pressure-induced softening. The observations are consistent with the ambient pressure Raman measurements indicating that SO(2) crystallizes in an acentric cell, but are inconsistent with a previously proposed interpretation that the structure of the high-pressure phase consists of (SO(2))(3) clusters. Dramatic changes in the Raman spectra are found above 17.5 GPa at room temperature. These indicate major changes in structure and possible formation of SO(2) clustering with an enlarged unit cell. The behavior at low temperature differs from that at room temperature. These findings provide constraints on the phase diagram of sulfur dioxide.     

Pressure-induced cubic to monoclinic phase transformation in erbium sesquioxide Er(2)O(3)

Cubic Er(2)O(3) was compressed in a symmetric diamond anvil cell at room temperature and studied in situ using energy-dispersive X-ray diffraction. A transition to a monoclinic phase began at 9.9 GPa and was complete at 16.3 GPa and was accompanied by a approximately 9% volume decrease. The monoclinic phase was stable up to at least 30 GPa and could be quenched to ambient conditions. The normalized lattice parameter compression data for both phases were fit to linear equations, and the volume compression data were fit to third-order Birch-Murnaghan equations of state. The zero-pressure isothermal bulk moduli (B(0)) and the first-pressure derivatives (B(0)') for the cubic and monoclinic phases were 200(6) GPa and 8.4 and also 202(2) GPa and 1.0, respectively. Ab initio density functional theory calculations were performed to determine optimized lattice parameters and atom positions for the cubic, monoclinic, and hexagonal phases of Er(2)O(3). The calculated X-ray spectra and predicted transition pressure are in good qualitative agreement with the experimental results.     

Influence of size and shape effects on the high-pressure solubility of n-alkanes: Experimental data, correlation and prediction

(Solid + liquid) phase equilibria (SLE) of (n-hexadecane, or n-octadecane + 3-methylpentane, or 2,2-dimethylbutane, or benzene) at very high pressures up to about 1.0 GPa have been investigated at the temperature range from T = (293 to 353) K. The thermostated apparatus for the measurements of transition pressures from the liquid to the solid state in two component isothermal solutions was used. The pressure-temperature-composition relation of the high pressure (solid + liquid) phase equilibria, polynomial based on the general solubility equation at atmospheric pressure was satisfactorily used. Additionally, the SLE of binary systems (n-hexadecane, or n-octadecane + 3-methylpentane, or 2,2-dimethylbutane, or benzene, or n-hexane or cyclohexane) at normal pressure was discussed. The results at high pressures were compared for every system to these at normal pressure. The influence of the size and shape effects on the solubility at 0.1 MPa and high pressure up to 600 MPa was discussed.The main aim of this work was to predict the mixture behaviour using only pure components data and cubic equation of state in the wide range of pressures, far above the pressure range which cubic equations of state are normally applied to. The fluid phase behaviour is described by the corrected SRK-EOS and the van der Waals one fluid mixing rules.     

Pressure induced phase transformation in U2O(PO4)2

The high pressure behavior of U₂O(PO₄)₂ has been investigated with the help of Raman scattering and X-ray diffraction measurements up to ∼14 and 6.5 GPa, respectively. The observed changes in the Raman spectra as well as the X-ray diffraction patterns suggest that U₂O(PO₄)₂ undergoes a phase transition at ∼6 GPa to a mixture of a disordered ambient pressure phase and a new high pressure phase. The new phase resembles the triclinic mixed-valence phase of uranium orthophosphate (U(UO₂)(PO₄)₂). On release of pressure the initial phase is not retrieved.