Chunli Ma

In Situ Observation of Multiple Phase Transitions in Low-Melting Ionic Liquid [BMIM][BF4] under High Pressure up to 30 GPa

In situ characterization of phase transitions and direct microscopic observations of a low-melting ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF(4)]), has been performed in detail by Raman spectroscopy. Compression of [BMIM][BF(4)] was measured under hydrostatic pressure up to ~30.0 GPa at room temperature by using a high-pressure diamond anvil cell. With pressure increasing, the characteristic bands of [BMIM][BF(4)] displayed nonmonotonic pressure-induced frequency shifts, and it is found to undergo four successive phase transitions at around 2.25, 6.10, 14.00, and 21.26 GPa. Especially, above a pressure of 21.26 GPa, luminescence of the sample occurs, which is connected with the most significant phase transition at around this pressure. It was indicated that the structure change under high pressure might be associated with a conformational change in the butyl chain. Upon releasing pressure, the spectrum was not recovered under a pressure up to 1.16 GPa, thereby indicating that this high-pressure phase remains stable over a large pressure range between 30 and 1.16 GPa in low-melting ionic liquid [BMIM][BF(4)]. Although the sample was kept under the normal pressure for 24 h, the spectrum was recovered, and it showed that the phase transition of [BMIM][BF(4)] was reversible. In other words, such a low-melting ionic liquid [BMIM][BF(4)] remains stable even after being treated under so a high pressure of up to 30 GPa.

Photoluminescence studies of Y2O3:Eu3+ under high pressure

The Eu-doped yttria (Y2O3:Eu3+) has been investigated by the in situ high-pressure angle dispersive synchrotron X-ray diffraction (XRD) and the photoluminescence (PL) spectroscopy. The red shift and intensity ratio variation of emissions with increasing pressure were observed and elucidated. It was found that the red shift of emissions is related to the expansion of the f orbit of the Eu3+ and the intensity ratio variation of emissions is ascribed to the change of the crystal field under high pressure. The pressure-induced changes in spectrum are related to the phase transition, which was confirmed by XRD pattern. The two high pressure phases were identified as the monoclinic (C2/m) phase and hexagonal (P-3m1) phase by the Rietveld refinement.

Pressure-driven variations of hydrogen bonding energy in ammonium azide (NH4N3): IR absorption and Raman scattering studies

In this study, high pressure infrared (IR) absorption and Raman scattering studies for ammonium azide (NH4N3) were carried out at room temperature up to 20 GPa and 22 GPa, respectively. For comparison and further assignment, the vibrational spectra at ambient conditions were calculated using CASTEP code, particularly for the far- and mid-IR modes. The recorded vibrational data consistently indicated a pressure-induced phase transition at 2.9 GPa. All observed vibrational modes maintained their identities at the high pressure phase, indicating that NH4N3 was still presented in the form of ammonium cations and azide anions linked by the hydrogen bond (N-H⋯N). Above 2.9 GPa, the relative magnitude of the torsional mode weakened and the N-H symmetric stretch displayed a redshift, indicating strengthened hydrogen bonding energy. The opposite effects were observed above 12 GPa, where the relative magnitude of the torsional mode strengthened and the N-H symmetric stretch reverted to a blueshift, indicating weakened hydrogen bonding energy. It can be concluded that the hydrogen bonding energy exhibited a weakening (0-2.9 GPa), strengthening (2.9-12 GPa), and then again weakening (12-22 GPa) phenomena with the increasing of compression. The hydrogen bonding energy changing with the increase of pressure can be ascribed to a phase transition at 2.9 GPa and a rotational or bending behavior of azide ions at 12 GPa.

Brillouin scattering study of liquid methane under high pressures and high temperatures.

Brillouin scattering measurements were performed on liquid methane using diamond anvil cell along five isotherms and at the pressures up to solidification points. Sound velocity, refractive index, and adiabatic bulk modulus of liquid methane as function of pressure were determined with the measurements from the platelet and backscattering geometries. The maximum pressure and temperature reached up to 5.12 GPa and 539 K. The sound velocity, refractive index, and adiabatic bulk modulus increased with pressure along each isotherm. The equation of state of liquid methane was determined from the present Brillouin results.

High pressure Raman scattering and X-ray diffraction studies of MgNb2O6

High pressure X-ray diffraction and Raman scattering studies have been carried out on magnesium niobate (MgNb2O6) in a diamond anvil cell (DAC) at room temperature up to 40 GPa. A pressure-induced phase transition was observed at pressures above 10 GPa accompanied by a softening of the internal ν9 (B3g) modes. Raman scattering results reveal that the distortion of the NbO6 octahedra decreases under high pressure. According to the X-ray diffraction data, the high pressure phase can be indexed with a monoclinic unit cell. The initial orthorhombic phase changes to a monoclinic phase possibly due to the rotation of the NbO6 octahedra, similar to some perovskite ABO3 structures.

Residual stress inspection by Eu3+ photoluminescence piezo-spectroscopy: An application in thermal barrier coatings

A non-destructive inspection technique was developed to measure the residual stresses in thermal barrier coatings (TBCs) by using Eu3+ photoluminescence piezo-spectroscopy. The new approach is based on the relationship between stress and the position of the main peak of 5D0→7F2 transition, which is built by the high-pressure techniques. The Eu3+ luminescent sublayer was applied in the current method to ensure that the detected position in TBCs can be well controlled. The laser used to detect Eu3+ luminescence gives a proper penetration depth and spatial resolution, which make this method suitable to detect the stresses concentrated near the interfaces between different layers. This method was successfully applied in detecting residual stress in plasma sprayed TBCs with a 8YSZ:Eu (1 mol. %) sublayer.

The pressure-induced amorphous state of acetonitrile

High-pressure Raman scattering studies are performed on acetonitrile in a diamond anvil cell up to 24.8 GPa at room temperature. The results show that liquid acetonitrile transforms into β phase at 0.3 GPa, and then into α phase at 0.8 GPa. With increasing pressure, the α to γ phase transition is observed at 9.8 GPa which results from the rearrangement of molecular structure. After further increasing the pressure to 21.5 GPa, the external Raman modes of acetonitrile completely disappear, and the result suggests that acetonitrile ultimately turns into an amorphous state accompanying the decreases of transmittance light. The high pressure behaviors of polar molecular acetonitrile will provide important information for the study of lattice dynamics of molecular crystals.

The acoustic velocity, refractive index, and equation of state of liquid ammonia dihydrate under high pressure and high temperature.

High-pressure and high-temperature Brillouin scattering studies have been performed on liquid of composition corresponding to the ammonia dihydrate stoichiometry (NH(3)·2H(2)O) in a diamond anvil cell. Using the measured Brillouin frequency shifts from 180° back- and 60° platelet-scattering geometries, the acoustic velocity, refractive index, density, and adiabatic bulk modulus have been determined under pressure up to freezing point along the 296, 338, 376, and 407 K isotherms. Along these four isotherms, the acoustic velocities increase smoothly with increasing pressure but decrease with the increased temperature. However, the pressure dependence of the refractive indexes on the four isotherms exhibits a change in slope around 1.5 GPa. The bulk modulus increases linearly with pressure and its slope, dB/dP, decreases from 6.83 at 296 K to 4.41 at 407 K. These new datasets improve our understanding of the pressure- and temperature-induced molecular structure changes in the ammonia-water binary system.

Ammonia molecule rotation of pressure-induced phase transition in ammonia hemihydrates 2NH3·H2O

High-pressure Raman scattering and synchrotron angle-dispersive X-ray diffraction studies have been performed on liquid ammonia hemihydrates (2NH3·H2O) at room temperature up to 41.0 GPa. The results demonstrate that liquid 2NH3·H2O transforms into a solid phase at 3.5 GPa. Upon increasing pressure, a solid-solid phase transition is observed at about 19.0 GPa. When pressure is increased up to 25.8 GPa, another solid-solid phase transition is obtained. The first solid-solid phase transition at about 19.0 GPa originates from the rotation of type II ammonia molecule via the O–H⋯N bond, and this phase transition is from orthorhombic to body-centered-cubic. High-pressure Raman scattering and X-ray diffraction results of 2NH3·H2O provide significant information for better understanding the physical properties of the ammonia–water binary system under extreme conditions, and further for the structure state of the outer planets and large satellites in the solar system.