Ling-Yun Tang

Phase transformations and vibrational properties of coronene under pressure

Both the vibrational and structural properties of coronene have been investigated upon compression up to 30.5 GPa at room temperature by a combination of Raman scattering and synchrotron x-ray diffraction measurements. The spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa where the high-pressure phases are identified as monoclinic and orthorhombic crystal structures with space groups of P2/m and Pmmm, respectively. A kink in the slope of the cell parameters as a function of pressure is associated with the disappearance of several internal Raman modes, which suggests the existence of structural distortions or reorganizations at approximately 6.0 GPa. Above 17.1 GPa, almost no evidence of crystallinity can be observed, indicating a possible transformation of coronene into an amorphous phase.

Structural and vibrational properties of phenanthrene under pressure

The structural and vibrational properties of phenanthrene are measured at high pressures up to 30.2 GPa by Raman spectroscopy and synchrotron X-ray diffraction techniques. Two phase transitions are observed in the Raman spectra at pressures of 2.3 GPa and 5.4 GPa which correspond to significant changes of intermolecular and intramolecular vibrational modes. Above 10.2 GPa, all the Raman peaks are lost within the fluorescence background; however, upon further compression above 20.0 GPa, three broad peaks are observed at 1600, 2993, and 3181 cm(-1), indicating that phenanthrene has transformed into amorphous phase. Using X-ray diffraction, the structures of corresponding phases observed from Raman spectra are indexed with space groups of P2(1) for phase I (0-2.2 GPa), P2/m for phase II (2.2-5.6 GPa), P2/m+Pmmm for phase III (5.6-11.4 GPa) which has a coexistence of structures, and above 11.4 GPa the structure is indexed with space group of Pmmm. Although phenanthrene has transformed to a hydrogenated amorphous carbon structure above 20.0 GPa, these amorphous clusters still show characteristic crystalline behavior based on our X-ray diffraction patterns. Our results suggest that the long-range periodicity and the local disorder state coexist in phenanthrene at high pressures.

Vibrational and structural properties of tetramethyltin under pressure

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Sn(CH(3))(4), have been investigated by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 49.9 GPa. Both techniques allow the obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 0.9 GPa for the liquid to solid and 2.8, 10.4, 20.4, and 32.6 GPa for the solid to solid. The foregoing solid phases are identified to have the orthorhombic, tetragonal, monoclinic crystal structures with space groups of Pmmm for phase I, P4/mmm for phase II, P2/m for phase III, respectively. The phases IV and V coexist with phase III, resulting in complex analysis on the possible structures. These transitions suggest the variation in the inter- and intra-molecular bonding of this compound.

High-pressure phases of a hydrogen-rich compound: Tetramethylgermane

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Ge(CH3)4, are studied by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 30.2 GPa. Both techniques allowthe obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 1.4 GPa for the liquid to solid and 3.0, 5.4, and 20.3 GPa for the solid to solid. The four high-pressure solid phases are identified to have the cubic, orthorhombic, monoclinic, and monoclinic crystal structures with space groups of Pa-3 for phase I, Pnma for phase II, P21/c for phase III, and P21 for phase IV, respectively. These transitions are suggested to result from the changes in the interand intramolecular bonding of this compound. The softening of some Raman modes on CH3 groups and their sudden disappearance indicate that Ge(CH3)4 might be an ideal compound to realize metallization and even high-temperature superconductivity at modest static pressure for laboratory capability.

Structural feature controlling superconductivity in compressed BaFe2As2

Superconductivity can be induced with the application of pressure but it disappears eventually upon heavy compression in the iron-based parent compound BaFe2As2. Structural evolution with pressure is used to understand this behavior. By performing synchrotron X-ray powder diffraction measurements with diamond anvil cells up to 26.1 GPa, we find an anomalous behavior of the lattice parameter with a S shape along the a axis but a monotonic decrease in the c-axis lattice parameter with increasing pressure. The close relationship between the axial ratio c/a and the superconducting transition temperature Tc is established for this parent compound. The c/a ratio is suggested to be a measure of the spin fluctuation strength. The reduction of Tc with the further increase of pressure is a result of the pressure-driven weakness of the spin-fluctuation strength in this material.

Pressure-induced phase coexistence in BaFe1.8Co0.2As2

We report high-pressure powder synchrotron X-ray diffraction measurements of overdoped BaFe1.8Co0.2As2 under quasi-hydrostatic pressure up to 40.1 GPa. Our results indicate that a tetragonal (T) to collapsed tetragonal (CT) phase transition occurs at 16.8 GPa and the two phase coexist until 30 GPa, which has not been previously observed in iron arsenide compounds. Both the lattice parameters a and c show discontinuous change for the T and CT phases. The decrease of the c lattice parameter is as large as 12.2% owing to the uniaxial pressure effect. The axial ratio c/a of the T phase exhibits similar features to the other 122-type compounds below 16.8 GPa, whereas there is a very small increase with increasing pressure in the two phase coexistence region. Because of the relationship between the axial ratio and superconductivity, the abnormal expansion may be related to the sudden increase of the strength of antiferromagnetic spin fluctuations.

Phase transitions in a hydrogen-rich compound: tetramethylsilane

High-pressure behavior of tetramethylsilane is investigated by synchrotron powder X-ray diffraction and Raman scattering at pressures up to 30 GPa and room temperature. Our results reveal the analogous phase transitions, though slight hysteresis for the certain phases. A new phase is found to appear at 4.2 GPa due to the disappeared Raman mode. These findings offer the possibility to understand the evolution of the H-H bonding with pressure in such hydrogen-rich compounds.