Liancheng Wang

Hydrogen bond symmetrization and superconducting phase of HBr and HCl under high pressure: An ab initio study

Ab initio calculations are performed to probe the hydrogen bonding, structural, and superconducting behaviors of HBr and HCl under high pressure. The calculated results show that the hydrogen bond symmetrization (Cmc2(1)-->Cmcm transition) of HBr and HCl occurs at 25 and 40 GPa, respectively, which can be attributed to the symmetry stretching A(1) mode softening. After hydrogen bond symmetrization, a pressure-induced soft transverse acoustic phonon mode of Cmcm phase is identified and a unique metallic phase with monoclinic structure of P2(1)/m (4 molecules/cell) for both compounds is revealed by ab initio phonon calculations. This phase preserves the symmetric hydrogen bond and is stable in the pressure range from 134 to 196 GPa for HBr and above 233 GPa for HCl, while HBr is predicted to decompose into Br(2)+H(2) above 196 GPa. Perturbative linear-response calculations predict that the phase P2(1)/m is a superconductor with T(c) of 27-34 K for HBr at 160 GPa and 9-14 K for HCl at 280 GPa.

Structural stability of polymeric nitrogen: A first-principles investigation

The search for the stable single-bonded (polymeric) solid nitrogen attracted much attention in view of its potential application as a high energy density material. In this study, the stability of different candidate polymeric structures of nitrogen has been studied using ab initio calculations based on density-functional theory for the first time, from the angles of thermodynamic stabilities, mechanical stabilities, and dynamical stabilities in the pressure range from 0 to 360 GPa, respectively. According to our results, only Cmcm, A7, rcg, cg, BP, P2(1)2(1)2(1), and Pba2 are competitive structures and more favorable than sc, ch, LB, and cw strcutrues; their stable pressure range were also presented. Among the competitive structures, BP, Pba2, and P2(1)2(1)2(1) are the novel ones for their enthalpies are lower than the cg structure above 170 GPa. We further identify that the P2(1)2(1)2(1) phase can transform to cg structure at pressure below 60 GPa. Also a new phase transition sequence with increasing pressure has been presented, which is from the molecular phase epsilon-N(2) to cg at 47 GPa, to Pba2 at 170 GPa, and then to P2(1)2(1)2(1) at 307 GPa.

Structural and dynamical properties of solid ammonia borane under high pressure

The structural and dynamical properties of solid ammonia borane were investigated by means of extensive density functional theory calculation up to 60 GPa. Molecular dynamics simulations suggest that the Cmc2(1) phase found by recent room-temperature x-ray diffraction experiments can be obtained from the Pmn2(1) structure at high pressure and low temperature. Two new high-pressure phases were found on further compression at room temperature. We also found that all three high-pressure phases have proton-ordered structures, and the separation of the NH(3) and BH(3) rotation observed in the simulations can be explained by their distinct rotational energy barriers. The role of dihydrogen bonds in the high-pressure phases is discussed.

Rotational dynamics of confined C60 from near-infrared Raman studies under high pressure.

Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C60 and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.

Brillouin scattering studies of liquid argon at high temperatures and high pressures

Brillouin scattering measurements were performed on liquid argon in a diamond anvil cell at various solidification points up to 503 K. With the measured results from the 60 degree platelet- and 180 degree back-scattering geometries, the sound velocity, refractive index, experimental equation of state, and adiabatic bulk modulus of liquid argon as a function of pressure were determined. The discrepancy between experimental and previous calculated equation of state indicates that the many-body contribution to the density of liquid argon increases with increasing pressure and decreases with increasing temperature. By analyzing the Brillouin spectra in the coexistence of liquid and solid phase regions, the volume change and latent heat of solid-liquid transformation along the equilibrium curve have been also obtained for the first time.

Order-disorder phase transition and dissociation of hydrogen sulfide under high pressure: Ab initio molecular dynamics study

The structural and dynamical properties of phase IV and V of hydrogen sulfide were investigated by means of extensive ab initio molecular dynamics simulations. Starting from an experimental proposal for the structure of phase IV, an Ibca symmetry with a stable hydrogen bonding network is found at 15 GPa and 100 K. Molecular dynamics simulations at increasing temperature and at the pressure of 15 GPa suggest that phase IV will transform to a proton disordered structure at 15 GPa and 350 K. The newfound structure has a hexagonal lattice of P63/mmc symmetry, which is believed to be the remaining crystalline structure of phase V. The high mobility of protons in phase V is believed to be the key point to the dissociation and decomposition of hydrogen sulfide.

Study of the hydrostatic pressure dependence of the raman spectrum of W/WS2 fullerene-like nanosphere with core-shell structure

The structural behavior of a W/WS2 fullerene-like nanosphere with a core–shell structure has been studied in the hydrostatic pressure range from atmospheric pressure to 18 GPa by Raman spectroscopy using a methanol–ethanol–water mixture (16:3:1) as the pressure transmitting medium (PTM). We found that it is interesting that the intensity ratio of the LA+TA mode and the A1g mode changes with increasing pressure. We attribute this change to the shape transformation of an inorganic fullerene-like IF-W/WS2 nanosphere under high hydrostatic pressure. By comparing the Raman spectra of an IF-W/WS2 nanosphere released from high pressure with that of the original one, we found that the change in morphology is reversible. This indicates that the spherical shape of the IF-W/WS2 has excellent behavior in resisting compression.

Structural, electronic, and optical properties of crystalline iodoform under high pressure: A first-principles study

The high pressure phases, electronic structure, and optical properties of iodoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density-functional theory. A new high pressure polar monoclinic structure with space group Cc, denoted as β phase, has been observed after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from α to β phase occurs at 40.1 GPa. Electronic structure calculated results showed that the insulator-metal transition in α phase due to band overlap is found at about 32 GPa. In addition, the calculated absorption spectra of iodoform are consistent with the experimental results.

New high-pressure phase of BaH2 predicted by ab initio studies

Pressure-induced phase transitions of BaH₂ have been studied by ab initio calculations. Our results show that BaH₂ transforms from the cotunnite structure to the InNi₂-type structure at about 2.3 GPa, which is in agreement with experimental results. The InNi₂ phase is predicted to be an insulator and transforms to a metallic phase with an AlB₂-type structure at about 34 GPa. Under higher pressure, a post-AlB₂ phase with the YbZn₂-type structure (space group Imma, 4 f.u./cell) is predicted, which is both dynamically and mechanically stable. Analysis of the enthalpies for both AlB₂ and YbZn₂ phases further supports the existence of this new phase. The [AlB₂ → YbZn₂] structural phase transition is identified as a second-order nature, driven by the softening of the transverse acoustic phonon mode at the L point (0.5, 0.0, 0.5).

Theoretical calculations of phase transitions and optical properties of solid iodine under high pressures

The structural stability and optical properties of solid iodine under pressure have been studied using the ab initio pseudopotential plane-wave method. The dependence of lattice parameters on pressure indicates that the first structural phase transition from phase I to phase V occurs at about 20 GPa. From the pressure dependence of our elastic constants for solid iodine in phase I, it is found that the first structural transformation from molecular phase I to the intermediate phase V occurs at about 20 GPa due to the softening of the elastic constant C44, which is very close to the transition pressure of 20 GPa obtained by geometry optimizations and 23.2 GPa obtained by experimental measurements. The optimized structure for phase V is a face-centered orthorhombic (fco) phase with equal interatomic distances d1 = d2 = d3, but this fco structure is mechanically unstable, with shear elastic stiffness coefficient C44<0. To understand the modulated phase V, we use a periodic crystal structure to mimic the incommensurate phase V and obtain some quantitative information. In our calculation, the modulated phase is thermodynamically and mechanically stable. It is believed that phase V is not a monatomic phase but an intermediate state between a molecular and a monatomic state.