Pressure-induced Molecular Dissociation Research Articles

Two intermediate incommensurate phases in the molecular dissociation process of solid iodine under high pressure

A high-pressure phase of solid iodine is identified in the pressure range of 16--23 GPa at room temperature by reanalyzing the previous powder x-ray diffraction data. The phase VI has a four-dimensional (4D) incommensurately modulated structure, in which ${\mathrm{I}}_{2}$ molecules and zigzag chains of three iodine atoms coexist. This structural model can successfully explain the Raman scattering data previously reported. Furthermore, our method clarifies that the structure of phase V, which was once analyzed as a 4D incommensurately modulated structure, can be better described by a five-dimensional incommensurately modulated tetragonal lattice wherein two modulation waves run in orthogonal directions. Our findings show that the pressure-induced molecular dissociation proceeds in multiple stages that are more complex than previously thought. In addition, it has become necessary to describe the molecular dissociation pressure separately for the starting pressure ${P}_{s}$ of 16 GPa and the finishing pressure ${P}_{f}$ of 29 GPa. This transition process has finally resolved the contradiction with the M\"ossbauer experiment.

Reexploration of Structural Changes in Element Bromine through Pressure-Induced Decomposition of Solid HBr**Supported by the National Natural Science Foundation of China under Grant Nos 51572108, 51632002, 11504127, 11674122, 11574112, 11474127 and 11634004, the 111 Project (No B12011), Program for Changjiang Scholars and Innovative Research Team in University (No IRT-15R23), and the National Fund for Fostering Talents of Basic Science (No

Simple molecular solids have been an important subject in condensed matter physics, particularly for research of pressure-induced molecular dissociation. We re-explore the structural changes of element bromine through pressure-induced decomposition of solid HBr. The phase changes in HBr are investigated by Raman spectroscopy and synchrotron x-ray diffraction up to 125 GPa at room temperature. By applying pressure, HBr decomposes into solid bromine in the pressure range of 18.7–38 GPa. The solid bromine changes from molecular phase to incommensurate phase at 81 GPa, and finally to monatomic phase at 91 GPa. During the process of pressure-induced molecular dissociation, the intermediate incommensurate phase of element bromine is confirmed for the first time from the x-ray diffraction studies. The decomposition of HBr is irreversible since HBr cannot form again upon pressure decompression.

High-pressure dissociation of selenium and tellurium.

Pressure-induced molecular dissociation provides significant insights into chemical bonding, governing the physical properties of molecular solids. Here the typical molecular solids, selenium and tellurium, have been explored experimentally to unearth the process of pressure-induced molecular dissociation. Before molecular dissociation, both commensurately and incommensurately modulated structures have been observed in these two elements. Upon compression, the successive commensurate-incommensurate phase transitions are proved by the soft amplitude (AMP) mode. The fitted critical pressure suggests that the molecular dissociation of Te is inferred to occur at 26.1 GPa in agreement with our XRD and Raman data. The current precise structural studies before pressure-induced molecular dissociation demonstrated in the elements selenium and tellurium will help in clarifying the mechanisms of molecular dissociation in related molecular solids.

High-pressure-activated carbon tetrachloride decomposition

The pressure-induced molecular dissociation as one of the fundamental problems in physical sciences has aroused many theoretical and experimental studies. Here, using a newly developed particle swarm optimization algorithm, we investigate the high-pressure-induced molecular dissociation. The results show that the carbon tetrachloride (CCl4) is unstable and dissociates into C2Cl6 and Cl2 under approximately 120 GPa and more. The dissociation is confirmed by the lattice dynamic calculations and electronic structure of the Pa3 structure with pressure evolution. The dissociation pressure is far larger than that in the case of high temperature, indicating that the temperature effectively reduces the activation barrier of the dissociation reaction of CCl4. This research improves the understanding of the dissociation reactions of CCl4 and other halogen compounds under high pressures.

Spiral chain O 4 form of dense oxygen

Oxygen is in many ways a unique element: It is the only known diatomic molecular magnet, and it exhibits an unusual O(8) cluster in its high-pressure solid phase. Pressure-induced molecular dissociation as one of the fundamental problems in physical sciences has been reported from theoretical or experimental studies of diatomic solids H(2), N(2), F(2), Cl(2), Br(2), and I(2) but remains elusive for molecular oxygen. We report here the prediction of the dissociation of molecular oxygen into a polymeric spiral chain O(4) structure (space group I4(1)/acd, θ-O(4)) above 1.92-TPa pressure using the particle-swarm search method. The θ-O(4) phase has a similar structure as the high-pressure phase III of sulfur. The molecular bonding in the insulating ε-O(8) phase or the isostructural superconducting ζ-O(8) phase remains remarkably stable over a large pressure range of 0.008-1.92 TPa. The pressure-induced softening of a transverse acoustic phonon mode at the zone boundary V point of O(8) phase might be the ultimate driving force for the formation of θ-O(4). Stabilization of θ-O(4) turns oxygen from a superconductor into an insulator by opening a wide band gap (approximately 5.9 eV) that originates from the sp(3)-like hybridized orbitals of oxygen and the localization of valence electrons.

中性子・放射光による構造物性研究, および結晶学の発展への貢献

In this article, I have reviewed a series of research on a various phase transitions such as (1) structural phase transitions of perovskite compounds driven by soft phonons, (2) pressure-induced molecular dissociation and metallization observed in solid halogens, and (3) the “Devil's Flower” type phase diagram observed in two compounds with frustrating interactions. Also commented is on the so-called “Small Science at Large Facility” typically symbolized by neutron and synchrotron radiation experiments like the present research.

A new phase of solid iodine with different molecular covalent bonds

There is a great interest in the behavior of diatomic molecular solids under extremely high-pressure conditions that lead to pressure-induced metallization, molecular dissociation, and formation of atomic phase. The consensus has been that the phase-transition sequence that happened in both solid bromine and iodine is from a molecular phase (phase I), to an incommensurate phase (phase V), and then to an atomic phase (phase II), with increasing pressure. However, a puzzle remains unresolved for both solids: pressure-induced X and Y bands were observed in the Raman spectra in the molecular phase at low pressures, even before the onset of phase V. Here, we suggest a phase for solid iodine in such a low-pressure range (designated as phase I') in which two different covalent intramolecular bonds coexist, based on first-principles calculations and later corroborated by x-ray diffraction experiments. The pressure dependence of the X and Y bands and other vibrational frequencies measured experimentally can be explained nicely by combining the vibrational modes of phase I and phase I'. These results help improve our understanding on the pressure-induced molecular dissociation and metallization in diatomic solids and may shed some light on the investigation of similar phenomena in solid H(2).

High-Pressure Dissociation of Crystalline para-Diiodobenzene: Optical Experiments and Car−Parrinello Calculations

We have investigated the high-pressure properties of the molecular crystal para-diiodobenzene, by combining optical absorption, reflectance, and Raman experiments with Car-Parrinello simulations. The optical absorption edge exhibits a large red shift from 4 eV at ambient conditions to about 2 eV near 30 GPa. Reflectance measurements up to 80 GPa indicate a redistribution of oscillator strength toward the near-infrared. The calculations, which describe correctly the two known molecular crystal phases at ambient pressure, predict a nonmolecular metallic phase, stable at high pressure. This high-density phase is characterized by an extended three-dimensional network, in which chemically bound iodine atoms form layers connected by hydrocarbon bridges. Experimentally, Raman spectra of samples recovered after compression show vibrational modes of elemental solid iodine. This result points to a pressure-induced molecular dissociation process which leads to the formation of domains of iodine and disordered carbon.

Structural phase transitions in iodine under high pressure

Abstract High-pressure powder X-ray diffraction experiments have been carried out on solid iodine at room temperature. A new intermediate phase V has been identified in the pressure range 24–28 GPa, in the close vicinity of the pressure-induced molecular dissociation. The structure is incommensurately modulated, a rare case for elemental solids. The nearest interatomic distances are distributed over a range 2.8–3.2 Å, which characterizes phase V as a transient state between the molecular and monatomic states. We discuss the overall change of the crystal structure of iodine with pressure, which gives an insight into the process of molecular formation and dissociation.

圧力誘起分子解離の最前線 固体ハロゲンの分子解離相転移と金属化

圧力誘起分子解離の最前線 固体ハロゲンの分子解離相転移と金属化

圧力誘起分子解離の最前線 分子性結晶の分子解離と金属化

圧力誘起分子解離の最前線 分子性結晶の分子解離と金属化

On the Frontier of the Pressure-Induced Molecular Dissociation

On the Frontier of the Pressure-Induced Molecular Dissociation

High-pressure dielectric measurements of solid hydrogen to 170 GPa

AT high pressure (>100 GPa), the valence–conduction band gap in solid hydrogen is predicted to decrease and eventually to close, transforming it from an insulator to a metal1. Recent experiments at pressures up to ∼300 GPa suggest a gradual closing of the direct gap at very high pressures whereas closure of the indirect gap may occur at lower pressures, perhaps below 200 GPa (refs 2–6). Measurements of the refractive index, n, and its frequency dependence (the dispersion dn/dω) as a function of pressure can provide information on the changes in electronic structure that occur under these conditions7,8. Here we report refractive-index measurements on solid hydrogen at visible frequencies at pressures up to 170 GPa. Unlike earlier studies9, we find no evidence for a divergence (dielectric instability) at 150 GPa, close to the low-temperature phase transition observed previously6and suggested as being associated with metallization4,9. Our results are consistent with closure of the indirect gap. A fit of our data to a dielectric model indicates that the onset of visible absorption owing to direct interband transitions should occur above 200 GPa, consistent with previous direct observations3. Pressure-induced molecular dissociation may occur before closure of the direct gap.

Pressure-induced molecular dissociation in iodine at low temperatures

Abstract Synchrotron x-ray diffraction experiments at low temperatures have revealed that even at 35K the molecular dissociation starts taking place at PS=21.5 and finishes at Pf=26.1GPa. An extrapolation of the phase boundary thus obtained at several temperatures intersects the T=4K line at PS=21.6 and Pf=26.3GPa. This result, inferring the monatomic structure to be realized at P=30GPa and T=4K, leads to a discrepancy against the recent Mössbauer result obtained at the same point in the P-T phase diagram.

Observation of Molecular Dissociation of Iodine at High Pressure by X-Ray Diffraction

Metallic iodine in the high-pressure phase above P/sub t/=21 Gpa is found to form a body-centered orthorhombic Bravais lattice. Hence, pressure-induced molecular dissociation takes place at P/sub t/.