R. J. Hemley

The pressure-temperature phase and transformation diagram for carbon; updated through 1994

Abstract In recent years, important advances in our understanding of the pressure-temperature phase and transformation diagram for carbon have occurred as a result of developments in both experimental and theoretical techniques. Graphite, diamond, liquid and vapor remain the major thermodynamically stable forms of carbon. However, due to the high activation energies for solid-state transformations and the specific effects of reaction paths, other metastable forms and a wide spectrum of complex hybrid forms may be generated, and possibly quenchedin, to survive metastably. This paper focuses primarily on developments since the last review of the carbon phase diagram published in 1989, but also includes references to the reliable older work. Some of the newer conclusions include the following: the Clapeyron slope of the diamond melting line, d T m d P , is positive; the liquid is metallic and there appears to be no evidence for a transformation between electrically conducting and non-conducting forms; melted droplets of carbon less than 0.2 μm in diameter quench to a giant fullerene structure even in the stability field of diamond; graphite transforms to a transparent phase on compression at room temperature; this phase reverts to graphite on decompression at this temperature from pressures as high as 100 GPa.

High-pressure crystal chemistry and amorphization of α-quartz

Single-crystal X-ray diffraction experiments on α-quartz at pressures to 15 GPa reveal structural instabilities that result in its gradual transition to an amorphous state. With increasing pressure the average SiO distance (≈ 1.61 ± 0.01 A) and SiO4 tetrahedral volume (≈ 2.14 ± 0.02 A3 remain constant. Compression of α-quartz results from a dramatic decrease in SiOSi angle and corresponding decrease in inter-tetrahedral (i.e., OO) distances. The onset of amorphization coincides with bending of all SiOSi angles to less than 120° and severe distortion of SiO4 tetrahedra, as oxygens approach a close-packed configuration.

Pressure-Induced High-Spin to Low-Spin Transition in FeS Evidenced by X-Ray Emission Spectroscopy

We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe Kb emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphidelike phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure. [S0031-9007(99)08946-2]

Strain/order parameter coupling in the ferroelastic transition in dense SiO2

New high-pressure measurements reveal the coupling of strain and order parameter in the pressure-induced ferroelastic transition in dense SiO2. Single-crystal X-ray diffraction measurements in quasi-hydrostatic media reversibly probe the spontaneous strain in the vicinity of the P42=mnm! Pnnm transition near 50 GPa, and indicate weak first-order character. A Landau model is developed that quantitatively relates all of the spectroscopic, elastic, structural, and thermodynamic data for the system. The elastic instability at the transition gives rise to anomalies in the Raman spectrum, which are expected to be a general feature of such pressure-induced transitions. q 2000 Elsevier Science Ltd. All rights reserved.

Structural transformation of molecular nitrogen to a single-bonded atomic state at high pressures.

The transformation of molecular nitrogen to a single-bonded atomic nitrogen is of significant interest from a fundamental stand point and because it is the most energetic non-nuclear material predicted. We performed an x-ray diffraction of nitrogen at pressures up to 170 GPa. At 60 GPa, we found a transition from the rhombohedral (R3c) epsilon-N(2) phase to the zeta-N(2) phase, which we identified as orthorhombic with space group P222(1) and with four molecules per unit cell. This transition is accompanied by increasing intramolecular and decreasing intermolecular distances. The major transformation of this diatomic phase into the single-bonded (polymeric) phase, recently determined to have the cubic gauche structure (cg-N), proceeds as a first-order transition with a volume change of 22%.

Critical behavior in the hydrogen insulator-metal transition.

The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals (1.5 megabars). The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower pressure insulator-metal transitions in doped V2O3 alloys.

Crystal structure of sulfur and selenium at pressures up to 160 GPa

Using advanced in situ X-ray diffraction techniques at high pressures and temperatures, we have resolved the long-standing problem of the phase transition sequence of sulfur in its non-metallic state. Our data show that there are only two phases of sulfur stable between 1.5 GPa and pressure of metallization of 86 GPa, S-II with triangular chain structure and S-III with novel squared chain structure. The same squared chain structure is formed in the heavier group-VI element Se at pressures of 20 GPa and temperatures of 450 K. Our X-ray diffraction data on metallic phases of sulfur above 83 GPa show that the S-IV phase has an incommensurately modulated monoclinic structure, the same as recently reported modulated structures of Te-III and Se-IV. S-IV is shown to transform to primitive rhombohedral β-Po phase at 153(3) GPa, the same transition is found in Se at pressure of around 80 GPa.

Vibrational dynamics of solid molecular nitrogen to megabar pressures

We report the results of Raman and synchrotron infrared absorption measurements of several molecular phases of solid nitrogen to pressures above 100 GPa (300 K). Low-temperature vibrational spectra to 45 GPa are also presented. The changes in Raman and infrared spectra at 60 GPa and 300 K are interpreted as arising from the e→ζ transition reported at low temperature. The character of splitting of the Raman vibron ν2 observed at 25 GPa and low temperatures differs from that previously reported, a difference that we ascribe to different experimental procedures employed and metastability of the low-temperature phase.

Infrared studies of the organic superconductor kappa-(BEDT-TTF)2Cu(SCN)2 under pressure

The organic superconductor -(BEDT-TTF)2 Cu(SCN)2 has been investigated at room temperature by polarized mid-infrared (600-7000 cm-1 ) reflectivity measurements at pressures of up to 1.7 GPa. The optical effective mass, m opt , decreases linearly with pressure, in contrast to the pressure dependence of the effective mass, m * , determined by magnetic quantum oscillation measurements (Caulfield J et al 1994 J. Phys.: Condens. Matter 6 2911-24; Caulfield J et al 1995 Synth. Met. 70 815-8). Most phonon modes are seen to exhibit a linear pressure dependence of 0.5-1% / GPa. The stronger pressure dependence of the central C = C mode of the BEDT-TTF molecule is discussed: it is thought to be due to the pressure dependence of the effective Coulomb repulsion in -(BEDT-TTF)2 Cu(SCN)2 . These measurements suggest that a change in the electron-electron interaction under pressure could be the relevant factor for the suppression of superconductivity under pressure in -(BEDT-TTF)2 Cu(SCN)2 .

Broken Symmetry Phase Transition in Solid HD: Quantum Behavior at Very High Pressures

The broken symmetry phase (BSP) transition in solid HD has been shown to be an example of quantum orientational melting. Anomalous features observed for the transition are a consequence of the symmetry properties of the system, namely, the fact that in HD all rotational states and transitions between them are allowed, in contrast to the behavior of the homonuclear H2 and D2.