Kanani K. M. Lee

Crystal structure of graphite under room-temperature compression and decompression

Recently, sophisticated theoretical computational studies have proposed several new crystal structures of carbon (e.g., bct-C4, H-, M-, R-, S-, W-, and Z-carbon). However, until now, there lacked experimental evidence to verify the predicted high-pressure structures for cold-compressed elemental carbon at least up to 50 GPa. Here we present direct experimental evidence that this enigmatic high-pressure structure is currently only consistent with M-carbon, one of the proposed carbon structures. Furthermore, we show that this phase transition is extremely sluggish, which led to the observed broad x-ray diffraction peaks in previous studies and hindered the proper identification of the post-graphite phase in cold-compressed carbon.

Slip Systems in MgSiO3 Post-Perovskite: Implications for D′′ Anisotropy

Slippery When Squeezed The behavior of seismic waves as they pass through Earths interior depends on the physical properties of major mineral phases at depth. If such minerals are anisotropic—that is, they influence seismic waves preferentially depending on crystallographic orientation—interpreting the structure of a region becomes more challenging. In the lowermost mantle, near the boundary with the outer core, deformation of MgSiO3 post-perovskite may affect anisotropy. Miyagi et al. (p. 1639) solved previous experimental limitations to show that, when squeezed at high pressures, MgSiO3 post-perovskite weakens and breaks along its (001) lattice plane. When modeled, this deformation pattern produces anisotropic structures that are consistent with seismic data collected from this region. The major mineral phase in the lower mantle deforms preferentially along one lattice plane. Understanding deformation of mineral phases in the lowermost mantle is important for interpreting seismic anisotropy in Earth’s interior. Recently, there has been considerable controversy regarding deformation-induced slip in MgSiO3 post-perovskite. Here, we observe that (001) lattice planes are oriented at high angles to the compression direction immediately after transformation and before deformation. Upon compression from 148 gigapascals (GPa) to 185 GPa, this preferred orientation more than doubles in strength, implying slip on (001) lattice planes. This contrasts with a previous experiment that recorded preferred orientation likely generated during the phase transformation rather than deformation. If we use our results to model deformation and anisotropy development in the D′′ region of the lower mantle, shear-wave splitting (characterized by fast horizontally polarized shear waves) is consistent with seismic observations.

Achieving high-density states through shock-wave loading of precompressed samples.

Materials can be experimentally characterized to terapascal pressures by sending a laser-induced shock wave through a sample that is precompressed inside a diamond-anvil cell. This combination of static and dynamic compression methods has been experimentally demonstrated and ultimately provides access to the 10- to 100-TPa (0.1–1 Gbar) pressure range that is relevant to planetary science, testing first-principles theories of condensed matter, and experimentally studying a new regime of chemical bonding.

Electronic conduction in shock-compressed water

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

COUPLING STATIC AND DYNAMIC COMPRESSIONS: FIRST MEASUREMENTS IN DENSE HYDROGEN

We demonstrate here a laser-driven shock wave in a hydrogen sample, pre-compressed in a diamond anvil cell. The compression factors of the dynamic and static techniques are multiplied. This approach allows access to a family of Hugoniot curves which span the P–T phase diagram of fluid hydrogen to high density. In this first-of-its-kind experiment, two hydrogen Hugoniot curves have been partially followed starting from pre-compression at pressures of 0.7 GPa and 1.2 GPa. Optical reflectance probing at two wavelengths reveals the onset of the conducting fluid state. The boundary line to conducting fluid hydrogen is suggested.

High‐pressure melting of MgO from (Mg,Fe)O solid solutions

Magnesium oxide (MgO) is a significant component of planetary interiors, particularly Earths mantle and other rocky planets within and beyond our solar system; thus its high-pressure, high-temperature behavior is important to understanding the thermochemical evolution of planets. Laser-heated diamond-anvil cell (DAC) experiments on (Mg,Fe)O ferropericlase up to ~40 GPa show that previous DAC experiments on MgO melting are too low, while previous multi-anvil experiments yield melting temperatures too high. Instead, our quasi-static experimental results are consistent with recent ab initio predictions as well as dynamic shock measurements. Extrapolated to the core-mantle boundary (CMB) of the Earth, MgO is expected to melt at ~8000 ± 500 K, much greater than expected geotherm temperatures.

Mapping temperatures and temperature gradients during flash heating in a diamond-anvil cell

Here, we couple two-dimensional, 4-color multi-wavelength imaging radiometry with laser flash heating to determine temperature profiles and melting temperatures under high pressures in a diamond-anvil cell. This technique combines the attributes of flash heating (e.g., minimal chemical reactions, thermal runaway, and sample instability), with those of multi-wavelength imaging radiometry (e.g., 2D temperature mapping and reduction of chromatic aberrations). Using this new technique in conjunction with electron microscopy makes a powerful tool to determine melting temperatures at high pressures generated by a diamond-anvil cell.

High pressures generated by laser driven shocks: applications to planetary physics

High power lasers are a tool that can be used to determine important parameters in the context of Warm Dense Matter, i.e. at the convergence of low-temperature plasma physics and finite-temperature condensed matter physics. Recent results concerning planet inner core materials such as water and iron are presented. We determined the equation of state, temperature and index of refraction of water for pressures up to 7 Mbar. The release state of iron in a LiF window allowed us to investigate the melting temperature near the inner core boundary conditions. Finally, the first application of proton radiography to the study of shocked material is also discussed.

From soft to superhard: Fifty years of experiments on cold-compressed graphite

In recent years there have been numerous computational studies predicting the nature of cold-compressed graphite yielding a proverbial alphabet soup of carbon structures (e.g., bct-C4, K4-, M-, H-, R-, S-, T-, W- and Z-carbon). Although theoretical methods have improved, the inherent nature of graphite (i.e., low-Z) and the subsequent room-temperature, high-pressure phase transition (i.e., low symmetry, nanocrystalline and sluggish), make experimental measurements difficult to execute and interpret even with the current technology of 3rd generation synchrotron sources. The room-temperature, high-pressure phase transition of graphite has been detected by numerous kinds of experiments over the past fifty years, such as electrical resistance measurements, optical microscopy, X-ray diffraction, inelastic X-ray scattering, and Raman spectroscopy. However, the identification and characterization of high-pressure graphite is replete with controversy since its discovery more than fifty years ago. Recent experiments confirm that this phase has a monoclinic structure, consistent with the M-carbon phase predicted by theoretical computations. Meanwhile, experiments demonstrate that the phase transition is sluggish and kinetics is important in discerning the phase boundary. Additionally, the post-graphite phase appears to be superhard with hardness comparable to that of diamond.

Efficient graphite ring heater suitable for diamond-anvil cells to 1300 K

In order to generate homogeneous high temperatures at high pressures, a ring-shaped graphite heater has been developed to resistively heat diamond-anvil cell (DAC) samples up to 1300 K. By putting the heater in direct contact with the diamond anvils, this graphite heater design features the following advantages: (1) efficient heating: sample can be heated to 1300 K while the DAC body temperature remains less than 800 K, eliminating the requirement of a special alloy for the DAC; (2) compact design: the sample can be analyzed with in situ measurements, e.g., x-ray, optical, and electrical probes are possible. In particular, the side access of the heater allows for radial x-ray diffraction (XRD) measurements in addition to traditional axial XRD.