Dylan K. Spaulding

Phase Transformations and Metallization of Magnesium Oxide at High Pressure and Temperature

Planetary Interiors Under Pressure The interiors of Earth and other rocky planets generally consist of a few common minerals. Depending largely on the size of the planet, the distribution and relative abundance of these minerals varies; for example, MgO is abundant in the mantles of Earth and large Earth-like planets, but is present in Jupiters core. The properties of MgO also vary with planetary size as a function of temperature and pressure. McWilliams et al. (p. 1330, published online 22 November) performed laser-shock experiments at pressures over three times higher than Earths inner core. MgO underwent two phase transformations, first to a solid with a modified crystal structure, and then to a conductive liquid. In terrestrial planets greater than eight Earth masses, MgO in the mantle could generate a magnetic field–generating dynamo such as those that typically found in planetary cores. Mantle minerals conductive at the high pressures and temperatures of planetary interiors could induce a magnetic field. Magnesium oxide (MgO) is representative of the rocky materials comprising the mantles of terrestrial planets, such that its properties at high temperatures and pressures reflect the nature of planetary interiors. Shock-compression experiments on MgO to pressures of 1.4 terapascals (TPa) reveal a sequence of two phase transformations: from B1 (sodium chloride) to B2 (cesium chloride) crystal structures above 0.36 TPa, and from electrically insulating solid to metallic liquid above 0.60 TPa. The transitions exhibit large latent heats that are likely to affect the structure and evolution of super-Earths. Together with data on other oxide liquids, we conclude that magmas deep inside terrestrial planets can be electrically conductive, enabling magnetic field–producing dynamo action within oxide-rich regions and blurring the distinction between planetary mantles and cores.

Pressure-induced chemistry in a nitrogen-hydrogen host–guest structure

New topochemistry in simple molecular systems can be explored at high pressure. Here we examine the binary nitrogen/hydrogen system using Raman spectroscopy, synchrotron X-ray diffraction, synchrotron infrared microspectroscopy and visual observation. We find a eutectic-type binary phase diagram with two stable high-pressure van der Waals compounds, which we identify as (N2)6(H2)7 and N2(H2)2. The former represents a new type of van der Waals host-guest compound in which hydrogen molecules are contained within channels in a nitrogen lattice. This compound shows evidence for a gradual, pressure-induced change in bonding from van der Waals to ionic interactions near 50 GPa, forming an amorphous dinitrogen network containing ionized ammonia in a room-temperature analogue of the Haber-Bosch process. Hydrazine is recovered on decompression. The nitrogen-hydrogen system demonstrates the potential for new pressure-driven chemistry in high-pressure structures and the promise of tailoring molecular interactions for materials synthesis.

Use of a multichannel collimator for structural investigation of low-Z dense liquids in a diamond anvil cell: validation on fluid H2 up to 5 GPa.

We report the first application of a multichannel collimator (MCC) to perform quantitative structure factor measurements of dense low-Z fluids in a diamond anvil cell (DAC) using synchrotron x-ray diffraction. The MCC design, initially developed for the Paris-Edinburgh large volume press geometry, has been modified for use with diamond anvil cells. A good selectivity of the diffracted signal of the dense fluid sample is obtained due to a large rejection of the Compton diffusion from the diamond anvils. The signal to background ratio is significantly improved. We modify previously developed analytical techniques for quantitative measurement of the structure factor of fluids in DACs [J. H. Eggert, G. Weck, P. Loubeyre, and M. Mezouar, Phys. Rev. B 65, 174105 (2002)] to account for the contribution of the MCC. We present experimental results on liquids argon and hydrogen at 296 K to validate our method and test its limits, respectively.

NEW OPTICAL DIAGNOSTICS FOR EQUATION OF STATE EXPERIMENTS ON THE JANUS LASER

We describe the configuration of two new optical diagnostics for laser‐driven dynamic‐compression experiments to multi‐Mbar pressures. A streaked optical pyrometer (SOP) has been developed to provide temporally and spatially‐resolved records of the thermal emission from shock‐compressed samples. In addition, temporally‐resolved broadband reflectivity is measured between 532 and ∼850 nm by supercontinuum generation in an optical fiber. These new tools expand capabilities to probe the thermal and electronic states of matter at high pressures and temperatures using the Lawrence Livermore National Laboratorys Janus laser.

Shock Experiments on Pre‐Compressed Fluid Helium

We summarize current methods and results for coupling laser‐induced shocks into pre‐compressed Helium contained in a diamond anvil cell (DAC). We are able to load helium, hydrogen, deuterium, and helium‐hydrogen mixtures into a DAC and propagate a laser‐generated shock into the pre‐compressed sample. This technique has allowed us to measure the Hugoniot for helium at initial densities ranging from 1 to 3.5 times liquid density. We have developed and used a methodology whereby all of our measurements are referenced to crystalline quartz, which allows us to update our results as the properties of quartz are refined in the future. We also report the identification and elimination of severe electro‐magnetic pulses (EMP) associated with plasma stagnation associated with ablation in a DAC.

Erratum: Evidence for a Phase Transition in Silicate Melt at Extreme Pressure and Temperature Conditions [Phys. Rev. Lett. 108 , 065701 (2012)]

This corrects the article DOI: 10.1103/PhysRevLett.108.065701.