Transition metal nitrides are of great technological and fundamental importance because of their strength and durability and because of their useful optical, electronic, and magnetic properties. We have evaluated a recently synthesized platinum nitride (PtN) that was shown to have a large bulk modulus, and we propose a structure that is isostructural with pyrite and has the stoichiometry PtN2. We have also synthesized a recoverable nitride of iridium under nearly the same conditions of pressure and temperature as PtN2. Although it has the same stoichiometry, it exhibits much lower structural symmetry. Preliminary results suggest that the bulk modulus of this material is also very large.

Raman spectroscopy in a laser heated diamond anvil cell and first principles molecular dynamics simulations have been used to study water in the temperature range 300 to 1500 K and at pressures to 56 GPa. We find a substantial decrease in the intensity of the O-H stretch mode in the liquid phase with pressure, and a change in slope of the melting line at 47 GPa and 1000 K. Consistent with these observations, theoretical calculations show that water beyond 50 GPa is "dynamically ionized" in that it consists of very short-lived (<10 fs) H2O, H3O+, and OH- species, and also that the mobility of the oxygen ions decreases abruptly with pressure, while hydrogen ions remain very mobile. We suggest that this regime corresponds to a superionic state.

The pentazolates, the last all-nitrogen members of the azole series, have been notoriously elusive for the last hundred years despite enormous efforts to make these compounds in either gas or condensed phases. Here we report a successful synthesis of a solid state compound consisting of isolated pentazolate anions N − 5 , which is achieved by compressing and laser heating cesium azide (CsN 3 ) mixed with N 2 cryogenic liquid in a diamond anvil cell. The experiment was guided by theory, which predicted the transformation of the mixture at high pressures to a new compound, cesium pentazolate salt (CsN 5 ). Electron transfer from Cs atoms to N 5 rings enables both aromaticity in the pentazolates as well as ionic bonding in the CsN 5 crystal. This work provides a critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high nitrogen content species.

Changes in the electronic configuration of iron at high pressures toward a spin-paired state within host minerals ferropericlase and silicate perovskite may directly influence the seismic velocity structure of Earth's lower mantle. We measured the complete elastic tensor of ferropericlase, (Mg(1-x),Fe(x))O (x = 0.06), through the spin transition of iron, whereupon the elastic moduli exhibited up to 25% softening over an extended pressure range from 40 to 60 gigapascals. These results are fully consistent with a simple thermodynamic description of the transition. Examination of previous compression data shows that the magnitude of softening increases with iron content up to at least x = 0.20. Although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most, if not all, of the lower mantle.

We describe the synthesis of nitrides of iridium and palladium using the laser-heated diamond anvil cell. We have used the in situ techniques of x-ray powder diffraction and Raman scattering to characterize these compounds and have compared our experimental findings where possible to the results of first-principles theoretical calculations. We suggest that palladium nitride is isostructural with pyrite, while iridium nitride has a monoclinic symmetry and is isostructural with baddeleyite.

We have directly resolved shock structures in pure aluminum in the first few hundred picoseconds subsequent to a dynamic load at peak stresses up to 43 GPa and strain rates in excess of 10(10)  s(-1). For strong shocks we obtain peak stresses, strain rates, and rise times. From these data, we directly validate the invariance of the dissipative action in the strong shock regime, and by comparing with data obtained at much lower strain rates show that this invariance is observed over at least 5 orders of magnitude in the strain rate. Over the same range, we similarly validate the fourth-power scaling of the strain rate with the peak stress (the Swegle-Grady relation).

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered iota and zeta' phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded eta-N.

Aerogel materials have myriad scientific and technological applications due to their large intrinsic surface areas and ultralow densities. However, creating a nanodiamond aerogel matrix has remained an outstanding and intriguing challenge. Here we report the high-pressure, high-temperature synthesis of a diamond aerogel from an amorphous carbon aerogel precursor using a laserheated diamond anvil cell. Neon is used as a chemically inert, near-hydrostatic pressure medium that prevents collapse of the aerogel under pressure by conformally filling the aerogel's void volume. Electron and X-ray spectromicroscopy confirm the aerogel morphology and composition of the nanodiamond matrix. Timeresolved photoluminescence measurements of recovered material reveal the formation of both nitrogen-and silicon-vacancy pointdefects, suggesting a broad range of applications for this nanocrystalline diamond aerogel.gigapascal | nanomaterials | phase transition | photonics | qubit A erogels are a fascinating class of high surface-area continuous solids with a broad range of both commercial and fundamental scientific applications (1-8). Both crystalline and amorphous structures have been synthesized. Amorphous carbon aerogel in particular has received a considerable amount of attention in recent years owing to its low cost, electrical conductivity, mechanical strength, and thermal stability (9). Numerous applications have been explored for this material including water desalination (10), electrochemical supercapacitors (11), as well as both thermal (12) and optical (13) insulation. Impressive advances have been made in the case of polycrystalline aerogels through the oxidative aggregation of chalcogenide quantum dots that preserve spectral signatures of quantum confinement (14). Furthermore, recent high-pressure, high-temperature (HPHT) experiments with mesoporous silica and periodic carbon have been employed to produce mesoporous coesite (15, 16) and diamond (17) structures.Despite much progress in achieving phase transitions in mesoporous materials, the transition from an amorphous to a crystalline phase in carbon aerogel materials has remained a challenge. The extremely low density and irregular pore structure in aerogel materials make preventing pore collapse a formidable obstacle. Such a nondestructive transition would nevertheless be desirable, because a porous, three-dimensional self-supporting structure consisting of diamonds would have unprecedented optical, thermal, and chemical properties while still maintaining the inherent advantages of aerogel morphology. For instance, diamond aerogel promises a widely tunable optical index of refraction (∼1 < n < 2.4) for antireflection coatings, biocompatibility, chemical doping, potentially enhanced thermal conduction, as well as electrical field emission applications while still maintaining the low-density, high surface area and self-supporting aerogel morphology. Such a material is particularly intriguing in light of the numerous scientific and technological applications of conventi...