Graphite resistive heated diamond anvil cell for simultaneous high-pressure and high-temperature diffraction experiments.

High-pressure and high-temperature experiments using a resistively heated diamond anvil cell have the advantage of heating samples homogeneously with precise temperature control. Here, we present the design and performance of a graphite resistive heated diamond anvil cell (GRHDAC) setup for powder and single-crystal x-ray diffraction experiments developed at the Extreme Conditions Beamline (P02.2) at PETRA III, Hamburg, Germany. In the GRHDAC, temperatures up to 2000 K can be generated at high pressures by placing it in a water-cooled vacuum chamber. Temperature estimates from thermocouple measurements are within +/-35 K at the sample position up to 800 K and within +90 K between 800 and 1400 K when using a standard seat combination of cBN and WC. Isothermal compression at high temperatures can be achieved by employing a remote membrane control system. The advantage of the GRHDAC is demonstrated through the study of geophysical processes in the Earth's crust and upper mantle region.

Hadean isotopic fractionation of xenon retained in deep silicates.

Our understanding of atmosphere formation essentially relies on noble gases and their isotopes, with xenon (Xe) being a key tracer of the early planetary stages. A long-standing issue, however, is the origin of atmospheric depletion in Xe1 and its light isotopes for the Earth2 and Mars3. Here we report that feldspar and olivine samples confined at high pressures and high temperature with diluted Xe and krypton (Kr) in air or nitrogen are enriched in heavy Xe isotopes by +0.8 to +2.3‰ per AMU, and strongly enriched in Xe over Kr. The upper +2.3‰ per AMU value is a minimum because quantitative trapping of unreacted Xe, either in bubbles or adsorbed on the samples, is likely. In light of these results, we propose a scenario solving the missing Xe problem that involves multiple magma ocean stage events at the proto-planetary stage, combined with atmospheric loss. Each of these events results in trapping of Xe at depth and preferential retention of its heavy isotopes. In the case of the Earth, the heavy Xe fraction was later added to the secondary CI chondritic atmosphere through continental erosion and/or recycling of a Hadean felsic crust.

Development of chemical and topological structure in aluminosilicate liquids and glasses at high pressure.

The high pressure structure of liquid and glassy anorthite (CaAl(2)Si(2)O(8)) and calcium aluminate (CaAl(2)O(4)) glass was measured by using in situ synchrotron x-ray diffraction in a diamond anvil cell up to 32.4(2) GPa. The results, combined with ab initio molecular dynamics and classical molecular dynamics simulations using a polarizable ion model, reveal a continuous increase in Al coordination by oxygen, with 5-fold coordinated Al dominating at 15 GPa and a preponderance of 6-fold coordinated Al at higher pressures. The development of a peak in the measured total structure factors at 3.1 Å(-1) is interpreted as a signature of changes in topological order. During compression, cation-centred polyhedra develop edge- and face- sharing networks. Above 10 GPa, following the pressure-induced breakdown of the network structure, the anions adopt a structure similar to a random close packing of hard spheres.

Structural change in molten basalt at deep mantle conditions.

Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth's interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.

Reactivity of xenon with ice at planetary conditions.

We report results from high pressure and temperature experiments that provide evidence for the reactivity of xenon with water ice at pressures above 50 GPa and a temperature of 1500 K-conditions that are found in the interiors of Uranus and Neptune. The x-ray data are sufficient to determine a hexagonal lattice with four Xe atoms per unit cell and several possible distributions of O atoms. The measurements are supplemented with ab initio calculations, on the basis of which a crystallographic structure with a Xe4O12H12 primitive cell is proposed. The newly discovered compound is formed in the stability fields of superionic ice and η-O2, and has the same oxygen subnetwork as the latter. Furthermore, it has a weakly metallic character and likely undergoes sublattice melting of the H subsystem. Our findings indicate that Xe is expected to be depleted in the atmospheres of the giant planets as a result of sequestration at depth.

Dissociative melting of ice VII at high pressure.

We have used x-ray diffraction to determine the structure factor of water along its melting line to a static pressure of 57 GPa (570 kbar) and a temperature of more than 1500 K, conditions which correspond to the lower mantle of the Earth, and the interiors of Neptune and Uranus up to a depth of 7000 km. We have also performed corresponding first principles and classical molecular dynamics simulations. Above a pressure of 4 GPa the O-O structure factor is found to be very close to that of a simple soft sphere liquid, thus permitting us to determine the density of liquid water near the melting line. By comparing these results with the density of ice, also determined in this study, we find that the enthalpy of fusion (DeltaH(f)) increases enormously along the melting line, reaching approximately 120 kJ/mole at 40 GPa (compared to 6 kJ/mole at 0 GPa), thus revealing significant molecular dissociation of water upon melting. We speculate that an extended two-phase region could occur in planetary processes involving the adiabatic compression of water.

Structural transition in compressed amorphous sulfur.

Properties of amorphous sulfur (a-S) were investigated by synchrotron x-ray diffraction up to 100 GPa between 40 and 175 K. Measurements of the structure factor yielded the radial distribution function and the densities of two amorphous forms. a-S undergoes a structural transition above 65 GPa, accompanied by density discontinuity of 7%. These results indicate the amorphous-amorphous transition, from a low-density to a high-density form, and open up the possibility for the direct measurements of density of liquid-amorphous materials at extreme conditions.

High P-T transformations of nitrogen to 170 GPa.

X-ray diffraction and optical spectroscopy techniques are used to characterize stable and metastable transformations of nitrogen compressed up to 170 GPa and heated above 2500 K. X-ray diffraction data show that varepsilon-N2 undergoes two successive structural changes to complex molecular phases zeta at 62 GPa and a newly discovered kappa at 110 GPa. The latter becomes an amorphous narrow gap semiconductor on further compression and if subjected to very high temperatures (approximately 2000 K) crystallizes to the crystalline cubic-gauche-N structure (cg-N) above 150 GPa. The diffraction data show that the transition to cg-N is accompanied by 15% volume reduction.

Synthesis of novel transition metal nitrides IrN2 and OsN2.

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.

Retention of xenon in quartz and Earth's missing xenon.

The reactivity of xenon with terrestrial oxides was investigated by in situ synchrotron x-ray diffraction. At high temperature (T > 500 kelvin), some silicon was reduced, and the pressure stability of quartz was expanded, attesting to the substitution of some xenon for silicon. When the quartz was quenched, xenon diffused out and only a few weight percent remained trapped in samples. These results show that xenon can be covalently bonded to oxygen in quartz in the lower continental crust, providing an answer to the missing xenon problem; synthesis paths of rare gas compounds are also opened.

Synthesis and characterization of a binary noble metal nitride.

There has been considerable interest in the synthesis of new nitrides because of their technological and fundamental importance. Although numerous metals react with nitrogen there are no known binary nitrides of the noble metals. We report the discovery and characterization of platinum nitride (PtN), the first binary nitride of the noble metals group. This compound can be formed above 45-50 GPa and temperatures exceeding 2,000 K, and is stable after quenching to room pressure and temperature. It is characterized by a very high Raman-scattering cross-section with easily observed second- and third-order Raman bands. Synchrotron X-ray diffraction shows that the new phase is cubic with a remarkably high bulk modulus of 372(+/-5) GPa.

High-pressure transformations in xenon hydrates.

A high-pressure investigation of the Xe*H(2)O chemical system was conducted by using diamond-anvil cell techniques combined with in situ Raman spectroscopy, synchrotron x-ray diffraction, and laser heating. Structure I xenon clathrate was observed to be stable up to 1.8 GPa, at which pressure it transforms to a new Xe clathrate phase stable up to 2.5 GPa before breaking down to ice VII plus solid xenon. The bulk modulus and structure of both phases were determined: 9 +/- 1 GPa for Xe clathrate A with structure I (cubic, a = 11.595 +/- 0.003 A, V = 1,558.9 +/- 1.2 A(3) at 1.1 GPa) and 45 +/- 5 GPa for Xe clathrate B (tetragonal, a = 8.320 +/- 0.004 A, c = 10.287 +/- 0.007 A, V = 712.1 +/- 1.2 A(3) at 2.2 GPa). The extended pressure stability field of Xe clathrate structure I (A) and the discovery of a second Xe clathrate (B) above 1.8 GPa have implications for xenon in terrestrial and planetary interiors.