Christophe L. Guillaume

Structural Diversity of Sodium

Sodium exhibits a pronounced minimum of the melting temperature at ∼118 gigapascals and 300 kelvin. Using single-crystal high-pressure diffraction techniques, we found that the minimum of the sodium melting curve is associated with a concentration of seven different crystalline phases. Slight changes in pressure and/or temperature induce transitions between numerous structural modifications, several of which are highly complex. The complexity of the phase behavior above 100 gigapascals suggests extraordinary liquid and solid states of sodium at extreme conditions and has implications for other seemingly simple metals.

Formation of transition metal hydrides at high pressures

Abstract Silane (SiH 4 ) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H 2 . The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane and reaction of hydrogen with Re gasket. From the data of a previous experimental report [M.I. Eremets, I.A. Trojan, S.A. Medvedev, J.S. Tse, Y. Yao, Science 319 (2008) 1506], the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.

Structure of sodium above 100 GPa by single-crystal x-ray diffraction

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

Anomalous optical and electronic properties of dense sodium

Synchrotron infrared spectroscopy on sodium shows a transition from a high reflectivity, nearly free-electron metal to a low-reflectivity, poor metal in an orthorhombic phase at 118 GPa. Optical spectra calculated within density functional theory (DFT) agree with the experimental measurements and predict a gap opening in the orthorhombic phase at compression beyond its stability field, a state that would be experimentally attainable by appropriate choice of pressure-temperature path. We show that a transition to an incommensurate phase at 125 GPa results in a partial recovery of good metallic character up to 180 GPa, demonstrating the strong relationship between structure and electronic properties in sodium.

On the structure of high-pressure high-temperature η-O2

In situ high-pressure high-temperature x-ray diffraction and optical studies have been conducted on solid oxygen between 10 and 20 GPa and up to 700 K. Optical observations and Raman spectroscopic studies have been utilized to confirm the existence of eta-O(2) and to identify phase behavior and phase boundaries of beta-, epsilon- and eta-O(2) at elevated temperatures. Subsequent single-crystal synchrotron x-ray diffraction studies yielded the structure of the eta-O(2) phase at 15.9 GPa and 625 K.

Synthesis of a Nitrogen-Stabilized Hexagonal Re3ZnNx Phase Using High Pressures and Temperatures

High pressure can induce profound changes in solids. A significant barrier to new alloys and ceramics, however, is that targeted starting materials may not react with each other, even with the help of pressure. We use nitrogen, in a new capacity, to incorporate two otherwise unreactive elements, Re and Zn, in the same structure when pressure alone does not suffice, without nitrogen altering the resulting backbone structure. Synthesis experiments up to 20 GPa and 1800 K show that while no Re-Zn alloy or solid solution is formed, a novel Re(3)ZnN(x) ordered solid solution is formed, at 20 GPa, with nitrogen occupying Re-coordinated cages. We put forth that unlike pure Re(3)Zn, our novel hexagonal Re(3)ZnN(x) structure is stabilized by nitrogen bond formation with rhenium. Pressure lifts the pronounced ambient Zn anisotropy, making it more compatible with Re and likely facilitating incorporation of the structure-stabilizing nitrogen anion. This methodology and result denote further options for removing impasses to material preparation, thus opening new avenues for synthesis. These can also be pursued with other ions including carbon, hydrogen, and oxygen, in addition to nitrogen.

Tuning between Mixing and Reactivity in the Ge−Sn System Using Pressure and Temperature

No bulk GeSn crystal existed prior to this work. Near 10 GPa the two elements resemble each other both electronically and structurally. Synthesis experiments at 10 GPa and 1500 K followed by annealing at 770 K using Ge and Sn starting materials and ex-situ analysis using transmission electron microscopy, scanning electron microscopy, and X-ray diffraction document the recovery of a Ge(0.9)Sn(0.1) solid solution (space group P4(3)2(1)2, a = 6.014 (1) A, c = 7.057 (1) A, Z = 12).

Correlation between structural and semiconductor-metal changes and extreme conditions materials chemistry in Ge-Sn.

High pressure and temperature experiments on Ge-Sn mixtures to 24 GPa and 2000 K reveal segregation of Sn from Ge below 10 GPa whereas Ge-Sn agglomerates persist above 10 GPa regardless of heat treatment. At 10 GPa Ge reacts with Sn to form a tetragonal P4(3)2(1)2 Ge(0.9)Sn(0.1) solid solution on recovery, of interest for optoelectronic applications. Using electron diffraction and scanning electron microscopy measurements in conjunction with a series of tailored experiments promoting equilibrium and kinetically hindered synthetic conditions, we provide a step by step correlation between the semiconductor-metal and structural changes of the solid and liquid states of the two elements, and whether they segregate, mix or react upon compression. We identify depletion zones as an effective monitor for whether the process is moving toward reaction or segregation. This work hence also serves as a reference for interpretation of complex agglomerates and for developing successful synthesis conditions for new materials using extremes of pressure and temperature.

Imaging of Mixing and Reaction in Group IV Systems Recovered from High Pressures and Temperatures

Unlike the complete solid solution of silicon–germanium, the temperature–composition phase diagram of neighbouring germanium–tin is characterised by virtually no bulk mutual solubility at ambient pressure. High pressures and temperatures, however, drastically change the electronic and structural characteristics of the liquid and solid states of these elements. This has recently been exploited to remove the ambient constraints leading to a novel tetragonal germanium–tin solid solution near 10 GPa. Scanning and field emission gun scanning electron microscopy as well as focused ion beam methods targeting specific regions of analysis of recovered products are employed, documenting here, crystallisation of germanium in a tin matrix below this pressure, reaction of germanium with tin near 10 GPa and formation of germanium–tin agglomerates above this pressure. We use these methodologies to prepare new silicon–germanium alloys as well.

Synthesis of Metal Nitrides Using High Pressures and Temperatures

Technologically, high density nitrides are showing promise for both ceramic and electronic applications. In a laser-heated diamond cell we prepare high density metal-nitrides by reaction of the nitrogen pressure medium with an elemental substrate. Two of our objectives are to develop criteria governing whether denser than ambient nitride phases will form, and to in particular establish the parameters required for synthesis in a multianvil press using elemental starting materials. We have already synthesized transition metal nitrides in a multianvil press using elemental starting materials, including hexagonal nickel nitride and alkali rhenium nitrides. Unlike previous metals, we also report that Cu does not form a nitride after heating with NaN3 at 2000 K and 20 GPa. Notably, Cu3N is a semiconductor exhibiting weak directional bonds, whereas the immediately adjacent lower atomic number systems are metallic interstitial nitrides. We also briefly mention our work on processing high pressure and temperature recovered reaction products with focused ion beam methods for tailored characterization using electron microscopy.