Session R4: MS: Phase Transitions I

Chemical Synthesis Under Extreme Pressures: Novel Condensed Matter Binary Compounds Containing Bismuth

Pressure is a fundamental thermodynamic variable that spans roughly 50 orders of magnitude throughout the universe, yet practically all of our chemical synthesis intuition is based upon results obtained near atmospheric conditions. At pressures on the order of millions of atmospheres, elemental properties that we consider fundamental become categorically altered. For example, atomic volumes drop sharply, valence orbital energies can fall below those of core orbitals, and electronegativities drift from their tabulated ambient pressure values. Even at relatively modest pressures of 10,000--100,000 atm, which are now readily accessible in the laboratory, these effects can lead to surprising new chemical bonding, structures, and properties, opening up a new frontier for chemical exploration. In this talk, I will show how we have harnessed pressure to exert thermodynamic control over the synthesis of novel binary bismuth intermetallic compounds that are impossible to synthesize using traditional methods. I will show how X-ray diffraction (both \textit{in situ} and \textit{ex situ}) can be used to solve the structures of the new phases that form, and I will give examples of high-pressure compounds that can be synthesized on the cubic millimeter scale to allow for bulk property measurements.   

Structural Transformations during Shock Compression/Release of Germanium

To examine the structural changes in germanium during dynamic loading conditions, in situ x-ray diffraction measurements were obtained in Ge (100) shocked to 40.6 GPa and released. The experimental results demonstrated that shocked Ge transforms from the ambient cubic diamond (cd) structure to the tetragonal $\beta$-Sn structure above 15.7 GPa, the phase observed under static loading conditions at comparable pressures, and to the liquid state beyond 31.5 GPa. The observation of a significant decrease in the $\beta$-Sn phase texture as the shock melting stress is approached suggests that texture loss in the high-pressure phase of shocked single crystals may serve as a general indication of the temperature being near the solid-liquid phase boundary. Upon full stress release (while maintaining uniaxial strain), high-pressure germanium phases (both solid and liquid) revert to the ambient cd phase indicating a reversible phase transformation under shock compression and release. These findings demonstrate that the cd to $\beta$-Sn phase change is reversible, and that recrystallization from the liquid phase can occur on nanosecond timescales during planar stress release. These results are consistent with the equilibrium Ge phase diagram determined from static compression experiments.   

In situ X-ray diffraction study of high-pressure phase transition in zinc oxide under shock loading

The wurtzite-to-rocksalt phase transformation in zinc oxide (ZnO) was investigated under shock loading using pulsed synchrotron x-ray diffraction at the Dynamic Compression Sector at the Advanced Photon Source. At ambient conditions, ZnO crystallizes in a tetrahedrally coordinated wurtzite structure. Static experiments have established a phase change to a rocksalt structure at 9 GPa. Continuum gas-gun studies identify a similar phase transition under shock loading between 12-15 GPa. This type of pressure-induced transition to a rocksalt structure is common to many semiconducting-wurtzite and zincblende compounds. New capabilities for time-resolved x-ray diffraction present unique opportunities to identify phases forming along the Hugoniot as well as to constrain orientation relations between parent and daughter phases. Various orientations of single-crystal ZnO as well sintered polycrystalline ZnO were shock compressed to 20 GPa using a two-stage light gas gun. In-situ x-ray diffraction data collected in the shock-compressed state confirm the high-pressure phase observed on the Hugoniot corresponds to the rocksalt structure. Furthermore, an analysis of the pre-impact Laue pattern along with the textured diffraction from the transformed material can place new constraints on the orientation relations between the starting wurtzite and high-pressure rocksalt phases.   

Solid-solid, melting, and solidification phase transitions in shock-compressed silicon

Solid-solid structural changes, melting, and recrystallization were examined in shock-compressed silicon using in situ synchrotron x-ray diffraction (XRD) measurements at the Dynamic Compression Sector. Flat-faced LiF(100) crystals impacted Si(100) samples resulting in impact stresses up to 52 GPa. Four x-ray diffraction measurements (100 ps duration; 153.4 ns between measurements) were recorded during the impact event providing time evolution of the structural changes at different stresses. For shock stresses less than $\sim$30 GPa, shock-compressed Si transformed to a highly textured simple hexagonal structure. Shock-melting, along the Hugoniot, was unambiguously established above $\sim$31-33 GPa by the disappearance of all crystalline diffraction peaks. Reshock from the melt boundary resulted in rapid (nanosecond) crystallization to the hexagonal-close-packed structure. These are the first direct in situ measurements showing recrystallization from the shock melted state.   

Shock response of nanodiamonds explored by dynamic x-ray diffraction shock compression.

Nanoparticles represent a unique class of material for the purposes of investigating both material properties and also computational simulations due to their size. The 4-5 nm sized structures have been shown to have a disordered diamond shell confining a compressed diamond core from past diffraction experiments by other groups. The strain between the core and the shell is a potential source for energy release and the response of this material is important to the understanding of failure of these systems. Shock experiments at the Dynamic Compression Sector of the Advance Photon Source provide a unique avenue to look at the structural response of these nanoparticles. We have done as series of dynamic x-ray diffraction shock experiments looking at the response of the core shell material and will present the results of our findings which speak to the changes in material response to compression and strain.   

Shock Compression of Graphite: Role of Orientational Order on the Graphite to Diamond Transformation

Past experiments on shock-compressed pyrolytic graphites - having different orientational orders - have shown very different responses below and above the reported transformation stresses. Well-defined two-wave structures, indicative of a rapid phase transformation, were reported for ZYB-grade highly oriented pyrolytic graphite (HOPG) samples. However, two-wave structures were not reported for the less oriented HOPG samples (ZYH-grade) and for as-deposited pyrolytic graphite (PG). The objectives of the present study are to determine if well-defined rapid phase transformation waves are possible in less oriented pyrolytic graphite samples and to better understand the role of orientational order on the phase change mechanisms. To address these objectives, plane shock wave experiments were performed on ZYB-grade HOPG, ZYH-grade HOPG, and PG samples. Using laser interferometry, transmitted wave profiles were measured at the graphite/LiF interface for the different samples. Our results show that rapid, well-defined phase change waves occur in each graphite type examined, regardless of orientational order. In contrast to previous reports, both HOPG grades have similar shock responses above the transformation. However, the PG response is different from both HOPG grades.