Session V25: High Pressure: Experiment

Kinetics studies across the phase transition of metals using \textit{dynamic}-DAC

We utilize the time-resolved synchrotron x-ray diffraction and in-situ optical spectroscopy to study the dynamic properties of several metals across the phase transition under different compression rates. The dynamic properties of metals across the pressure-induced phase transition, for example the mechanism of solidification or solid-solid transitions, are lacking. Obtaining the time-resolved structural evolutions of metals under rapid compression is critical to understanding material stability or metastability, and transition mechanism. In addition, the dynamical pressure changes can dramatically influence the microstructure and even phase boundaries, further affecting the properties of metals, such as toughness and strength.   

Phase Transition Kinetics of GST at High Pressures

The phase change material Ge2-Sb2-Te5 (GST) undergoes an amorphous to ordered phase transition in nanoseconds. This ultra-fast phase transition kinetics in conjunction with drastic electrical property changes makes GST an ideal candidate for next generation optical and digital storage media. The origins of this fast transition have thus far been traced back to the atomic configurations of both the amorphous and ordered phases. However the precise configurations of both the ordered and amorphous phases are still unknown with advances from simulations constantly altering our understanding of these structures. To discover the structural dependence of the phase transition kinetics of GST, we have performed in-situ synchrotron X-ray diffraction experiments on GST using a new Hydrothermal Diamond Anvil Cell under a range of pressures and observed new patterns of phase transition of GST at elevated temperatures. Ab initio simulations have been performed to interpret the structural evolutions of GST phases at different pressures, with an emphasis on the role of vacancies in the phase transition. Our results provide new insight into the mechanism of the fast phase transition kinetics of GST.   

Pressure-induced phase transitions in GeS under high pressures

We have studied the pressure-induced structural and electronic phase transitions of layered GeS (\textit{Pnma}) to 30 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that of metal at around 18 GPa. The visual appearance of GeS supports the insulator-metal transition: initially black GeS becomes opaque and eventually reflective with increasing pressure. The Raman result indicates that the metallization is preceded by a structural phase transition, presumably to the previously predicted \textit{Cmcm} structure.   

High pressure behavior of Cr$_2$O$_3$ to 62~GPa

Corundum-structured oxides are of interest for a broad range of reasons, including their mineralogical occurrences and technological uses. The high pressure behavior of Cr$_2$O$_3$ is of particular interest due to the widespread use of ruby, (Al,Cr)$_2$O$_3$, as a pressure standard in diamond anvil cells experiments. Although there have been a number of high pressure studies on Cr$_2$O$_3$, discrepancies still exist among the different data sets. Here we present synchrotron X-ray diffraction data on the structure and compressional behavior of Cr$_2$O$_3$ to 62~GPa. Although no change in crystal structure is detected within the resolution of the measurements, a change in compressional behavior occurs near 30~GPa where Cr$_2$O$_3$ changes color from red to orange.   

Amorphous diamond -- A high-pressure superhard carbon allotrope

Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully \textit{sp}$^{3}$-bonded amorphous structure and diamond-like strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced \textit{sp}$^{2}$-to-\textit{sp}$^{3}$ bonding change, while x-ray diffraction confirmed the perseverance of non-crystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of $>$70 GPa has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope. The nanoscale transmission x-ray microscopy is being utilized for accurate pressure-volume determination of glassy carbon and its high-pressure phase.   

Comparison of the existing internally consistent pressure scales at high pressures and high temperatures

There have been several efforts to determine internally consistent pressure scales for static diamond anvil high pressure study. We decide to extend the choice of pressure scales to include W and Cu. A recent study of Cu claims that electronic theory can constrain cold curve and possibly room temperature isotherm (Greeff et al., 2006, JPCS). We will present our comparison of 6 different pressure scales in regards with the suggested Cu EOS. We have measured angle-dispersive x-ray diffraction of Au, Pt, W, Cu, Ne, and NaCl to directly compare with the current existing EOS. We will also discuss discrepancies in the precise determination of pressure of phase transformations.   

Simultaneous measurement of pressure evolution of crystal structure and superconductivity in Fese$_{0.8 }$ using designer diamonds

Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on (P4/nmm) PbO type $\alpha $-FeSe$_{0.89}$ compound to a pressure of 44 GPa and at low temperatures down to 4 K using a synchrotron source and designer diamond anvils technique. At ambient temperature, a structural phase transition from the tetragonal (I4/nmm) phase to orthorhombic (Pbnm) is observed at 11 GPa and persist up to 75 GPa. The superconducting transition temperature increases rapidly with pressure in a parabolic manner reaching a maximum of $\sim $40 K at $\sim $ 11GPa. It then decreases at higher pressures. We also performed a complimentary pressure dependence x-ray diffraction simultaneously with resistance measurement at low temperatures and observe superconductivity only in the low pressure orthorhombic phase (Cmma) of $\alpha $-FeSe$_{0.89}$ Upon increasing pressure at 10 K, structural phase change from a mixed phase of orthorhombic (Cmma) and hexagonal (P63/mmc) to a high pressure orthorhombic phase (Pbnm) is observed at around 12 GPa where Tc is maximum.   

Melting Studies of Metals in a Paris Edinburgh Cell via Radiography

Equation-of-state measurements of amorphous solids and liquids suffer from the lack of distinct X-ray diffraction patterns that can be indexed and used for a precise and accurate volume determination. For relatively high Z materials, however, a possible remedy might be found in the application of X-ray radiography. The presentation will describe current efforts with regard to cerium and alloys of noble metals in the liquid state. This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. The X-ray studies were performed at HPCAT (Sector 16), APS/ANL. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from~DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.   

Homoepitaxial Boron Doped Diamond Anvil as Heating Element in a Diamond Anvil Cell

Recent advances in designer-diamond technology have allowed for the use of electrically and thermally conducting homoepitaxially-grown layers of boron-doped diamond (grown at 1200\r{ }C with a 2{\%} mixture of CH$_{4}$ in H, resulting in extremely high doping levels $\sim $ 10$^{20}$/cm$^{3})$ to be used as heating elements in a diamond anvil cell (DAC). These diamonds allow for precise control of the temperature inside of the diamond anvil itself, particularly when coupled with a cryostat. Furthermore, the unmatched thermally conducting nature of diamond ensures that no significant spatial gradient in temperature occurs across the culet area. Since a thermocouple can easily be attached anywhere on the diamond surface, we can also measure diamond temperatures directly. With two such heaters, one can raise sample temperatures uniformly, or with any desired gradient along the pressure axis while preserving optical access. In our initial experiments with these diamond anvils we report on the measurement of the thermal conductivity of copper-beryllium using a single diamond heater and two thermocouples. We augment these measurements with measurements of sample pressure via ruby fluorescence and electrical resistance of the sample and diamond heater.   

High pressure study of mixed valence compound CsAuI$_{3}$

CsAuI$_{3}$, chemically described as Cs$_{2}$Au$^{I}$Au$^{III}$I$_{6}$, is a mixed valence compound in the family of CsAuX$_{3}$ (X = Cl, Br, I), resembling the high Tc superconductor parent compound BaBiO$_{3}$. At ambient conditions it adopts a distorted perovskite structure with compressed Au$^{I}$X$_{6}$ octahedra and elongated Au$^{III}$X$_{6}$ octahedra along the crystallographic c-axis. The compound undergoes a pressure-induced transition into a new tetragonal phase comprising nearly equivalent AuX$_{6}$ octahedra around 5.3 GPa, which agrees with the valence transition previously reported using M\"{o}ssbauer spectroscopy between 8-12 GPa. We present a thorough high pressure studies of CsAuI3 through X-ray diffraction and Raman spectroscopy, confirming a pressure-induced band Jahn-Teller effect associated with the 5d$^{9}$ Au$^{II}$ ion. We will also report the reversible amorphization above 15 GPa.   

Pressure tuning of the thermal conductance of weak interfaces

We use high pressure to reveal the dependence of interfacial heat transport on the stiffness of interfacial bonds. The combination of time-domain thermoreflectance and SiC anvil techniques is used to measure the pressure-dependent thermal conductance $G(P)$ of clean and modified Al/SiC interfaces at pressures as high as $P$=12 GPa. We create low-stiffness, van der Waals bonded interfaces by transferring a monolayer of graphene onto the SiC surface before depositing the Al film. For such weak interfaces, $G(P)$ initially increases approximately linearly with $P$, consistent with results of molecular dynamics simulations. At high pressures, $P>$8 GPa, the thermal conductance of weak interfaces approaches the high values characteristic of strongly-bonded, clean interfaces. The results provide new insight demonstrating that interface stiffness dominates thermal transport at weak interfaces, but plays a minor role for strong interfaces with stiffness similar to that in the bulk of the materials.   

Target tracking in a dusty plasma: phase transitions and equations of state

A dusty plasma is a low-density ionized gas containing micron-sized charged dust particles. Laboratory dusty plasmas can be used as kinematic simulators of condensed matter systems, displaying phenomena such as phase transitions. The motion of individual particles is resolvable using a high-speed video camera. We use recursive state estimation (target tracking) techniques to track the dust kinematics, from which we calculate an equation of state for the toy condensed matter system.