Session G39: Matter at Extreme Conditions: Experiment

Powder X-ray diffraction of dynamically-compressed tantalum and lead in the terapascale pressure regime

We will present advances in powder x-ray diffraction methods for measuring crystal structure in the Terapascale pressure regime on laser ramp-compressed solids, and will show results for dynamically compressed tantalum up to 750 GPa and lead up to 600 GPa. Both of these systems show signatures of high pressure phase transitions not yet seen in static high pressure studies. We will discuss the possible effects of temperature and kinetics on high pressure phase transitions in ramp-compressed materials. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Kinetics studies across the melting line of metals using \textit{dynamic}-DAC

We utilize the time-resolved synchrotron x-ray diffraction and \textit{in-situ }optical spectroscopy to study the dynamic properties of several metals across the melting lines under different compression rates at different temperatures. The dynamic properties of metals across the pressure-induced liquid-solid transitions, such as the nucleation time and the mechanism of recrystallization are lacking. Time scales for metal nucleation and growth rates are challenging to obtain. X-ray diffraction under rapid compression will provide unique insight to understand the melting and crystallization mechanisms. In addition, the dynamical pressure changes can dramatically influence the microstructure and even phase boundaries, further affecting the properties of metals. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 11-ERD-046.   

Experimental Measurement of Speeds of Sound in Liquid Carbon Monoxide and Development of High-Pressure, High-Temperature Equations of State

We report the adiabatic sound speeds for liquid carbon monoxide along two isotherms, from 0.17 to 2.13 GPa at 297 K and from 0.31 to 3.2 GPa at 600 K. The carbon monoxide was confined in a resistively heated diamond-anvil cell and the sound speed measurements were conducted \textit{in situ} using a recently reported variant of the photoacoustic light scattering effect. The measured sound speeds were then used to parameterize a polarized exponential-6 intermolecular potential for carbon monoxide. P$\rho $T thermodynamic states, sound speeds, and shock Hugoniots are calculated using the newly parameterized intermolecular potential and compared to previously reported experimental results. Additionally, we present an analytical equation of state for carbon monoxide that was generated by fitting to a grid of calculated P$\rho $T states over a range of 0.1-10 GPa and 150-2000 K. * This work was performed under the auspices of the U.S. Department of Energy jointly by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

More than doubled ambient superconducting transition in a heavily compressed aromatic hydrocarbon

Exploring superconductivity at higher transition temperatures $T_{\mathrm{c}}$s in light elements such as hydrogen and carbon and their organic compounds has long been an attractive issue. Cation-doped aromatic hydrocarbons have been discovered to be superconductive with an increasing $T_{\mathrm{c}}$ by adding more hydrocarbon rings. Here we present a discovery of an enhancement of $T_{\mathrm{c}}$ from the ambient 4.8 K to 12.2 K in compressed Ba$_{\mathrm{1.5}}$Phenanthrene by magnetic susceptibility measurements up to 61 GPa. In contrast to the existence of superconductivity within a very narrow pressure range in fullerides, we find that this organic compound maintains superconductivity at more than doubled ambient $T_{\mathrm{c}}$ even at 61 GPa. A phase transition in the region between 3.0 and 5.4 GPa and an orientational disorder at around 28 GPa are identified using synchrotron X-ray diffraction technique. A nice correction between $T_{\mathrm{c}}$ and the angle between two crystal axes indicates the essential role of electronic correlations.   

Stable External Heating of Diamond Anvil Cell: Examples and Issues

While laser heating has been applied to successfully study materials at extreme conditions, external heating also has been extensively developed and applied for material studies at moderate temperature below $\sim$1000 K at high pressures. We have tested various external heating methods to accomplish stable heating at high pressures. Experimental measurements using two mini coil heaters at 900 K and 580 K to 100 GPa and 185 GPa, respectively and isobaric heating at 95 GPa up to 1000 K will be presented. New measurements using a graphite gasket heater will be compared along with internal heating methods. We will present comparison among different external heating methods and different temperature measurements using various examples. HP-CAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOE-NNSA, DOE-BES, and NSF. The APS is supported by DOE-BES under Contract No. DE-AC02-06CH11357.   

Homoepitaxial Boron Doped Diamond Anvils as Heating Elements 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 $^{\circ}$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 lateral 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 continuing set of benchmark experiments, we use two newly created matching heater anvils with 500$\mu $m culets to analyze the various fluorescence emission lines of ruby microspheres, which show more complicated behavior than traditional ruby chips. We also report on the temperature dependence of the high-pressure Raman modes of paracetamol (C$_{8}$H$_{9}$NO$_{2})$ up to 20 GPa.   

Phase transition dynamics in high-pressure VO$_{2}$

Vanadium dioxide VO$_{2}$ is a prototypical strongly correlated material which presents an insulator-metal transition at both ambient and high pressures. We use synchrotron X-ray diffraction combined with a diamond anvil cell to determine the pressure-temperature phase diagram of VO$_{2}$. We also use ultrafast coherent phonon spectroscopy to study its phase transition dynamics at high pressure. We find that, in contrast with ambient pressure experiments where strong photoexcitation promptly changes the lattice potential symmetry, at pressures as high as P$=$11 GPa the coherent phonons are still observed upon the photo-driven phase transition to the metallic state. The mechanism of the phase transition dynamics will be discussed.   

Vanadium and V-Ti alloys at high pressure

Experimental studies of vanadium found that during compression it undergoes a phase transition from the low pressure body centered cubic crystal structure to a rhombohedral phase at 65 GPa when compressed under quasihydrostatic conditions (PRB 83, 054101). Theoretical studies are in reasonable agreement with the transition pressure and predict that upon further compression above 200 GPa the bcc phase becomes stable again. The latest study (PRL 103, 235501) predicts that alloying vanadium with small amounts of the neighboring elements can increase or decrease the stability of the bcc phase relative to the rhombohedral phase. We performed powder x-ray diffraction experiments in diamond anvil cell of pure vanadium and V-Ti alloys at ambient temperature to very high pressures. We will discuss our results, including the equation of state and the stability of the rhombohedral phase at high pressures.   

Nuclear magnetic resonance at pressures of up to 10.1 GPa detects an electronic topological transition in aluminum metal

We present high sensitivity $^{27}$Al nuclear magnetic resonance (NMR) measurements on metallic aluminum under high pressures of up to 10.1 GPa. The measured Knight shift and spin-lattice relaxation rate indicate an unexpected negative curvature in the pressure dependence of the electronic density of states (DOS) that violates a free electron behavior. Based on a careful analysis of the Fermiology of aluminum metal with numerical LDA calculations we attribute the observed change in the DOS to a pressure induced electronic topological transition. We discuss an unexpected increase of the NMR linewidth above 4.2 GPa that is not in agreement with the metal's cubic symmetry.   

Ultrasonic Investigation of Cerium under High Pressure

The contribution of the lattice to the famous volume collapse transition in cerium is re-evaluated using a unique combination of several techniques available at sector 16 BMB / HPCAT. These eliminate any indirect /iterative procedures employed previously: Energy dispersive X-ray scattering provides the pressure of the sample (as well as quality control about the state of the sample), X-ray radiography delivers a shadow image allowing a precise length measurement and the ultrasound pulse overlap method gives the transit time of the longitudinal and transverse pulses. Our preliminary analysis indicates a larger contribution by the lattice as previously thought. 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.   

High Pressure Crystalline Structure and Resistance of Vanadium Dioxide to 13.5 GPa

We have investigated the insulator-to-metal transition in thin film vanadium dioxide as a function of pressure at ambient temperature using a designer diamond anvil cell (DAC). Four-point probe resistance measurements show a monotonic decrease over the entire pressure range studied with no significant discontinuity. High-pressure X-ray diffraction measurements observe an $\mathrm{M_1}$ ($\mathrm{P2_1/c}$) phase at 0 GPa, an $\mathrm{M_2}$ (C2/m) phase from 0.8 GPa to 1.1 GPa, and a reentrant $\mathrm{M_1}$ phase from 1.1 GPa to 13.5 GPa. Crystal refinement above 1.1 GPa shows a monotonically decreasing $a$, $b$ and $c$ lattice constants and a minimum in the monoclinic angle, $\beta$, near 8.5$\pm$0.5 GPa. The atomic positions show that the first V-V nearest neighbor distance ($d$) decreases over the entire pressure range, the second nearest neighbor distance ($s$) increases until 5 GPa after which it is constant with $s$$\approx$$f$$\approx$3.2 {\AA}. The next most closely spaced V-V distance ($f$), which corresponds to V atoms in different unit cells, is approximately constant across the entire pressure range measured.   

Unraveling Convoluted Structural and Electronic Transitions in SnTe at High Pressure

The longstanding uncertainty in high-pressure structural evolution of SnTe has greatly impeded the understanding of its complex electronic properties. Here we unravel the convoluted high-pressure phase transitions of SnTe using angle-dispersive synchrotron x-ray diffraction combined with first-principles structural search. We identify three coexisting intermediate phases of Pnma, Cmcm, and GeS type structure and establish the corresponding phase boundaries. We further unveil the intricate pressure-driven evolution of the energetics, kinetics and lattice dynamics of SnTe to elucidate its distinct phase-transition mechanisms. Subsequent electronic band calculations reveal pressure-induced metallization, superconductivity and topological phase transition in SnTe. These findings resolve structures and predict intriguing properties of SnTe, which have broad implications for other IV-VI semiconductors that likely harbor similar novel high-pressure phases and properties.   

Compression of HgCr$_2$S$_4$ and HgCr$_2$Se$_4$ spinels

The family of ACr$_2$X$_4$ spinels constitutes a prototype system for studying magnetism in solids [1]. More recently, members of this series were found to exhibit multiferroicity [2]. The origin of the ferroic properties is unknown; the role of the structure, however, appears to be important [3]. Given the strong interplay between structural and ferroic properties in these systems, structural tuning by pressure can provide valuable hints for multiferroicity. We have performed high-pressure structural investigations on the multiferroic HgCr$_2$S$_4$ and the HgCr$_2$Se$_4$ compounds. HgCr$_2$S$_4$ exhibits three structural transitions: the starting cubic phase adopts a tetragonal structure at 20 GPa, at 27 GPa an orthorhombic distortion occurs, and a third transition takes place above 37 GPa. As for HgCr$_2$Se$_4$, our studies detect a structural transition at 14 GPa, near the theoretically predicted band gap closure [4]. We discuss the possible mechanisms for the observed phase transitions for both Cr-spinels.\\[4pt] [1] T. Rudolf \textit{et al}., N. J. Phys. 9, 27 (2007) and refs. therein\\[0pt] [2] S. Weber \textit{et al}., PRL 96, 157202 (2006)\\[0pt] [3] V. Gnezdilov \textit{et al}., PRB 84, 045106 (2011)\\[0pt] [4] S. Guo \textit{et al}., JPCM 24, 045502 (2012)   

Electron-phonon interaction of GaAs nanowires under pressure

We present resonant Raman scattering (RRS) investigation of wurtzite and zinc-blende phase GaAs nanowires under hydrostatic pressure up to 30 GPa. The Raman spectra are excited by 532 nm and 488 nm laser lines. High order longitudinal optical modes 2LO and 3LO are observed under the resonant conditions. Pressure dependence of band gap of WZ and ZB nanowires has been obtained from the corresponding resonant pressures, and band gap of WZ nanowires is found to be larger than that of ZB nanowires. When applying pressure at 21 GPa, Raman signals of WZ and ZB phases disappear, manifesting phase change to a high-pressure phase.   

Response of Aluminum under ramp Compression to Mbar

Laser-Produced X-ray drive is an important tool for ramp compression to very high pressure. Its application was often limited by the length of rise pulse, the peak pressure was not higher than 400GPa for metal steps. A new method was developed using heavy reservoir film, that can absorb high energy Au M-band x rays generated within the halfraum which otherwise could preheat the step sample. Meanwhile, heavy reservoir can also produce higher pressure peak and longer rise time.Results from this reservoir shot (4.5Mbar) at the SG-III prototype are presented. Al/LiF interface velocities versus time for multiple sample thicknesses were measured and converted to p-v relations using backward integration.