Session R3: NT.1 Novel Techniques: Diamond Anvil Cells I

From microns to millimeters: New Diamond Cells for Multi-Megabar pressures and for Neutron Diffraction

We developed new diamond cells for two extreme applications. One cell was designed to routinely study hydrogen above 2 Megabar (200 GPa) [2] by adopting the principle of deflecting plates reported earlier [1]. Neutron diffraction requires millimeter-sized samples even for the very high neutron fluxes available at the Oak ridge National Laboratory. We develop a new diamond cell capable of routinely reaching pressures of 80 GPa with culets of 1.5 mm. The diamonds were of only 4 mm diameter with conical design [3] using strongly supported seats made of polycrystalline diamond. We present new, high-quality data for D$_{2}$O showing signs of symmetrisation in ice [4]. Tests using very large CVD diamonds are in progress.\\[4pt] [1] Zha, C., Z. Liu, M. Ahart, R. Boehier, and R. Hemley (2013) \textit{Phys. Rev.Lett. in press}.\\[0pt] [2] Boehler, R. (2006), \textit{Rev of Sci Inst}, \textit{77}, 115103.\\[0pt] [3] Boehler, R., and K. De Hantsetters (2004) \textit{High Pressure Research}, \textit{24}, 391-396.\\[0pt] [4] M. Guthrie, R. Boehier, C. Tulk, A. M. dos Santos, K. Li, J.J. Molaison, and R. Hemley (2013), \textit{PNAS submitted}.   

A New Way of Generating Load at Cryogenic Temperatures for Neutron Studies

Pressure generation at cryogenic temperatures presents a problem for a wide array of experimental techniques, particularly for neutron studies due to the volume of sample required.\footnote{Bailey, I. F. (2003). Z. Kristallogr., 218(2-2003), 84-95.} This challenge has been previously tackled by using a modified Bridgman-seal in a Paris-Edinburgh cell.\footnote{S. Klotz, B. Padmanabhan, J. Philippe, and T. Str\"assle, High Pressure Res. 28, 621 (2008).} We present a novel design of a pressure cell in which load is generated by a bellows driven by helium gas which ensures leak-free operation of the device. The bellows is custom-designed to generate the load of 80~kN at the maximum operational gas pressure of 350~bar. For opposed anvils with 3~mm diameter working surface, for example, this load converts into an average pressure of 11~GPa across the culets. The cell has four large windows for the scattered beam and the setup allows control of pressure in a wide (P,T)-range in which helium is in gas or liquid state. The cell has been used at the WISH beamline of the ISIS Pulsed Neutron Source with sapphire anvils. The device will be presented in detail, along with pressure loading curves and initial experimental data.   

X-ray imaging in the large volume Paris-Edinburgh press

Synchrotron X-ray tomography is a non-destructive 3D imaging/microanalysis technique selective to a wide range of properties such as density, chemical composition, chemical states, structure, and crystallographic perfection; with extremely high sensitivity and spatial resolution. We describe a new device, based on the V7 Paris-Edinburgh press, to extend this technique to high-pressure high-temperature conditions. It consists of two opposed conical anvils to pressurize the sample encased in an X-ray transparent boron epoxy gasket. Both anvils can rotate, with no limitation in the rotating angles, through two sets of gear reducer and thrust bearings located at the end of each anvil. The accurate and simultaneous rotation of the top and bottom anvils is monitored by stepper motors and encoders. This enables the collection of data at small angular steps over 180$^{\circ}$ rotation required for a complete 3D tomographic reconstruction. The potentials of this new equipment will be illustrated on two examples: (1) the determination of the volumetric properties of materials by absorption contrast tomography, and (2) the characterisation of ill-ordered materials under HP-HT by X-ray diffraction tomography.   

Limitations and possibilities of AC calorimetry in diamond anvil cells

Dynamic laser heating or internal resistive heating could allow for the determination of calorimetric properties of samples that are held statically at high pressure. However, the highly non-adiabatic environment of high-pressure cells presents several challenges. Here, we quantify the errors in AC calorimetry measurements using laser heating or internal resistive heating inside diamond anvil cells, summarize the equipment requirements of supplying sufficient power modulated at a high enough frequency to measure specific heats and latent heats of phase transitions, and propose two new experiments in internally-heated diamond anvil cells: an absolute measurement of specific heat (with $\sim$10\% uncertainty) of non-magnetic metals using resistive heating at $\sim$10 MHz, and a relative measurement to detect changes in either the specific heat of metals or in the effusively (the product of specific heat, density and thermal conductivity) of an insulator.   

High pressure modifications of the liquid structure of the light alkaline metals

The alkali group elements are considered as textbook examples of free-electron metals because of the single $s$ electron in the valence band. However, when these metals are subjected to compression they exhibit unexpected complexity suggesting extraordinary liquid states at extreme conditions. The analysis of the liquid structures of the light alkali metals has not been completed because of the lack of experimental data. Only X ray diffraction (XRD) data at room pressure are available. The major difficulty with liquid diffraction at high pressure is the large scattering background signal generated by the diamond anvil cell giving a signal over background ratio of around only 1 to 5 per cent. All these points explain the lack of experimental data of liquid alkali metals at high pressure. We performed the first quantitative measurements of the liquid structure factor of light alkali metals up to 100 GPa using XRD. We explored the P-T diagram in order to obtain quantitative structure factor, radial-distribution function and density of liquid alkali metals up to 100 GPa. We confirmed the existence of a different liquid structure at the minimum of the melting curve compared with the one at room pressure.   

Kinetics of Phase Transitions in Simple Materials using \textit{dynamic}-DAC

The pressure-induced phase transition is often limited by diffusion and occurs at the intermediate time-scale ($\mu $s to ms) of shock wave and static DAC high pressure experiments, which can be obtained precisely and in controlled ways using \textit{dynamic}-DAC. Coupling it with time-resolved optical spectroscopy and time-resolved synchrotron x-ray diffraction, we have recently studied high-pressure kinetics of phase transitions in an array of materials including Bi, Ga, Fe, H$_{2}$O, and methane hydrates. In this presentation, we will first describe the experimental methods and then present the results of (i) solidification of H$_{2}$O and Ga, (ii) phase transitions in Bi and Fe, and methane hydrates, and (iii) solid-state reactions in methane hydrates. As such, we will demonstrate the significance of obtaining the time-resolved structural information to understand the phase meta/stability, transition mechanisms, and diffusion-controlled crystal growth and interfacial reactions in solids.\\[4pt] In collaboration with Hyunchae Cynn, Lawrence Livermore National Laboratory; Choong-Shik Yoo, Department of Chemistry and Institute for Shock Physics, Washington State University; and William Evans, Lawrence Livermore National Laboratory.