Session E17: Matter in Extreme Environments: Novel Approaches to Pressure and Synthesis

Matter in Extreme Environments: Materials synthesis and crystallography at extreme pressure - revealing some remarkable materials properties

During last decades, the impact of high-pressure studies on fundamental physics and chemistry, and especially on Earth and planetary sciences, has been enormous. Modern science and technology rely on the vital knowledge of matter which is provided by crystallographic investigations. The most reliable information about crystal structures of solids and their response to alterations of pressure and temperature is obtained from single-crystal diffraction experiments. Advances in diamond anvil cell (DAC) techniques, designs of double-stage DACs, and in modern X-ray instrumentation and synchrotron facilities have enabled structural research at multimegabar pressures.
We have developed a methodology for performing single-crystal X-ray diffraction experiments in double-side laser-heated DACs and demonstrated that it allows the crystal structure solution and refinement, as well as accurate determination of thermal equations of state above 200 GPa at temperatures of thousands of degrees. Application of this methodology resulted in discoveries of novel compounds with unusual chemical compositions and crystal structures, uncommon crystal chemistry and physical properties. It has been successful in investigations of various classes of solids - elemental materials, oxides, carbides, borides, carbonates, nitrides, and silicates. In this contribution we will report the results of our single-crystal diffraction studies of phase relations in the Fe-O system, transition metals carbonates, silicates, and nitrides at simultaneous high pressures and high temperatures. Perspectives of materials synthesis and crystallography at extreme conditions will be outlined.
  

Investigation of Pressure Induced Formation of Diamondene

Pressure is a convenient thermodynamic parameter for obtaining new materials that cannot be synthesized under ambient conditions. For instance, theoretical calculations show that when two layers of graphene are compressed at high-pressures in the presence of specific chemical groups, they can be turned into a 2D diamond called diamondene: a ferromagnetic semiconductor with spin-polarized bands. Efforts to experimentally demonstrate this structure are in the initial stages, yet we have already obtained robust results [1]. In this initial work, we obtained indirect evidence of diamondene formation at room temperature by compressing two layers of graphene using water as a pressure transmitting medium (PTM) in a diamond anvil cell (DAC). The phase transition was identified by measuring the G band dispersion with laser energy as a function of pressure. We will report our progress in investigating this phase transition with new experimental evidences.


[1] Martins, Luiz Gustavo Pimenta, et al. Nature communications 8.1 (2017): 96.   

X-Ray Diffraction Crystallization Studies on a Zr-Based Bulk Metallic Glass under High-Pressure and High-Temperature

X-ray diffraction crystallization studies were conducted on bulk metallic glass, Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8. Each sample was compressed using a Paris-Edinburgh Press at Beamline 16-BM-B, HPCAT, of the Advanced Photon Source, and heated to 800°C. MgO was used as a pressure standard for this experiment, and two constant heating rates, 2.9°C/min and 6.0°C/min were invesitgated. A heating rate dependence was seen in the crystallization temperature, with a faster heating rate corresponding to a higher crystallization temperature for the full pressure range tested.   

Graphene as a Diffusion Barrier in High-Temperature Electronics

The development of high-temperature semiconductor technology for use in applications such as power plants, nuclear reactors, and on hot planets like Venus (~460°C) is severely limited by the failure of essential metal contacts in otherwise thermally robust devices. This failure, which manifests as bubbling on the metal contact, is thought to be due to inter-diffusion of metal and semiconductor layers. To test this hypothesis, we constructed a monolayer graphene barrier to see if bubble formation would be reduced. Auger electron spectroscopy was used to characterize the inter-diffusion of material layers in Pd/AlGaN/GaN and Pd/Graphene/AlGaN/GaN Schottky diodes subjected to thermal treatments over a range of temperatures from 100 to 450°C. Graphene was shown to stop bubble formation in high-temperature metal contacts up to 425°C, supporting the inter-diffusion hypothesis and demonstrating promise for use on, for example, the planet Mercury (where temperatures range from approximately -180 to 430°C).   

In Situ Solid State Laser Refrigeration at GPa Pressures

Solid state laser refrigeration can cool a host lattice through the emission of anti-Stokes upconverted photons from rare-earth dopant ions. Recently, laser refrigeration has also been demonstrated in condensed phases, including liquid water¹ using single-beam laser tweezers. Laser refrigeration within a diamond anvil cell would allow researchers to explore in situ temperature-dependent properties of materials without external hardware. To date, laser refrigeration at extreme pressures conditions has not yet been demonstrated. Here, we demonstrate the cooling of 10%Yb³⁺:LiYF₄ (Yb:YLF, I₄₁/a space group) micro-crystals at elevated pressures in a diamond anvil cell. In addition, we investigate the impact of a scheelite to fergusonite phase transition on laser refrigeration at pressures >10.5 GPa. Varying the irradiance of a 1020nm continuous wave laser shows that Yb:YLF can be effectively cooled with a linear dependence on laser irradiances. Cooling temperatures are quantified using a ratiometric thermometry approach involving a Boltzmann fit to f-f transitions from the Yb³⁺ ions that occur between the Stark levels within the ²F_5/2 and ²F_7/2 manifolds.
1. Roder, P.B. et al. (2015) Laser refrigeration of hydrothermal nanocrystals in physiological media. PNAS 112: 15024-15029   

Magnetometry and Stress Tomography in Diamond Anvil Cells using Nitrogen Vacancy Centers

The Nitrogen Vacancy (NV) center in diamond has emerged as a promising candidate for the nanoscale sensing of temperature, strain, electric and magnetic fields. The integration of NV-based sensing into diamond anvil cells (DAC), a workhorse of high pressure science, offers a means not only for making spatially resolved measurements of relevant sample properties but also for monitoring the stress distribution in the diamond anvil itself. Compared to conventional high pressure probes, key advantages of NV sensing include diffraction limited spatial resolution (~ 1 um) and versatility, thus enabling exploration of novel phases of matter and the transitions between them, with pressure as a tuning parameter. Additionally, imaging the stress distribution inside DACs can provide insight into the mechanical failure of anvils and inform improvements in anvil design. We describe two main results: 1) we generate a layer of NVs near the tip of the diamond anvil and use DC magnetometry to study pressure-driven magnetic phase transitions and 2) using a carefully applied bias magnetic field we map the tensorial stress distribution within the diamond anvil itself.   

Sterically Controlled Solid-State Mechanochemistry Under Hydrostatic Pressure

Mechanical stress can modify the energy landscape of chemical reactions and enable new reaction pathways. Mechanochemical mechanisms under tensile stress have been extensively studied in one-dimensional polymers. However, bond activation has not been possible with hydrostatic pressure in three-dimensional solids. Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components consisting of a compressible mechanophore and incompressible ligands. In these ‘molecular anvils’, isotropic stress leads to anisotropic deformation of the compressible mechanophore and activating bonds. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in crystalline metal-organic chalcogenides, where bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds. These results reveal an unexplored mechanism and enable new possibilities for high-specificity mechanosynthesis.   

The toroidal diamond anvil cell for detailed measurements under extreme static pressures.

The upper pressure achievable in the conventional Diamond Anvil Cell (DAC) had been limited to approximately 400 GPa since the early 2000s. We show that by sculpting the diamond anvil tip into a toroidal shape with a Focussed Ion Beam, the maximum pressure can be extended toward the terapascal pressure range. Measurements similar to standard DACs are so possible in terms of precision, reproducibility, sample dimensions and type of materials from gas to solids and with any atomic number. Our toroidal-DAC design will be compared to other innovative schemes recently proposed to extend the pressure limit of the DAC , such as the double-stage DAC.
  

Toroidal diamond anvils for static compression experiments beyond 5 megabar

The diamond anvil cell has been around for over 50 years and has been the primary tool for routinely studying materials up to pressures of ~3 Mbar. Experiments over 4 Mbar with in situ pressure determination have been reported, however these reports are scarce. This indicates that these experiments are challenging, and that the success rate of these experiments is quite low. However, critical for developing accurate fundamental physics and chemistry models, with possible applications in modeling interiors of large planets. In this presentation I will show that focused ion beam crafted toroidal single-crystal diamond anvils with ~9.0 μm culets are capable of producing pressures over 5.0 Mbar. The toroidal surface prevents gasket outflow and provides a means to stabilize the central culet. We have reached a maximum pressure of ~6 Mbar using Re as in-situ pressure marker, a pressure regime typically accessed only by double-stage diamond anvils and dynamic compression platforms.   

Single Crystal X-ray Diffraction in Laser Heated DACs

Until recently, all DAC laser-heating systems were stationary and could not be used for single-crystal structural studies aimed not only at determining lattice parameters, but also at structural refinements, which require measuring X-ray diffraction intensities and at least partial rotation of the DAC during experiments with monochromatic radiation. The beam of a stationary laser enters the cell at a fixed angle, so that the rotation leads to departure of the crystal from the focus position and scattering the powerful laser light in arbitrary directions by the diamond anvils that might be dangerous.
In order to overcome this problem we have developed a portable laser heating system and diamond anvils with round tables (DARTs) and implemented them in our experiments at verious diffraction beamlines (ID15 and ID27 at ESRF, and P02 at PETRA III). The performance and potential of this approach have been demonstrated in simultaneous high-pressure and high-temperature powder and single-crystal diffraction studies of chemical and phase relations in transition metals oxides, carbides, hydrides, carbonates, and silicate perovskites.   

Carbon-based clathrates

Clathrate structures are found throughout nature in tetrahedral systems including water, silicates, silicon, germanium, tin and even colloids. Given similarities in bonding, it is natural to presume the existence of clathrate frameworks based on carbon, but attempts to prepare these materials have been unsuccessful thus far. If synthesized, these materials are expected to exhibit diamond-like mechanical properties due to the nature of covalent bonding, as well as strong electron-phonon coupling that could lead to high superconducting transition temperatures. We develop a stabilization principle based on the substitution of boron within the polyhedral carbon cages. Using this approach, we predict and synthesize the first carbon-based clathrate comprised of truncated octahedral C₁₂B₁₂ cages that trap Sr atoms in the “type-VII” structure. The mechanical properties, electronic structure and superconducting properties of this material will be discussed, as well as the potential for different structures and compositions within this new family of carbon-based materials.