Hydrogen is the simplest element in the Periodic Table of Elements, yet in the solid state it has a complex phase diagram as a function of temperature and pressure. Synthesis of metallic hydrogen (MH) is considered one of the top problems for physics in the 21^st century. There are two pathways for creating MH under high pressures: through intermediate pressures (~150 GPa) and high temperatures (above 2000K) and through extreme pressures (~500 GPa) and low temperatures (5 K). I investigated the second path using infrared absorption spectroscopy in DAC. This research faces a number of challenges such as hydrogen diffusion into diamond, high compressibility of hydrogen, early failure of the diamonds due to inner defects, etc. MH in this form has probably never existed on Earth or in the Universe; it may be a room temperature superconductor and is predicted to be metastable. If metastable it will have an important technological impact.

Thermal transport properties of minerals and melts at high pressures and temperatures is of central importance to the evolution and dynamics of planets. The pressure of the Earth’s interior continuously increases with the depth from the surface of the Earth: several hundred MPa at the region of the crust, ∼20 GPa at the upper mantle and ∼130 GPa at the lower mantle. Precise data of thermal conductivity and/or thermal diffusivity for minerals at elevated pressure make it possible to estimate the heat budget in the Earth.

In the Earth’s core and mantle, thermal conductivity of minerals containing dominantly iron-bearing silicates and iron alloys defines the heat and energy flow as well as the geodynamo over the Earth’s history. Direct measurements of thermal conductivity of Earth minerals at extreme pressures and temperatures are very challenging; the available data are limited, inconclusive, and not corresponding to the theoretical calculations. Therefore, there is a lack of experimental data, which is currently substituted by extrapolations and estimations.

In the present work, I have studied the direct measurements of thermal conductivity of different materials at different high pressure and temperature conditions up to 130 GPa using diamond anvil cells. In the group we have developed a laser system for these measurements. We used a continuous 100-Watt infrared laser for background heating from both sides and a pulsed 300-Watt infrared laser to heat one side in microsecond pulses. Fast measurements allowed us to resolve the heating response through the sample and understand the tendency of materials behavior within the mantle and core. Recent addition of a Pockels cell allows us to shape the pulse to a nearly square nearly 1 mm long time-dependence, which simplifies the already complicated calculations.

Thermal conductivity can be determined by measuring temperature of sample radiometrically and fitting the results with final element calculations, but the sample thickness needs to be determined, which was obtained from interferometric measurements through the KCl pressure transmitting medium; the refractive index of KCl was measured in a separate high-pressure experiment. Ir was used for absorption of the laser if the absorption of the sample was small.

Cubic spinels (e.g., γ-Si₃N₄) have great potential for industrial applications as an abrasive material. γ-Si₃N₄ was also considered to be of interest for opto-electronic applications. In this project I synthesized nitrides of the XIV group of the Periodic Table (C, Si, Ge, Sn) with cubic spinel structure, γ-M₃N₄ (where M = Si, Ge, and Sn), using another type of LVP called multi-anvil apparatus (MAA) and performed chemical and structural characterization of the products using the micro-focus X-ray diffraction, TEM, Raman spectroscopy, electron microprobe, elemental analysis. The goal was to obtain solid solutions in Si-Ge and Ge-Sn ranges and investigate their probable superhard properties.

Boron, the 5^th element of the Periodic Table, is a nonmetallic, hard material with high melting point and boiling temperature. Despite decades of extensive investigations, boron and boron-rich solids remain in the focus of modern research, because there are many outstanding fundamental questions related to boron chemistry, crystal-chemistry, bonding, polymorphism, and physical properties of boron allotropes and compounds. There are also a number of theoretical predictions, which require experimental verification. New knowledge is gained by contemporary progress in material synthesis and methods of their investigations.

My research is aimed at the development of high-pressure high-temperature (HPHT) synthesis of single crystals of boron allotropes and boron-rich compounds, which can be used further for precise investigations of their structures, properties, and behavior at extreme conditions. The HPHT synthesis using the large-volume-press technique yields single crystals of especially high purity and quality. I performed the synthesis of inorganic materials at high pressures and high temperatures (boron-rich compounds, iron carbides, nanodiamonds, aluminum-nickel alloys) using the large-volume press with toroidal anvils (toroidal press) recently installed in the Laboratory of Crystallography. I contributed to the development of the toroidal cell assembly and conducted pressure and temperature calibrations of this new press.

Diamond anvil cell (DAC) technique was used for in situ studies of single crystals at high pressures and room temperature, as well as at extreme HPHT conditions with double-sided laser heating of samples in DACs. Various analytical techniques I used include synchrotron and in house XRD, Raman and infra-red (IR) spectroscopy, SEM and TEM.

In this study, I focused on high-pressure investigations of α-B, β-B and stoichiometric boron carbide, B₁₃C₂. The present work resulted in the HPHT synthesis of the previously unknown non-icosahedral boron allotrope, ζ-B, for the first-time and confirmed earlier theoretical predictions. Structural stability of α-B and β-B in the Mbar pressure range and B₁₃C₂ up to 73 GPa was experimentally proven. Careful tracing of the bond lengths in structures of these solids under compression shed light into mechanics of their HP behavior.