Research Summary
High pressure and high/low temperature chemistry
The overall objectives of my work in materials science are to

· Elucidate the behavior of materials exposed to extreme conditions, notably high pressures (HP) and high and low temperatures (HT and LT);
· Create new functional materials with interesting and useful properties, such as extreme hardness (diamonds, boron nitrides, boron carbides) and superconductivity (magnesium boride); and
· Develop new designs for experimental approaches.

By investigating the HPHT and HPLT behaviors of elemental materials and compounds, my research sheds light on bonding evolution, phase transformations, and development of PT-phase diagrams of fundamental importance in materials synthesis and their applications. In particular, my state-of-the-art research focuses on developing new materials that are as strong as steel, but of lower density than water, by combining various methods of synthesis at extreme conditions using large-volume-press techniques, diamond anvil cells (DACs), and double-sided laser heating that can generate temperatures above 1200 K. This work will additionally entail honing existing synthetic and analytical methods to produce the kinds of chemical reactions conducted at extreme conditions of HPHT and HPLT that can trigger structural, electronic, and magnetic changes in matter and exhibit behaviors that are distinct from that at ambient conditions.

Applications of this research include new energy-efficient and environment-friendly materials, and new materials for possible use in aircraft production and space exploration.

Chemistry of Rare-Earth Elements Based Materials at Extreme Conditions

Novel materials can be synthesized under high pressure with enhanced properties, such as high energy density, superhardness, and high Tc superconductivity. Although materials synthesized under high-pressure conditions are not the most stable form at ambient conditions, they could be metastable upon pressure release due to a large energy barrier for the chemical transformation. This notion is in line with most previous high-pressure studies, where increased temperature is required to promote the synthesis of compounds at elevated pressures to overcome the reverse energy barrier between the ambient and sought-after high-pressure stoichiometry.

One of the great challenges in the field is to devise materials that will be metastable upon pressure release and to recover these high-pressure materials with alternate stoichiometry at ambient conditions for possible useful applications. A well-known elemental analogue is the mass production of synthetic diamonds under high-pressure conditions, with various practical applications such as machining and cutting tools. Although the stable form of carbon at ambient conditions is graphite, synthetic diamonds are metastable upon pressure release due to the large energy barrier of the sp3 to sp2 conversion. A similar method is used in the synthesis of cubic boron nitride (also metastable at ambient), where c-BN is produced by treating hexagonal boron nitride at high pressure and temperature.
Applying high pressure to metal (primarily Rare-Earth Elements) borides will uncover chemical reactions not possible at ambient conditions and open the boundaries of the metal-boron system doped with carbon, nitrogen, and hydrogen with other element of similar radii, and illuminate reaction mechanisms and behavior, leading to the creation of materials with the desired chemistry and properties.

Light elements such as boron, nitrogen, carbon and hydrogen are in the forefront of modern research, because many long-standing, fundamental questions related to their chemistry, crystal chemistry, bonding, polymorphism, and physical properties of their compounds remain to be resolved. Currently boron compounds are widely used as superhard materials, superconductors, dielectrics, B-doped semiconductors, and reinforcing chemical additives. Composites on the base of boron are characterized by extreme strength and low density, making them useful as filaments for advanced aerospace structures and in personal security (bullet-proof vests). As nitrogen has strong covalent bonding with boron, part of the project will be dedicated to the synthesis of ternary compounds with nitrogen.

The synthesis and investigation of metal borides under extreme conditions are of great interest for material science and technology, given the interesting properties of these compounds, namely superconductivity, low compressibility and high hardness, high melting points, good thermal, and chemical stability.

Main focus is on compounds that can be synthesized at low pressure and temperature aiming to achieve mass production of the material using thermodynamic conditions that can be reproduced (scaled up) by large volume presses.

Chemical laws

under extreme conditions

Substantial additional work is needed to elucidate the behavior of chemical compounds under extreme conditions, which extend our understanding of traditional chemistry. It has long been recognized that the valence electrons of an atom dominate the chemical properties, while the inner-shell electrons or outer empty orbital do not participate in chemical reactions. It is also well known that pressure, as a fundamental thermodynamic variable, plays an important role in the synthesis of new materials. Recent experiments, however, have used pressure to stabilize a series of unconventional stoichiometric compounds with new oxidation states, in which the inner-shell electrons or outer empty orbital become chemically active.

Therefore, there is a strong demand for the systematic investigation of chemical elements in the periodic table under pressure and the unusual chemical reactions these generate. Elucidating these reactions mechanisms, furthermore, will lay the foundation for establishing a basic theory of chemical reaction under high pressure.

Due to challenges in high pressure experiments—notably strong oxidizing or reducing agents (e.g., F and Li), which are harmful to experimental instruments—most studies have been carried out from the standpoint of theoretical calculations. More experimental research is thus needed. My lab accordingly focuses on performing systematic experiments on targeting elements groups both to obtain new “designed” materials with interesting and demanding properties, and to illuminate the chemical forces that drive the elements to become these materials through those phase transformations and chemical reactions. Low temperature can facilitate some of this research, as it allows for a full or partial freezing of the phase transformation and chemical reaction.