Title：The change of basic chemical behavior of elements under high pressure
Speaker：Maosheng MIAO, Assistant Professor
Department of Chemistry and Biochemistry, California State University Northridge
Beijing Computational Science Research Center, Beijing, China
Time：July 9, 2019 (Tuesday) at 9:30 am
Venue：333 Lecture Hall, Physics Building
Abstract：
The chemistry at ambient condition has implicit boundaries rooted in the atomic shell structure: the inner-shell electrons and the unoccupied outer-shell orbitals do not involve as major component in chemical reactions and in chemical bonds. The chemical properties of atoms are determined by the electrons in the outermost shell; hence, these electrons are called valence electrons. These general rules govern our understanding of chemical structures and reactions.
Using first principles calculations, we demonstrate that under high pressure, the above doctrines can be broken. We show that both the inner shell electrons and the outer shell empty orbitals of Cs and other elements can involve in chemical reactions. In the presence of fluorine and under pressure, the formation of CsFn (n > 1) compounds containing neutral or ionic molecules is predicted. Their geometry and bonding resemble that of isoelectronic XeFn molecules, showing a cesium atom that behaves chemically like a p-block element under these conditions. Furthermore, we find that under high pressure Hg in Hg-F compounds transfers charge from the d orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to + 4 and + 3 yielded the thermodynamically stable compounds HgF4 and HgF3. The former consists of HgF4 planar molecules. HgF3 is metallic and ferromagnetic, with a large gap between its partially occupied and unoccupied bands under high pressure.
In other works, we find that Xe, Kr, and Ar can form thermodynamically stable compounds with Mg at high. The resulting compounds are metallic and the noble gas atoms are negatively charged, suggesting that chemical species with a completely filled shell can gain electrons, filling their outermost shell(s). Similarly, we predicted that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond -1.
Furthermore, we show that the quantized orbitals of the enclosed interstitial space may play the same role as atomic orbitals, an unprecedented view that led us to a unified theory for the recently observed high-pressure electrode phenomenon. In the last example for high-pressure chemistry, we demonstrate that He can form stable compounds with ionic crystals. The driving force for these reactions is not the local chemical bonds but rather the alternation of the long-range Coulomb interactions among ions while incorporating He atoms in the lattice.
Brief Bio:
Dr. Maosheng Miao obtained his Ph.D. in physical chemistry at Jilin University in China. He has more than 20 years working experience and background in first principles calculations of the atomic and electronic structures of materials and solid-state chemistry. He has worked in many areas of computational materials science, ranging from semiconductor defects, surfaces and interfaces, functional oxides, solid-state lighting materials, two-dimensional materials, high pressure physics and chemistry, to method developments, including structure search methods and local pseudopotentials. He has published more than 100 papers in top journals and has an H-index of 32. His discovery of new chemical bonds formed by Cs 5p electrons was reported by Scientific American and highlighted by Nature Chemistry and Nature China. His work provided a unified mechanism for the striking phenomenon of high-pressure electrides and was highlighted by JACS and Nature Chemistry. His work on Hg-F compounds in which Hg behaves as a transition metal was assessed as top 5% by Angewandte Chemie and was selected as a back cover. His work proposing a method to improve the performance of high electron mobility transistors (HEMT) was reported by Compound Semiconductor.
