Title：Phase sequence in metal dioxides as an evolution of metal-centered polyhedrons under pressure

Speaker：Shengbai Zhang，Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute

Time：

May 28, 2019 10:00

Venue：333 Lecture Hall, Physics Building

Abstract：

A crystal is often described as a stacking of a repetitive unit, i.e., the unit cell, with a specific symmetry. While such a description is tremendously successful in describing the physical properties of a crystal, unfortunately, it is often powerless in revealing the relationship between compressed phases of different symmetries. Taking the family of metal dioxides (MO2) as an example, the structural evolution, subject to a fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here, I will present an alternative way to treat the compressed crystals as a stacking of coordination polyhedrons from which a unified structural transition pattern emerges. Our combined X-ray diffraction (XRD) experiments and first-principles calculations reveal that the increase of the coordination number happens often only at one of the apexes of the polyhedron in an orderly fashion, leaving the basal plane and other apexes topologically intact. The chemistry of MO2 evolves accordingly by giving away its ionicity in exchange for covalency under pressure, in line with the increased polyhedron sharing.

[1] S.-Y. Xie, et al., J. Phys. Chem. Lett. 2018, 9, 2388-2393.

Brief Bio:

Prof. Zhang’s research involves first-principles calculations, theory, and modeling of structural and electronic properties of condensed matter by using the petaflop computational facilities at RPI’s Center for Computational Innovations (CCI). Research directions are largely clean-energy and electronic-applications driven. In particular, he pioneered first-principles theory of defects in semiconductors in the late eighties and since then he has studied numerous defects, especially those in photovoltaic semiconductors. He pioneered first-principles theory of unconventional coordination chemistry of molecular hydrogen on open metal sites as a possible means to store hydrogen on board. He also has a longtime exposure to the physics and chemistry of semiconductor nanostructures and surfaces. In recent years, He studies water splitting, ultrafast phase change memory, topological insulators, Weyl semimetals, nano-catalysis, two-dimensional materials, chalcogenide perovskites, monolayer-thick high-temperature superconductor films, and real-time carrier dynamics. He has published 350 peer-reviewed papers in journals including 58 Physical Review Letters, with over 27,900 total citations (Hirsch Index = 77) by Google Scholar's count.