报告题目 (Title)：A new symmetry-mode based descriptor of chirality. Applications to chiral organic-inorganic hybrid metal-halides
报告人 (Speaker)：Prof. Branton J. Campbell（Brigham Young University, USA）
报告时间 (Time)：2024年6月20日（周四）19:00-21:00
报告地点 (Place)：校本部 D413
邀请人 (Inviter)：任伟 教授
主办部门：理学院物理系
报告摘要：
A structure is said to be chiral when it has no improper symmetry operations (e.g. mirror planes and inversion centers). Chiral hybrid organic-inorganic perovskites (HOIP) have been reported to possess a variety of exciting electronic, magnetic, optical, and electro-magneto-optical properties, which appear to be the result of chiral framework distortions. The desire to engineer and optimize these properties motivates us to understand how the chiral organic molecule induces chiral distortions of the inorganic framework. Here, we present the use of group-theoretic symmetry-mode analysis to identify and classify all types of chiral distortions of a given layered-HOIP framework. As an example, the single-layered chiral HOIP compound, (MPA)2CuCl4, proves to be incapable of any distortions belonging to its chiral irrep. Remarkably, it exhibits a hybrid-improper chiral distortion, which can be quantified as the pseudo-scalar product ξ=p⋅r of an in-plane ferroelectric moment p and an octahedral rotational moment r. By treating the existence of the chiral molecule as an order parameter θ of the chiral irrep, we obtain an invariant trilinear coupling term of the form θpr in the free energy, which correctly predicts that reversing the chirality of the MPA molecule causes the ferroelectric moment to switch when the sense of the octahedral rotation is fixed.
报告人简介：
Professor Branton Campbell earned BS degrees (1993) from Brigham Young University (BYU) in both Physics and Mathematics. At the University of California at Santa Barbara, he received an M.A. in Physics (1995) and a PhD in Materials Science (1999) with Sir Anthony K. Cheetham, exploring structure-property relationships in zeolites and structure-determination from X-ray and neutron powder diffraction. As a post-doctoral researcher (1999-2002) in the Materials Science Division at Argonne National Laboratory, he became interested in single-crystal X-ray diffuse scattering methods for characterizing local structure-property relationships in a wide variety of functional materials. Since joining the Physics & Astronomy faculty at Brigham Young University (2002-present), he has collaborated with Harold Stokes to develop a wealth of crystallographic infrastructure for employing group representation theory (GRT) to characterize solid-state phase transitions, which is publically available through the ISOTROPY Software Suite. These tools treat both magnetic and non-magnetic structures at both commensurate and incommensurate wavevectors. He spent one-year (2014-2015) as a Fulbright Scholar working in the Department of Chemistry Durham University in the UK, where he and John S. O. Evans developed GRT methods for crystallographic tructure determination. As the Chair of the IUCr Commission on Magnetic Structures (2011-2023), he led the development of the magnetic CIF standard for communicating magnetic structures, which has now been widely adopted. Other active research interests include topological domain-intersection defects in crystals, cooperative rigid-unit modes in flexible frameworks, and chiral order in hybrid organic-inorganic perovskites. He served as member and Chair of the US National Committee on Crystallography for many years (2013-2023) and co-organized the 2019 International School of Crystallography on Magnetic Crystallography in Erice, Italy.