The Mott Lecture is a traditional plenary speech at the ICANS, in the honor of Sir Nevill Mott, winner of the Nobel Physics prize in 1977. Sir Nevil Mott was one of the pioneers of the ICANS. 

This year's Mott Lecture will be given to Prof. Stephen Elliott. The title of his lecture will be "Coordination Defects in Chalcogenide Glasses: 50 Years on".

Prof. Stephen Elliott

    Stephen Elliott has been a Visiting Professor in the Physical and Theoretical Chemistry Laboratory at Oxford since January 2021. Prior to that, he was Professor of Chemical Physics in the Department of Chemistry at the University of Cambridge until he retired in 2019, having been, in turn, a Demonstrator (Assistant Lecturer), Lecturer and Reader there since 1979. He is now a Life Fellow at Trinity College, Cambridge, having previously been a Professorial Fellow until retiring in 2020, and previously a Prize (Research) Fellow, and then a Teaching Fellow in Physics and Chemistry since 1977. In addition, he was Professor of Physics at the Ecole Polytechnique, Palaiseau, France, from 1998 to 2000. He was an undergraduate at Trinity College, Cambridge, and obtained a Bachelor degree in Theoretical Physics from the University of Cambridge, followed by a PhD on theoretical and experimental studies of amorphous solids at the Cavendish Laboratory, Cambridge. His work has been recognised by a number of awards and prizes, including a Nuffield Foundation Science Research Fellowship in 1991, the 1992 W.H. Zachariasen Prize, the 2001 Stanford R. Ovshinsky (inaugural) Award, the Chancellor’s Medal, University of Pardubice, Czech Republic, awarded in 2012, the 2014 George W. Morey Award of the American Ceramic Society, the 2017 Royal Society of Chemistry John B. Goodenough Award, a Kavli-Winton Fellowship, University of Berkeley, USA (2017), Doctor of Science (ScD), University of Cambridge (2024), and the 2025 Varshneya Frontiers of Glass Science Lecture (American Ceramic Society).

Coordination Defects in Chalcogenide Glasses: 50 Years on

    Some 50 years ago, Bob Street and Nevill Mott (SM) [1], then Mott, together with Ted Davis and Street (MDS) [2], and, independently, Marc Kastner, Dave Adler and Hellmut Fritzsche (KAF) [3], proposed models for point defects in (S, Se-based) chalcogenide glasses which explained why they are electron-spin-paired and diamagnetic (i.e. electron-spin-resonance inactive), in accord with experiment, rather than having unpaired spins and being paramagnetic, as might be expected for the homolytic breaking of covalent bonds. It was proposed that the repulsive Hubbard Coulomb energy incurred in placing an additional electron on a paramagnetic centre, to make it negatively charged and diamagnetic, was outweighed by a large lattice relaxation associated with the formation of the complementary positively charged centre, following Phil Anderson's suggestion of the notion of a negative effective Hubbard energy [4]. KAF [3] clarified this general picture in terms of specific bonding interactions, involving the formation of under- and over-coordinated defects ('valence-alternation pairs', VAPs).

    In this talk, I will discuss three origins for the formation of over-coordinated defects in chalcogenide glasses: i) dative bonding, involving the overlap of a p lone pair (LP) on one atom with an empty bonding orbital on another (as in the original KAF model [3]); ii) sp3 hybridization; and iii) 'hyperbond' formation, i.e. the overlap of a p-LP on one atom with an empty antibonding orbital on another. It will be shown that the latter novel mechanism can account for the formation of the unusual over-coordinated As4- defect found in molecular-dynamics (MD) simulations of glassy As2S3 [5], and the neutral Se4-Se3 defect complex recently found in MD simulations of glassy Se under high electric fields, in connection with its use as an Ovshinsky Threshold Switch (OTS) [6]. A discussion will also be given of the nature of electronic defects in phase-change-memory glassy tellurides (e.g. Ge2Sb2Te5), which consist of clusters of over-coordinated atoms, not point defects [7].

[1] RA Street and NF Mott, Phys. Rev. Lett. 35, 1293 (1975)

[2] NF Mott, EA Davis and RA Street, Phil. Mag. 32, 961 (1975)

[3] M Kastner, D Adler and H Fritzsche, Phys. Rev. Lett. 37, 1504 (1976)

[4] PW Anderson, Phys. Rev. Lett. 34, 953 (1975)

[5] SI Simdyankin, TA Niehaus, G Natarajan, T Frauenheim and SR Elliott, Phys. Rev. Lett. 94, 086401 (2005)

[6] Y Sun, T Gotoh, J Zhao, M Zhang, S Shi, H Zhang, Z Liu, J Shen, R Dronskowski, Z Song, SR Elliott and M Zhu, Nat. Mat. To be published

[7] K Konstantinou, FC Mocanu, T-H Lee and SR Elliott, Nat. Comm. 10, 3065 (2019)

History of Mott Lecture

Mott Lecture (20:40-21:20, 23rd Aug，Wuhan Lecture Hall (武汉报告厅))

Research Home Page: https://www.chem.ox.ac.uk/people/stephen-elliott

Abstract for Presentation

Biography