Chemical bonding in phase-change materials: the role of multi-centre hyperbonding
T-H. Lee1 , S.R. Elliott2,3*
1 Department of Engineering, University of Cambridge, UK
2 Department of Chemistry, University of Cambridge, UK
3 Physical and Theoretical Chemistry Laboratory, University of Oxford, UK
EXTENDED ABSTRACT: Phase-change random-access memory (PCRAM) is one of the most promising candidates for next-generation, non-volatile computer-memory technology. Binary information is stored as two metastable structural states of the memory material (typically tellurides, e.g. Ge2Sb2Te5 (GST)), which can be interchangeably switched, reversibly and ultra-rapidly (~ns), by the application of appropriate Joule-heating voltage pulses in 2-terminal memory-cell devices. One memory state is an electrically-resistive glassy phase (“0”), and the other is an electrically-conducting, metastable crystalline phase, with typically a defective rocksalt structure (“1”). However, in order to optimize the memory performance by ‘designing’ new, improved PCRAM materials (in terms of switching speed, resistivity contrast between phases, memory endurance etc), it is essential to understand the nature of the chemical bonding in these chalcogenide materials. In this work, we have carried out DFT-based molecular-dynamics simulations and electronic-structure calculations on a range of monovalent chalcogens and binary and ternary chalcogenides (including GST) in order to characterize, and understand the nature of the chemical bonding in these materials [1]. We find that in telluride glassy (g-) materials, normal two-centre/two-electron (2c/2e) covalent bonds coexist with weaker 3c/4e hyperbonds – a form of dative (coordinate) bond formed by the overlap of a Te p lone pair with an antibonding orbital associated with a neighbouring Ge/Sb-Te bond (in GST). Such hyperbonds are dominant in the octahedrally-coordinated crystalline (c-) phase, and are more polarizable than normal covalent bonds, thereby accounting for the property contrast between g- and c-phases of PCRAM materials.
REFERENCES
[1] T-H. Lee and S.R. Elliott, Adv. Mat. 32, 2000340 (2020); Phys. Stat. Sol. – RRL. 15, 2000516 (2021).
Prof. Stephen Elliott is an Emeritus Professor at Cambridge and a Visiting Professor at Oxford. He has published 416 papers on disordered/glassy materials, in Nature, Science, Nature Materials, PNAS, Advanced Materials, Angew. Chemie, PRL etc, and is the author of 3 books.