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Complex materials: A journey from disappearing ice phases and ‘pink’ phosphorus to stacking disordered silver iodide

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The research activities of our group focus on the structural characterisation of complex materials including disordered crystals, amorphous, glassy and nanomaterials as well as liquids. Our keen ambition is to establish links between the atomic structure of materials, and their chemical and physical properties and performances. Building on this, we also investigate complex phase transitions, crystallisation phenomena, chemical reactions in confinements as well as the chemical functionalisation of nanomaterials including carbon nanotubes and graphene. In this talk, I will present an overview over our most recent work including the following topics: (1) Doping-induced disappearance of ice II from the phase diagram. Ammonium fluoride acts as a ‘magic ingredient’ that enables us to let ice II disappear from the phase diagram in a highly selective fashion.[1] A detailed understanding of the underlying mechanisms and thermodynamics is presented, and we argue that our new finding has wider implications that enables us to understand some of the anomalies of water’s phase diagram including the anomalous properties of liquid water. The selective disappearance of a particular phase with the aid of a dopant highlights the exciting possibility of potentially discovering new phases of ice but also other materials in the future using specific impurities. (2) New 1D allotropes of phosphorus and arsenic. The pnictogen nanomaterials, including phosphorene and arsenene, display remarkable electronic and chemical properties. Yet, the structural diversity that these main group elements are capable of is still poorly explored. We filled single-wall carbon nanotubes with elemental phosphorus and arsenic from the vapour phase.[2,3] Using electron microscopy, chains of highly reactive P4 and As4 molecules were found as well as new one-dimensional allotropes: a single-stranded zig-zag chain and a double-stranded zig-zag ladder. These linear structures represent important intermediates between the gas-phase clusters of the pnictogens and the extended 2D sheets of phosphorene and arsenene. Remarkably, band-gap calculations predict that the insulating P4 and As4 chains become semiconducting, once converted to the zig-zag ladder, and form fully metallic allotropes in the form of the zig-zag chain. (3) Stacking disorder everywhere! Stacking-disordered materials consist of structurally well-defined layers that are stacked on top of one another in a disordered fashion. Naturally, stacking disorder is found for a wide range of layered materials such as graphite or molybdenum sulphide. Using our MCDIF FaX program, we can model the diffuse diffraction features that arise from stacking disorder and obtain quantitative insights into the extents of different types of stacking as well as memory effects within the stacking sequences. Following extensive work on stacking disorder in ice and diamond, we have now identified a first system, silver iodide, where the stacking disorder can be controlled in a quantitative fashion.[4] This now offers the fascinating prospect of being able to fine-tuning the physical and chemical properties of a material between the extreme polytypic endmembers.

[1] J.J. Shephard, B. Slater, P. Harvey, M. Hart, C.L. Bull, S.T. Bramwell, C.G. Salzmann, Nat. Phys., 14 (2018) 569–572 [2] M. Hart, E.R. White, J. Chen, C.M. McGilvery, C.J. Pickard, A. Michaelides, A. Sella, M.S.P. Shaffer, C.G. Salzmann, Angew. Chem. Int. Ed. 56 (2017) 8144-8148 [3] M. Hart, J. Chen, A. Michaelides, A. Sella, M.S.P. Shaffer, C.G. Salzmann, Angew. Chem. Int. Ed. 57 (2018) 11649-11653 [4] R.L. Smith, M. Vickers, M. Rosillo-Lopez, C.G. Salzmann, Cryst. Growth Des. 19 (2019) 2131-2138

This talk is part of the Materials Chemistry Research Interest Group series.

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