Discovering new crystalline forms of metal oxides


Matthew J. Lyle, Chris J. Pickard, and Richard J. Needs

High performance computing is an essential part of many areas of scientific research. Materials discovery, in particular, is especially suited to computational simulation. Here we use an ab-initio random structure searching (AIRSS) algorithm to generate thousands of possible atomic arrangements for our system of choice. Constraints such as symmetry and stoichiometry can be incorporated to help our search towards particular structures we may be after. These candidate structures are energetically ranked, usually using density functional theory (DFT), and low-energy or other interesting structures are further investigated.

My projects have focussed on finding new crystalline forms of metal oxides, including alumina (Al2O3) and titania (TiO2). Alumina exists in a number of distinct crystalline forms that are widely used in industry; annual global production is 100 Mtonne, valued at approximately U.S.$32 billion. Conventional crystalline aluminas exhibit negligible porosity, though many of their industrial applications require high porosities and surface areas. This is usually achieved through the fashioning of mesoporous channels, though often at the expense of crystallinity which is known to increase alumina stability. Moreover, crystalline microporous silicates and aluminosilicates, e.g. zeolites, have long been used as adsorbents, ion exchangers, and catalysts.


In this project we performed an extensive first-principles computational search for new crystalline forms of alumina that are stable at ambient pressure. We found 147 unique structures with energies intermediate between those of the conventional alumina phases and identified a new class of crystalline aluminas which are up to 43% less dense. We attribute these low densities to significant amounts (up to 100%) of fivefold Al coordination and the formation of highly microporous zeolite-like channels. These are the first crystalline aluminas exhibiting such extensive fivefold Al coordination and surface areas, suggesting a new paradigm for the processing of alumina for industrial use.


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