A new polymorph of lithium bromide is predicted to be metastable — and is made by a systematic exploration of synthesis conditions
Throwing chemicals in a crucible, heating them, and seeing what comes out is all very well, but there are more rational and subtle approaches to solid-state synthesis that might uncover new compounds. Many inorganic compounds can exist in a variety of structural forms — polymorphs — but some of them are extremely difficult to make because they are only metastable under accessible conditions. Now, Martin Jansen and colleagues at the Max Planck Institute for Solid State Research in Stuttgart have used1 an 'energy landscape concept' to predict the stability of a new polymorph of lithium bromide, which they went on to make.
They predicted that LiBr would be more stable in the wurtzite structure rather than in other forms such as that of caesium chloride or rock salt. To make this metastable phase, however, they had to use a low-temperature deposition method. In the more commonly used techniques such as molecular beam epitaxy or layer-by-layer deposition the substrate is heated, but Jansen's method uses a cooled substrate. This enables the starting material to be evenly distributed and mixed on an atomic level. The crucial consequence of this method is that the atoms are close together, enabling metastable phases to form at low temperatures.
The team synthesised bulk thick-layer samples of wurtzite LiBr, confirming their predictions that the compound can exist in this metastable form. Exploring the computational energy landscape could offer new and exotic targets for solid-state chemists to make.
Liebold-Ribeiro, Y., Fischer, D. & Jansen, M. Experimental substantiation of the “energy landscape concept” for solids: synthesis of a new modification of LiBr. Angew. Chem. Int. Ed. 10.1002/anie.200800333 (2008).
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Withers, N. Keeping your cool. Nature Chem (2008). https://doi.org/10.1038/nchem.12