Gallium: out of the box

What does it look like:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

This is gallium, one of the few elements that doesn’t have a cubic or hexagonal structure at room temperature. Gallium is orthorhombic: the cell angles are all 90o, but the a,b and c axes are different lengths. It turns out that a and c are very close, but this doesn’t make gallium “almost tetragonal”, because it’s missing the required symmetry. Gallium has the space group C m c a, possibly the only one of the 230 space groups to have almost had a Village People song written about it.

All elements are interesting in their own way, and gallium doesn’t disappoint. It has a melting point near 30oC, so it will sit on the lab bench but melt in your hand. It’s heavily used in the semiconductor industry (mainly in the form of GaAs, gallium arsenide). And it has the bizarre feature of being able to penetrate metals such as steel along grain boundaries, making them brittle. Surely that’s got to be the making of a plot twist in a TV crime show.

Where does the structure come from?

This structure of gallium is #9008085 in the Crystallography Open Database

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