John Carlos Baez on Nostr: Here's a longer version: Blue bronze is molybdenum oxide K₀.₃MoO₃ — a ...

Here's a longer version:

Blue bronze is molybdenum oxide K₀.₃MoO₃ — a deep-blue, metallic-looking crystal in the family of alkali molybdenum bronzes first prepared by Wold, Kunnmann, Arnott and Ferretti in 1964. As far as we can tell, it doesn't exist in nature. The name "bronze" is jargon inherited from the sodium tungsten bronzes Wöhler made back in 1825. It refers to the brassy luster of these compounds, not to the copper-tin alloy.

Blue bronze is built from parallel chains of molybdenum and oxygen atoms threading through the crystal, with potassium ions tucked between the layers. The electrons free to carry current are essentially trapped on these chains, like cars on a one-lane highways with no exits. This makes blue bronze a textbook example of a quasi-one-dimensional metal.

This is the setup for a remarkable result Rudolf Peierls established in the 1930s: a one-dimensional metal cannot stay metallic in its ground state! Instead, its atoms spontaneously bunch themselves into pairs, raising a barrier that stops electrons from flowing freely and turning the metal into an insulator. Simultaneously, there is a slight periodic rippling in the density of conduction electrons — a charge density wave. But the wavelength of the charge density wave can be an irrational multiple of the spacing between atom pairs! And in blue bronze, this does happen. It's thus an example of an 'aperiodic crystal'.

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