Niobium selenide crystals are usually synthesized by heating niobium and selenium in a sealed quartz tube until long whiskers or ribbons form. Satoshi Tanda and co-workers at Hokkaido University in Sapporo have modified this crystal-growth process by creating a large temperature gradient across the tube, which allows the selenium to exist as both liquid droplets and a gas.

The Hokkaido team found three different crystal forms – rings, Möbius strips and figures-of-eight. As the ribbon-shaped crystals grow, they wrap around the selenium droplets, due to the surface tension of the liquid, until they eventually form a perfect seamless ring. Möbius strips are more difficult to produce because of the half-twist involved.

However, the low symmetry of the monoclinic niobium selenide crystals encourages the ribbons to both twist and bend, while rotating droplets may also help. Meanwhile, the figure-of-eight structures form if the ribbons circle the droplet twice. Tanda and co-workers are extending the technique to other materials, including tantalum selenide and tantalum sulphide.