A15 Phases - History

History

The first time the A15 structure was observed in 1931 when an electrolytically deposited layer of tungsten was examined. The discussion if the β–tungsten structure is an allotrope of tungsten or a the structure of a tungsten suboxide was long-standing and in 1998 still articles about the discussion were published. In the end it seems most likely that the material is a true allotrope of tungsten.

The first inter-metallic compound with the typical A₃B composition was the chromium silicide Cr₃Si, discovered in 1933. Several other compounds of the A15 structure were discovered in the following years. No large interest existed in the research on those compounds. This changed with the discovery that Vanadium silicide V₃Si showed superconductivity at around 17 K in 1953. In the following years several other A₃B superconductors were found. Niobium-germanium held the record for the highest temperature of 23.2 K from 1971 till the discovery of the cuprate superconductors in 1986. It took time before the method to produce wires from the very brittle A15 phase materials was established. This method is still complicated. Though some A15 phase materials can withstand higher magnetic field intensity and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most of the applications due to the easier manufacturing. Nb₃Sn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers.

The Voronoi diagram of the A15 phase is known to have the least surface area among point sets in three-dimensional Euclidean space. This partition, also known as the Weaire-Phelan structure, is often present in clathrate hydrates.