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Boron: a Hunt for Superhard Polymorphs

Boron is a unique element, being the only element, all known polymorphs of which are superhard, and all of its crystal structures are distinct from any other element. The electron-deficient bonding in boron explains its remarkable sensitivity to even small concentrations of impurity atoms and allows boron to form peculiar chemical compounds with very different elements. These complications made the study of boron a great challenge, creating also a unique and instructive chapter in the history of science. Strange though it may sound, the discovery of boron in 1808 was ambiguous, with pure boron polymorphs established only starting from the 1950s-1970s, and only in 2007 was the stable phase at ambient conditions determined. The history of boron research from its discovery to the latest discoveries pertaining to the phase diagram of this element, the structure and stability of beta-boron, and establishment of a new high-pressure polymorph, gamma-boron, is reviewed.