Two good metals make a semiconductor: A potassium-nickel compound under pressure

We predict a potassium-nickel intermetallic compound ${\mathrm{K}}_{2}\mathrm{Ni}$ at high pressure and identify it as the long-sought structure of the only known K-Ni compound to date [Parker et al., Science 273, 95 (1996)]. Although both constituent elements are metallic, ${\mathrm{K}}_{2}\mathrm{Ni}$ exhibits a semiconducting ground state with an indirect band gap of 0.65 eV. Electron instability due to the degeneracy at the Fermi level arises from the particular motif of the structure, which in turn induces symmetry-breaking Peierls distortion and a nonmetallic ground state. The results indicate that the chemical properties of elements can change dramatically under extreme conditions and have significant implications for the postulation that potassium is incorporated in Earth's core.