Face-centered-cubic K3B80 and Mg3B80 metals: Covalent and ionic bondings

By means of first-principles calculations within the density-functional theory, we find that stable face-centered- cubic (fcc) K3B80 and Mg3B80 solids can be formed. For both solids, two possibly stable geometrical phases (identified as phase A and phase B) with different lattice parameters can exist, where phase A has a lattice parameter smaller than phase B. In phase A, B-80 clusters are significantly distorted and two or four intercluster covalent bonds are formed for K3B80 or Mg3B80, respectively. In phase B, B-80 units are slightly distorted and no intercluster covalent bonds exist. The phase A of Mg3B80 bears the largest cohesive energy among them and is more stable than the fcc B-80 solid. The charge population analysis shows that K and Mg are ionized and donate electrons to the other boron atoms of K3B80 and Mg3B80 solids. The different ionic radii of K and Mg lead to major geometrical differences between K3B80 and Mg3B80 solids and the competition of the covalent and ionic bondings could explain the emergence of two different geometrical phases for both. The electronic structural calculations reveal that both fcc K3B80 and Mg3B80 solids are metals.