Qing-Bo Yan, Qing-Rong Zheng, and Gang Su *
College of Physical Sciences, Graduate University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
(Received 28 October 2008; revised manuscript received 16 August 2009; published 23 September 2009)

Abstract:

By means of first-principles calculations within the density-functional theory, we find that stable face-centered-cubic (fcc) K ₃ B ₈₀ and Mg ₃ B ₈₀ 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    K ₃ B ₈₀ or Mg ₃ B ₈₀ , respectively. In phase B, B ₈₀ units are slightly distorted and no intercluster covalent bonds exist. The phase A of Mg ₃ B ₈₀ 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 K ₃ B ₈₀ and Mg ₃ B ₈₀ solids. The different ionic radii of K and Mg lead to major geometrical differences between K ₃ B ₈₀ and Mg ₃ B ₈₀ 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 K ₃ B ₈₀ and Mg ₃ B ₈₀ solids are metals.