Electronic structures, mechanical and thermodynamic properties of cubic alkaline-earth hexaborides from first principles calculations

The electronic structures, mechanical and thermodynamic properties of alkaline-earth hexaborides MB6 (M = Ca, Sr or Ba) are calculated from first principles using density functional theory combined with the quasi-harmonic approximation. These three alkaline-earth hexaborides are semiconductors with a slightly increased trend for their band gaps as M orders from Ca to Ba. Their band gaps depend sensitively on the values of lattice constant a and internal parameter z. The polycrystalline values of the elastic constants and bulk, shear and Young's moduli are consistent with those determined experimentally. All alkaline- earth hexaborides have strongly anisotropic elastic properties in the order of CaB6 > SrB6 > BaB6. By using the phonon calculations, the thermodynamic properties are investigated. The obtained phonon dispersion relations for CaB6, SrB6, and BaB6 show similar features and there are LO/TO splitting lines in the range of 5-10 THz. Finally, the thermal conductivities of CaB6, SrB6 and BaB6 are evaluated via Clarke's model and Cahill's model. (C) 2015 Elsevier B.V. All rights reserved.