The contribution of phonons to the thermal expansion of some simple cubic hexaboride structures: SmB 6 , CaB 6 , SrB 6 and BaB 6 .

We have performed first-principles calculations of the structure and lattice dynamics in the metal hexaborides SmB ₆ , CaB ₆ , SrB ₆ and BaB ₆ using Density Functional Theory in an attempt to understand the negative thermal expansion in the first of these materials. The focus is on the role of Rigid Unit Modes involving rotations of the B ₆ octahedra similar to the rotations of structural polyhedra connected by bonds in Zn(CN) ₂ , Prussian Blue and Si(NCN) ₂ . However, it was found that there is very low flexibility of the network of connected B ₆ octahedra, and the lattice dynamics do not support negative thermal expansion except possibly at very low temperature. Thus the negative thermal expansion observed in SmB ₆ probably has an electronic origin.