XN (X= Bi, and Sb): Atom intercalation influence on transport anisotropy*

The electronic structure and lattice thermal conductivity (κ) of hexagonal anti-perovskite Ba₃XN compounds are studied by employing first-principles density functional theory and Boltzmann transport calculations. For Ba₃N we find significant intrinsic anisotropy (κ_c/κ_a ~ 7.5) in thermal conductivity, with the cross-plane direction having the higher conductivity. Interestingly, introducing Bi/Sb atoms (Ba₃SbN/Ba₃BiN) has little effect on the in-plane thermal conductivity (κ_a), however κ_c decreases significantly, reducing the anisotropy κ_c/κ_a ~ 3.5. The anisotropy in κ for these systems originates from different bonding along the crystallographic directions, specifically from relative weak bonding between the layers in the hexagonal plane, which leads to anisotropic phonon group velocities. These results will be discussed in terms of structural bonding, group velocities and phonon life times. In particular the unusual behavior of thermal conductivity for Bi/Sb compounds will be discussed.