Optic phonons and anisotropic thermal conductivity in hexagonal Ge₂Sb₂Te₅

The lattice thermal conductivity (κ) of hexagonal Ge₂Sb₂Te₅ (h-GST) is studied via direct first-principles calculations. We find significant intrinsic anisotropy (κ_a/κ_c~2) of κ in bulk h-GST, with the dominant contribution to κ from optic phonons, ~75%. This is extremely unusual as the acoustic phonon modes are the majority heat carriers in typical semiconductors and insulators. The anisotropy derives from varying bonding along different crystal directions, specifically from weak interlayer bonding along the c-axis, which gives anisotropic phonon dispersions. The phonon spectrum of h-GST has very dispersive optic branches with higher group velocities along the a-axis as compared to flat optic bands along the c-axis. The large optic mode contributions to the thermal conductivity in low-κ h-GST is unusual, and development of fundamental physical understanding of these contributions may be critical to better understanding of thermal conduction in other complex layered materials.