Slices of the iso-energy surfaces of (a) ZA and (b) ZO ′ phonon in A–Γ–M plane. The plane is divided into 6 sections by dash dotted lines. according to angle θ ( q , z ) between wave vector and Γ–A direction. Group velocities denoted by arrows are divided into 6 groups identified by different colors with legend shown below the Figure according to angle θ ( v , z ) between velocity direction and Γ–A direction.