The κ/τ CRTA of the oxychalcogenides are larger than for the Bi 2 Ch 3 and SnS/SnSe. In addition to the chemical bonding and atomic masses, the harmonic function also depends sensitively on the number of atoms n a in the primitive unit cell and the crystal symmetry (spacegroup). The I4/mmm oxychalcogenide structure has the same n a = 5 as the R 3m structure adopted by Bi 2 Se 3 and Bi 2 Te 3 and is lower symmetry, yet has 4 -5× larger κ/τ CRTA . We therefore attribute the larger group velocities in the oxychalcogenides primarily to the lighter O atoms and stronger chemical bonding.