As for the electrical properties, the κ latt is anisotopic and larger along the a = b direction than the c direction (Figs S19-S21, Section S2.4 of the SI). For Bi 2 SO 2 and Bi 2 SeO 2 the anisotropy is relatively small, with the in-layer κ latt being ∼ 50% larger than the cross-layer transport. For Bi 2 TeO 2 , on the other hand, the anisotropy is much larger and the κ latt along the a = b direction is around 4× larger than along the c direction. With reference to the anisotropy in the electrical properties discussed in the previous section, the a = b direction therefore corresponds to the "easy axis" for both electrical and thermal transport in all three systems.