Finally, as for the electrical properties the ZT is anisotropic (Section S2.5, SI). For n-type Bi 2 SO 2 and Bi 2 SeO 2 , the largest and smallest ZT are predicted to occur respectively along the a = b and c directions, but the ZT along the two independent directions are similar and close to the average due to a balance of a larger σ along the a/b direction and a smaller κ latt along the c direction. For n-type Bi 2 TeO 2 , on the other hand, the combination of a reasonable σ and a much lower κ latt along the c direction, together with a smaller anisotropy in the Seebeck coefficient, results in a much higher ZT > 2.5 at the intrinsic n ≈ 10 18 cm -3 and high T . This suggests that Bi 2 TeO 2 could achieve a still larger ZT max if the growth orientation could be controlled. With p-type doping, on the other hand, in all three systems the much larger electrical conductivity more than offsets the larger κ latt along the a/b direction, resulting in ZT up to 20× larger than along the c axis. In this case, the tetragonal symmetry means that, despite the large anisotropy, these values remain similar to the averages. (We note that in this case we calculate the average ZT from the averaged S, σ and κ el /κ latt and not by averaging the ZT along the three crystal axes, because this is more representative of how the power factor and ZT are calculated from experimental measurements.)