For the HSE06 calculations, the semi-core Ge 3d electrons were subsumed into the pseudopotential cores, which we found had very little impact on the electronic-structure calculations but significantly reduced the computational cost of the hybrid calculations. Whereas the k k k-point meshes used for the geometry optimisations were sufficient to converge the E g and ε ε ε ∞ of the Pnma phases, explicit testing indicated that denser sampling was required in calculations on the R3m phases. For these systems, we therefore employed the non-self-consistent approach documented in previous work, after verifying that the E g and ε ε ε ∞ obtained using both methods and the "base" k k k-point meshes gave quantitatively similar results. Convergence tests and a comparison of the self-consistent and non-self-consistent HSE06 calculations can be found in the ESI. 3 Results and discussion