Supercell finite-displacement phonon calculations were performed using the Phonopy package, and the lattice thermal conductivities were computed using the Phono3py code. The second-order (harmonic) force constants were obtained using a displacement step of 10 -2 Å with 4 × 2 × 4, 4 × 4 × 2 and 3 × 3 × 3 supercell expansions of the Pnma, R3m and rocksalt conventional cells, respectively, containing 256, 192 and 216 atoms. The thirdorder (anharmonic) force constants for the Pnma and R3m structures were obtained using a displacement step of 3 × 10 -2 Å with 2 × 1 × 2 and 3 × 3 × 1 supercells containing 32 and 54 atoms. (As explained below, we did not attempt thermal-conductivity calculations on the rocksalt structures due to the presence of imaginary modes in the phonon spectra.) During post-processing, transformation matrices were applied to the R3m and rocksalt structures to transform to the primitive cells.