Phonon and κ latt calculations were performed using the Phonopy and Phono3py codes. The second-order (harmonic) force constants were obtained using a displacement step of 10 -2 Å and 2 × 2 × 2 supercell expansions with 512 atoms. The third-order (anharmonic) force constants were computed using a step of 3 × 10 -2 Å and the 64-atom unit cells. Phonon dispersion curves were obtained by interpolating the frequencies along q-point paths between the high-symmetry points in the P2 1 3 Brillouin zone. Atom-projected density of states (PDoS) curves were obtained from frequencies evaluated on uniform Γ-centered q-point grids with 24 × 24 × 24 subdivisions. The "particle-like" contribution to the κ latt was determined by solving the phonon Boltzmann transport equation (BTE) within the single-mode relaxation-time approximation (RTA), using modal properties computed on 10 × 10 × 10 Γ-centered q-point grids. Testing on smaller 8 × 8 × 8 meshes confirmed the RTA and full linarised BTE solution are quantitatively similar (Fig. , ESI † ). The "wave-like" intraband tunnelling contribution to the κ latt was additionally obtained by solving the Wigner transport equation. The S, σ and κ el were determined by solving the electron BTE within the momentum relaxation-time approximation (MRTA) as implemented in the AMSET code. Accurate electronic bandgaps, E g , and uniform band structures were obtained using non-self-consistent HSE06. Energy-and momentumdependent electron relaxation times were estimated from the rates of acoustic deformation potential (ADP), ionized impurity (IMP), piezoelectric (PIE), and polar optical phonon (POP) scattering. The deformation potentials were determined from single-point calculations on deformed structures generated by AMSET. The elastic constant matrices C, high-frequency/static dielectric constants ε ∞ /ε S , Born effective-charge tensors Z * and piezoelectric tensors e were obtained using the finite differences and density-functional perturbation theory (DFPT) approaches in VASP. All properties were computed with PBEsol+D3, apart from the ε ∞ , which were computed using nonself-consistent HSE06. The POP frequencies ω po were calculated by combining the phonon frequencies at q = Γ from Phonopy with the Z * from DFPT. k-point convergence tests and validation of non-self-consistent HSE06 are provided in Table . † Additional details of the calculations and analysis performed in this work, including key equations, are provided in the ESI. †