bitals were modelled using plane-wave basis sets with kineticenergy cutoffs of 400-500 eV (Table ). The electronic Brillouin zones were integrated using the Γ-centered Monkhorst-Pack k k kpoint meshes shown in Table . Both parameters were chosen by explicit testing to converge the total energies to < 1 meV atom -1 and the external pressure to < 1 kbar (0.1 GPa). Projector augmented-wave (PAW) pseudopotentials were used to model the ion cores, with valence configurations of: Se -4s 2 4p 4 ; Te -5s 2 5p 4 ; Ge -3d 10 4s 2 4p 2 ; and Sn -4d 10 5s 2 5p 2 . Each of the structures were fully optimised to tolerances of 10 -8 eV and 10 -2 eV Å -1 on the electronic total energy and atomic forces respectively. The PAW projections were performed in real space, and the precision of the charge density grids was set automatically to avoid aliasing errors. Lattice-dynamics and thermal-conductivity calculations were set up and post-processed using the Phonopy and Phono3py codes. The second and third-order force constants Φ Φ Φ /Φ Φ Φ were determined using the supercell finite-difference approach with the supercell expansions and reduced k k k-point sampling listed in Table and displacement step sizes of 10 -2 and 3 × 10 -2 Å respectively. Phonon density of states (DoS) and atom-projected DoS (PDoS) curves g(ν) were computed by interpolating frequencies onto uniform Γ-centered grids with 24 × 24 × 24 (DoS) and 16 × 16 × 16 subdivisions (PDoS) with a Gaussian smearing of width σ = 0.032 THz. Phonon dispersion curves ν j (q q q) were computed by interpolating the frequencies along strings of q q qpoints passing through the high-symmetry wavevectors of the respective Brillouin zones, with transformation matrices applied to project to the primitive cells where required. The lattice thermal conductivities were computed using the single-mode relaxationtime approximation (SM-RTA) from modal properties evaluated on the Γ-centered q q q-point grids listed in Table . The calculated κ κ κ latt were analysed using the procedures outlined in our previous work. 3 Results and Discussion