Representative phonon spectra of the four structure types are shown in Fig. arranged by decreasing symmetry from Fm 3m → R3m → Cmcm → Pnma. Phonon dispersion and DoS curves for all ten systems are provided in the ESI. The number of atoms n a in the primitive cells increases with decreasing symmetry (c.f. Table ), which results in the number of bands at each wavevector q q q increasing from 3n a = 6 in the Fm 3m systems to 3n a = 24 in the Pnma structures. Comparison of the Table Optimised lattice parameters of the ten structures examined in this work. The percentage differences to literature values where available are shown in parentheses. We also show for each composition the average formula mass m and mass difference ∆m. In Fm 3m SnTe and Cmcm SnSe, the PDoS curves show that the lower and upper-frequency bands are dominated by the tetrel and chalcogen atoms, respectively (Fig. ). On the other hand, this is not the case for R3m GeTe and Pnma GeSe, for which both atoms make similar contributions across the full frequency spectrum (Fig. ). Considering the mass differences in Table , this behaviour cannot be put down to the difference in atomic masses of the tetrel and chalcogen, and instead indicates a difference in the bonding in the Ge and Sn systems. Indeed, the relative contributions of the tetrel and chalcogen atoms to the phonon spectra appear to depend on both the composition and the crystal symmetry. All three Fm 3m structures show some level of split contributions, which are most prominent in GeSe. R3m GeTe shows roughly equal contributions from both elements across the frequency spectrum, whereas the spectrum of R3m SnTe shows unequal contributions that are particularly prominent in a feature between ∼2.5-3.5 THz. With the exception of SnSe, the Pnma phases also show roughly equal contributions from the tetrel and chalcogen atoms across the full phonon spectrum, whereas tin selenide shows larger contributions from Sn at low frequencies and larger contributions from Se at higher frequencies. This is also evident in the phonon spectrum of Cmcm SnSe. The lower coordination number and more directional bonding in the othorhombic Pnma and Cmcm phases may indicate a higher degree of covalent character compared to the higher-symmetry Fm 3m and R3m phases, which could partially explain differences in the atomic contributions to the phonon spectra. However, the different contributions of the tetrel and chalcogen atoms to the lowerand higher-frequency parts of the spectra of the two orthorhombic phases of SnSe suggests there are some deeper subtleties that may require a more detailed analysis of the bonding to understand.