Using the SM-RTA we predict room-temperature κ latt from "particle-like" transport, κ p , of 0.26 and 0.18 W m -1 K -1 for π SnS and SnSe. These are well below the averaged values of 0.96-5.01 W m -1 K -1 for IV-VI chalcogenides in the other structure types in Fig. obtained from similar calculations. We previously showed that low κ latt in the IV-VI chalcogenides is a balance of low phonon group velocities and a large "phase space" of energy-and momentum-conserving phonon scattering pathways, favoured by structure types with large primitive cells, and strong three-phonon interactions, favoured in structures where the T 2+ are constrained to locally-symmetric environments. Separating the κ p into harmonic (group velocity) and weighted-average lifetime components, κ/τ CRTA and τ CRTA , confirms the large unit cell of the π structure reduces the group velocities by ∼75-80% compared to the Pnma phases (Fig. ). The τ CRTA of π SnS and SnSe are also around 30 and 50% smaller above 300 K (Fig. ). We stress that we only use the CRTA model to determine a weighted-average lifetime, and we do not use this approximation to calculate the κ latt . Comparison of the "phase space" functions N2 (ω) indicate that the π phases show comparable numbers of scattering pathways to the Pnma phases, but with higher spectral weight at low frequency (Fig. ). This does not follow the trend with unit-cell size in our previous study, indicating that the larger numbers of energy-conserving scattering channels enabled by the denser phonon dispersions are offset by momentum conservation restrictions from the higher cubic symmetry. The calculated Pλ place the anharmonicity in the π phases between the Pnma and R3m phases (Fig. ), which can be ascribed to 25% of the Sn atoms being in more constrained rhombohedral-Fig. Analysis of the "particle-like" thermal conductivity, κ p , of π SnS and SnSe using the models employed in our previous studies. The P in (d) are taken from our previous study, but include the P for Pnma SnS from Ref. . like environments (c.f. Table