The tin sulfides
represent a materials platform for earth-abundant
semiconductor technologies. We present a first-principles study of
the five known and proposed phases of SnS together with SnS<sub>2</sub> and Sn<sub>2</sub>S<sub>3</sub>. Lattice-dynamics techniques are
used to evaluate the dynamical stability and temperature-dependent
thermodynamic free energy, and we also consider the effect of dispersion
forces on the energetics. The recently identified π-cubic phase
of SnS is found to be metastable with respect to the well-known orthorhombic <i>Pnma</i>/<i>Cmcm</i> equilibrium. The <i>Cmcm</i> phase is a low-lying saddle point between <i>Pnma</i> local
minima on the potential-energy surface and is observed as an average
structure at high temperatures. Bulk rocksalt and zincblende phases
are found to be dynamically unstable, and we show that whereas rocksalt
SnS can potentially be stabilized under a reduction of the lattice
constant the hypothetical zincblende phase proposed in several previous
studies is extremely unlikely to form. We also investigate the stability
of Sn<sub>2</sub>S<sub>3</sub> with respect to SnS and SnS<sub>2</sub> and find that both dispersion forces and vibrational contributions
to the free energy are required to explain its experimentally observed
resistance to decomposition
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