## 2 Structures and Physical Properties of Tin(II) Sulfide  
The most commonly utilised and studied crystalline phase of SnS is the orthorhombic Herzenbergite phase, with the space group Pnma (also reported as Pmcm, see Figure 1). In this structure, the Sn\({}^{2+}\) ion coordinates to three S\({}^{2-}\) ions, with the remaining Sn electrons occupying the vacant position of the tetrahedral geometry as a lone pair of electrons. Bulk orthorhombic SnS has a density of 5.22 g cm\({}^{-3}\) and a melting point of 882 \({}^{\circ}\)C [25]. Single crystals of SnS typically exhibit p-type conductivity in the range 10\({}^{-1}\)-10\({}^{-4}\)\(\Omega\) cm\({}^{-1}\)[26], although depending on the Sn content, SnS may change its charge-carrying characteristics from p-type to n-type. Bulk band gap energies ranging from 1.20 to 1.37 eV [27] have been reported, with some outlying values as low as 1.07 eV [28]. SnS possesses a thermal conductivity around 1 W m\({}^{-1}\) K\({}^{-1}\) and a Seebeck coefficient in the range 300-500 uV K\({}^{-1}\)[29; 30].  
Other SnS phases of interest and their respective space groups are Fm\(\overline{3}\)m (rock salt phase), Cmcm (orthorhombic high temperature phase) and F\(\overline{4}\)3m (zinc blende phase). The structural and thermodynamic properties of orthorhombic SnS and other polymorphs have been assessed from first-principles calculations. These calculations show good agreement with experimental data, with the exception of zinc blende SnS. This polymorph is predicted to be thermodynamically and kinetically unstable, with a significant discrepancy observed between the calculated and measured lattice parameters for the F\(\overline{4}\)3m structure [31]. The Cmcm phase could not be stabilised, because it underwent a second-order phase transition to the thermodynamically stable orthorhombic Pnma structure, which is observed experimentally at 878 K [32]. At normal temperature and pressure, SnS also exhibits a low symmetric stable \(\alpha\)-SnS phase. Upon increasing the temperature, \(\alpha\)-SnS undergoes a phase transition to a high symmetry \(\beta\)-SnS phase. In simpler terms, orthorhombic SnS undergoes a phase change to a tetragonal structure, with a = 4.23 and c = 11.51 A [33]. It is reported that the low temperature phase crystallises in a germanium sulfide (GeS)-type structure with the space group Pbnm (D\({}_{2\text{h}}\)16), whereas the higher temperature phase crystallises in the thallium iodide (TII) type structure with space group Cmcm (D\({}_{2\text{h}}\)17), with the movement of Sn and S along the reference No. 100 direction caused by thermal expansion [34; 35]. At a temperature of 878 K (phase transition temperature) the axial ratio of lattice-parameter decreases continuously from a/c >1 to a/c <1. SnS also exists in a cubic \(\pi\)-SnS form, which has been reported by [PERSON] et al. The structure was initially determined via electron diffraction methods due to the size and small quantities produced. The cubic lattice parameter was determined to be 11.7 A [36]. Subsequently, larger scale quantities of \(\pi\)-SnS were prepared by [PERSON] et al. Further analysis of this phase via powder XRD showed a cubic lattice parameter \(a\) = 11.595 A. Optical absorption spectroscopy determined an indirect band gap energy of 1.53 eV, in good agreement with density functional theory (DFT) calculations. The DFT calculations also showed that \(\pi\)-SnS is energetically similar to \(\alpha\)-SnS, but with a higher stability than the ideal rock salt structure. Raman data showed a shift towards higher energies along with a splitting of the 96 cm\({}^{-1}\) peak [37]. \(\pi\)-SnS has been utilised in the production of thin film solar cells by [PERSON] et al., with a band gap of 1.74 eV and a conductivity of 10\({}^{-6}\)\(\Omega\)\({}^{-1}\) cm\({}^{-1}\)[38]. Table 1 shows compiled