b–c plane) with strong Sn–Se bonding within the plane of the slabs, which are then linked with weaker Sn–Se bonding along the a direction. The structure contains highly distorted SnSe7 coordination polyhedra, which have three short and four very long Sn–Se bonds, and a lone pair of the Sn2+ sterically accommodated between the four long Sn–Se bonds. The two-atom-thick SnSe slabs are corrugated, creating a zig-zag accordion-like projection along the b axis. The easy cleavage in this system is along the (100) planes. While cooling from its high-temperature, higher symmetry phase (space group Cmcm, #63), SnSe undergoes a displacive (shear) phase transition at ~750–800 K, resulting in a lower symmetry Pnma (#62) space group. Owing to this layered, zig-zag accordion-like structure, SnSe demonstrates low anharmonicity and an intrinsically ultralow lattice thermal conductivity, making SnSe one of the world’s least thermally conductive crystalline materials. The fundamental mechanism of the low thermal conductivity has been elaborated in this “soft” accordion-like layered structure and verified due to a abnormally strong phonon renormalization at room temperature. Use in energy harvesting Tin(II) selenide may be soon used in energy harvesting. Tin(II) selenide has demonstrated the ability to convert waste heat into electrical energy. SnSe has exhibited the highest thermoelectric material efficiency, measured by the unitless ZT parameter, of any known material (~2.62 at 923 K along the b axis and ~2.3 along the c axis). When coupled with the Carnot efficiency for heat conversion, the overall energy conversion efficiency of approximately 25%. In order for this thermoelectric process to work, a thermoelectric generator must take advantage of the temperature difference experienced by two legs of a thermocouple junction. Each leg is composed of a specific material that is optimized at the operating temperature range of interest. SnSe would serve as the p-type semiconductor leg. Such a material needs to have low total thermal conductivity, high electrical conductivity, and high Seebeck coefficient according to the thermoelectric figure of merit ZT. Even though the record-high efficiency is most likely due to low thermal conductivity of the crystal, the electronic structure may have as important role: SnSe has highly anisotropic valence band structure, which consists of multiple valleys that act as independent channels for very mobile, low effective-mass charge transport within, and heavy-carrier conductivity perpendicular to the layers. While, historically, lead telluride and silicon-germanium have been used, these materials have suffered from heat conduction through the material. At