Cu 2 SnS 3 has recently attracted attention as a low-cost, earthabundant absorber material for thin-film solar cells. Unfortunately, multiple Cu 2 SnS 3 structures exist and the conditions under which different structures form are poorly defined. The structures of tetragonal and monoclinic Cu 2 SnS 3 phases are clarified by use of solid-state nuclear magnetic resonance spectroscopy (ssNMR). The monoclinic Cu 2 SnS 3 phase is shown to form under Sn-rich, Cu-poor conditions and the tetragonal Cu 2 SnS 3 phase forms under Cu-rich, Sn-poor conditions, indicating that there is a composition difference between the structures that has not previously been reported. Furthermore, the monoclinic phase is destabilized at temperatures between 200 and 345 °C, when the material becomes tetragonal.

Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal, and superconducting properties. Here, we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ∼4.5°) in the metallic kagomé MgCo 6 Ge 6 at T = 100(2) K via X-ray diffraction measurements, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ∼300 to 100 K, no evidence for a phase transition was found in physical property measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge 2 ) dimers that connect adjacent layers. Further modeling of the displacive disorder in the crystal structure shows the presence of a second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits interlayer twisting, but with a different pattern, consistent with the change in electron count due to the removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials.

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