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Cu 2 MnSnS 4 shares several promising properties with the widely investigated Cu 2 ZnSnS 4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorption characteristics for absorber materials. Thin film Cu 2 MnSnS 4 samples with various cation compositions are co-sputtered reactively followed by a high temperature anneal. Formation of Cu 2 MnSnS 4 and co-existence of several secondary phases is verified by XRD and Raman. Our investigation of the crystal structure based on first-principles DFT confirms that stannite crystal structure is preferred over kesterite, although, further verification considering cation disorder is needed. The direct band gap of Cu 2 MnSnS 4 is calculated as 1.52 eV (1.62 eV) for stannite (kesterite), which coincides with the range of the measured band gaps from spectrophotometry of 1.42-1.59 eV. After further annealing treatments below 240 C, the absorption shows reversible changes: the band gap blue-shifts and the Urbach tail energy is reduced. It is concluded that, just like Cu 2 ZnSnS 4 , disorder also occurs in Cu 2 MnSnS 4 . The implications of our findings are discussed and related to the current understanding of cation disorder in Cu 2 ZnSnS 4 and related compounds. Furthermore, for the first time first-principles DFT investigations are presented for the thiospinel Cu 2 MnSn 3 S 8 which is observed experimentally as a secondary phase in Sn-rich Cu 2 MnSnS 4 thin films.

A new structural phase is discovered for M2CO2 MXenes with M = Sc, Y, La, Lu, Tm, and Ho. The hexagonal carbon layer sandwiched between M atoms, typical for MXenes, is transformed into C3 trimers with anionic electrons localized in quasi zero‐dimensional lattice spaces in‐between the C3 units, so the systems can be described as [M6 C3 O6]+II : 2e− electrides. The systems are readily ionized into [M6 C3 O6]+II with very low ionization energy via an anti‐doping mechanism. It is shown that this new structure of Sc2CO2 can bind multiple lithium atoms, with low migration barriers. The findings indicate that these M2CO2 MXenes with unusual carbon trimers are a new family of 2D electride insulators with the potential for charge storage applications, thermal field emission, and as anode material in lithium batteries.

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