Cs2MIIMIV 3Q8 (Q = S, Se, Te): An Extensive Family of Layered Semiconductors with Diverse Band Gaps

Flame-melting rapid-cooling reactions were used to synthesize a number of pure phases of the Cs2MIIMIV 3Q8 family (MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs2MIIMIV 3Q8 compounds presented here crystallize in the noncentrosymmetric space group P212121, except for Cs2ZnGe3S8, which crystallizes in the centrosymmetric space group P21/n. The structures contain chains of corner-sharing MIIQ4 and MIVQ4 tetrahedra linked by edge-sharing MIV 2Q6 dimers to give a two-dimensional structure. All phases are structurally similar to the AMIIIMIVQ4 (A = alkali metal, Tl; MIII = Al, Ga, In; MIV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered MII and MIV sites as opposed to the occupational disorder of MIII and MIV over all tetrahedral sites present in AMIIIMIVQ4. The structural trends of the Cs2MIIMIV 3Q8 family are discussed, along with a systematic study of their optical properties. Density functional theory (DFT) electronic structure calculations were performed using the projector augmented wave method to further investigate the trends in the band gaps of the Cs2MIIMIV 3Se8 (MII = Mg, Zn; MIV = Ge, Sn) compounds. The experimental diffuse reflectance UV–vis spectroscopy results show that the Mg compounds have smaller band gaps than those containing Zn for both the Ge and the Sn families whereas the DFT calculations show the opposite trend. Cs2HgSn3Se8 was studied as a representative example of this family using differential thermal analysis and melts congruently at 595 °C. Crystal growth of this compound using the Bridgman method resulted in a polycrystalline ingot from which plate crystals ∼2 mm × 3 mm could be cleaved. The band gap of the compounds varies from a narrow 1.07 eV for Cs2ZnGe3Te8 to a wide 3.3 eV for Cs2ZnGe3S8 and Cs2CdGe3S8 making this family a potentially useful source of materials for a variety of electronic applications. Cs2HgSn3Se8 crystals exhibit photoconductivity response where the photoexcited electron and hole show mobility-lifetime products on the order of 3.69 × 10–5 cm2/V and (μτ)h∥ = 7.78 × 10–5 cm2/V, respectively.