Cs₂M^IIM^IV₃Q₈ (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 Cs 2 M II M IV 3 Q 8 family (M II = Mg, Zn, Cd, Hg; M IV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs 2 M II M IV 3 Q 8 compounds presented here crystallize in the noncentrosymmetric space group P 2 1 2 1 2 1, except for Cs 2 ZnGe 3 S 8, which crystallizes in the centrosymmetric space group P 2 1 / n . The structures contain chains of corner-sharing M II Q 4 and M IV Q 4 tetrahedra linked by edge-sharing M IV 2 Q 6 dimers to give a two-dimensional structure. All phases are structurally similar to the AM III M IV Q 4 (A = alkali metal, Tl; M III = Al, Ga, In; M IV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered M II and M IV sites as opposed to the occupational disorder of M III and M IV over all tetrahedral sites present in AM III M IV Q 4 . The structural trends of the Cs 2 M II M IV 3 Q 8 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 Cs 2 M II M IV 3 Se 8 (M II = Mg, Zn; M IV = 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. Cs 2 HgSn 3 Se 8 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 Cs 2 ZnGe 3 Te 8 to a wide 3.3 eV for Cs 2 ZnGe 3 S 8 and Cs 2 CdGe 3 S 8 making this family a potentially useful source of materials for a variety of electronic applications. Cs 2 HgSn 3 Se 8 crystals exhibit photoconductivity response where the photoexcited electron and hole show mobility-lifetime products on the order of 3.69 × 10 –5 cm 2 /V and ( μτ ) h∥ = 7.78 × 10 –5 cm 2 /V, respectively.