Incorporation of A2Q into HgQ and Dimensional Reduction to A2Hg3Q4 and A2Hg6Q7 (A = K, Rb, Cs; Q = S, Se). Access of Li Ions in A2Hg6Q7 through Topotactic Ion-Exchange

The synthesis of the one-dimensional K 2 Hg 3 Q 4 (Q = S, Se) and Cs 2 Hg 3 Se 4 and the three-dimensional A 2 Hg 6 S 7 (A = K, Rb, Cs), and A 2 Hg 6 Se 7 (A = Rb, Cs) in reactive A 2 Q x fluxes is reported. Pale yellow, hexagonal plates of K 2 Hg 3 S 4 crystallize in space group Pbcn, with a = 10.561(5) Å, b = 6.534(3) Å, and c = 13.706(2) Å, V = 945.8(7) Å, 3 d calc = 5.68 g/cm 3, and final R = 5.7%, R w = 6.3%. Red, hexagonal plates of K 2 Hg 3 Se 4 crystallize in space group Pbcn, with a = 10.820(2) Å, b = 6.783(1) Å, and c = 14.042(2) Å, V = 1030.6(5) Å, 3 d calc = 6.42 g/cm 3, and final R = 7.7%, R w = 8.4%. Orange yellow, hexagonal plates of Cs 2 Hg 3 Se 4 crystallize in space group Pbcn, with a = 12.047(4) Å, b = 6.465(2) Å, and c = 14.771(6) Å, V = 1150.4(7) Å, 3 d calc = 6.83 g/cm 3, and final R = 5.5%, R w = 6.2%. Black needles of K 2 Hg 6 S 7 crystallize in space group P 4̄2 1 m, with a = 13.805(8) Å and c = 4.080(3) Å, V = 778(1) Å, 3 d calc = 6.43 g/cm 3, and final R = 3.1%, R w = 3.6%. Black needles of Rb 2 Hg 6 S 7 crystallize in space group P 4 2 nm, with a = 13.9221(8) Å and c = 4.1204(2) Å, V = 798.6(1) Å, 3 d calc = 6.65 g/cm 3, and final R = 4.3%, R w = 5.0%. Black needles of Cs 2 Hg 6 S 7 crystallize in space group P 4 2 nm, with a = 13.958(4) Å and c = 4.159(2) Å, V = 810.2(8) Å, 3 d calc = 6.94 g/cm 3, and final R = 4.3%, R w = 4.4%. Black needles of Cs 2 Hg 6 Se 7 crystallize in space group P 4 2 nm, with a = 14.505(7) Å and c = 4.308(2) Å, V = 906(1) Å, 3 d calc = 7.41 g/cm 3, and final R = 3.6%, R w = 4.0%. The A 2 Hg 3 Q 4 compounds contain linear chains. The A 2 Hg 6 Q 7 compounds display noncentrosymmetric frameworks with A + cations residing in tunnels formed by both tetrahedral and linear Hg atoms. K 2 Hg 6 S 7, Rb 2 Hg 6 S 7, Cs 2 Hg 6 S 7, Rb 2 Hg 6 Se 7, and Cs 2 Hg 6 Se 7 display room-temperature bandgaps of 1.51, 1.55, 1.61, 1.13, and 1.17 eV, respectively. Bandgap engineering through S/Se solid solutions of the type Rb 2 Hg 6 Se 7 - x S x and Cs 2 Hg 6 Se 7 - x S x is possible in these materials. All A 2 Hg 6 Q 7 melt congruently, with melting points of 556 ± 10 °C, except for Rb 2 Hg 6 Se 7 which degrades. Rb 2 Hg 6 S 7 can undergo ion exchange reactions with LiI to give Li 1.8 Rb 0.2 Hg 6 S 7 .