Journal of the American Chemical Society, Vol.120, No.1, 124-136, 1998
Incorporation of A(2)Q into HgQ and dimensional reduction to A(2)Hg(3)Q(4) and A(2)Hg(6)Q(7) (A = K, Rb, Cs; Q = S, Se). Access of Li ions in A(2)Hg(6)Q(7) through topotactic ion-exchange
The synthesis of the one-dimensional K(2)Hg(3)Q(4) (Q = S, Se) and Cs2Hg3Se4 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 K2Hg3S4 crystallize in space group Pbcn, with a = 10.561(5) Angstrom, b = 6.534(3) Angstrom, and c = 13.706(2) Angstrom, V = 945.8(7) Angstrom(3), d(calc) = 5.68 g/cm(3), and final R = 5.7%, R-W = 6.3%. Red, hexagonal plates of K2Hg3Se4 crystallize in space group Pbcn, with a = 10.820(2) Angstrom, b = 6.783(1) Angstrom, and c = 14.042(2) Angstrom, V = 1030.6(5) Angstrom(3), d(calc) = 6.42 g/cm(3), and final R = 7.7%, R-W = 8.4%. Orange yellow, hexagonal plates of Cs2Hg3Se4 crystallize in space group Pbcn, with a = 12.047(4) Angstrom, b = 6.465(2) Angstrom, and c = 14.771(6) Angstrom, V = 1150.4(7) Angstrom(3), d(calc) = 6.83 g/cm(3), and final R = 5.5%, R, = 6.2%. Black needles of K2Hg6S7 crystallize in space group , with a = 13.805(8) Angstrom and c = 4.080(3) Angstrom, V = 778(1) Angstrom(3), d(calc) = 6.43 g/cm(3), and final R = 3.1%, R-W = 3.6%. Black needles of Rb2Hg6S7 crystallize in space group P4(2)nm, with a = 13.9221(8) Angstrom and c = 4.1204(2) Angstrom, V = 798.6(1) Angstrom(3), d(calc) = 6.65 g/cm(3), and final R = 4.3%, R-W = 5.0%. Black needles of Cs2Hg6S7 crystallize in space group P4(2)nm, with a = 13.958(4) Angstrom and c = 4.159(2) Angstrom, V = 810.2(8) Angstrom(3), d(calc) = 6.94 g/cm(3), and final R = 4.3%, R-W = 4.4%. Black needles of Cs2Hg6Se7 crystallize in space group P4(2)nm, with a = 14.505(7) Angstrom and c = 4.308(2) Angstrom, V = 906(1) Angstrom(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 Af cations residing in tunnels formed by both tetrahedral and linear Hg atoms. K2Hg6S7, Rb2Hg6S7, Cs2Hg6S7, Rb2Hg6Se7, and Cs2Hg6Se7 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 Rb2Hg6Se7-xSx and Cs2Hg6Se7-xSx is possible in these materials. All A(2)Hg(6)Q(7) melt congruently, with melting points of 556 +/- 10 degrees C, except for Rb2Hg6Se7 which degrades. Rb2Hg6S7 can undergo ion exchange reactions with LiI to give Li1.8Rb0.2Hg6S7.