The synthesis of the one-dimensional
K2Hg3Q4 (Q = S, Se) and
Cs2Hg3Se4 and the
three-dimensional
A2Hg6S7 (A = K, Rb, Cs), and
A2Hg6Se7 (A = Rb, Cs)
in reactive A2Q
x
fluxes is reported.
Pale yellow,
hexagonal plates of K2Hg3S4
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,
d
calc = 5.68 g/cm3, and final
R = 5.7%, R
w = 6.3%. Red,
hexagonal plates
of K2Hg3Se4 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,
d
calc = 6.42 g/cm3, 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) Å,
b = 6.465(2) Å, and c = 14.771(6)
Å, V
= 1150.4(7) Å,
d
calc =
6.83 g/cm3, and final R = 5.5%,
R
w = 6.2%. Black needles of
K2Hg6S7
crystallize
in space group P4̄21
m, with
a = 13.805(8) Å and c = 4.080(3)
Å, V = 778(1) Å,
d
calc = 6.43 g/cm3, and
final
R = 3.1%, R
w = 3.6%. Black
needles of Rb2Hg6S7
crystallize in space group P42
nm,
with a = 13.9221(8)
Å and c = 4.1204(2) Å, V =
798.6(1) Å,
d
calc = 6.65
g/cm3, and final R = 4.3%,
R
w = 5.0%. Black needles
of Cs2Hg6S7 crystallize in
space group P42
nm, with a
= 13.958(4) Å and c = 4.159(2) Å,
V = 810.2(8) Å,
d
calc = 6.94 g/cm3, and final
R = 4.3%, R
w = 4.4%. Black
needles of Cs2Hg6Se7
crystallize in space group
P42
nm, with a =
14.505(7) Å and c = 4.308(2) Å, V
= 906(1) Å,
d
calc = 7.41
g/cm3, and final R = 3.6%,
R
w = 4.0%. The
A2Hg3Q4 compounds contain
linear chains. The
A2Hg6Q7 compounds display
noncentrosymmetric frameworks with A+ 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
-
x
S
x
and
Cs2Hg6Se7
-
x
S
x
is possible in these materials. All
A2Hg6Q7 melt congruently, with
melting
points of 556 ± 10 °C, except for
Rb2Hg6Se7 which degrades.
Rb2Hg6S7 can undergo ion
exchange reactions
with LiI to give
Li1.8Rb0.2Hg6S7.