The composition-dependent optical and electronic tunability of the
sodalite analogues with
stoichiometries
Zn8X2[BO2]12 (X
= O, S, Se) and
[Cd
y
Zn(1-y)]8X2[BeSi
x
Ge(1-x)O4]6
(X = S, Se
and Te) have been demonstrated. The materials strongly
photoluminesce, and a comparison
of the photoluminescence behavior of the single crystals with the
as-synthesized powder
analogues shows that the visible emission is intrinsic to the sodalite
analogue and not due
to impurities such as bulk semiconductor. The emission maxima of
these materials can be
varied by as much as 100 nm with subtle modifications in the host
sodalite framework
composition and excitation energies can be stored over, at minimum,
several minutes. The
materials can be prepared either hydrothermally or by high-temperature
solid-state reactions.
The local and average long-range structures of composition
Zn8X2[BO2]12 (X
= O, S, Se) and
M8X2[BeSi
x
Ge1-x
O4]6
(M = Zn, Cd; X = S, Se, Te) are described based on the results
of
polycrystalline X-ray diffraction, multinuclear solid-state MAS NMR,
CdL3 XANES, UV/visible and photoluminescence spectroscopic measurements.
Additionally, the crystal
structures of the synthetic helvite solid solutions
Zn8S2[BeSi
x
Ge1-x
O4]6
(x = 0.03, 0.37, 0.74)
are presented based on refinement of single-crystal X-ray diffraction
and selected area
electron diffraction data; these materials crystallize in the acentric,
cubic space group P23
(No. 195) with unit-cell parameters a = 8.250(4),
8.221(2), and 8.163(1) Å, respectively. In
all these sodalite analogues, the anionic, rigid sodalite framework
encloses [M4X]6+ tetrahedra
in a crystalline cubic array, separating each tetrahedron from its
eight nearest neighbors
by at least 6.4 Å, center to center. This electrostatic isolation
results in dramatic high-energy shifts in the optical absorption spectra of the materials and
low-frequency shifts in
the MAS NMR spectra of the cage center anions, relative to the bulk
semiconductors. 113Cd
MAS NMR and CdL3 XANES spectra show that the influence of
the anion type on the
electronic structure at the Cd2+ ions in the cadmium
sodalites is small.
