Hybrid Organic–Inorganic Halide Derivatives of the 2H Hexagonal Perovskite Structure

Four quaternary hybrid halide perovskites have been synthesized
in hydrohalic acid solutions under hydrothermal conditions. The structures
of (CH3NH3)2AgRhX6 and
(CH3NH3)2NaRhX6 (X = Cl–, Br–) consist of infinite one-dimensional
chains of face-sharing metal halide octahedra. The structure is closely
related to the 2H hexagonal perovskite structure, but the space group
symmetry is lowered from hexagonal P63/mmc to trigonal P3m1 by site ordering of the Rh3+ and Ag+/Na+ cations. All compositions demonstrate broad-spectrum
visible light absorption with optical transitions arising from rhodium
d-to-d transitions and halide-to-rhodium charge transfer transitions.
The bromides show a 0.2 eV red-shift in the optical transitions compared
to the analogous chlorides. Crystal field splitting energies were
found to be 2.6 and 2.4 eV for the chloride and bromide compositions,
respectively. Band structure calculations for all compositions give
rather flat valence and conduction bands, suggesting a zero-dimensional
electronic structure. The valence bands are made up of crystal orbitals
that are almost exclusively Rh 4d–Cl 3p (Br 4p) π* in
character, while the conduction bands have Rh 4d–Cl 3p (Br
4p) σ* character.