Four quaternary hybrid halide perovskites have been synthesized in hydrohalic acid solutions under hydrothermal conditions. The structures of (CH 3 NH 3 ) 2 AgRhX 6 and (CH 3 NH 3 ) 2 NaRhX 6 (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 P6 3 /mmc to trigonal P3m1 by site ordering of the Rh 3+ 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.
Using the tools of group theory, we have mapped out the symmetry consequences of structural distortions in hybrid layered perovskites. These distortions include octahedral tilting and cation ordering within the inorganic layers as well as orientational ordering of organic cations in the organic layers. This analysis is coupled with synthesis and crystal structure determination of a wide variety Ruddlesden-Popper phases. The combined analysis shows that certain modes of octahedral tilting are strongly favored in large part because they lead to favorable hydrogen bonding interactions between the halide ions of the inorganic layers and the ammoniumtype protons of the organic cations. This information is used to guide our search for new lead-free ferroelectrics and multiferroics among this large class of compounds.
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