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The synthesis, crystal structures, and optical properties of four ternary and six quaternary halides containing the Rh3+ ion are reported here. Rb3RhCl6 adopts a monoclinic structure with isolated [RhCl6]3− octahedra. Rb3Rh2Cl9, Cs3Rh2Cl9, and Cs3Rh2Br9 crystallize in a vacancy ordered variant of the 6H hexagonal perovskite structure, which contains isolated Rh2X93− (X = Cl, Br) dimers of face-sharing octahedra. Cs2AgRhCl6 and Cs2NaRhCl6 adopt the 12R rhombohedral perovskite structure, featuring [M2RhCl12]7− face-sharing octahedral trimers, connected to one another through rhodium-centered octahedra. A4AgRhCl8 and A4AgRhBr8 (A = CH3CH2CH2CH2NH3+, (CH3)2CHCH2CH2NH3)+) crystallize in a cation-ordered variant of the n = 1 Ruddlesden Popper structure, which features layers of corner-connected octahedra with a chessboard ordering of Ag+ and Rh3+ ions separated by double layers of organic cations. The diffuse reflectance spectra of all compositions studied feature peaks in the visible that can be attributed to spin-allowed d-to-d transitions and peaks in the UV that arise from charge transfer transitions. Electronic structure calculations reveal moderate Rh–X–Ag hybridization when rhodium- and silver-centered octahedra share corners, but minimal hybridization when they share faces. Many of the compositions studied have an electronic structure that is effectively zero-dimensional, but Cs2AgRhCl6 is found to possess a two-dimensional electronic structure. The results are instructive for controlling the electronic dimensionality of compositionally complex halide perovskite derivatives. 

Hybrid n = 1 Ruddlesden-Popper perovskites with aromatic ammonium cations like benzylammonium (BzA) and phenethylammonium (PEA) have been shown to adopt polar structures and exhibit ferroelectricity, but many of the examples discovered thus far contain either Pb or Cd. Here, we describe the synthesis and structural characteriza-tion of four layered halide double perovskites: (BzA)4AgBiBr8, (PEA)4AgBiBr8, (BzA)4AgInCl8, and (PEA)4AgInCl8. In all four compounds the inorganic layers exhibit a chessboard ordering of Ag+ and Bi3+/In3+ and the layers stack in a coherent pattern that maintains the ordering over three-dimensional space. The octahedra sur-rounding Ag+ show a large axial compression, which results in much shorter bonds to the terminal halide ions than to the bridging halide ions, whereas the bismuth- and indium-centered octahedra show only small distortions. There appears to be a competition between polar distortions of the octahedra and octahedral tilting, both of which can optimize hydrogen bonding interactions between the ammonium cations and the inorganic layers. Unlike the Pb- or Cd-containing analogs, the double perovskites seem to favor patterns of octahedral tilting that suppress po-lar ordering of the organic cations. The packing of the organic cations depends on both their conformational flexi-bility and the lateral dimensions of the inorganic layer. These forces favor intra-layer edge-to-face interaction be-tween aromatic rings in the three of the four compounds. The lone exception is (PEA)4AgBiBr8, which forms weak inter-layer edge-to-face interactions between aromatic rings and slip-stacked packing within each organic layer. 

A symmetry mode analysis yields 47 symmetrically distinct patterns of octahedral tilting in hybrid organic–inorganic layered perovskites that adopt then= 1 Ruddlesden–Popper (RP) structure. The crystal structures of compounds belonging to this family are compared with the predictions of the symmetry analysis. Approximately 88% of the 140 unique structures have symmetries that agree with those expected based on octahedral tilting alone, while the remaining compounds have additional structural features that further lower the symmetry, such as asymmetric packing of bulky organic cations, distortions of metal-centered octahedra or a shift of the inorganic layers that deviates from thea/2 +b/2 shift associated with the RP structure. The structures of real compounds are heterogeneously distributed amongst the various tilt systems, with only 9 of the 47 tilt systems represented. No examples of in-phase ψ-tilts about theaand/orbaxes of the undistorted parent structure were found, while at the other extreme ∼66% of the known structures possess a combination of out-of-phase ϕ-tilts about theaand/orbaxes and θ-tilts (rotations) about thecaxis. The latter combination leads to favorable hydrogen bonding interactions that accommodate the chemically inequivalent halide ions within the inorganic layers. In some compounds, primarily those that contain either Pb²⁺or Sn²⁺, favorable hydrogen bonding interactions can also be achieved by distortions of the octahedra in combination with θ-tilts. 

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 P3 ̅m1 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 eV 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. 

Two novel ternary compounds from the pseudobinary CH3NH3X–AgX (X = Br, I) phase diagrams are reported. CH3NH3AgBr2 and CH3NH3Ag2I3 were synthesized via solid state sealed tube reactions and the crystal structures were determined through a combination of single crystal and synchrotron X-ray powder diffraction. Structurally, both compounds consist of one-dimensional ribbons built from silvercentered tetrahedra. The structure of CH3NH3AgBr2 possesses orthorhombic Pnma symmetry and is made up of zig-zag chains where each silver bromide tetrahedron shares two edges with neighboring tetrahedra. The tetrahedral coordination of silver is retained in CH3NH3Ag2I3, which has monoclinic P21/m symmetry, but the change in stoichiometry leads to a greater degree of edge-sharing connectivity within the silver iodide chains. With band gaps of 3.3 eV (CH3NH3Ag2I3) and 4.0 eV (CH3NH3AgBr2) the absorption onsets of the ternary phases are significantly blue shifted from the binary silver halides, AgBr and AgI, due in part to the decrease in electronic dimensionality. The compounds are stable for at least one month under ambient conditions and are thermally stable up to approximately 200 1C. Density functional theory calculations reveal very narrow valence bands and moderately disperse conduction bands with Ag 5s character. Bond valence calculations are used to analyze the hydrogen bonding between methylammonium cations and coordinatively unsaturated halide ions. The crystal chemistry of these compounds helps to explain the dearth of iodide double perovskites in the literature.