Structure Directing Forces in Hybrid Layered Double Perovskites Containing Aromatic Organic Cations

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 characterization 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 surrounding 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 polar ordering of the organic cations. The packing of the organic cations depends on both their conformational flexibility and the lateral dimensions of the inorganic layer. These forces favor intralayer edge-to-face interaction between aromatic rings in three of the four compounds. The lone exception is (PEA)4AgBiBr8, which forms weak interlayer edge-to-face interactions between aromatic rings and slip-stacked packing within each organic layer.