The structural versatility of proton sponge bismuth halides

Hybrid halometalates containing lead, tin, bismuth and antimony and organic cations have recently shown a bevy of interesting photophysical properties. Aiming at finding chemically stable and thermally inert species, three halobismutate species of this class, crystallized with proton sponge-derived cations (PRSH), have been isolated as microcrystalline powders by mixing 1,8-bis(dimethylamino)-naphthalene (proton sponge, or PRS) and bismuth oxide in concentrated HX acids (X = Cl, Br and I). The two isomorphous (PRSH)3Bi2X9 (X = Br, I) species, containing isolated [Bi2X9]3- anions, are triclinic at room temperature and convert upon heating into a monoclinic structure through a displacive phase transformation, fully reversible for X = I and only partially for X = Br. At variance, (PRSH)BiCl4 is polymeric, and contains extended zigzagging 1D chains formed by edge-sharing BiCl6 octahedra. These species were extensively studied by synchrotron and laboratory X-ray powder diffraction measurements, which enabled to detect the evolution toward the two high-temperature beta-phases (X = Br, I), to derive the structure of five different (significantly complex) species and to assess the thermal strain tensors in the different regimes. Additional thermal, spectroscopic and computational analyses completed the characterization of these materials.