Phase transitions and structural motifs of inorganic-organic lead halide hybrids

Lemmerer, Andreas
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Abstract Layered inorganic-organic hybrid compounds have been widely studied as new potential sources of semiconductors and other optical devices. They simulate natural quantum well materials, where the inorganic part acts as semiconductors, separated by an organic part. This class of hybrid materials has no covalent bonds between the inorganic and organic parts; instead, weak hydrogen bonds and van der Waals forces bind and stabilise the overall structure. The inorganic part is made up of layers of corner-sharing metal halide octahedra, MX6, where the metal must be in a divalent state and the halides are Cl, Br or I. The 2-D layers extend infinitely in two directions and are separated themselves by layers of primary ammonium cations, with only one ammonium group at one end of the chain, [(R-NH3)2MX4], or two ammonium groups at either of the chain, [(H3N-R-NH3)MX4]. Due to its similarity to the cubic perovskite structure, this inorganic motif is referred to as "layered perovskite-type". Depending on the choice of the organic ammonium cation, the materials can display phase transitions and / or have optical and electronic properties. Various investigations of inorganic-organic hybrids have concentrated on the phase transitions of the hybrids of general formula [(CnH2n+1NH3)2MX4] and [(NH3CnH2nNH3)MX4] (n = 1-18; X = Cl, Br, I; M = Cu2+, Mn2+, Cd2+) to elucidate their mechanism. There are two types of displasive transitions, a minor one were small conformational changes within the alkylammonium chain occurs, and a major one, when the entire alkylammonium chain becomes disordered along its long axis. The interlayer spacing between the inorganic layers increases with temperature and during the major phase transition. The methods used to identify the temperatures and the enthalpies of the phase transitions are Differential Scanning Calorimetry (DSC); and Single Crystal X-ray Diffraction (SC-XRD) as well as Powder X-Ray Diffraction (P-XRD) to follow the structural changes. In contrast, only a few reports on investigations of the lead iodide hybrids, [(CnH2n+1NH3)2PbI4] were found in the literature, with only two single crystal structures previously reported. Due to the difficulty in growing good quality crystals, the previous studies on the lead iodide hybrids have been only researched using DSC and P-XRD. The phase transition behaviour has been found to show the same trends as the previous hybrids. The primary aim of this study was to follow the same phase transitions via SC-XRD, ideally single-crystal to single-crystal, and to determine the detailed structural changes with the hopes of elucidating their detailed phase transition mechanism. A secondary aim was to synthesize as many inorganic-organic hybrids as possible using a variety of primary ammonium cations to find different inorganic motifs apart from the layered perovskite-type. Other inorganic motifs can have purely corner-, edge or face-sharing octahedra or combinations thereof to give 2-D net-type networks or 1-D extended chains. The effect that the identity of the ammonium cation has on the type of inorganic motif and the effect on the detailed structural geometry within the inorganic motif are investigated. Examples of structural geometries within the layered perovskite-type inorganic motif that can differ from compound to compound are the relative positions of the inorganic and organic moieties; the N---H….X hydrogen bonding geometry between the halides and the ammonium group; and the relative positions of successive inorganic layers.
x-ray diffraction, phase transition, inorganic-organic hybrids