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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5390 |
| Date | 15 August 2008 |
| Creators | Lemmerer, Andreas |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 29811881 bytes, application/pdf, application/pdf |
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