Synthetic and crystallographic study of various functional low-dimensional perovskites

Abstract

The discovery and development of new low-dimensional materials have attracted extensive attention over the past decades for the applications in functional devices. The work in this thesis focuses on the structural characterisation and thermal evolution of inorganic layered perovskites, and the design, synthesis, characterisation of new hybrid organic-inorganic lead halide 2D layered perovskites and 1D perovskite-related materials. The preliminary study of physical properties such as UV-Vis spectroscopy and photoluminescence measurements are also explored in this thesis.

The thermal evolution and high-temperature phase behavior of the ferroelectric layered perovskite, Bi₄Ti₃O₁₂, a n = 3 member of Aurivillius family, has been re-examined by high-resolution powder neutron diffraction. Both experimental and theoretical studies previously, suggested various and conflicting structural models and phase transition sequences. In Chapter 3, we confirm that Bi₄Ti₃O₁₂ undergoes two distinct structural transitions from the parent I4/mmm tetragonal phase to the room temperature ferroelectric B1a1 phase. A previously unconsidered and unusual intermediate paraelectric phase is confirmed to exist above TC, with a tetragonal symmetry of P4/mbm. This phase displays a unique type of octahedral tilting, which has been rationalised by the bonding requirements of the Bi³⁺ cations within the triple perovskite blocks.

Five new hybrid lead(Ⅱ) halide layered perovskites are presented in Chapter 4: TGPbBr₄, IGPbBr₄, T₂PbBr₄, A₂PbBr₄ and ITPbBr₄, with the organic species of 1,2,4-triazolium (T), guanidinium (G), imidazolium (I) and acetamidinium (A). Four of them adopt (110)- oriented layered perovskite structures, three of them are ʺ2 × 2ʺ (110)-oriented structure types (TGPbBr₄, IGPbBr₄ and A₂PbBr₄) and one of them is a ʺ3 × 3ʺ (110)-oriented structure type (ITPbBr₄), whereas T₂PbBr₄ adopts a (001)-oriented layered perovskite structure. The structural differences and the possible formation mechanism of these structure types in terms of size effects and H-bonding preferences are also discussed. In addition, the differences between the templating molecular species, contrasting physical properties such as bandgaps and photoluminescent behaviours of these structures are also illustrated in this Chapter.

Four new 1D chain structure type hybrid organic-inorganic lead(Ⅱ) halide compounds are presented in Chapter 5: IQPbBr₃, QPbBr₃, QPbI₃ and IQPbI₃. Templated by large organic amines, isoquinoline (IQ) and its isomer quinolone (Q). All four compounds possess the same generic formula as cubic perovskite, ABX₃, but adopt totally different structures. IQPbBr₃, adopts a 1D face-sharing single chain hexagonal perovskite structure type, and the other three, QPbBr₃, QPbI₃ and IQPbI₃, adopt a non-perovskite structure which is built from 1D edge-sharing octahedral double chains. All four compounds were carefully analysed by single crystal X-ray diffraction, powder X-ray diffraction and CHN elemental analysis. The differences between these structures in terms of templating molecular species and bandgaps of each compound were also discussed. Three of them have lower bandgaps than the other reported materials with the same structure type, indicating the promising application of photovoltaics for these types of materials.