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No description available.

No description available.

No description available.

No description available.

No description available.

Polyoxometalates (POMs) are anionic oxides of the early transition metals in their highest oxidation state (i.e. V(V), Nb(V), Ta(V), Mo(VI) and W(VI)) which possess a wide structural diversity and an equally broad range of physical properties. Transition metal substituted polyoxometalates (TMSPs) are the subset of the POM family which members can be described as metal complexes of purely inorganic, highly nucleophilic, POM based ligands and oxophilic transition metals. TMSPs are usually synthesized in solution via multi-step reaction procedures; however the processes controlling their formation are poorly understood and usually described as "self-assembly". Hence, the design of synthetic methods aimed at producing a desired structure is based on a mixture of empirical observations and general considerations about the reactivities of both ligand and transition metal in solution. The judicious exploration of the synthetic parameter space (e.g. pH, added electrolytes, etc.) have resulted in the discovery of a high number of complexes, a process which contributes to a better understanding of the relationship between synthetic conditions and final structure. However, and despite the vast number of TMSPs reported in the past couple of decades, this connection between synthetic variables and the structure of the cluster is far from being fully established. In particular, the effect of high pH values of the reaction mixture and the presence of alkaline metal ions does not seem to have been fully explored and therefore a potentially high number of clusters may lay undiscovered. A section of this thesis is devoted to that exploration of the parameter space of TMSP synthesis involving four different lacunary POM ligands ({γ-SiW10}, {γ-GeW10}, {A-α-PW9} and {α-P2W15}) and late first-row transition metal ions. The synthetic variables scanned during this programme were primarily two: the increase of the pH, by means of the addition of several inorganic bases, and the addition of alkaline metal salts to the reaction mixtures. A number of novel TMSPs were thereby discovered which, thanks to their relevant structural features, represent a significant contribution to the final goal of rationally design pathways to produce these complex inorganic architectures. The remarkable physical properties of POMs can be exploited by their integration into suitable composite materials. In particular, their ability to undergo several reversible electrochemical reductions whilst retaining their structural integrity, a property which is usually coupled to a pronounced change of optical absorbance, make these clusters ideal candidates for electrochromic applications. However, their performance in terms of optical contrast and switching time is typically lower than state-of-the-art electrochromic materials. A strategy employed to exploit and enhance the electrochromic response of POMs is to disperse them in an electrochromic organic polymer and hence produce an improved material in terms of its optical contrast. However, mostly classic POMs, with just one type of transition metal in their elemental composition and of relatively simple structure, have been examined for this application while only a small number of TMSPs have been evaluated for the same purpose. V(IV/V)-substituted POMs, for example, have not been used to fabricate this type of materials in spite of their electrochromic properties. A section of this thesis is devoted to the deposition and characterization of a number of TMSP-based hybrid films in terms of morphology, electrochemistry and electrochromic response. Electrodeposited films containing an electrochromic organic polymer and a dimeric V(V) TMSP, {V(V)6P4W30}, showed a higher optical contrast than films containing the monomeric equivalent, {V(V)3P2W15}, or POM-free films. This result constitutes a significant insight into the effect of the structure of the cluster on the electrochromic performance of POM-based hybrid materials.

Rb-IV is the stable high-pressure phase of rubidium between 16 and 21 GPa. The structure of Rb-IV has long been known to be complex, but it is only recently that it has been solved as being an incommensurate host-guest composite structure, comprising a tetragonal host framework containing chains of "guest" atoms that form structures incommensurate with the host along their common c- axis. While similar composite structures have been observed in a number of other elemental metals, Rb-IV is unique in that on pressure decrease below 16.7 GPa at 300 K, the chains of guest atoms become disordered and liquid-like. This thesis is a detailed structural study of Rb-IV. High-pressure, combined with high-temperature powder diffraction techniques, have been used to map the P-T phase diagram of rubidium between 15 GPa and 20 GPa and between 298 K and 600 K. The results show that the guest order-disorder transition pressure is strongly temperature dependent, and that the disordered phase is observed to the highest temperatures. Technical developments, which have made it possible to extract reliable modulation reflection intensities from a Rb-IV single crystal, are described. The resulting data are used for a full modulated structure refinement of Rb-IV, revealing a saw-tooth shaped modulation of the guest structure, from which new information on the host-guest interactions has been extracted. Inelastic X-ray scattering techniques have been used to measure the longitudinal acoustic (LA) phonons in a Rb-IV single crystal. Two LA-like phonon branches, one for each of the two composite subsystems, are observed along the common c-axis. The sound velocities in the host and guest structures are determined and the pressure dependence is shown to differ by a factor of two. Finally, developments that will enable future combined high-pressure high- temperature single-crystal diffraction studies, and single-crystal diffraction studies at pressures above 100 GPa, will be presented.

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This thesis details the physical and electronic structure of several technologically important transition metal chalcogenides (TMCs) using a combination of transmission electron microscopy (TEM) and surface science experimental techniques. The materials studied include CuxTe and CdTe, which find application in high efficiency, low weight photovoltaic devices. CuxTe alloys are frequently used as an electrical back contact in high efficiency CdTe photovoltaics. Here, we examine the alloying of Te on the Cu(111), polycrystalline Cu and Cu(643) surfaces. Chapter 3 of this thesis shows that the alloying of Te and Cu(111) is facile at room temperature, contrary to previous reports. Two distinct surface phases exist, depending on Te surface concentration. Below a coverage of 0.33 monolayers (MLs) of Te a surface substitutional alloy (SSA) is found to exist, where a Te adatom substituted for a surface Cu atom. For Te coverages greater than 0.66 ML, an unusual Cu3Te2 alloy continually grows on the surface, stabilised by a good lattice match to the Cu(111) substrate. The surface alloying of the Cu-Te system displays an intriguing dependence on the surface termination of the Cu substrate. Of the three Cu substrates studied here, Cu(111), Cu(643)R and polycrystalline Cu, a 1 ML film of Te gave ordered alloy structures with stoichiometries of Cu3Te2, CuTe and Cu2Te, respectively. In chapter 4, the study of thin film photovoltaics is extended to the deposition of CdTe onto Cu and CuxTe substrates. CdTe is observed to grow three dimensionally on Cu(111), Cu3Te2 and Cu2Te. Cu+ diffusion, crucial for photovoltaic performance, is detected for CdTe thicknesses greater than 2 ML and is assigned, predominately, to Cu2Te crystallites forming within the CdTe layer, with a minor amount of Cu residing in interstitial sites in the host CdTe structure. Chapter 5 describes the alloying of Te with a intrinsically chiral surface, Cu(643)R, the first study of its kind. The results of this study reveal that step mediated alloying occurs between Cu and Te with significant faceting of the surface. Two ordered CuTe alloy phases were observed for sub-monolayer Te coverages. The low coverage alloy exists for Te coverages between 0.18 ML and 0.45 ML and has a chiral unit cell. The high coverage alloy exists for Te coverages between 0.45 ML and 1.5 ML and has an achiral unit cell. The atomic positions of these surface alloys are tentatively interpreted from the scanning tunnelling microscopy (STM) images. In contrast to the thin film experiments in chapters 3-5, chapter 6 describes a study of TaS3 nanoribbons. These studies reveal that the nanoribbons have a distinct core-shell type structure. Characterisation with surface science techniques shows that the shell is nonstoichiometric and amorphous while TEM shows a crystalline core to the material. Interestingly, the TaS3 are observed to be unstable when interfaced on a Au substrate, with the shell persistently losing S to the substrate, which have potential implications in device integration.