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Materials Chemistry

There is currently much interest in b-barium borate (BBO) for non-linear optical applications and, in particular, recent attention has turned to methods of producing thin films of this material. Based on preliminary results from this laboratory, this project was undertaken to carry out fundamental investigations into structure and reactivity that would enable rational approaches to the design and optimisation of molecular precursors for the chemical solution deposition (CSD) of BBO thin films.

In order to facilitate deposition studies in our laboratory, a controlled atmosphere "clean box" was designed and installed, thereby eliminating the need for access to external clean room facilities. Air sensitive compounds prepared in our laboratory can be handled and stored inside the "clean box" using standard inert atmosphere techniques, preventing sample degradation. In addition to a small vacuum/nitrogen manifold, the box contains a spinner for the production of thin films and a hotplate for intermediate heating. Firing is carried out in a furnace which is connected directly to the box with access through a door in the box wall.

This facility has been highly successful and is now being used for a range of studies in materials chemistry.

In order to determine the types of molecule formed by interactions between boron alkoxides and group 2 alkoxides, a series of metal phenoxoborates have been synthesised and structurally characterised. Structures of fourteen of these compounds have been correlated with their solution behaviour by ¹¹B NMR studies and this information has enabled us to characterise solutions of molecular precursors which cannot be obtained as crystalline materials. In this way, the thermal processing behaviour of precursors can be related to their chemical nature, which can then be modified to optimise processing conditions.

We have shown that the phenoxoborate precursors do not decompose cleanly to BBO, whereas a precursor prepared from barium and boron methoxyethoxides provides higher quality films with preferential 001 orientation. Solution studies on this precursor showed that tetrahedral boron is present, possibly as a molecular mixed alkoxide of the type [BaB₂(OR)₈]. Further optimisation of molecular structure is now possible in order to reduce the activation energy for thermal decomposition.

The "clean box" deposition facility funded by this grant has enabled us to use CSD to prepare the first oriented thin films of bismuth tungstate, a piezoelectric material with potential applications in the area of microengineered mechanical systems (MEMS). A significant advantage of this method is that precursor solutions of mixed tungsten and bismuth alkoxides provide crystalline material at relatively low temperatures (400-500 ^oC), eliminating problems due to higher temperature phase transitions.