Investigation of the incorporation of trigonal planar oxyanions in inorganic oxide materials

Deakin, Joshua (2022). Investigation of the incorporation of trigonal planar oxyanions in inorganic oxide materials. University of Birmingham. Ph.D.

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Abstract

The work presented in this thesis focuses on the potential incorporation of carbonate into different materials, showing how care must be given to consider its possible inclusion when synthesising materials ≤1000°C. Other oxyanion investigations are also presented for the likes of sulphate and borate.

A new layered perovskite structure was synthesised with the formula Ba $_3$ Yb $_2$ O $_5$ CO $_3$ . The presence of carbonate was first confirmed by Raman spectroscopy and the new structure was determined by the Rietveld refinement using Neutron diffraction data. It was then proven that this system could be extended to a range of other rare earths. Further investigation shows how this layered structure is also seen with the partial inclusion of sulphate, provided there was stabilisation from the addition of zinc.

Carbonate incorporation was also determined in the cobalt deficient Ba $_2$ Co $_{1-x}$ O $_{4-δ}$ systems, where a structural change is seen from monoclinic to orthorhombic-like upon increasing the amount of deficiency. This carbonate incorporation may also be seen in the iron equivalent Ba $_2$ Fe $_{1-x}$ O $_{4-δ}$ systems, whilst sulphate incorporation into these iron systems was also demonstrated leading to materials with higher thermal stability. The inclusion of carbonate/sulphate was confirmed through Raman spectroscopy and structure refinement.

Various smaller investigations into the potential carbonate incorporation of other systems are also presented, including TiO $_2$ , and Nb $_2$ O $_5$ . Although not conclusive, the results suggest that carbonate could also be present at low temperature in polymorphs of these compounds.

Interesting borate systems, Sr $_{4-x}$ Ba $_x$ Na(BO $_3$ ) $_3$ , Sr $_{4-x}$ Ba $_x$ Li(BO $_3$ ) $_3$ , were synthesised. These systems have been previously reported, but we show for the first time here that they have perovskite-like superstructures. In these materials the borate groups are shown to provide all the oxygen atoms in the structure, illustrating the flexibility of the perovskite structure to accommodate oxyanion groups. The new Sr $_{4-x}$ Ba $_x$ Na $_{0.5}$ Li $_{0.5}$ (BO $_3$ ) $_3$ series was also prepared and whilst investigating other potential isostructural systems, a new strontium/calcium A site cation ordered Sr $_{2.2}$ Ca $_{1.8}$ Li(BO $_3$ ) $_3$ system was discovered.