The synthesis, and structural studies of advanced functional materials

This research project is divided into two sections. The first section is an investigation into the stability of photovoltaic hybrid perovskites under various annealing conditions. Synthesis of MAPbI3 and related systems has been performed using a solution precipitation method. The compounds have then been divided in many smaller batches and thermally treated at different temperatures under air, vacuum and in the presence of the gettering agent, Zn. The temperatures and conditions were chosen to coincide with those typically implemented in the manufacture of photovoltaic devices, with the aim of understanding how these annealing conditions affect the structure. Firstly, we show that annealing in air increases both the tetragonal a and c lattice parameters, but reduces the c/a ratio, as a result in improved packing and establishing more hydrogen bonding. Secondly, annealing in a vacuum at 90 oC for up to 73 hours showed an increase in both the tetragonal a and c lattice parameters up to 5 hours but then there is a reduction to form a plateau around 40 hours, demonstrating that the annealing period is critical to tuning the MA packing and disorder in the structure. Finally, we show that annealing in a vacuum in the presence of Zn metal significantly reduces the lattice parameters, in contrast to the air and vacuum only environment, demonstrating that Zn can be used as a gettering agent to tune the composition during annealing. This work sheds some light on the wide variation in photovoltaic properties of MAPbI3 that are reported in the literature, and the importance in determining the correlation of photovoltaic properties with annealing procedures on these very flexible hybrid perovskites. The second section is on the synthesis and characterisation of magnetically frustrated mixed metal fluoride pyrochlores. RbFe2F6 is a charged ordered defect pyrochlore, that is better described by the formula, RbFe2+Fe3+F6. We have demonstrated that the average antiferromagnetic structure remains the same down to 1.5 K, despite recent inelastic neutron scattering studies showing a change in the dynamics. We have synthesised a number of related systems with the general formula AFeMF6, where M = Cr, Zn and A = Rb and Cs. We have isolated a previously unreported phase, RbFe1.3Cr0.7F6, that possesses the same charge ordered orthorhombic structure as RbFe2F6 with the space group Pnma. SQUID magnetometry shows RbFe1.3Cr0.7F6 has a magnetic transition at ~10 K with divergence between zero field cooled and field cooled magnetic susceptibility. Neutron scattering confirms the system is a spin glass, and no long ranged magnetic order is observed. The magnetically dilute pyrochlore, RbFeZnF6 was synthesised and showed Curie-Weiss behaviour down to 2 K.