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Mixed-metal oxides were prepared via hydrothermal synthesis and characterised using various analytical techniques. Three different class of materials were studied, namely perovskites, hexagonal perovskites and doped TiO2. By using hydrothermal synthesis, control on the crystal size, morphology and phase purity was achieved, which are difficult by conventional methods. A new titanate perovskite solid solution with nominal composition, NaCe1-xLaxTi2O6, containing three different metals on the A-site was produced by a single step hydrothermal synthesis. Rietveld analysis of powder X-ray and neutron diffraction data enabled the space group R c to be assigned for the whole series. The particle size and morphology can be changed by varying the solvent and NaOH concentration. 23Na MAS NMR showed that a genuine solid solution was produced, with no A-site ordering present, while 2H MAS NMR of the samples prepared in D2O showed the inclusion of D2O in the lattice in place of Na, greatest for the cerium-containing materials, in which oxidation to Ce4+ can occur. The preparation of B-site substituted perovskites was possible via hydrothermal synthesis. The effect of substituting Zr for Ti was studied for two different perovskites namely NaLaTi2O6 and NaBiTi2O6. For NaLaTi2O6, Zr substitution had a drastic effect on the crystal morphology. Other factors that can affect the morphology such as the solvent, reaction time, temperature and NaOH concentrations were also investigated. In the case of NaBiTi2O6, the effect of Zr substitution on ferroelectric and piezoelectric properties was studied. Pure hexagonal YMnO3 was achieved by a …