Piezoelectric materials, such as BaTiO3 (BT), are of considerable research interest owing to their capacity to couple electrical input/output with mechanical displacement. Materials based on Na0.5Bi0.5TiO3 (NBT) are environmentally friendly lead-free piezoelectric materials that exhibit electrical and piezoelectric properties that are sensitive to nonstoichiometry. Compositional regulation of the stoichiometric ratios is used widely to control the phase assemblage, defect equilibria, and resultant applications. NBT-based materials also have been reported to offer dual photocatalytic and piezocatalytic functionality for degradation of organic pollutants, catalytic water splitting, CO oxidation, and CO2 reduction. The present work is comprised of four discrete sections that report investigations of the effects of compositional regulation on NBT and the effects of defects on BaTiO3. The NBT was synthesized by solid-state reaction at 1000°C for 24 h (phase equilibria) and hydrothermal synthesis at 180°C for 24 h (Bi/Ti and Na/Ti compositional control) while defects in BT were induced in commercial material by the application of different atmospheres at 800°C for 12 h (comparable study). Characterisation included XRD, Raman, SEM, TEM, AFM, PFM/KPFM, XPS/ARXPS, ICP-MS, DSC/TGA, and UV/Vis spectrophotometry. Testing included various approaches to photocatalysis, piezocatalysis, and photoelectrochemistry. In chapter 4 Two revised binary phase diagrams and a complete isothermal section of the ternary system Na2O-Bi2O3-TiO2 at 1000°C were constructed on the basis of (1) comprehensive literature survey of published phases diagrams, (2) experimental XRD and SEM data, and (3) thermodynamic calculations. The latter were enabled by the generation of new thermodynamic data for the twelve phases for which these data were unavailable. In chapte 5 The study on the effects of variable Bi/Ti ratio during hydrothermal synthesis were conducted. As the Bi/Ti ratio decreased, there was a noticeable shift in the phase composition from principally BO (Bi2O3) to BTO (Bi4Ti3O12) to NBT. Additionally, the principal grain shapes altered from equiaxed to platy to spherical. Samples with Bi/Ti ratio of 12/12 exhibited the highest methylene blue (MB) photodegradation of 92% under UV irradiation for 2 h. This excellent photocatalytic performance was attributed to the formation of a heterojunction of BTO on NBT with advantageous band alignment. In chapter 6 Alteration of the Na/Ti ratio through control of the NaOH precursor concentration (2.5 M, 7.5 M, 12.5 M) revealed the formation of a spherical secondary phase Na0.5Bi4.5Ti4O15 (NBT4) dispersed in a continuous matrix of NBT, resulting in the formation of a type-Ⅱ heterojunction (confirmed by DFT modelling). Increasing NaOH concentration facilitated substitution of Bi3+ by Na+ in the NBT solid solution, resulting in the formation of charge-compensating oxygen vacancies (["V" _"O" ^"••" ]). The resultant energy band structure was favorable for the hydrogen evolution reaction (HER). Piezo/photocatalytic hydrogen production showed very efficient HER rates up to 140 μmol/g/h for DI water, 68 μmol/g/h for simulated seawater, and 58 μmol/g/h for natural seawater. In chapter 7 BT was used in a comparable study, albeit for a commercial product whose defect states were enhanced through exposure at O2, N2, Ar, or H2. Optimal results for maximal reduction by H2 were obtained. ARXPS revealed the formation of graded surface-to-bulk reduction, which exposed for the first time a discrete interface between the surface and subsurface. The associated "V" _"O" ^"••" formation resulted in the establishment of an energy band structure favorable to the HER by piezo/photocatalysis, yielding rates up to 132.4 μmol/g/h for DI water, 63.4 μmol/g/h for simulated seawater, and 48.7 μmol/g/h for natural seawater.