Hydrothermal synthesis of Bi-based pyrochlore oxides: a review of synthesis strategies, composition, morphology and photocatalytic applications.

The development of efficient, controllable, and sustainable synthesis strategies is a central goal in modern materials science. Among the available routes for inorganic material preparation, liquid-phase methods particularly hydrothermal synthesis offers superior control over crystal growth, morphology, and composition under relatively mild conditions. Hydrothermal synthesis enables the formation of highly crystalline materials, precise particle-size control, and access to metastable phases that are often difficult to obtain through conventional solid-state reactions. This method has been widely applied to the synthesis of complex oxide structures, including perovskite- and pyrochlore-type compounds. Pyrochlore oxides (A2B2O7) have attracted considerable interest due to their broad range of functional properties, such as photocatalysis, energy storage, electrocatalysis, dielectric behavior, and ion conductivity. In particular, Bi-based pyrochlores exhibit enhanced visible-light absorption arising from the Bi3+ 6s2 lone-pair electrons, making them promising semiconductor materials. This review summarizes recent advances in the hydrothermal synthesis of Bi-based pyrochlore oxides, highlighting synthesis strategies, compositional variations, morphology and photocatalytic applications. This review paper is systematically divided into several sections to provide a clear and coherent discussion. These sections cover hydrothermal synthesis methods and fundamental aspects of pyrochlore structures, followed by detailed discussions of bismuth-based alkali metals, alkaline earth metals, transition metals, and group 14 and 15 elements pyrochlore oxide systems synthesized by hydrothermal method. This classification allows for systematic analysis of synthesis routes, compositional variations, morphology and their photocatalytic properties.