The hydrothermal method is an interesting synthesis process for obtaining new phases. Many bismuth oxides with trivalent (Bi3+), pentavalent (Bi5+), and mixed-valence bismuth (Bi3+ and Bi5+) have been synthesized by hydrothermal reactions using NaBiO3·nH2O as the starting material. This exciting starting compound has produced a variety of crystal structures via hydrothermal reactions. The hydrothermal products may adopt PbSb2O6-, ilmenite-, tritutile-, pyrochlore-, fluorite-, and perovskite-type structures, which are never achieved by high-temperature solid-state reactions. Some of them exhibit interesting properties such as photocatalytic activity, superconductivity, and magnetic frustration. It is not too much to say that the hydrothermal route employing NaBiO3·nH2O opens a new world of crystal chemistry of bismuth oxides.
