Preparation of bismuth oxides with mixed valence from hydrated sodium bismuth oxide

Hydrothermal treatment up to 200 °C of the hydrated sodium bismuth oxide, NaBiO3.nH2O, resulted in the formation of some bismuth oxides. The major phase is α-Bi2O3 above 190 °C, a bismuth oxide with mixed valence around 160 °C and monoclinic Bi2O4 around 140 °C. Monoclinic Bi2O4 is a new bismuth oxide with mixed valence, Bi3+Bi5+O4, isostructural with β-Sb2O4. Its lattice parameters area = 12.3668(2),b= 5.1180(1),c = 5.5670(1)A˚and β = 107.838(1) ° in the monoclinic system. Both bismuth oxides containing Bi5+ change to α-Bi2O3 on heating in air at 600 °C, accompanied by weight loss caused by reduction of Bi5+ to Bi3+, monoclinic Bi2O4 via an intermediate phase during the course of pyrolysis.