| Abstract: | The TT-, T-, B-, P-, and poorly ordered M-forms of niobium(V) oxide were crystallized hydrothermally by treating niobic acid with pure water and acids such as hydrochloric acid, nitric acid, and sulfuric acid at 250–750°C and 15–100MPa. Triniobium chloride heptaoxide was hydrolyzed in pure water and acids solutions at 250–500°C and 15–98MPa, producing the P-, R-, and B-forms. The formation of the above polymorphs involved either dissolution-precipitation or the solid-phase rearrangement of the structures of amorphous solids. The B-form was shown to be the most stable phase below 760°C by an isothermal-conversion method. The relative stabilities of seven polymorphs of niobium(V) oxide were estimated by observing the rates of conversion of metastable forms into the B-forms in 1.2 mol/dm3 hydrochloric acid. |
