Synthesis of transparent mixed vanadia/niobia gels and their decomposition to a new metastable VNb9O25 phase

Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)3, and niobium pentaethoxide, [Nb(OEt)5]2, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb9O25. At lower temperatures, metastable solid solutions with TT-Nb2O5 structure (600 °C) and M-Nb2O5 structure (700 °C) are formed from the amorphous xerogel. The new VNb9O25 phase is structurally related to M-Nb2O5. The solid solution with M-Nb2O5 structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb9O25. The full transformation of this metastable phase to the well known tetragonal VNb9O25 requires a annealing temperature of about 1000 °C.