A mechanochemical route for the synthesis of VNbO5 and its structural re-investigation using structure solution from powder diffraction data

Abstract

A new and solvent-free synthesis route has been adopted and optimized to prepare crystalline VNbO₅ from the mechanochemical reaction between Nb₂O₅ and V₂O₅ as starting reagents. The substantially amorphous mixture of equimolar pentoxide V and Nb metals observed after extended mechanical treatment transforms into a crystalline powder following calcination under mild conditions at 710 K. The structure solution of the X-ray diffraction pattern using a global optimization approach, combined with Rietveld refinement, points to a space group P2₁2₁2₁ (no. 19) different from Pnma (no. 62) previously proposed in the literature assuming it to be isostructural to VTaO₅. The new space group helps to describe weak peaks that remained previously unaccounted for and allows more reliable determination of atomic fractional coordinates and interatomic distance distribution. The as-prepared VNbO₅ has been tested as a dopant (5 wt%) for the purpose of solid state hydrogen storage, decreasing significantly the release of hydrogen of MgH₂/Mg (620 K) and further enhancing the hydrogen sorption kinetic properties.