Computational and experimental investigation of the transformation of V2O5 under pressure

It has previously been reported that under high-pressure V2O5 (alpha-V2O5) transforms into a layered polymorph beta-V2O5, consisting of V5+O6 octahedra instead of V5+O5-square pyramids. Both polymorphs have a good performance as positive electrode for lithium batteries. In this work, we investigate the pressure-induced (alpha -> beta transformation combining first principles and experimental methods. Density functional theory (DFT) predicts that (alpha-V2O5 transforms to beta-V2O5 at 3.3 GPa with a 11% volume contraction; experiments corroborate that at a pressure of 4 GPa, V2O5 (d = 3.36 g/cm(3)) transformed into a well-crystallized -V2O5, with a much denser structure (d = 3.76 g/cm(3)). beta-V2O5 can be also prepared at 3 GPa, although with a substantial degree of amorphization. The calculated bulk modulus of (alpha-V2O5 (18 GPa) indicates that this is a very compressible structure; this being linked to the contraction along its b-axis (interlayer space) and to a significant decrease of a long V-O distance (V-O approximate to 2.9 A). As a result, the vanadium coordination increases from five (square pyrmamid) in (alpha-V2O5 to six (distorted octahedron), leading to the stabilization of the high-pressure 0) polymorph. This change of the coordination environment of vanadium ions also affects the electrical conductivity. The calculated density of states shows a narrowing of 0.5 V in the band gap for the P polymorph, in comparison to the ambient-pressure material; the measured resistivities at room temperature (10 000 Omega cm in alpha-polymorph and 400 Omega cm in beta-polymorph) reveal that beta-V2O5 is indeed a better electronic conductor than (alpha-V2O5. In view of these results, similar transformations at moderate pressures are expected to occur in other V5+ frameworks, suggesting an interesting way to synthesize novel V5+ compounds with potential for electrochemical devices.