The high pressure phases β- and δ-V2O5: structure refinement, electrical and optical properties, thermal stability

The results of comprehensive investigation of V2O5 high pressure modifications are presented in this paper. The transformation of α -V2O5 has been studied using a ‘toroid’-type high pressure chamber at pressures of 6.0–8.5 GPa in the temperature region 600–1100°C. δ-V2O5 was obtained after treatment at P=8.0, 8.5 GPa, and T=700–1100°C for 2–10 min. The structure was refined by the Rietveld method from the X-ray diffraction data: a=11.9695(1), b=4.69915(5), c=5.32642(6) Å, β=104.327(1)°, V=290.276(4) Å3, Z=4, space group C2/c. The structural parameters are in good agreement with the values obtained by Balog et al. [V2O5 phase diagram revisited at high pressures and high temperatures. J Alloys Compd. 2007;429:87–98], indicating their independence from the synthesis method. The temperature dependence of the electrical resistivity of δ-V2O5 was studied and compared with the electrical resistivity of α - and β -phases. IR absorption and Raman spectra of the α -, β- and δ-V2O5 were measured. These measurements revealed very low stability of the δ -phase under the laser beam. The thermal stability at ambient pressure of δ-V2O5 was investigated and compared with the stability of β -V2O5.