Fig. 4
RAS and XRD of \(\hbox {V}_{2}\hbox {O}_{5}\) crystals annealed at 600, 700, 800 and 900 K in (a–d). (a) 600 K; Significant changes in the RAS induced by the initial 20 minute anneal. Minima-maxima structure (2.7–3.1 eV) in 4 and 6 h spectra. Optical anisotropy is reduced with additional annealing. XRD is characterised by \(\hbox {V}_{2}\hbox {O}_{5}\)<010> and \(\hbox {V}_{4}\hbox {O}_{9}\)<001>. (b) 700 K; Initial 20 min anneal induces a sharp negative structure at 2.8 eV. Cumulative anneals of 1, 2 and 3 h characterised by minima-maxima structure (2.3–2.9 eV). Additional annealing reduce the optical anisotropy. XRD characterised by strong \(\hbox {V}_{2}\hbox {O}_{5}\)<010> and \(\hbox {V}_{4}\hbox {O}_{9}\)<001> and comparatively weak \(\hbox {V}_{6}\hbox {O}_{13}\)<001>. (c) 800 K; Strong IR structure in all spectra. XRD dominated by \(\hbox {V}_{2}\hbox {O}_{5}\)<010> and \(\hbox {V}_{6}\hbox {O}_{13}\)<001>. \(\hbox {V}_{4}\hbox {O}_{9}\)<001> only observed for the initial 20 minute anneal. (d) 900 K; Considerable positive IR structure and negative structure around 2.5 eV. Spectra is \(\hbox {V}_{2}\hbox {O}_{5}\)<010>-like after 2 and 3 h of cumulative annealing. XRD dominated by \(\hbox {V}_{2}\hbox {O}_{5}\)<010> and \(\hbox {V}_{6}\hbox {O}_{13}\)<001>.
