Figure 3.

Absorption spectra and schematic energy level diagrams. (a) Experimental absorption spectra. Black lines correspond to β-Cu₂V₂O₇ at 0.11 GPa. Red lines correspond to γ-Cu₂V₂O₇ at 0.43 GPa. Dashed and dotted lines correspond to the linear fits determining the band gap energy. The band gap energies of β- and γ-Cu₂V₂O₇ were experimentally determined to be 1.91 and 1.73 eV respectively. Dashed lines correspond to Gaussian fits determining the d-d transition energy. Inset: the experimental band gap as a function of pressure. The individual fitted spectra are shown in Figure S2. (b) Schematic energy level diagrams of the elongated octahedral CuO₆ (nearly D_4h) and elongated pyramidal CuO₅ (nearly C_4v) complexes.