The effect of pressure on the band-gap energy in FePO4 and FeVO4

In this work, we have studied the electronic structure of FePO4 and FeVO4 under high pressure by means of optical-absorption measurements and first-principles calculations. Samples of both compounds have been pressurized up to 6 GPa within a diamond-anvil cell to be characterized in the ultraviolet-visible-near infrared range. The optical-absorption measurements corroborated previously reported phase transitions: berlinite (P3121) -* CrVO4-type (Cmcm) in FePO4, and FeVO4-I (P1 ) -* FeVO4-I' (P1 ) -* FeVO4-II' (aMnMoO4-type, C2/m) in FeVO4. According to our experiments, FePO4 presents direct electronic band-gaps in both phases. In contrast, the low- and high-pressure phases of FeVO4 have indirect band-gaps. In both compounds, the bandgap energy decreases with pressure. The computed electronic band structure revealed that the population at valence band maxima (VBM) and conduction band minima (CBM) changes with pressure in both FePO4 and FeVO4. Pressure-induced changes in Fe coordination modulate the occupation at VBM and CBM, being more pronounced the consequences in FePO4 due to the shift from tetrahedral (FeO4) to octahedral (FeO6) coordination. We have also analyzed the charge density within the quantum theory of atoms in molecules (QTAIM) methodology to complement the electronic structure results by computing the Bader effective charges, the Laplacian, and ellipticity at the bond critical points for both compounds as a function of pressure.