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 (P1̄) ⟶ FeVO₄-I’ (P1̄) ⟶ FeVO₄-II’ (α-MnMoO₄-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 band-gap 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.