Pressure-induced structural and electronic transitions in FeOOH from first principles

Using density-functional theory, we investigate the stability, structural, magnetic, and electronic properties of the iron oxyhydroxide polymorphs [alpha-, beta-, gamma-, and hp(epsilon)-FeOOH] under hydrostatic pressure. At ambient conditions goethite (alpha) is the lowest energy phase, consistent with recent calorimetric measurements. Around 6-7 GPa we predict a transformation to the high-pressure hp(epsilon) phase. This structural transformation is followed by a high-spin to low-spin transition at 7.7 GPa, at much lower pressure than for other currently discussed iron-bearing minerals. While in the ground state the Fe(3+) ions are coupled antiferromagnetically, at high pressures a strong competition to a ferromagnetic alignment is found in hp(epsilon)-FeOOH. Concerning the electronic properties, including an on-site Coulomb repulsion parameter U (LDA/GGA+U method) improves the size of the zero-pressure band gaps substantially but shifts the spin transition to higher pressure (56.5 GPa). The predicted spin crossover is associated with a blueshift of 0.4 eV.