Otte, Katrin; Pentcheva, Rossitza; Schmahl, Wolfgang W.; Rustad, James R.
Pressure-induced structural and electronic transitions in FeOOH from first principles.
In: Physical Review B, Vol. 80, No. 20
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Using density-functional theory, we investigate the stability,structural, magnetic, and electronic properties of the iron oxyhydroxidepolymorphs [alpha-, beta-, gamma-, and hp(epsilon)-FeOOH] underhydrostatic pressure. At ambient conditions goethite (alpha) is thelowest energy phase, consistent with recent calorimetric measurements.Around 6-7 GPa we predict a transformation to the high-pressurehp(epsilon) phase. This structural transformation is followed by ahigh-spin to low-spin transition at 7.7 GPa, at much lower pressure thanfor other currently discussed iron-bearing minerals. While in the groundstate the Fe(3+) ions are coupled antiferromagnetically, at highpressures a strong competition to a ferromagnetic alignment is found inhp(epsilon)-FeOOH. Concerning the electronic properties, including anon-site Coulomb repulsion parameter U (LDA/GGA+U method) improves thesize of the zero-pressure band gaps substantially but shifts the spintransition to higher pressure (56.5 GPa). The predicted spin crossoveris associated with a blueshift of 0.4 eV.