Rethinking the magnetic properties of lepidocrocite: A density functional theory and cluster expansion study

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The iron oxy-hydroxide lepidocrocite (𝛾γ-FeOOH) is an abundant mineral critical to a number of chemical and technological applications. Of particular interest are the ground state and finite temperature magnetic order and the subsequent impact this has upon crystal properties. The magnetic properties investigated in this work are governed primarily through superexchange interactions and have been calculated using density functional theory and cluster expansion methods. Quantification of these exchange terms has facilitated the determination of the ground state magneto-crystalline structure and subsequent calculation of its lattice constants, elastic moduli, cohesive enthalpy, and electronic density of states. Based upon the morphology and coupling constants, the Heisenberg quasi-1D spin 1/2 AFM chain model is justified. The resulting magnetic heat capacity vs temperature has been studied and the Néel temperature is obtained and in good agreement with experimental values. This resolves a long-standing discrepancy between the experimentally measured behavior and what might be expected from this class of mineral.