Brucite (Mg(OH)2) under small perturbation: A combined first principles and synchrotron X-ray diffraction study

Abstract

In this work, we discuss the discrepancy observed in previous experimental and theoretical studies in determining hydrogen's position in brucite (Mg(OH)2) at ambient conditions with the aid of temperature dependent synchrotron x-ray powder diffraction (XRPD) and first-principles calculation based on density functional theory. To date, the literature reports two competing descriptions of brucite's ground state structural configuration based on P-3m1 and P-3 symmetry without a conclusive choice for the best solution. Our study confirms that Mg(OH)2 exists in P-3m1 structure at 0 GPa and 0 K from its lattice dynamical and mechanical stability. Transition from P-3m1 to P-3 symmetry occurs under small structural perturbation causing a volume compression. We show with the aid of temperature dependent XRPD that perturbations causing a small volume compression can lead to a transition in brucite from hydrogen ordered (P-3m1 symmetry) to hydrogen disordered (P-3 symmetry) structure. DFT calculations with different exchange-correlation potentials show that the contradicting results among previous static calculations can be resolved by the use of a van der Waals density functional. Further, we report temperature dependent thermal expansion of brucite in the temperature range 110–400 K.