Ab initio study of the stability and elasticity of brucite

Brucite [Mg(OH)₂] is a mineral of great interest owing to its various applications and roles in geological processes. Its structure, behavior under different conditions, and unique properties have been the subject of numerous studies and persistent debate. As a stable hydrous phase in subduction zones, its elastic anisotropy can significantly contribute to the seismological properties of these regions. We performed ab initio calculations to investigate brucite's stability, elasticity, and acoustic velocities. We tested several exchange-correlation functionals and managed to obtain stable phonons for the P3̄ phase with r²SCAN at all relevant pressures up to the mantle transition zone. We show that r²SCAN performs very well in brucite, reproducing the experimental equation of state and several key structure parameters related to hydrogen positions. The room-temperature elasticity results in P3̄ reproduces the experimental results at ambient pressure. These results, together with the stable phonon dispersion of P3̄ at all relevant pressures, indicate P3̄ is the stable candidate phase not only at elevated pressures but also at ambient conditions. The success of r²SCAN in brucite suggests this functional should be suitable for other challenging layer-structured minerals, e.g., serpentines, of great geophysical significance.