Crystal structure, stability, and electronic properties of hydrated metal sulfates MSO₄(H₂O)_n (M=Ni, Mg; n=6, 7) and their mixed phases: A first principles study

Removal of Mg from hydrated Ni sulfates has long been a problem in the industrial purification process of hydrated Ni sulfates. In this work, we have investigated this industrial problem using state-of-the-art molecular simulations. Periodic Density Functional Theory (DFT) and cluster DFT calculations are used to study the crystal structures and phase stability of the hexahydrated and heptahydrated Ni and Mg sulfates and their mixed phases. The calculated lattice parameters of MSO₄(H₂O)_n (M=Ni, Mg; n=6, 7) crystals are in good agreement with available experimental data. The relative energy differences of the mixed phase for both hexahydrated and heptahydrated Ni/Mg sulfates obtained from both the periodic and cluster DFT calculations are generally less than kT (25.8meV, T=300K), indicating that a continuous solid solution is formed. We also investigated the Bader charges and electronic structures of the hexahydrated and heptahydrated Ni/Mg sulfates using the periodic DFT calculations. The energy band gaps of the hexahydrated and heptahydrated Ni and Mg sulfates were predicted by first-principles calculations. Large energy band gaps of about ~5.5eV were obtained from the DFT-GGA calculations for hydrated Mg sulfates, and band gaps of about ~5.1eV were obtained by the DFT-GGA+U calculations for hydrated Ni sulfates.