Ab initio quantum-mechanical study of the effects of the inclusion of iron on thermoelastic and thermodynamic properties of periclase (MgO)

We calculated the thermodynamic and thermoelastic properties of periclase and ferropericlase, the latter having a stoichiometric composition of (Fe0.03Mg0.97)O, at pressures and temperatures which are typical of the Earth’s lower mantle. The static lattice energies and vibrational frequencies were derived through ab initio calculations carried out at the hybrid HF/DFT level. The thermodynamic properties were calculated by following a standard statistical-thermodynamics approach, within the limit of the quasi-harmonic approximation. A third-order Birch–Murnaghan equation of state fit to the static E(V) data of periclase yielded K 0 = 163.8 GPa, K′ = 4.3, and V 0 = 75.09 Å3. The fit at 300 K and 0.1 MPa on the P(V) data yielded K 0 = 160.1 GPa, K′ = 4.2, and V 0 = 75.99 Å3. Such results successfully reproduced the best available experimental and previous computational data. The presence of iron with low-spin configuration in the structure had the effects (1) to reduce the cell volume, both at the static (74.19 Å3) and at the ambient conditions (75.14 Å3); (2) to increase the bulk modulus (respectively 172.2 GPa at the static limit, and 167.4 GPa at 298 K and 0.1 MPa), and (3) to decrease the thermal expansion (2.79 * 10^−5 K−1 for periclase and 2.60 * 10^−5 K−1 for ferropericlase at 300 K). Since the discussed parameters were also calculated at high pressure and temperature conditions simultaneously, the reliability of the quasi-harmonic approximation was tested by evaluating the shape of the potential energy curve, at conditions which simulate those of the Earth’s lower mantle. Such test confirmed the applicability of this approximation over all the P/T range considered.