(88e) Theoretical Investigation of Iron Spin Crossover Pressure in Fe-Bearing MgO

The ferropericlase (Fp), MgO containing ~20 mol% FeO, is the second abundant mineral in the lower mantle (LM). Within the LM pressure range, Fe in Fp will undergo a spin transition, from high-spin sate to low-spin state, which can notably affect Fp properties. It is important to clarify the pressure range, where these spin-transition induced changes occurring, for understanding the LM properties sufficiently. However, even under room-temperature (T), experimental reported spin-transition pressure ranges show large controversy, while theoretical predicted room-T values including only T effects but not contribution from Fe distribution by assuming ideally periodic Fe substitution in small supercells (~100 atom) are generally less than half of experimental reports. Moreover, existing evidences show that Fe-distribution may play an important role in determining the spin-transition pressure ranges, since spin-transition pressure strongly depends on Fe concentration and local distributions. However, to date, Fe distribution features in MgO as functions of iron concentration and temperature are still missing. By performing Based on the first-principles calculations together with the updated framework, the combination of the cluster expansion approach and the Monte Carlo method, it has been properly described the Fe distribution feature in Fe-bearing MgO as functions of T (300 K – 1,800 K) and Fe concentration (3.125 mol% – 50 mol%) in the supercell containing 93,312 atoms. The results show that at ~900 K, Fe substituted in MgO will experience an inhomogeneous to homogeneous transition, and homogeneous Fe substitution acquiring normal-distribution feature can induce broad spin-transition pressure range comparable to experimental observations. Together with the T effects, it is predicted that Fe spin-transition will occur in a very broad spin-transition pressure range in LM environments, where a continuous and smooth Fp property changes induced by Fe spin-transition can be expected.