[S-IT27] [EE] Carbon in Planetary Interiors

[SIT27-03] Spin crossover of iron in (Mg,Fe)CO₃ ferromagnesite and beyond

*Han Hsu¹、Sheng-Chieh Huang¹ (1.National Central University)

キーワード：spin crossover, carbonate, magnesite, first-principles, Mossbauer

Ferromagnesite [(Mg,Fe)CO₃], an iron-bearing carbonate stable up to 100-115 GPa, is believed to be the major carbon carrier in the earth's lower mantle and play a key role in the earth’s deep carbon cycle. Starting ~100 GPa, ferromagnesite goes through a complicated structural transition. The detail of this transition and the atomic structures of high-pressure (Mg,Fe)CO₃ phases are still highly debated. Here, we use the local density approximation + self-consistent Hubbard U (LDA+U_sc) method to study the iron spin crossover in ferromagnesite with a wide range of iron concentration (12.5-100%). Our calculation shows that this mineral undergoes a crossover from the high-spin (HS) (S = 2) to the low-spin (LS) (S = 0) state at around 45-50 GPa, regardless of the iron concentration [1]. The intermediate-spin (S = 1) state is energetically unfavorable and not involved in spin crossover. The anomalous changes of volume, density, and bulk modulus accompanying the spin crossover obtained in our calculation are in great agreement with experiments. Our calculation also predicts that an abrupt change of the iron nuclear quadrupole splitting, from 2.8 mm/s to 0.3 mm/s, can be observed in Mossbauer spectra at 45-50 GPa as a signature of the HS-LS crossover. Based on these accurate results, I will also discuss why the LDA+U_sc method is necessary to make reliable predictions regarding the structural transition of iron-bearing carbonates in the earth's deep lower mantle, a subject of great geophysical and geochemical interest.

[1] Han Hsu and Sheng-Chieh Huang, Phys. Rev. B 94, 060404(R) (2016).