Iron spin crossover in the NAL phase and ferromagnesite

Spin crossover (SCO) in iron-bearing minerals has attracted tremendous attention, as SCO leads to anomalous changes of the physical properties of these minerals. The local density approximation $+$ self-consistent Hubbard $U$ (LDA$+U_{sc})$ method, with the $U$ parameters computed self-consistently, has elucidated SCO in many lower-mantle minerals. In this talk, two recent LDA$+U_{sc}$ studies of SCO in earth minerals are presented: the new hexagonal aluminous (NAL) phase [1] and (Mg,Fe)CO$_{3}$ ferromagnesite [2]. The former is considered as a main aluminum host in the subducted basalt, and the latter is believed to be the major carbon carrier in the Earth's lower mantle and play a key role in the deep carbon cycle. For both minerals, the abrupt change of iron quadrupole splitting and the volume/elastic anomalies accompanying the SCO obtained in our calculations are in great agreement with experiments. Our calculations also suggest that the spin transition pressure $P_{T}$ in the NAL phase is not very sensitive to temperature, due to its three nearly degenerate low-spin (LS) states, in contrast with (Mg,Fe)O ferropericlase and (Mg,Fe)CO$_{3}$ ferropericlase, in which $P_{T}$ significantly increases with temperature. [1] H. Hsu, submitted. [2] H. Hsu and S.-C. Huang, Phys. Rev. B \textbf{94}, 060404(R) (2016).