Using density functional theory plus Hubbard U calculations, we show that
the ground state of (Mg,Fe)(Si,Fe)O3​ perovskite, a major mineral phase in
the Earth's lower mantle, has high-spin ferric iron (S=5/2) at both the
dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site
iron undergoes a spin-state crossover to the low-spin state (S=1/2), while
the A-site iron remains in the high-spin state. Our calculation shows that the
B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied
by a noticeable volume reduction and an increase in quadrupole splitting,
consistent with recent X-ray diffraction and M\"ossbauer spectroscopy
measurements. The volume reduction leads to a significant softening in the bulk
modulus, which suggests a possible source of seismic velocity anomalies in the
lower mantle.Comment: 11 pages, 4 figures, 1 tabl