The optical properties of (Mg0.8125, Fe0.1875)O ferropericlase without and with O ionic vacancy at the pressure of the Earth’s lower mantle are studied by using the first-principles calculations. The perfect-crystal absorption data indicate that, similar to predictions from the crystal-field theory, the electronic spin transition of iron in ferropericlase under high pressure causes the evident blue-shift of its optical-absorption spectrum, leaving the near-infrared region transparent. However, interestingly, the optical-absorption results calculated in defective crystal are consistent with experimental observations (unlike predictions from the crystal field theory): the spin transition causes the enhancement in the optical absorption at the near-infrared region. Meanwhile, refractive-index data in defect crystal show that the effects of pressure, wavelength, and spin-transition on the high-pressure refractive-index of lower-mantle ferropericlase are relatively evident.
