铁的电子自旋相变对下地幔方镁铁矿光学性质的影响

Effects of the electronic spin-transition in iron on optical properties of lower-mantle ferropericlase

摘要:

本文采用第一性原理方法，计算了(Mg0.8125，Fe0.1875)O方镁铁矿理想晶体和含氧离子空位点缺陷晶体在下地幔压力范围内的光学性质.理想晶体吸收谱数据表明，与晶体场理论预测的结果一致，铁在高压下的电子自旋相变将导致方镁铁矿吸收光谱显著蓝移，从而使得近红外区域出现了透明的现象.然而，令人感兴趣的是，含空位缺陷晶体的结果与晶体场理论的结果有本质差别，却与实验观测一致：电子自旋相变将使得近红外区的吸收性显著增强.同时，含空位缺陷晶体的折射率结果显示，压力、波长以及电子自旋相变对下地幔方镁铁矿的折射率有较明显的影响。

Abstract:

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.