Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, USA.
Phys Rev Lett. 2011 Mar 18;106(11):118501. doi: 10.1103/PhysRevLett.106.118501. Epub 2011 Mar 14.
Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and Mössbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.
利用密度泛函理论加 Hubbard U 计算，我们表明地幔下部主要矿物相（Mg，Fe）（Si，Fe）O3 钙钛矿的基态具有高自旋三价铁（S=5/2），位于十二面体（A）和八面体（B）位。随着压力的增加，B 位铁在 40 到 70 GPa 之间经历自旋态交叉到低自旋态（S=1/2），而 A 位铁保持高自旋态。这种 B 位自旋态交叉伴随着明显的体积减小和四极分裂的增加，与最近的 X 射线衍射和穆斯堡尔光谱测量结果一致。异常的体积减小导致在交叉过程中体弹模量显著软化，这表明地幔下部地震速度异常可能的来源。
