Ab initio calculations of the elasticity of hcp-Fe as a function of temperature at inner-core pressure
Ab initio calculations of the elasticity of hcp-Fe as a function of temperature at inner-core pressure
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DOI：
10.1016/j.epsl.2009.10.015
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发表时间：
2009-11
影响因子：
5.3
通讯作者：
L. Vočadlo;D. Dobson;I. Wood
中科院分区：
文献类型：
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作者：
L. Vočadlo;D. Dobson;I. Wood
利用从头算有限温度分子动力学模拟方法，计算了六方密堆积(HCP)Fe在~300 GPa处的弹性常数随温度的变化关系。纵向模数c11随着温度的升高而减小，这与以前的计算结果完全相反，但与在常压下对其他过渡金属的实验观察结果一致。C33和C44也随温度升高而降低，而C12和C23略有升高。当这些模数被用来计算通过晶体的P波速度时，各向异性的感觉是VPi沿c轴最快，直到5000K；然而，到5500K，各向异性反转，Vp在a-b平面上变得更快。这表明，对于以沿c轴方向取向的hcp-Fe晶体为主的内核来说，观察到的各向同性外内核可能是由于hcp-Fe处于接近熔融的温度，此时平行于c轴和垂直于c轴的轴向波速变得相似，而在内核更深的地方，铁离熔化越远，各向异性越强，平行于极轴的P波速度越快。因此，不需要其他机制，例如成分或晶体排列的变化，来解释观测到的地球核心地震各向异性随深度的变化。
Ab initio finite temperature molecular dynamics simulations have been used to calculate the elastic constants of hexagonal-close-packed (hcp) Fe as a function of temperature at ~300GPa. The longitudinal modulus c11decreases with temperature, in stark contrast to previous calculations, but in agreement with experimental observations on other transition metals at ambient pressures. c33and c44also decrease with temperature, while c12and c23slightly increase. When these moduli are used to calculate P-wave velocities through the crystal, the sense of the anisotropy is such that VPis fastest along the c-axis up to 5000K; however, by 5500K the anisotropy reverses with VPbecoming faster in the a–b plane. This suggests that, for an inner core dominated by crystals of hcp-Fe aligned with the c-axis in the polar direction, the observed isotropic outer–inner core could result from the hcp-Fe being at a temperature close to melting where the axial wave velocities parallel and perpendicular to the c-axis become similar, while at greater depths in the inner–inner core, where iron is further from melting, stronger anisotropy is achieved with the faster P-wave velocities parallel to the polar axis. No other mechanisms, such as changes in composition or crystal alignment, are therefore required to account for the observed change in seismic anisotropy of the Earth's inner core with depth.