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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The elastic properties of hcp-Fe1-xSix at Earth's inner-core conditions
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The elastic properties of hcp-Fe1-xSix at Earth's inner-core conditions

机译：hcp-Fe1-xSix在地球内核条件下的弹性

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The density of the Earth's inner core is less than that of pure iron and the P-wave velocities and, particularly, the S-wave velocities in the inner core observed from seismology are lower than those generally obtained from mineral physics. On the basis of measurements of compressional sound velocities to similar to 100 GPa in diamond-anvil cells, extrapolated to inner-core pressures, it has been suggested that both the inner-core density and P-wave velocity can be matched simultaneously by the properties of a hexagonal-close-packed (hcp) Fe-Si or Fe-Ni-Si alloy. In this paper we present the results of ab initio molecular dynamics simulations of hcp-Fe-Si alloys at 360 GPa and at temperatures up to melting. We find that although the inner-core density can be readily matched by an Fe-Si alloy, the same is not true for the wave velocities. At inner-core temperatures, the P-wave velocity in hcp-Fe-Si remains equal to, or slightly above, that of hcp-Fe and shows little change with silicon content. The S-wave velocity is reduced with respect to that of pure hcp-iron, except for temperatures immediately prior to melting, where the velocities are almost equal; this is a consequence of the fact that the strong temperature dependence of the shear modulus that was seen in similar simulations of hcp-Fe just prior to melting was not found in hcp-Fe-Si, and so in this temperature range the reduced S-wave velocity of pure iron closely matches that of the alloy. Our results show that for an hcp-Fe-Si alloy matching the inner-core density, both the P-wave and the S-wave velocities will be higher than those observed by seismology and we conclude, therefore, that our calculations indicate that inner core velocities cannot be explained by an hcp-Fe-Si alloy. The opposite conclusion, obtained previously from experimental data measured at lower pressures, is a consequence of: (i) the necessarily large extrapolation in pressure and temperature required to extend the experimental results to inner-core conditions and (ii) the use of a velocity-density relationship for pure hcp-iron that is now considered to be incorrect. (C) 2016 The Authors. Published by Elsevier B.V.
机译：地球内核的密度小于纯铁的密度，并且特别是从地震学观察到的内核中的S波速度低于通常从矿物物理学获得的S波速度。根据在金刚石-铁砧室中近似于100 GPa的压缩声速的测量值，外推到内芯压力，已表明内芯密度和P波速度可以通过这些特性同时匹配六方密堆积（hcp）Fe-Si或Fe-Ni-Si合金。在本文中，我们介绍了hcp-Fe-Si合金在360 GPa和高达熔融温度下的从头算分子动力学模拟的结果。我们发现，尽管内芯密度可以很容易地由Fe-Si合金来匹配，但对于波速却并非如此。在内核温度下，hcp-Fe-Si中的P波速度保持等于或略高于hcp-Fe的速度，并且随硅含量的变化很小。相对于纯六方铁，S波的速度有所降低，但熔化之前的温度几乎是相同的;这是由于以下事实的结果：在hcp-Fe-Si中未发现类似在熔炼之前的hcp-Fe模拟中所见的剪切模量与温度的强相关性，因此在此温度范围内，还原S纯铁的波速与合金的波速非常接近。我们的结果表明，对于匹配内核密度的hcp-Fe-Si合金，P波和S波的速度都将高于地震学观察到的速度，因此我们得出的结论是，我们的计算表明堆芯速度不能用hcp-Fe-Si合金来解释。相反的结论是先前从较低压力下测得的实验数据得出的，其结果是：（i）将实验结果扩展到内芯条件所需的必要的压力和温度外推法;以及（ii）使用速度纯hcp铁的密度关系现在被认为是不正确的。 （C）2016作者。由Elsevier B.V.发布

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