地球内核条件下六方晶系铁的轴比
The axial ratio of hcp iron at the conditions of the Earth's inner core
摘要：
我们提出了从头计算的高温轴向c/a比的六方密堆积（hcp）铁在地球的核心压力，以帮助解释所观察到的地震各向异性的内核。计算是基于密度泛函理论，这是众所周知的预测高压铁的性能具有良好的准确性。c/a的温度依赖性是通过在固定体积和温度下最小化亥姆霍兹自由能来确定的，其中由于晶格振动使用谐波理论计算的热贡献。非谐修正的谐波预测估计从计算的热平均应力从从头算分子动力学模拟六方晶系铁在内核的条件。我们发现轴比随温度的增加而逐渐增加。这种增加比早期计算中发现的要小得多，但与最近的高压，高温衍射测量结果合理一致。这一结果对早期对内核地震各向异性的解释提出了质疑。
英文摘要：
We present ab initio calculations of the high-temperature axial c/a ratio of hexagonal-close-packed (hcp) iron at Earth's core pressures, in order to help interpret the observed seismic anisotropy of the inner core. The calculations are based on density functional theory, which is known to predict the properties of high-pressure iron with good accuracy. The temperature dependence of c/a is determined by minimising the Helmholtz free energy at fixed volume and temperature, with thermal contributions due to lattice vibrations calculated using harmonic theory. Anharmonic corrections to the harmonic predictions are estimated from calculations of the thermal average stress obtained from ab initio molecular dynamics simulations of hcp iron at the conditions of the inner core. We find a very gradual increase of axial ratio with temperature. This increase is much smaller than found in earlier calculations, but is in reasonable agreement with recent high-pressure, high-temperature diffraction measurements. This result casts doubt on an earlier interpretation of the seismic anisotropy of the inner core.
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