铁方镁石中铁自旋交叉的声子色散
Phonon dispersions throughout the iron spin crossover in ferropericlase
摘要：
铁长石（Fp），（Mg $_\mathrm{1-x}$ Fe $_\mathrm{x}$）O，是地球下地幔中第二丰富的相。在相关的压力-温度条件下，Fp中的铁经历高自旋（HS），S=2到低自旋（LS），S=0的状态变化。这种现象的本质现在已经很好地理解了，但是在这种铁态变化期间，关于结构稳定性和软声子模式的存在仍然存在基本问题。存在一般理论来解释体积减小、显著的热弹性异常以及这种HS-LS交叉的广泛性质。这些理论广泛使用准调和近似（QHA）。因此，动力和结构稳定性对其有效性至关重要。在这里，我们调查的振动光谱Fp在整个自旋交叉使用$\textit{ab initio}$ DFT+Usc计算。我们解决振动模式与孤立和（第二）最近邻铁离子进行HS-LS状态变化。正如预期的那样，这种固溶体的声学模式是弹性的，而光学模式是最受影响的。我们发现，有没有软声子模式在这个HS-LS交叉，和FP是动态稳定的在所有相关的压力。
英文摘要：
Ferropericlase (Fp), (Mg$_\mathrm{1-x}$Fe$_\mathrm{x}$)O, is the second most abundant phase in the Earths lower mantle. At relevant pressure-temperature conditions, iron in Fp undergoes a high spin (HS), S=2, to low spin (LS), S=0, state change. The nature of this phenomenon is quite well understood now, but there are still basic questions regarding the structural stability and the existence of soft phonon modes during this iron state change. General theories exist to explain the volume reduction, the significant thermo-elastic anomalies, and the broad nature of this HS-LS crossover. These theories make extensive use of the quasi-harmonic approximation (QHA). Therefore, dynamical and structural stability is essential to their validity. Here, we investigate the vibrational spectrum of Fp throughout this spin-crossover using $\textit{ab initio}$ DFT+Usc calculations. We address vibrational modes associated with isolated and (2nd) nearest neighbor iron ions undergoing the HS-LS state change. As expected, acoustic modes of this solid solution are resilient while optical modes are the most affected. We show that there are no soft phonon modes across this HS-LS crossover, and Fp is dynamically stable at all relevant pressures.
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