Melting of compressed iron-alloys by monitoring atomic dynamics

地震学和宇宙化学观测表明，地核的主要成分是铁，据认为是由~10 wt%的镍和一些轻元素（如S、Si、O、C、H）组成的合金。地核由一个由液体外核包围的固体内部区域组成。铁在高压下的熔融温度为地核温度分布提供了一个重要的参考点，并影响了与该区域有关的许多重要的地球物理量：穿过核幔边界（CMB）的热流、CMB上方热边界层内的温度梯度、相位关系和候选相位的预期地震波速度。更具体地说，液体外核和固体内核交界处的铁熔点提供了该界面温度的上限，因为迄今为止的研究表明，所有可能的外核液体都与相应的内核合金固体共存于纯铁的熔点或以下。以前，铁在高压下的熔化研究是通过冲击压缩、电阻和激光加热在金刚石砧细胞中使用视觉观察或同步加速器x射线衍射和理论方法进行的。然而，铁相对于其合金的熔化曲线仍然是不确定的，特别是在压力高于70gpa时。PI开发了一种新的测量方法，用于在高压下使用57Fe同步加速器Mössbauer光谱（SMS）检测含铁材料的固液相边界，也称为核正向散射。聚焦同步辐射以1mev的带宽穿过钻石砧细胞内激光加热的含57fe样品。当熔化发生时，特征短信时间签名消失。这个过程用Lamb-Mössbauer因子来描述，这个量与铁原子的均方位移直接相关。因此，我们测量材料中原子的动力学，而不是静态衍射测量。由于该方法监测原子的动力学，SMS技术为高压下材料的熔点测定提供了一种新的、独立的方法。我们已经成功地在高达82 GPa的纯铁上进行了这种熔化研究。与以前的研究相比，我们的数据定义了不同的熔化趋势，从而为在更高压力下进一步使用该方法研究铁和铁合金提供了重要的理论依据。熔化实验将在阿贡国家实验室先进光子源的3-ID-B区进行，合金的压力至少为80 GPa，纯铁的压力高于80 GPa。通过约束这些铁合金在高pt条件下的相对熔化曲线，我们将为选择轻元素合金对铁的影响提供新的约束。S熔化行为。这些结果将代表着理解富铁合金在类地行星核心条件下熔化的重要一步，从而为它们各自的内部提供必要的温度约束。

Seismological and cosmochemical observations indicate that the main constituent in Earth's core is iron, which is thought to be alloyed with ~10 wt% nickel and some light elements (e.g. S, Si, O, C, H). Earth's core consists of a solid inner region surrounded by a liquid outer core. The melting temperature of iron at high-pressure provides an important reference point for the temperature distribution within Earth's core and affects a number of important geophysical quantities related to this region: heat flow across the core-mantle boundary (CMB), the temperature gradient within the thermal boundary layer above the CMB, the phase relations, and expected seismic wave speeds of candidate phases. More specifically, the melting point of iron at the boundary between the liquid outer core and solid inner core provides an upper bound of the temperature at that interface, because studies thus far indicate that all plausible outer core liquids coexist with corresponding inner core alloy solids at or below the melting point of pure iron. Previously, melting studies of iron at high-pressures were performed by shock-compression, resistive- and laser-heating in diamond anvil cells using visual observations or synchrotron x-ray diffraction, and theoretical methods. However, the melting curve of iron with respect to its alloys remains uncertain, especially at pressures above 70 GPa. The PI has developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using 57Fe synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated 57Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure. We have successfully performed such melting investigations on pure iron up to 82 GPa. Our data define a different melting trend compared with previous studies, thus providing important rationale to proceed further with this method on iron and iron-alloys at higher pressures. The melting experiments will be conducted to pressures of at least 80 GPa for the alloys and higher than 80 GPa for pure iron, at sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. By constraining the relative melting curves of these iron-alloys at high-PT conditions, we will provide a new constraint on the effect of select light-element alloying to iron?s melting behavior. These outcomes will represent a significant step towards understanding the melting of iron-rich alloys at conditions of terrestrial-type planetary cores and thus provide necessary temperature constraints of their respective interiors.