Theoretical Investigations of Mantle and Core Materials

我们将使用新的电子结构方法来更好地了解铁在地幔和地核中矿物中的作用。我们将限制对我们了解地球很重要的物理性质，包括导致密度、声速、电导率和导热性变化的相变。过渡金属和过渡金属氧化物的性质预测是准确预测地球材料性质的关键问题。直到现在才发展出包含成功理论的基本要素的技术。这是一个深刻的问题，30多年来，理解这些材料的实验和理论一直是研究的活跃领域。这项工作的目标是：(1)为固体地球建模提供有用的预测；(2)更好地理解矿物行为并帮助解释实验数据；(3)为过渡金属氧化物和含过渡金属钙钛矿等高度相关材料开发和测试方法；(4)为实验设计提供指导。术语“第一性原理”意味着没有直接使用实验数据；性质是从基础物理学中计算出来的，使用原子核及其电荷的位置作为基本输入。含有铁和其他过渡金属的材料在密度泛函理论中存在问题，而密度泛函理论是材料第一性原理研究的标准。例如，w<s:1> stite （FeO)，重要的下地幔相镁（Mg,Fe）O）的端元，根据常规能带理论预测为金属，但它是绝缘体。LDA+U模型对反铁磁（AFM）有序菱面体w<e:1>和镁ow<e:1>结构给出了一个空白，但对室温或高温顺磁立方结构不能给出空白，也不能正确预测绝缘体到金属的转变。铁金属本身也有问题的迹象。人们普遍认为地球的内核是由六方密排铁（hcp）和少量的轻元素组成的。理论预测铁的AFM基态低于50 GPa，但到目前为止还没有实验证实hcp-Fe中的磁序。铁-镍的理论和实验存在明显的差异，理论预测可观察到超精细场，但同步加速器Mössbauer实验没有观察到磁性的迹象。我们最近的工作表明，hcp铁在内核条件下的弹性毕竟不能解释观测到的地震各向异性。因此内核不可能是纯hcp铁合金。Ni和其他微量元素稳定了fcc结构，使内核可能由两大相组成，从而解释了地震观测到的非均质性。我们将利用动态平均场理论（DMFT）结合密度泛函理论（DFT）来解决矿物和铁金属中的铁问题。与标准能带理论和LDA+U不同，DMFT包括动态量子和热波动，这被认为是正确描述过渡金属氧化物的关键。

We will use new electronic structure methods to better understand the effects of iron in minerals in Earth's mantle and core. We will constrain physical properties important for our understanding of the Earth, including phase transitions that are responsible for changes in density, sound speeds, and electrical and thermal conductivity. Predicting properties of transition metals and transition metal oxides is a remaining key problem in accurate prediction of properties of Earth materials. Only now have the techniques been developed that include the basic ingredients of a successful theory. This is a deep problem, and experiments and theory to understand these materials have been active areas of research for over 30 years. The goal of this work is to (1) make predictions useful for modeling of the solid Earth (2) better understand mineral behavior and help in interpreting experimental data, (3) develop and test methods for highly correlated materials such as transition metal oxides and transition metal bearing perovskites, and (4) provide guidance for the design of experiments. The term 'first-principles' means that no experimental data are directly used; properties are computed from fundamental physics using as basic input the positions of the atomic nuclei and their charges.Materials containing iron and other transition metals are problematic for density functional theory, the standard for first-principles studies of materials. For example, wüstite (FeO), end-member of the important lower mantle phase magnesiowüstite ((Mg,Fe)O), is predicted to be a metal by conventional band theory, but is an insulator. The LDA+U model, with which much progress has been made, gives a gap for antiferromagnetically (AFM) ordered rhombohedral wüstite and magnesiowüstite, but cannot give a gap for the room temperature or high temperature paramagnetic cubic structure, nor does it correctly predict insulator to metal transitions. There are also indications of problems for iron metal itself. Earth's inner core is widely believed to consist of hexagonal close-packed (hcp) iron with a few percent light elements. Theory predicts an AFM ground state of iron below 50 GPa, but to date there is no experimental confirmation of magnetic order in hcp-Fe. There is a clear discrepancy in theory and experiment for Fe-Ni, where theory predicts observable hyperfine fields, but synchrotron Mössbauer experiments observe no sign of magnetism. Our most recent work shows that the elasticity of hcp iron at inner core conditions does not, after all, explain the observed seismic anisotropy. The inner core thus cannot be pure hcp iron alloy. Ni and other minor elements stabilize the fcc structure, so that the inner core may consist of two major phases, thus explaining the seismically observed heterogeneity. We will address the problem of Fe in minerals and iron metal using dynamical mean field theory (DMFT) integrated with density functional theory (DFT). Unlike standard band theory and LDA+U, DMFT includes dynamical quantum and thermal fluctuations, which are believed to be crucial in correctly describing transition metal oxides.