Local Atomic Arrangements and Phase Stability in Ultrathin Metal Alloy Films

超薄金属合金薄膜中的局部原子排列和相稳定性

• 批准号: 0076063
• 负责人: Mark Asta
• 金额: $ 45万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2000
• 资助国家: 美国
• 起止时间: 2000-09-01 至 2004-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0076063&HistoricalAwards=false
• 关键词: Local; Atomic; Arrangements; Phase; Stability

In this research surface-diffuse-scattering experiments and atomic-scale computational modeling are closely coupled in order to investigate the nature of the interatomic interactions governing the energetics and thermodynamic properties of ultra-thin metal-alloy films. The long-term goal is the development of a theoretical framework for predicting how the atomic structure and phase stability of specific alloy systems are affected by variations in temperature, the composition and thickness of the film, and the chemistry and crystalline geometry of the substrate. This experimental effort focuses on the investigation of chemical short-range order and local atomic displacements in thin-film alloys through measurements of surface x-ray scattering diffuse intensities. These experiments are carried out primarily at the Advanced Photon Source, ANL. In order to derive quantitative structural information from these experiments it is necessary to measure the diffuse intensity on an absolute scale. For this purpose, a number of new techniques are explored for providing the intensity of the direct beam for surface diffuse scattering, which is the evanescent wave traveling along the surface. Once data is obtained on an absolute scale at many points in reciprocal space, chemical short-range-order and atomic-displacement contributions to the diffuse scattered intensity of ultra-thin metallic alloy solid solutions can be extracted. Such detailed structural information provides important insights into the nature of the interatomic interactions governing phase stability in thin-film alloys. The theoretical effort focuses on the development of a microscopic framework for calculating both chemical and elastic contributions to the energetics of ordered and disordered structures in ultra-thin epitaxial alloy films. This approach makes use of the results of electronic structure calculations of the total energies and relaxed atomic structures of ordered alloy films to extract interatomic interaction parameters and force constants from first principles. These parameters form the basis for efficient Monte-Carlo simulations from which thin-film alloy structural and thermodynamic properties are calculated. In order to validate and further guide the development of the computational approach, detailed comparisons are performed between the structural information obtained from these simulations and from surface diffuse-scattering measurements. %%%Bulk composition-temperature phase diagrams provide only limited insight into the atomic structures which form in ultra-thin (i.e., a single to a few monolayers in thickness) alloy films. In particular, it has been observed commonly that two elements, which are immiscible in bulk crystalline phases, form stable mixed alloy phases in ultra-thin films. Furthermore, it has been demonstrated recently that ultra-thin films consisting of alloys of immiscible metals display a unique form of lateral, nanometer-scale compositional ordering not observed in the bulk. The potentially novel catalytic and magnetic properties associated with these new metallic alloy phases have given rise to increasing interest in their atomic structures and thermodynamic stability. ***

在这项研究中，表面扩散散射实验和原子尺度计算模型紧密耦合，以研究控制超薄金属合金薄膜的能学和热力学性质的原子间相互作用的本质。长期目标是开发一个理论框架，用于预测特定合金系统的原子结构和相稳定性如何受到温度变化、薄膜的成分和厚度以及衬底的化学和晶体几何形状的影响。本实验通过测量表面X射线散射扩散强度来研究薄膜合金中的化学短程有序和局域原子位移。这些实验主要在ANL的高级光子源上进行。为了从这些实验中获得定量的结构信息，有必要在绝对尺度上测量漫射强度。为此，探索了许多新的技术来提供用于表面漫反射的直接光束的强度，即沿表面传播的逝去波。一旦在倒易空间的多个点上获得了绝对尺度的数据，就可以提取化学短程有序和原子位移对超薄金属合金固溶体扩散散射强度的贡献。这种详细的结构信息对控制薄膜合金中相稳定性的原子间相互作用的性质提供了重要的见解。理论工作的重点是发展一个微观框架，用于计算超薄外延合金薄膜中有序和无序结构的能量学的化学和弹性贡献。该方法利用有序合金薄膜总能量和弛豫原子结构的电子结构计算结果，从第一性原理中提取原子间相互作用参数和力常数。这些参数构成了有效的蒙特卡罗模拟的基础，由此计算薄膜合金的结构和热力学性质。为了验证和进一步指导计算方法的发展，对从这些模拟获得的结构信息和从表面漫反射测量获得的结构信息进行了详细的比较。%体相成分-温度相图仅提供了对在超薄(即，厚度为单到几个单分子层)合金膜中形成的原子结构的有限了解。特别是，人们普遍观察到，在超薄膜中，两种不相容的元素在块状晶相中形成了稳定的混合合金相。此外，最近已经证明，由不相容的金属合金组成的超薄膜显示出一种独特的横向纳米级成分有序化，这在整体上是没有观察到的。与这些新的金属合金相关联的潜在的新颖的催化和磁性引起了人们对它们的原子结构和热力学稳定性的越来越大的兴趣。***