Local Atomic Arrangements and Phase Stability in Ultrathin Metal Alloy Films

在这项研究中，表面漫散射实验和原子尺度的计算建模紧密结合，以调查的性质的原子间相互作用的超薄金属合金薄膜的能量学和热力学性质。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、薄膜组成和厚度以及衬底化学和晶体几何形状的影响。这项实验工作的重点是通过测量表面x射线散射扩散强度的化学短程秩序和薄膜合金中的局部原子位移的调查。这些实验主要在先进光子源（ANL）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，探索了许多新技术，用于提供用于表面漫散射的直射光束的强度，所述表面漫散射是沿表面沿着行进的倏逝波。一旦在倒易空间中的许多点处获得绝对尺度上的数据，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的漫散射强度的贡献。这种详细的结构信息提供了重要的见解的性质，原子间的相互作用，在薄膜合金相稳定性。 理论上的努力集中在一个微观框架的发展计算化学和弹性的贡献，在超薄外延合金薄膜的有序和无序结构的能量。这种方法利用有序合金薄膜的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，从该模拟中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，从这些模拟和表面漫散射测量获得的结构信息之间进行详细的比较。 本体组成-温度相图仅提供了对超薄（即，厚度为单个到几个单层）合金膜。特别地，已经普遍观察到，在体晶相中不混溶的两种元素在超薄膜中形成稳定的混合合金相。此外，最近已经证明，由不混溶金属的合金组成的超薄膜显示出在体中未观察到的独特形式的横向的纳米级组成有序。与这些新的金属合金相相关的潜在的新颖的催化和磁性特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。***

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. ***