The Structure and Entropy of Alloys Studied by Mossbauer Diffractometry and Nuclear Resonant Scattering

35年来，穆斯堡尔效应在材料科学中的几乎所有应用都利用了它作为光谱学的能力。许多研究者已经从穆斯堡尔原子的不同晶体相或化学环境中鉴定和量化了光谱成分。然而，穆斯堡尔散射也可以是相干的，这使得它可以用于第四种材料衍射实验（其他三种是x射线、电子和中子衍射）。穆斯堡尔衍射的独特之处在于它的光谱选择性。本程序利用穆斯堡尔衍射的化学环境选择性来研究金属合金中的无序性。穆斯堡尔光谱法的化学灵敏度用于选择特定化学环境中的原子，然后从具有该特定环境的原子中测量衍射图案。目前已有一种基于CCD照相机的区域探测器，并要求为法兰克福的一个小组开发的一种新型充满气体的区域探测器提供资金。对化学顺序不完善的57Fe3Al和57FeRh合金进行了研究。在这些研究中，测量了不规则化学环境的空间周期性（除DO3和B2有序结构的铁位点外的铁位点）。使用法兰克福探测器，应该可以测量准晶Al-Cu-Fe合金中不同铁环境下的衍射模式。如果探测器的问题得到解决，将在研究过程中进行穆斯堡尔衍射的同步加速器实验。在过去的几年里，第三代同步加速器源的进步使得测量伴随声子激发的核激发成为可能。这些非弹性谱可以用来获得材料中铁原子态的声子偏密度。这项关于非弹性核共振散射的工作将展示铁原子的振动如何对不同合金相的振动熵做出贡献。非弹性核共振散射最近表明，Fe3Al的振动熵几乎完全取决于化学短程序（而不是远程序）。该提案描述了对Pt3Fe和FeRh中铁原子的振动如何依赖于化学顺序的测量，铁原子的振动如何依赖于FeAl中的点缺陷浓度，以及准晶和结晶Al-Cu-Fe中铁原子的振动如何不同。在大约25,000篇关于材料穆斯堡尔光谱研究的出版物之后，穆斯堡尔效应已经很好地建立起来，并且非常适合于材料科学的许多研究。它的扩展到非弹性核共振光谱法，对于研究像薄膜这样的小样品的原子振动是有用的，因为中子方法是不实用的。三种衍射方法（x射线、电子和中子）的互补性使它们在材料科学中得到了广泛的应用。穆斯堡尔衍射法的第四种方法是有发展空间的。

9816617FultzFor 35 years, almost all applications of the Mossbauer effect in materials science have utilized its capabilities as a spectroscopy. Many investigators have identified and quantified spectral components from different crystallographic phases or chemical environments of the Mossbauer atom. Mossbauer scattering can also be coherent, however, enabling its use for a fourth type of diffraction experiment on materials (the other three being X-ray, electron, and neutron diffraction). The unique feature of Mossbauer diffraction is its spectroscopic selectivity. This program studies disorder in metallic alloys by the chemical environment selectivity of Mossbauer diffraction. The chemical sensitivity of Mossbauer spectrometry is used to select an atom in a particular chemical environment, and a diffraction pattern is then measured from atoms having that particular environment. An area detector based on a CCD camera is now available, and funds are requested for a new type of gas-filled area detector developed by a group in Frankfurt. Studies are on alloys of 57Fe3Al and 57FeRh having imperfect chemical order. In these studies, measurements are made of the spatial periodicities of irregular chemical environments (Fe sites other than the Fe sites of the DO3 and B2 ordered structures). With the Frankfurt detector, it should be possible to measure diffraction patterns from different Fe environments in a quasicrystalline Al-Cu-Fe alloy. If detector issues are resolved, some synchrotron experiments on Mossbauer diffraction will be performed during the course of this research. In the last couple of years, advances at third generation synchrotron sources have made it possible to measure nuclear excitations accompanied by phonon excitations. These inelastic spectra can be used to obtain the phonon partial densities of states of Fe atoms in the material. This work on inelastic nuclear resonant scattering will show how vibrations of Fe atoms contribute to the vibrational entropy of different alloy phases. Inelastic nuclear resonant scattering recently showed that the vibrational entropy of Fe3Al depends almost entirely on chemical short-range order (as opposed to long-range order). This proposal describes measurements to be made on how the vibrations of Fe atoms depend on chemical order in Pt3Fe and FeRh, how Fe vibrations depend on point defect concentrations in FeAl, and how Fe vibrations differ for quasicrystalline and crystalline Al-Cu-Fe.%%%After approximately 25,000 publications on Mossbauer spectrometry studies of materials, the Mossbauer effect is well established and well suited for many studies in materials science. Its extension to inelastic nuclear resonant spectrometry should be useful for studies of atom vibrations in small samples such as thin films for which neutron methods are not practical. The complementary nature of the three diffraction methods (X-ray, electron, and neutron) has sustained their widespread use in materials science. There is room for a fourth method of Mossbauer diffractometry.