Untersuchung der elektrischen Hyperfeinwechselwirkung in M n+1 AX n -Phasen mittels der gestörten γ-γ-Winkelkorrelation
Untersuchung der elektrischen Hyperfeinwechselwirkung in M n+1 AX n -Phasen mittels der gestörten γ-γ-Winkelkorrelation
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γ-γ-Winkelkorrelation 中的 M n+1 AX n-Phasen-Phasen 中的超电子干扰分析
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发表时间：
2014
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通讯作者：
D. Jürgens
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作者：
D. Jürgens
Mn+1AXn相是具有金属和陶瓷性质的纳米层状三元碳化物和氮化物。字母M代表前过渡金属，字母A代表来自IIIA-VIA族的A-元素，并且X是碳和/或氮。M原子形成八面体，X原子在它们的中心。指数n描述嵌入在两个六方A层之间的Mn+1AXn层的厚度。这些材料的特殊性能源于其微观结构。为了获得原子水平上的洞察力，扰动γ-γ角关联（PAC）的方法被应用。通过离子注入和/或中子活化将放射性探针111 In/111 Cd和181 Hf/181 Ta插入到主体材料中，以测量作用于探针核位点的电场梯度（EFG）。
工作的第一个主题是寻找最佳的退火参数，使高比例的探针感测到相同的局部环境。下一步是确定MAX结构中探针的晶格位置。结果发现，111 In在In和Al基MAX相中占据A位，而181 Hf在Hf 2 InC中占据M位。作为一个令人惊讶的结果，这项研究表明，PAC方法是敏感的，不同的堆积序列在具有相同的成分，但不同的化学计量比的阶段。
实验进行了比较，广泛的计算的基础上的密度泛函理论（DFT），提出了几乎所有的MAX-化合物的家庭成员的第一次。的DFT计算再现测量的EFG具有高精度和支持的假设，即探针驻留在预测的晶格网站。
Mn+1AXn phases are thermodynamically stable nanolaminated ternary carbides and nitrides showing both metallic and ceramic properties. The letter M stands for an early transition metal, the letter A represents an A-element from group IIIA - VIA, and X is carbon and/or nitrogen. The M-atoms form octahedra with X-atoms in their centers. The index n describes the thickness of the Mn+1AXn-layer which is embedded between two hexagonal A-layers. The exceptional properties of these materials have their origin in the microstructure. In order to gain insight on the atomic level, the method of perturbed γ-γ angular correlation (PAC) is applied. The radioactive probes 111In/111Cd and 181Hf/181Ta are inserted into the host materials by ion implantation and/or neutron activation to measure the electric field gradient (EFG) acting on the site of the probe nuclei.
The first topic of the work is the search for optimal annealing parameters, so that a high a fraction of probes senses the same local environment. The next step is the determination of the lattice site of the probes in the MAX structure. As a result, it is found that 111In occupies in the In- and Al-based MAX phases the A-site, whereas 181Hf occupies in Hf2InC the M-site. As a surprising result this study demonstrates that the PAC method is sensitive to different stacking sequences in phases with the same constituents but different stoichiometric ratios.
The experiments are compared to extensive calculations based on the density functional theory (DFT) which are presented for nearly all members of the family of MAX-compounds for the first time. The DFT calculations reproduce the measured EFGs with high accuracy and support the hypothesis that the probes reside on the predicted lattice sites.