The structure of planar defects and defect phases in Laves phase alloys and their influence on hydrogen storage properties
The atomic arrangements in extended planar defects in different types of Laves phases is studied by high-resolution scanning transmission electron microscopy. To understand the role of such defect phases for hydrogen storage, their interaction with hydrogen will be investigated.
Compositional deviations from the ideal stoichiometry of compounds result in defects of the crystal lattice. Laves phases are one of the most frequent types of compounds with a comparably simple crystal structure and an AB2 stoichiometric composition. In this case, deviations from the ideal AB2 composition are mainly compensated by anti-site atoms. Only near the boundaries of their composition ranges, where the number of excess atoms is greatest, extended planar defects occur, which are stabilized by the excess atoms. Interestingly, it has been recently found [1] that in the case of Nb-rich NbFe2 Laves phase, these planar faults contain structural motifs that are characteristic of the crystal structure of the neighboring phase in the system (Fig. 1). For the Fe-Nb system, this is the µ phase Nb6Fe7, which - similar to the Laves phase - is a topologically close-packed (tcp) phase and has a crystal structure that is structurally related to that of the Laves phase. Therefore, in this case the occurrence of structural motifs from the µ phase as part of the planar defects could be understandable. However, these observations raise the question of whether the atomic arrangement in such planar defects will always resemble that of the neighboring phases, or whether motifs of the µ phase structure must also be expected in systems in which such a µ phase does not exist. In order to answer this fundamental question about the structural arrangement of atoms in planar defects, three systems with Laves phases (Cr-Nb, Mn-Zr, Fe-Ti) were chosen in which no µ phase exists. NbCr2 and ZrMn2 are examples where the Laves phase is the only intermetallic compound of the system, i.e., the neighboring phases for example on the Nb- and Zr-rich side are the solid solutions bcc (Nb) and hcp (αZr), respectively. In the case of TiFe2, the neighboring phase on the Ti-rich side is the intermetallic compound FeTi with a bcc-based B2-type crystal lattice, which is not a tcp structure and is crystallographically not related to the Laves phase structure.
Hydrogen interaction with such kind of extended defects is a second topic of the project. Laves phase alloys are known since long as promising materials for hydrogen storage. The planar defects and defect phases in off-stoichiometric Laves phases are expected to have profound effects on the hydrogen storage properties. In cooperation with the group Advanced Transmission Electron Microscopy (Christian Liebscher), various types of such planar faults are investigated by HRSTEM (high resolution scanning transmission electron microscopy) and the local hydrogen distribution in hydrogen-charged samples will be studied by atom probe tomography.
![Fig. 1: Atomically resolved STEM images of three different variants of extended planar defects in the matrix of a Nb-rich NbFe2 Laves phase. The layer stacks marked on the right side by upward and downward triangles indicate structural units characteristic of the Laves phase, while the circles mark Nb-rich layer stacks with atomic arrangements that occur in the µ phase Nb6Fe7. White arrows mark the positions of the local twinning mirror planes. The viewing direction is [11-20] for all images and the vertical axis is the basal plane normal (figure taken from [1].](/4961201/original-1712818231.jpg?t=eyJ3aWR0aCI6ODQ4LCJmaWxlX2V4dGVuc2lvbiI6ImpwZyIsIm9ial9pZCI6NDk2MTIwMX0%3D--40df645ad91dc9b6e8c808e6989a8bbc735fffa3 "Fig. 1: Atomically resolved STEM images of three different variants of extended planar defects in the matrix of a Nb-rich NbFe2 Laves phase. The layer stacks marked on the right side by upward and downward triangles indicate structural units characteristic of the Laves phase, while the circles mark Nb-rich layer stacks with atomic arrangements that occur in the µ phase Nb6Fe7. White arrows mark the positions of the local twinning mirror planes. The viewing direction is [11-20] for all images and the vertical axis is the basal plane normal (figure taken from [1].")
![Fig. 1: Atomically resolved STEM images of three different variants of extended planar defects in the matrix of a Nb-rich NbFe2 Laves phase. The layer stacks marked on the right side by upward and downward triangles indicate structural units characteristic of the Laves phase, while the circles mark Nb-rich layer stacks with atomic arrangements that occur in the µ phase Nb6Fe7. White arrows mark the positions of the local twinning mirror planes. The viewing direction is [11-20] for all images and the vertical axis is the basal plane normal (figure taken from [1].](/4961201/original-1712818231.jpg?t=eyJ3aWR0aCI6MjQ2LCJvYmpfaWQiOjQ5NjEyMDF9--81eeaaa8b428f7fe8a5e59486969dd8b4a6d1e96)
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