Stacking Faults and a Novel Structural Polytype in a Hydrogen-Storage (La<SUB>0.8</SUB>Mg<SUB>0.2</SUB>)Ni<SUB>3.5</SUB> Alloy with Block-Stacking Superstructures

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We investigated the microstructure of a hydrogen-storage (La 0.8 Mg 0.2 )Ni 3.5 alloy with block-stacking superstructures by electron diffraction and Z-contrast scanning transmission electron microscopy (STEM), particularly focusing on the type of stacking fault structures and possible occurrence of novel structural variants. It was found that two major phases coexist in the alloy, which are of 5:19-2H type and 5:19-3R type superstructures that are constructed by the common structural blocks but with different stacking sequences. A high density of stacking faults were often observed, most of which were of inter-block-layer type that does not change the intra-block structure but simply alters local stacking sequence of the blocks. As a minor phase in the alloy, we identified a novel polytype structure represented as 5:19-12R, whose long-period block-stacking sequence is described as ABCA'CABC'BCAB'.