An investigation was made of the microstructure of a H-storage alloy, (La0.8Mg0.2)Ni3.5, with block-stacking superstructure using electron diffraction and Z-contrast scanning transmission electron microscopy, with particular focus on the type of stacking fault structures and possible occurrence of novel structural variants. It was found that 2 major phases coexisted in the alloy, which were of 5:19-2H type and 5:19-3R type superstructures that were constructed by the common structural blocks but with different stacking sequences. A high density of stacking faults was often observed, most of which was of inter-block-layer type that did not change the intra-block structure but simply altered the local stacking sequence of the blocks. As a minor phase in the alloy, a novel polytype structure was identified and represented as 5:19-12R, whose long-period block-stacking sequence was described as ABCA'CABC'BCAB'.

Stacking Faults and a Novel Structural Polytype in a Hydrogen-Storage (La0.8Mg0.2)Ni3.5 Alloy with Block-Stacking Superstructures. R.Ishikawa, E.Abe: Materials Transactions, 2009, 50[5], 943-7