DOI: 10.1039/C4SC00499J

An unprecedented re-entrant mechanism in which HfNCl host layers transform to an alternative geometry and then revert to their original structure, has been discovered during the synthesis of superconducting Na_0.5HfNCl by ultra-slow electrochemical intercalation of sodium. Both the host and final product contain hexagonal β-HfNCl layers although the stacking sequence changes during a complex reaction sequence involving four intermediate phases. Restacking occurs through a displacive mechanism in which β-type layers transform to a different arrangement, most likely the rectangular layers found in the α-HfNCl polymorph, and then retransform to β-layers. These results reveal that intercalation reactions may proceed by very different mechanisms to those expected in the conventional ‘slab-gliding’ picture.