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RESEARCH PRODUCT

Lithium intercalation chemistry, microstructure and superconductivity in zirconium and hafnium nitride halides

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Abstract Lithium intercalation in β-MNX (M=Zr, Hf; X=Cl, Br) leads to superconducting compounds with critical temperatures between 12 and 24 K. The lithium uptakes after treatment of the host materials with n-butyllithium/hexane solutions are ca. 0.2 atoms per formula for β-ZrNCl and β-ZrNBr, and between 0.07 and 0.67 for β-HfNCl. Electrochemical lithiation experiments agree with the results obtained with chemical methods, as samples with larger capacity on discharge are also those having larger lithium contents after chemical lithiation. Variations exist in the electrochemical profiles of different batches for the three compounds indicating differences in the intercalation reaction pathway. Both Zr and Hf compounds show poor electrochemical reversibility indicating that these materials are not suitable for electrochemical applications. High resolution electron microscopy images confirm the structural model previously reported, isotypic to SmSI. The major part of crystals from Zr compounds as well as from the β-HfNCI samples showing a high lithium intercalation degree show a regular stacking of the [X–M–N–N–M–X] layers, being almost free of defects. Hf samples exhibiting low lithium uptakes show a high proportion of crystals with a HfO 2 layer at their thin edges. This constitutes a physical barrier that obstruct the lithium diffusion through the van der Waals gap and hence the induction of superconductivity. In agreement with these results, magnetic measurements for Li x HfNCl show, in contrast to Li x ZrNX compounds, small superconducting fractions and very broad transitions indicating a distribution of critical temperatures and a heterogeneous nature of the samples.