Intercalation chemistry of superconducting zirconium and hafnium nitride halides

Abstract

Lithium intercalation in MNX (M=Zr, Hf; X=Cl, Br) may lead to superconducting compounds with critical temperatures up to 24 K. 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 these compounds indicating differences in the intercalation reaction pathway. High-resolution electron microscopy images confirm the structural model isotypic to SmSI for β-MNX phases. The major part of crystals from ZrNX as well as from the HfNCl samples showing a high lithium intercalation degree exhibit a regular stacking of the layers, being almost free of defects. In contrast, samples with low lithium uptakes show a high proportion of crystals with a HfO₂ layer at their thin edges. In agreement with these results, magnetic measurements for Li_xHfNCl indicate, in contrast to Li_xZrNX compounds, small superconducting fractions and very broad transitions consistent with a distribution of critical temperatures and a heterogeneous nature of the samples.