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

Lithium intercalation in beta-MNX (M = Zr, Hf; X = Cl, Br) leads to superco nducting compounds with critical temperatures between 12 and 24 K. The lith ium uptakes after treatment of the host materials with n-butyllithium/hexan e solutions are ca. 0.2 atoms per formula for beta-ZrNCl and beta-ZrNBr, an d between 0.07 and 0.67 for beta-HMCl. Electrochemical lithiation experimen ts agree with the results obtained with chemical methods, as samples with l arger capacity on discharge are also those having larger lithium contents a fter 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 electroc hemical reversibility indicating that these materials are not suitable for electrochemical applications. High resolution electron microscopy images co nfirm the structural model previously reported, isotypic to SmSI. The major part of crystals from Zr compounds as well as from the beta-HfNCl 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 l ow lithium uptakes show a high proportion of crystals with a HfO2 layer at their thin edges. This constitutes a physical barrier that obstruct the lit hium diffusion through the van der Waals gap and hence the induction of sup erconductivity. Tn agreement with these results, magnetic measurements for LixHfNCl show, in contrast to LixZrNX compounds, small superconducting frac tions and very broad transitions indicating a distribution of critical temp eratures and a heterogeneous nature of the samples. (C) 2000 Editions scien tifiques et medicales Elsevier SAS. All rights reserved.