Structural study on novel two-dimensional superconductor NaxHfNCl

The structure of the novel two-dimensional superconductor, NaxHfNCl (x = 0. 29, T-c = 20 K) is determined by powder neutron diffraction measurement. Na xHfNCl is isomorphous to Li0.16ZrNCl, which also exhibits superconductivity with T-c = 15 K, The nearest interatomic distance of Hf-Hf (3.24 Angstrom at 4 K) is longer than that of Zr-Zr (3.10 Angstrom at 4 K) in Li0.16ZrNCl. This structural change corresponds to the decrease in the transfer integra l of the d bands in the Hf system, i.e. t(Hf)/t(Zr) similar to 0.85, indica ting the enhancement of the density of states in NaxHfNCl. (C) 1999 Elsevie r Science Ltd. All rights reserved.