Chemical Transport Synthesis, Electrochemical Behavior, and Electronic Structure of Superconducting Zirconium and Hafnium Nitride Halides

• Published in: Inorganic chemistry 1999-10, Vol.38 (20), p.4530-4538
• Main Authors: Vlassov, Mikhail, Palacín, M. Rosa, Beltrán-Porter, Daniel, Oró-Solé, Judith, Canadell, Enric, Alemany, Pere, Fuertes, Amparo
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic9903127

The layered nitrides β-MNX (M = Zr, Hf; X = Cl, Br) crystallize in the space group R3̄m with a hexagonal cell of dimensions a = 3.6031(6) Å, c = 27.672(2) Å for β-ZrNCl, a = 3.5744(3) Å, c = 27.7075(9) Å for β-HfNCl, and a = 3.6379(5) Å, c = 29.263(2) Å for β-ZrNBr. Lithium intercalation using n-buthyllithium in hexane solutions leads to solvent free superconductors of formula Li0.20ZrNCl, Li0.42HfNCl, Li0.67HfNCl, and Li0.17ZrNBr showing critical temperatures of 12, 18, 24, and 13.5 K, respectively. Whereas several samples of β-ZrNBr and β-ZrNCl showed reproducibility in the lithium uptake and in the corresponding critical temperatures, different samples of β-HfNCl subjected to the same treatment in n-buthyllithium showed lithium uptakes ranging from 0.07 to 0.67, and corresponding critical temperatures between 0 and 24 K. A linear dependence of T c versus the lithium content is observed when all the superconducting samples are considered. The results obtained from electrochemical lithiation are consistent with those obtained with chemical methods, as samples with larger capacity on discharge are also those found to have larger lithium contents after chemical lithiation. Most samples present a reduction step around 1.8 V vs Li0−Li+ whose origin is still unclear. The electrochemical capacity on discharge for β-HfNCl and β-ZrNBr depends on the milling time spent in the preparation of the electrodes, with long milling times resulting in lower intercalation degree. Possible causes for this effect are either the creation of structural defects (e.g., stacking faults) or some sample decomposition induced by local heating. The same phenomena are proposed to account for the different behavior of β-HfNCl samples, although additional aspects such as the presence of hydrogen, oxygen, or extra hafnium atoms in the structure have to be considered. Tight-binding band structure calculations for β-MNX (M = Zr, X = Cl, Br; M = Hf, X = Cl), ZrCl, and Y2C2Br2 are reported. The density of states and Fermi surfaces of the β-MNX phases as well as the relationship between the electronic structure of the β-ZrNCl and ZrCl are discussed. Despite the structural relationships, the electronic structures near the Fermi level of the β-MNX and Y2Br2C2 phases are found to be very different.