title: Chemical Transport Synthesis, Electrochemical Behavior, and Electronic Structure of Superconducting Zirconium and Hafnium Nitride Halides.
abstract: The layered nitrides beta-MNX (M = Zr, Hf; X = Cl, Br) crystallize in the space group R&thremacr;m with a hexagonal cell of dimensions a = 3.6031(6) Å, c = 27.672(2) Å for beta-ZrNCl, a = 3.5744(3) Å, c = 27.7075(9) Å for beta-HfNCl, and a = 3.6379(5) Å, c = 29.263(2) Å for beta-ZrNBr. Lithium intercalation using n-buthyllithium in hexane solutions leads to solvent free superconductors of formula Li(0.20)ZrNCl, Li(0.42)HfNCl, Li(0.67)HfNCl, and Li(0.17)ZrNBr showing critical temperatures of 12, 18, 24, and 13.5 K, respectively. Whereas several samples of beta-ZrNBr and beta-ZrNCl showed reproducibility in the lithium uptake and in the corresponding critical temperatures, different samples of beta-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 Li(0)-Li(+) whose origin is still unclear. The electrochemical capacity on discharge for beta-HfNCl and beta-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 beta-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 beta-MNX (M = Zr, X = Cl, Br; M = Hf, X = Cl), ZrCl, and Y(2)C(2)Br(2) are reported. The density of states and Fermi surfaces of the beta-MNX phases as well as the relationship between the electronic structure of the beta-ZrNCl and ZrCl are discussed. Despite the structural relationships, the electronic structures near the Fermi level of the beta-MNX and Y(2)Br(2)C(2) phases are found to be very different.
authors.affiliation: Institut de Ciència de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain, Institut de Ciència de Materials de la Universitat de València, P.O. Box 2085, 46071 València, Spain, and Departament de Química-Física, Facultat de Química, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Vlassov
authors.forename: M. Rosa
authors.initials: MR
authors.lastname: Palacín
authors.forename: Daniel
authors.initials: D
authors.lastname: Beltrán-Porter
authors.forename: Judith
authors.initials: J
authors.lastname: Oró-Solé
authors.forename: Enric
authors.initials: E
authors.lastname: Canadell
authors.forename: Pere
authors.initials: P
authors.lastname: Alemany
authors.forename: Amparo
authors.initials: A
authors.lastname: Fuertes