Chemical transport synthesis, electrochemical behavior, and electronic structure of superconducting zirconium and hafnium nitride halides

The layered nitrides beta-MNX (M = Zr, Hf; X = Cl, Br) crystallize in the s pace group R (3) over bar m with a hexagonal cell of dimensions a 3.6031(6) Angstrom, c = 27.672(2) Angstrom for beta-ZrNCl, a 3.5734(3) Angstrom, c = 27.7075(9) Angstrom for beta-HfNCl, and a 3.6379(5) Angstrom, c 29.263(2) Angstrom for beta-ZrNBr. Lithium intercalation using n-buthyllithium in hex ane solutions leads to solvent free superconductors of formula Li0.20ZrNCl, Li0.42HfNCl, Li0.67HfNCl, and Li0.17ZrNBr showing critical temperatures of 12, is, 24, and 13.5 K, respectively. Whereas several samples of beta-ZrNB r and beta-ZrNCl showed reproducibility in the lithium uptake and in the co rresponding critical temperatures, different samples of beta-HFNCl subjecte d to the same treatment in n-buthyllithium showed lithium uptakes ranging f rom 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 a ll the superconducting samples are considered. The results obtained from el ectrochemical lithiation are consistent with those obtained with chemical m ethods, as samples with larger capacity on discharge are also those found t o have larger lithium contents after chemical lithiation. Most samples pres ent 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 d epends on the milling time spent in the preparation of the electrodes, with long milling times resulting in lower intercalation degree. Possible cause s for this effect are either the creation of structural defects (e.g., stac king faults) or some sample decomposition induced by local heating. The sam e phenomena are proposed to account for the different behavior of beta-HfNC l samples, although additional aspects such as the presence of hydrogen, ox ygen, 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 = H f, X = Cl), ZrCl, and Y2C2Br2 are reported. The density of states and Fermi surfaces of the beta-MNX phases as well as the relationship between the el ectronic structure of the beta-ZrNCl and ZrCl are discussed. Despite the st ructural relationships, the electronic structures near the Fermi level of t he beta-MNX and Y2Br2C2 phases are found to be very different.