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.

Superconducting nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

“…b-polymorphs for all elemental combinations of interest can be prepared by highpressure synthesis (see paragraph 2.3). A further development of the gas-solid reaction was made by the group of Fuertes et al [41][42][43] in 1999 when they prepared the nitride halides just by annealing of elemental Zr or Hf with NH 4 X (X ¼ Cl, Br) at 850 C. Hence, application of an ammonia stream is not necessary, but hydrogen is still present in the products [43]. When the reaction is performed in fused silica tubes, the produced hydrogen according to Eq.…”

The staging influence on the electronic structure and transport of superconducting sodium-doped hafnium nitride chloride

“…those responsible for the transport, may be associated with a relatively low effective mass for low band fillings (essentially, around the K point) but with a high effective mass for higher band fillings (essentially, around the C point). Thus, as noted in several theoretical studies 7,[27][28][29] there may be important changes in the transport properties provided that the region associated with the flat part of the band is attained. At this point the following question comes to the fore: does staging, with the associated different occupation of the sites in the van der Waals gaps, influence the degree of intercalation necessary to reach these levels?…”

“…[1][2][3][4][5] Special attention among those which have recently been the object of much interest and debate is deserved by the intercalated zirconium and hafnium nitride halides. [6][7][8][9] Although the pristine material is a band gap insulator, sodium intercalated hafnium nitride chloride has been shown to be superconducting with critical temperatures of 20-24 K, [10][11][12][13] which rank among the highest for inorganic non-oxide materials. Hafnium nitride chloride exhibits two different polytypes, a and b, 14 the second of which leads to the superconducting compounds.…”

“…1a) as a stacking of hexagonal [Cl-Hf-N-N-Hf-Cl] double layers along the c axis. [6][7][8][9] In the intercalated samples the sodium atoms reside in the van der Waals gaps in between the layers. One of the factors which quite often does not receive attention enough when trying to correlate the degree of intercalation, i.e.…”

10 Titanium, zirconium and hafnium

“…The lithium uptake of HfNCl depends on conditions, with lithium uptakes vaying from 0.07 to 0.67 and critical temperatures showing a linear dependence on Li content. 20 , with products such as [ZrNCl(CoCp 2 ) 0.10 ] being superconductors below 14 K, the highest transition temperature yet observed for a metallocene intercalation compound. 21 Oxides and other binary and related compounds A logical but neat outcome of interest in TiO 2 as a photocatalyst is the ¢nding of photocatalytic oxidation of trichloroethylene using TiO 2 coated optical ¢bres.…”