Superconducting Metallocene Intercalation Compounds of β-ZrNCl

Abstract

The metal nitride halides, MNX (M ) Ti, Zr; X ) Cl, Br, I), were first reported by Juza and Heners in 1964 along with a high temperature -polymorph for ZrNCl and ZrNBr. 1 Subsequently, Ohashi et al. have shown that an alternative chemical vapor transport synthesis of -ZrNCl and -ZrNBr yields more highly crystalline samples of these compounds. 2, 3 We have recently reported that these host lattices are in fact isostructural with rhombohedral SmSI. 4,5 In this structure each Zr atom is seven-coordinate and is best described as being in a distorted monocapped octahedral arrangement with three halide atoms and three N atoms adopting a facarrangement with a fourth capping nitrogen above the N 3 face. The octahedra are distorted by the displacement of the Zr atom from the center toward the capped N 3 face to such an extent that the Zr and N atoms become essentially coplanar. These monocapped octahedral units are arranged in an edge-sharing manner, forming the XZrNNZrX layer structure (Figure 1).The intercalation of lithium into -ZrNCl was first reported in 1984 and subsequent studies showed that other alkali metals could also be intercalated into this lattice. 6,7 Interest in these compounds was recently increased following reports that they become superconducting at temperatures below 15K. 8,9 In a more recent study, intercalation of lithium ions in the isostructural host lattice -HfNCl was shown to yield Li 0.48 (THF) yHfNCl, which exhibits a superconducting transition temperature of 25.5K. 10 Metallocenes and in particular cobaltocenes have previously been intercalated into a range of host lattices including MX 2 (M ) Ti, Zr, Nb, Ta, Sn; X ) S, Se), 11 MOCl (M ) Fe, V, Ti), 12 MPS 3 (M ) Mn, Cd, Fe), 13 and Zr(HPO 4 ) 2 ‚H 2 O. 14 These intercalates have been widely studied with respect to their structural, electronic, and magnetic properties. In this study we report the metallocene intercalation chemistry of -ZrNCl.-ZrNCl was prepared under conditions reported previously and intercalation was achieved by using excess guest solutions in THF of cobaltocene {Co(Cp) 2 ; Cp ) η-C 5 H 5 }, 1,1′-dimethylcobaltocene {Co(Cp′) 2 ; Cp′ ) η-C 5 H 4 Me}, and decamethylcobaltocene {Co(Cp*) 2 ; Cp* ) η-C 5 Me 5 } at 60°C. 2,3 In each case, complete intercalation was confirmed by the absence of Bragg reflections characteristic of -ZrNCl in the XRD pattern with the stoichiometry of the guest determined by elemental analysis. The characterizing data are summarized in Table 1. As the interlayer separations of the cobaltocene and 1,1′-dimethylcobaltocene are the same and less than that observed for the decamethylcobaltocene intercalate, it can be deduced that the guest molecules are orientated with their principal axes parallel to the host layers. This orientation is the same as has been observed for cobaltocene intercalates of other layered systems and is shown schematically in Figure 1. [15][16][17][18][19] We have also attempted intercalation reactions with other organometallic guests with similar redox potentials, including chro...

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

“…By-products in synthesis at low temperatures are assigned to constitute of (NH 4 ) 2 [ZrCl 6 ] [21] and the a-phase of ZrNCl [19]. This gas-solid reaction technique could be expanded to b-ZrNBr [5,29,30] and especially to b-HfNCl [7,31], while the a-phase is the product for the other group IV metal nitride halides [32]. In most reports it is noted that the metal dihydride was reacted with NH 4 X at 650 C (M ¼ Zr, Hf; X ¼ Cl, Br) to avoid a decomposition of the product into ZrNH 0.6 [33] or ZrN [5,19].…”

Transition Metal Pnictide-Halides: A Class of Under-Explored Compounds

“…A true structural description proved difficult due to the layered nature of the samples giving rise to preferred orientation, which complicates analysis by X-ray diffraction. Neutron diffraction, performed by Shamoto et al, supported by several X-ray diffraction studies provided the currently accepted crystal structure of b-ZrNCl [42][43][44][45][46]. The space group was identified as R " 3m (a = 3.5974(2) Å , c = 27.5480(20) Å , c = 120°).…”

A new intermediate intercalate in superconducting sodium-doped hafnium nitride chloride

“…Superconductivity in ZrNX or HfNX is induced by intercalation of alkaline metals or Lewis bases as cobaltocene or pyridine into the van der Waals gap. [1][2][3]5,7 For sodium intercalated hafnium nitride chloride a superconducting phase has been reported with critical temperatures of 20, 23 or 24 K 8-11 and in lithium intercalated HfNCl, critical temperatures of 18, 20, 24 and 26 K have been observed in different samples showing in some cases co-intercalated organic molecules. 2,12,13 In both systems as well as in doped zirconium nitride chloride that shows lower critical temperatures (close to 12 K), the influence on the critical temperature of factors such as the nature of the transition metal, the co-intercalated molecules and the doping level have still not been completely understood.…”