Chemical Transport Synthesis, Electrochemical Behavior, and

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Inorg. Chem. 1999, 38, 4530-4538

Chemical Transport Synthesis, Electrochemical Behavior, and Electronic Structure of Superconducting Zirconium and Hafnium Nitride Halides Mikhail Vlassov,†,‡ M. Rosa Palacı´n,† Daniel Beltra´ n-Porter,§ Judith Oro´ -Sole´ ,† Enric Canadell,† Pere Alemany,⊥ and Amparo Fuertes*,† Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain, Institut de Cie`ncia de Materials de la Universitat de Vale`ncia, P.O. Box 2085, 46071 Vale`ncia, Spain, and Departament de Quı´mica-Fı´sica, Facultat de Quı´mica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain ReceiVed March 18, 1999 The layered nitrides β-MNX (M ) Zr, Hf; X ) Cl, Br) crystallize in the space group R3hm 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 Tc 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.

Introduction The recently discovered layered intercalated compounds MxZrNCl, MxZrNBr, and MxHfNCl (M ) alkaline ion) represent a new class of ionocovalent superconductors with record critical temperatures for inorganic non-oxide materials.1-3 The structure of the host lattices of those phases was first reported by Juza et al. in 1964,4 and recently redetermined in a supercell with the new space group R3hm.3,5,6 The layered compounds β-MNX (M †

Institut de Cie`ncia de Materials de Barcelona (CSIC). Permanent address: Earthcrust Research Institute, St. Petersburg University, Russia. § Institut de Cie ` ncia de Materials de la Universitat de Vale`ncia. ⊥ Universitat de Barcelona. (1) Yamanaka, S.; Hohetama, K.; Kawaji; H. Nature 1998, 392, 580582. (2) (a) Yamanaka, S.; Kawaji, H.; Hotehama, K.; Ohashi, M. AdV. Mater. 1996, 8, 771. (b) Kawaji, H.; Hotehama, K.; Yamanaka, S. Chem. Mat. 1997, 9, 2127. (3) Fuertes, A.; Vlassov, M.; Beltra´n-Porter, D.; Alemany, P.; Canadell; Casan˜-Pastor, N.; Palacı´n, M. R. Chem. Mater. 1999, 11, 203. (4) Juza, R.; Friedrichsen, H. Z. Anorg. Allg. Chem. 1964, 332, 173178. (5) Fogg, A. M.; Evans, S. O.; O’Hare, D. Chem. Commun. 1998, 2269. (6) Shamoto, S.; Kato, T.; Ono, Y.; Miyazaki, Y.; Ohoyama, K.; Ohashi, M.; Yamaguchi, Y.; Kajitani, T. Physica C 1998, 306, 7. ‡

) Hf, Zr; X ) Cl, Br) crystallize in the rhombohedral SmSI structure.7 They are built up by double layers of composition -X(MNNM)X- stacking along c that are separated by a van der Waals gap where different species can be intercalated.1-3,8 All atoms are located in 6c sites (0,0,z) with approximate z of 0.12, 0.20, and 0.39 for M, N, and X, respectively. The cell parameters are a = 3.6 Å and c = 27.7 Å (for X ) Cl) and 29.3 Å (for X ) Br). The Zr or Hf metal atoms are bonded to three halide atoms from outside the layer (d(M-X) = 2.7 Å for X ) Cl and 2.9 Å for X ) Br), three nitrogen atoms inside the layer (d(M-N) = 2.1 Å), and one additional nitrogen from the second N sheet (d(M-N) = 2.2-2.3 Å) (Figure 1a).3 The coordination polyhedra described by these ligands is a single capped trigonal antiprism. The new structure may be considered as a result of the intercalation of N atoms in the tetrahedral sites of the ZrCl structure. ZrCl shows the same space group R3hm and similar cell parameters (a ) 3.424(2) Å and c ) 26.57(4) Å) (see Figure 1b).9,10 The structure of the layered mono(7) Hulliger, F. In Structural Chemistry of Layer-Type Phases; Le´vy, F.; Ed.; D. Reidel Publishing Co.: Dordrecht/Boston, 1976; Vol. 5, p 263. (8) Fogg, A. M.; Green, V. M.; O’Hare, D. Chem. Mater. 1999, 11, 216. (9) Adolphson, D. G.; Corbett, J. D. Inorg. Chem. 1976, 15, 1820.

10.1021/ic9903127 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999

Zirconium and Hafnium Nitride Halides

Figure 1. Crystal structures of (a) the host lattice for the superconductors LixMNX (M ) Zr, Hf; X ) Cl, Br)3 and (b) ZrCl.9 Black and white spheres represent metal and halide atoms, respectively. Nitrogen atoms are placed at the center of the tetrahedra.

halides MX (M ) Sc, Zr, Hf, Y, Ln; X ) Cl, Br) consists of cubic-close-packed layers of metal and halide atoms stacked in pairs, yielding the sequence X-M-M-X with relative orientations AbcA. They span metallic behavior from the physical and chemical points of view. The basic structure of the monohalide is preserved after oxidative chemical reactions, leading to M2X2Z2 or M2X2Z (M ) Sc, Y, Ln; Z ) H, C) and MXZ or MXZ0.5 (M ) Zr, Hf; Z ) H, C, O, N), where the nonmetal atoms (Z) occupy interstitial sites in the double metal layers.11-13 In the zirconium halide derivatives, the anions O2- (in ZrClO0.5),13 C4- (in ZrClC0.5),14 H- (in Zr2X2H),15 and N3- (in β-ZrNX)3 occupy the tetrahedral sites, whereas in the lanthanide derivatives the Z anions occupy either the tetrahedral or the octahedral sites. The insertion of anions modifies the band population and affects the electronic properties, the result being normal valence or semiconducting compounds. As in the alkaline-intercalated β-MNX compounds, superconductivity has also been recently reported for the ternary lanthanide carbide halides Ln2C2X2 (Ln ) La, Y, Lu; X ) Br, I) as well as for their thorium-substituted or sodium-intercalated derivatives.16 The intercalation of alkaline ions in β-ZrNCl either chemically or electrochemically has been investigated since 1984 by Yamanaka et al. because of the potential application of this compound as insertion electrode or electrochromic material.17-19 More recently, the same authors reported superconductivity below 12 K for the lithium-inserted compound Li0.2ZrNCl, and below 25 K for the intercalated hafnium derivative LixHfNCl.2,1 Co-intercalation of organic molecules takes place in polar (10) Ford, J. E.; Corbett, J. D.; Hwu, S. Inorg. Chem. 1983, 22, 2789 and references therein. (11) Corbett, J. D.; McCarley, R. E. New Transition Metal Halides and Oxides. Crystal Chemistry and Properties of Materials with QuasiOne-Dimensional Structures; Rouxel, J., Ed.; Reidel Publishing: Dordrecht/Boston, 1986. (12) Meyer, G.; Hwu, S.; Wijeyesekera, S.; Corbett, J. D. Inorg. Chem., 1986, 25, 4811. (13) Seaverson, L. M.; Corbett, J. D. Inorg. Chem. 1983, 22, 3202. (14) Ford, J.; Corbett, J. D.; Hwu, S. Inorg. Chem. 1983, 22, 2790. (15) Wijeyesekera, S. D.; Corbett, J. D. Inorg. Chem. 1986, 25, 4709. (16) Henn, R. W.; Schnelle, W.; Kremer, R. K. and Simon, A. Phys. ReV. Lett. 1996, 77, 374, and references therein. (17) Yamanaka, S.; Kawaji, S.; Sumihara, M.; Hattori, M. Chem. Lett. 1984, 1403. (18) Ohashi, M.; Yamanaka, S.; Sumihara, M.; Hattori, M. J. Incl. Phenom. 1984, 2, 289. (19) Ohashi, M.; Shigeta, T.; Yamanaka, S.; Hattori, M. J. Electrochem. Soc. 1989, 136, 1086.

Inorganic Chemistry, Vol. 38, No. 20, 1999 4531 solvents such as tetrahydrofuran (THF), dimethylformamide (DMF), or propylene carbonate (PC). This can be done either during the intercalation reaction of alkaline ions or by additional treatment in the organic solvent of the intercalated phases.1,2 The co-intercalation process allows an increase of the alkali metal uptake because of the concomitant larger separation between the layers in the van der Waals gap, but the doping mechanism and the physical properties of the resulting superconductors may be different from those of the solvent free materials. Cobaltocene intercalation on β-ZrNCl leads to superconducting materials with the same doping levels and critical temperatures as those observed in the alkali metal intercalates.8 In a recent communication3 we reported the crystal structures for the host compounds β-HfNCl, β-ZrNCl, and β-ZrNBr, as well as superconductivity below 13.5 K for Li0.17ZrNBr and preliminary band structure calculations. In the same paper superconductivity below 24 K in solvent free LixHfNCl was also reported for the first time. In this paper we present a comparative study of the synthesis, chemical lithiation and electrochemical behavior for different samples of the solvent free superconductors LixZrNCl, LixHfNCl, and LixZrNBr. We also report electronic band structure calculations for the three corresponding host compounds, and comment on the relationship between their electronic structures and that of the parent compound ZrCl. Experimental Section Synthesis of Zirconium and Hafnium Nitride Halides. β-HfNCl, β-ZrNCl, and β-ZrNBr were prepared by the reaction of Hf (Aldrich, 99.5%) or Zr (Alfa, 99.9%) with NH4Cl (Aldrich, 99.9%) or NH4Br (Aldrich, 99.999%) at 850 °C, followed by recrystallization via vapor transport under conditions reported previously.3,20 Whereas for β-ZrNCl and β-ZrNBr we obtained reproducibility in the electrochemical or chemical intercalation behavior, samples with different intercalation capacities were obtained for β-HfNCl using similar conditions in the synthesis (hereafter labeled HfNCl-I to HfNCl-V). The sample HfNClII was recrystallized just by treatment at 850 °C without chemical vapor transport. Chemical Intercalation. Chemical lithiation reactions were done between room temperature and 50° C by dispersion of the powders in a 0.1 M solution of n-buthylithium in hexane (Aldrich), using evacuated Ar-sealed glass tubes. The lithiation time was 140 h. Lithium contents were determined by atomic emission analysis. Before lithiating the samples, elemental analysis was performed to determine N and H contents. Electrochemical Intercalation. Electrochemical tests were made with Swagelok cells. The positive electrode was prepared by manually grinding the sample with 30 wt % black carbon (SP kindly supplied by MMM, Belgium). To investigate the effect of grinding on the capacity for discharge, different milling times were used in some cases for two independent portions of the same sample. Two sheets of Whattman GF/D borosilicate glass fiber, soaked with 1 M LiPF6 in 1:1 EC/DMC (ethylene carbonate/dimethyl carbonate) electrolyte, were used as a separator. The negative electrode consisted of lithium metal foil (0.38 mm thick). Cells were tested on an Arbin BT2042 cycler operating in a galvanostatic low-current mode between 3 and 1.25 V and at a rate of 1 Li atom/200 h. Handling of the lithiated samples and cell preparation were done in an Ar-filled glovebox. X-ray Diffraction, Electron Diffraction, and Transmission Electron Microscopy. X-ray diffraction patterns were taken in Seifert 3000 and Siemens D-5000 diffractometers using Cu KR radiation. Profile refinements, using the Rietveld method, were carried out with the help of the program FULLPROF.21 Electron diffraction patterns, electron (20) Ohashi, M.; Yamanaka, S.; Sumihara, M.; Hattori, M. J. Solid State Chem. 1988, 75, 99. (21) Rodrı´guez-Carvajal, J. Program FULLPROF, Version 2.5, April 1994, Institut Laue-Langevin, unpublished.

4532 Inorganic Chemistry, Vol. 38, No. 20, 1999

Vlassov et al.

Table 1. Exponents and Parameters Used in the Calculations atom

orbital

Hii (eV)

ζ1

Zr

5s 5p 4d 6s 6p 5d 5s 5p 4d 3s 3p 4s 4p 2s 2p 2s 2p

-6.41 -3.77 -6.97 -6.61 -3.84 -6.90 -6.01 -3.62 -5.74 -26.64 -14.22 -25.21 -12.96 -25.37 -13.90 -19.65 -11.13

1.82 1.78 3.84 2.21 2.17 4.36 1.74 1.70 1.56 2.18 1.73 2.59 2.13 1.95 1.95 1.63 1.63

Hf Y Cl Br N C a

ζ2

c1a

c2a

1.51

0.6210

0.5769

1.71

0.6967

0.5322

3.55

0.8213

0.3003

Figure 2. Observed and calculated X-ray diffraction patterns for β-ZrNCl.3

Contraction coefficients used in the double-ζ expansion.

microscopy images, and XEDS analyses were obtained in a JEOL 1210 transmission electron microscope operating at 120 kV, equipped with a side-entry 60/30° double tilt GATHAN 646 analytical specimen holder and a Link QX2000 XEDS element analysis system. The specimens for electron microscopy were prepared by dispersion of the powders in n-heptane and deposition of a droplet of this suspension on a carboncoated holey film supported on an aluminum grid. The particle sizes were estimated by observation of low-resolution transmission electron images for a minimum of 50 crystals of each sample. Magnetic Susceptibility. Magnetic susceptibility measurements were performed in a Quantum Design SQUID magnetometer down to 5 K, on the lithiated powder samples double sealed under Ar atmosphere, in zero-field-cooled and field-cooled conditions (Hmeasd ) 30 G). Band Structure Calculations. The tight-binding band structure calculations use an extended Hu¨ckel Hamiltonian22 and a modified Wolfsberg-Helmoltz formula23 to calculate the nondiagonal Hij matrix elements. Except otherwise stated, a rigid band scheme was used to analyze the effect of the lithiation. The exponents and parameters24 used in the calculations are summarized in Table 1.

Results and Discussion Synthesis, Crystal Chemistry, and Superconducting Properties. All the samples that were purified by vapor transport in the last step of the synthesis (HfNCl-I, HfNCl-III, HfNCl-IV, HfNCl-V, β-ZrNCl, and β-ZrNBr) showed impurity free X-ray diffraction patterns that can be indexed in a hexagonal cell of dimensions a ) 3.6031(6) Å, c ) 27.672(2) Å for β-ZrNCl, a ) 3.5744(3), c ) 27.7075(2) Å for β-HfNCl, and a ) 3.6379(5), c ) 29.263(2) Å for β-ZrNBr, with the space group R3hm.3 Figure 2 shows the observed and calculated X-ray diffraction patterns for β-ZrNCl performed with the program FULLPROF.3 Similar figures corresponding to the pure compounds β-HfNCl and β-ZrNBr are reported in ref 3. The X-ray diffraction pattern of the sample HfNCl-II showed small impurities of HfO2 and Hf2ON2 (see Figure 3), but its crystallinity degree and purity were found to be significantly higher than those of the nontransported samples, i.e., those obtained after the reaction of Hf or Zr with ammonium halide, without further recrystallization. Table 2 shows the lithium intercalation degrees, hydrogen contents, particle sizes, and critical temperatures for the different samples. For β-ZrNCl and β-ZrNBr compounds, reproducibility in the synthesis and in the intercalation behavior was observed. Indeed, (22) Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 6093. (23) Ammeter; J.; Bu¨rgi; H.-B; Thibeault, J.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 3686. (24) Vela; A.; Ga´zquez, J. L. J. Phys. Chem. 1988, 92, 5688.

Figure 3. Powder X-ray diffraction patterns of β-HfNCl samples as prepared after the reaction of Hf and NH4Cl, recrystallized at 850 °C (HfNCl-II), and chemical vapor transported.

different samples obtained in the same synthesis conditions led to the same lithium uptakes upon treatment with n-buthyllithium, similar electrochemical behaviors, and the same critical temperatures. In contrast, β-HfNCl samples showed significant differences in behavior. The amount of lithium intercalated after 140 h of treatment with n-buthyllithium was e0.11 per formula unit for HfNCl-IV and HfNCl-V whereas HfNCl-I and HfNClIII showed a significantly higher lithium uptake, 0.67 and 0.42, respectively. The color of the samples changed to dark gray or black upon lithium intercalation. Lithiated HfNCl-IV and HfNCl-V did not show any superconducting transition down to 4 K, whereas lithiated HfNCl-I, HfNCl-II, and HfNCl-III showed critical temperatures of 24, 18, and 18 K, respectively. The observation of a Tc of 18 K for a lithium content lower than 0.67 differs remarkably from that found for samples treated with naphthyllithium in the previous work from Yamanaka et al.1 In that case, the solvent tetrahydrofuran (THF) was systematically co-intercalated with lithium in the van der Waals gap, and consequently their samples were formulated as Lix(THF)yHfNCl. In these samples the critical temperatures varied only from 25.5 to 24.4 K for a wide range of lithium contents, i.e., between y ) 0.13 and y ) 0.97. This difference in behavior suggests that the co-intercalated molecules may play a role in the charge-transfer process, and that a different doping mechanism may operate in the superconducting solvent free compounds. Compared to LixHfNCl, the solvent free zirconium samples allowed a lower intercalation level. For the two compounds LixZrNCl and LixZrNBr only one superconducting

Zirconium and Hafnium Nitride Halides

Inorganic Chemistry, Vol. 38, No. 20, 1999 4533

Table 2. Particle Sizes, Chemical Analysis, and Critical Temperatures for MNX sample MNX

color

particle size (for ∼70% of crystals) (µm)

HfNCl-I HfNCl-II HfNCl-III HfNCl-IV HfNCl-V β-ZrNCl β-ZrNBr

light gray light gray light gray dark gray, metal brightness dark gray, metal brightness yelow greenish golden