3174 Chem. Mater. 2010, 22, 3174–3182 DOI:10.1021/cm100243v
Tunable Defect Structure in the Li-Mg-N Ternary Phase System: A Powder Neutron Diffraction Study Andrew S. Bailey,†,^ Peter Hubberstey,† Robert W. Hughes,‡ Clemens Ritter,§ and Duncan H. Gregory.*,‡ †
‡
School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, United Kingdom, and § Institut Laue-Langevin, 6, rue Jules Horowitz, BP 156, F-38042 Grenoble Cedex 9, France. ^ Current address: AWE, Aldermaston, Reading, Berkshire RG7 4PR, United Kingdom. Received January 25, 2010. Revised Manuscript Received March 22, 2010
Defect structures in the Li-Mg-N system can be tuned by control of composition and temperature. A series of compounds in the Li3N-Mg3N2 system have been synthesized via solid state reaction of the binary nitrides. The resulting ternary nitrides have been structurally characterized by variable temperature powder neutron diffraction. LiMgN is orthorhombic (space group Pnma), at room temperature. It undergoes a structural transition above 673 K to a simple cubic antifluorite structure (space group Fm3m). Nitrogen-deficient, magnesium-rich Li0.24Mg2.76N1.92 is isostructural with Mg3N2 and retains a cubic anti-bixbyite structure, I213, from room temperature to 873 K. Li1.11Mg0.89N0.96 retains a simple cubic anti-fluorite structure, (space group Fm3m), over the entire temperature range investigated. However, Li1.09Mg0.91N0.97 crystallizes with the orthorhombic Pnma LiMgN-type structure, providing evidence for the accommodation of variable metal and nitrogen stoichiometry in that structure and expansion of the structure stability field beyond a LiMgN “line phase”. Introduction In the 21st century a key area of materials study is the search for alternative power generation and energy storage means. Dwindling supplies of fossil fuels and the atmospheric pollution associated with their use are the driving force behind this search. In terms of mobile applications, hydrogen is seen as the main contender as, for example, the means of running a fuel cell driven car. The exploitation of hydrogen as a fuel provides challenges in itself. A major challenge is the storage of hydrogen in a practicable manner. Pressurized gaseous or cryogenically liquefied hydrogen systems are considered too bulky and heavy to be used in vehicle applications and can also carry an energy cost to enable the hydrogen to be stored. An alternative is a solid state hydrogen store, allowing sorption into a solid medium, which would weigh less, occupy less space, demand less energy, and be a more practical solution for mobile applications.1,2 There are various types of system that can be employed as storage materials for hydrogen. Until the early part of this decade, the majority of hydrogen storage research had been concerned with either hydrogen in alloys (of which LaNi5 was a classic example) or the physisorption of hydrogen in porous materials (of which various forms *To whom correspondence should be addressed. E-mail: d.gregory@ chem.gla.ac.uk.
(1) Schlapbach, L.; Z€ uttel, A. Nature 2001, 414, 353. (2) Mandal, T. K.; Gregory, D. H. Ann. Rep. Prog. Chem., Sect. A 2009, 105, 21.
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of carbon are examples). The publication in 2002 by Chen et al. on hydrogen storage in lithium nitride signified a new direction in the research into hydrogen storage in complex hydrides.3 In this relatively simple Li-N-H system, the hydrogen is stored by transformation between interconvertible, chemically distinct phases through breaking and making of nitrogen-hydrogen bonds. Much research has been performed on modifications to this system to either increase capacity or to decrease the desorption temperature.4,5 Since the initial report on the Li-N-H system, one of the largest areas of research has focused on the pseudoquaternary nitride-imide-amide system of lithium and magnesium. Various combinations of either MgH2 and LiNH2 or Mg(NH)2 and LiH have been assessed for their storage capacity,6-8 with the highest calculated capacity of 9.1 wt % (experimentally, ∼7 wt % H2 evolved at 527 C) found for a 3:12 ratio of Mg(NH)2 to LiH.9 The outcomes of these reactions vary with different Li:Mg ratios and many combinations result in mixtures of magnesium and (3) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (4) Gregory, D. H. J. Mater. Chem. 2008, 18, 2321. Gregory, D. H. Chem. Rec. 2008, 8, 229. (5) Orimo, S.; Nakamori, Y; Eliseo, J. R.; Z€ uttel, A.; Jensen, C. M. Chem. Rev. 2007, 107, 4111. Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1238. (6) Nakamori, Y.; Orimo, S. Mater. Sci. Eng., B 2004, 108, 48. (7) Nakamori, Y.; Orimo, S. J. Alloys Compd. 2004, 370, 271. (8) Luo, W. J. Alloys Compd. 2004, 381, 284. (9) Nakamori, Y.; Kitahara, G.; Miwa, K.; Ohba, N.; Noritake, T.; Towato, S.; Orimo, S. J. Alloys Compd. 2005, 404 - 406, 396.
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lithium binary phases, whereas others produce single ternary phases (for example Mg(NH2)2 þ 2LiH T Li2Mg(NH)2 þ 2H2).10 A recent publication reported hydrogen storage capacity in LiMgN; formed by the 1:1 reaction of MgH2 and LiNH2, releasing approximately 8.1 wt % H2.11 On rehydrogenation, LiMgN absorbs ∼8 wt % to form Mg(NH2)2, MgH2, and LiH, and hence, in principle, this pseudoreversal system could be a very attractive one. An independent study however claims the true experimental hydrogen capacity of MgH2 þ LiNH2 to be 3.4 wt %, with the remaining weight change attributed to ammonia.12 Furthermore, the final observed product in this latter study is Li2Mg2(NH)3 rather than LiMgN. The end members of the Li3N-Mg3N2 system are wellknown and well-characterized. Li3N has a unique structure consisting of layers of edge-connected Li8N hexagonal bipyramids.13-16 This connectivity produces hexagonal [Li2N] planes which can contain 1-2% lithium vacancies and provide a mechanism for superionic Liþ conductivity. Magnesium nitride, Mg3N2, crystallizes in the anti-bixbyite structure, a defect variant of the (anti-)fluorite structure with the metal occupying 75% of the tetrahedral holes in an approximately cubic close packed array of N atoms.17,18 The 1:1 Li/Mg ternary nitride, LiMgN, has been previously studied by both Juza and Hund19 and Yamane et al.20 The former reported a simple cubic, cationdisordered anti-fluorite structure for LiMgN, whereas the latter reported an orthorhombic structure, isostructural to LiCaN.21 Like that of anti-bixbyite, the LiCaN structure is anti-fluorite related, with an approximately cubic, close packed array of nitrogen atoms. Here, however, half the tetrahedral holes are occupied by M2þ (Ca or Mg), while the lithium ions are slightly displaced from the centers of the remaining tetrahedral holes. Yamane et al. also reported structures for two other compositions in the Li3N-Mg3N2 phase space. In the lithium-rich part of the phase diagram, a phase with a simple cubic antifluorite structure was reported, whereas in the magnesium-rich section of the system, a distorted anti-bixbyite structure is formed with a very close relationship to that of the binary nitride, Mg3N2. Thus as Li replaces Mg, the Li-Mg-N system exhibits rich defect chemistry from one end member, Mg3N2 to the other, Li3N. By replacing Mg by Li, for example, in LiMgN, it could become possible, in principle, to increase the gravimetric uptake (10) Xiong, Z.; Wu, G.; Hu, J.; Chen, P. Adv. Mater. 2004, 15, 1522. (11) Lu, J.; Fang, Z. Z.; Choi, Y. J.; Sohn, H. Y. J. Phys. Chem. C 2007, 111, 12129. (12) Osborn, W.; Markmaitree, T.; Shaw, L. L. J. Power Sources 2007, 172, 376. (13) Rabenau, A.; Schulz, H. J. Less-Common Met. 1976, 50, 155. (14) Schulz, H.; Schwarz, K.-H. Acta Crystallogr., Sect. A 1978, 34, 999. (15) Schulz, H.; Thiemann, K. H. Acta Crystallogr., Sect. A 1979, 35, 309. (16) Gregory, D. H.; O’Meara, P. M.; Gordon, A. G.; Hodges, J. P.; Short, S.; Jorgensen, J. D. Chem. Mater. 2002, 14, 2063. (17) Partin, D. E.; Williams, D. J.; O’Keefe, M. J. Solid State Chem. 1997, 132, 56. (18) Reckeweg, O.; DiSalvo, F. J. Z. Anorg. Allg. Chem. 2001, 627, 371. (19) Juza, R.; Hund, F. Naturwissenschaften 1946, 33, 121. (20) Yamane, H; Okabe, T. H.; Ishiyama, O.; Waseda, Y.; Shimada, M. J. Alloys Comp. 2001, 319, 124. (21) Cordier, G.; Gudat, A.; Kniep, R.; Rabenau, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1702.
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of hydrogen. Further, by analogy to the Li-N-H system,22-24 these defects provide a potential means to modify and control hydrogen uptake and release. Here we report a detailed study of the structure of the ternary Li-Mg-N nitrides as a function of composition and temperature. The temperatures used represent the range and extremes of conditions that might be used in service in a practical hydrogen storage system. In situ powder neutron diffraction has allowed us to investigate the phase behavior, order-disorder relationships and the evolution of (defect) structure with temperature in the Li-Mg-N system for the first time. These findings should have important implications with respect to lithium (and hydrogen) ionic mobility and hence design of new nonstoichiometric hydrogen storage materials in the lithium-magnesium nitrideimide-amide system. Experimental Section Synthesis. Li3N and Mg3N2 reagents were prepared by reaction of the pure metal with nitrogen using a sodium flux as a solvent (Li, Aldrich 99.9%; Mg, Alfa 99.9%; Na, Riedel-de :: Haen>99%).25 All manipulations were carried out in an inert atmosphere. In an argon-filled glovebox a piece of lithium or magnesium (approximately 15 g) was cut from a larger ingot and the covering oxide layer was removed with a file. The clean metal was submerged in molten sodium contained within a stainless steel crucible. The crucible was then sealed inside a stainless steel reaction vessel fitted with a cold finger. The vessel was heated for 48 h at 773-973 K. Upon cooling, the vessel was placed under a vacuum of 10-4 Torr and heated for 24 h at 723 K to remove the sodium. Once cooled to room temperature, the vessel was loaded into an evacuable glovebox and opened in a nitrogen filled atmosphere. Li3N was obtained as a purple-black crystalline solid. Mg3N2 was obtained as a yellow-green solid. Powder X-ray diffraction confirmed phase purity by comparison to ICDD card numbers 30-759 and 35-778, respectively. Li-Mg-N phases were synthesized by the reaction of appropriate molar ratios of Li3N and Mg3N2 based upon the reaction conditions described by Yamane et al.20 All manipulations were performed in an evacuable glovebox filled with nitrogen (1, 2) or argon (3). Mixtures of Li3N and Mg3N2 were intimately mixed and pelletized. In the case of LiMgN (1) and Li0.24Mg2.76N1.92 (2) the pellets were placed in an alumina crucible and sealed inside a stainless steel vessel under a static nitrogen atmosphere. The vessel was then heated to 900 K for 24 h before being furnace cooled. For Li1.10Mg0.90N0.97 (3), the pellets were wrapped in a molybdenum foil liner before being sealed (arc welded) within a stainless steel crucible under argon. The crucible was fired at 1000 K for 48 h under a flow of argon to prevent aerial oxidation before being quenched in ice-water. The crucible was then opened in a nitrogen-filled glovebox using pipe cutters. (22) David, W. I. F.; Jones, M. O.; Gregory, D. H.; Jewell, C. M.; Johnson, S. R.; Walton, A.; Edwards, P. P. J. Am. Chem. Soc. 2007, 129, 1594. (23) Weidner, E.; Bull, D. J.; Shabalin, I. L.; Keens, S. G.; Telling, M. T. F.; Ross, D. K. Chem. Phys. Lett. 2007, 444, 76. (24) Bull, D. J.; Weidner, E.; Shabalin, I. L.; Telling, M. T. F.; Jewell, C. M.; Gregory, D. H. Ross, D. K. Phys. Chem. Chem. Phys. 2010, 12, 2089. (25) See, for example: Barker, M. G.; Blake, A. J.; Edwards, P. P.; Gregory, D. H.; Hamor, T. A.; Siddons, D. J; Smith, S. E. Chem. Commun. 1999, 1187.
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Figure 2. Rietveld refinement profile for LiMgN at room temperature. Crosses indicate observed data, the upper continuous line shows calculated profile, the lower continuous line the difference. Tick marks show reflection positions.
Figure 1. Crystal structure of LiMgN at room temperature, viewed down the c axis, showing alternating LiN4 and MgN4 tetrahedra.
Powder X-ray Diffraction (PXD). All products were initially characterized by PXD. Data were collected using a Philips X-pert diffractometer operating with copper KR radiation. Scans were carried out from 5 to 80 in 2θ with a step size of 0.02 over 50 min. Because of the air-sensitive nature of the products, a dedicated airtight aluminum sample holder with Mylar windows was employed.26 Data were analyzed by Philips Automated Powder Diffraction software. Phase identification was carried out using Philips PC-IDENTIFY to access the ICDD PDF database. Lattice parameters were refined by least-squares fitting. Powder Neutron Diffraction (PND). Neutron diffraction data were collected on the D1A instrument at the Institut LaueLangevin, Grenoble, France. Samples of approximately 2 g were sealed inside 10 mm diameter vanadium sample cans using gold seals. A germanium monochromator was used to select wavelengths. Data for LiMgN were collected at 1.4 A˚ at room temperature for 20 h, and at 1.91 A˚ for measurements at 473, 673, and 873 K for 6 h each. Data for the magnesium-rich phase were collected at 1.4 A˚ at room temperature (25 h) and at 1.91 A˚ for high temperature runs at 473, 673, and 873 K (5 h each). Diffraction data for the lithium-rich phase were collected at 1.91 A˚ at room temperature, 473 and 873 K (3 h each). All hightemperature measurements were performed using the standard D1A top-loading furnace. Rietveld Refinement. The structures of 1-3 were refined taking the respective crystal structures reported by Yamane et al. as initial models. Rietveld refinements were performed using GSAS with the ExpGUI interface.27,28 The background and (26) Barker, M. G.; Begley, M. J.; Edwards, P. P.; Gregory, D. H.; Smith, S. E. J. Chem. Soc., Dalton Trans. 1996, 1. (27) Larson, A. C.; von Dreele, R. B. The General Structure Analysis System; Los Alamos National Laboratories: Los Alamos, NM, 2000. (28) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210.
scale factor were refined in initial cycles. The background was modeled with a Chebyschev polynomial function (GSAS Function 1). The cell parameters and zero point were subsequently refined. The latter part of the refinement dealt with atomic positions, peak widths, profile coefficients, isotropic temperature factors, and site occupancies. In the final cycles, anisotropic temperature factors were refined. Peak profiles were modeled using the Thompson-Cox-Hastings pseudo-Voigt function (GSAS Function 2).
Results and Discussion LiMgN (1). A 1:1 Li/Mg sample of nominal stoichiometry LiMgN was prepared, and the identity of the phase was confirmed by PXD. The pattern could be indexed to an orthorhombic cell, a=7.159(3) A˚, b=3.507(1) A˚, and c=5.013(2) A˚, which is in close agreement with published values.20 Rietveld refinement against the neutron data resulted in a good fit to the literature model, a cationordered anti-fluorite in space group Pmna (Figure 1). A plot of the fit to the profile can be seen in Figure 2 and Table 1 contains important refined crystal structure parameters. (Additional anisotropic TDPs can be found in the Supporting Information, Table SI1.) Attempts to vary the occupancies of the sites reduced the quality of the fit and in some cases led to physically meaningless values, strongly suggesting full occupancy on all sites. Selected bond lengths are summarized in Table 2. The Li atoms are situated in a distorted nitrogen tetrahedron with three relatively short bonds (2.139(3) 2 and 2.159(6) A˚) and one long bond (2.391(6) A˚). These short Li-N bonds are of a similar length to those bonds observed in the Li2N layers of Li3N (2.130(1) A˚).13 The bonding geometry of the Li atoms is similar to that observed in the isostructural compound LiCaN with one notable exception. In LiCaN the Li atoms are coordinated to three nitrogen atoms and form planar 1 ¥[LiN3/3] bands in [010] direction. The fourth Li-N bond is very long at ∼3.57 A˚ (cf., 2.391 A˚ in LiMgN) and is not considered a true bond. The nearest comparable Li - N bond lengths to the fourth bond of LiMgN reported are in Li3Ba2TaN4 and Li3Ba2NbN4, which have values of
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Table 1. Crystallographic Parameters and Rietveld Refinement Results for LiMgN at Room Temperature from PND Compound Space Group a/A˚ b/A˚ c/A˚ V/A˚3 Calculated density, Fx/g cm-3 Ue 100 /A˚2 x z No. of observations No. of parameters Rwp Rp Reduced χ2
Li (x, 1/4, z) Mg (x, 1/4, z) N (x, 1/4, z) Li Mg N Li Mg N
LiMgN (1) Pnma (No. 62) 7.1531(2) 3.5045(1) 5.0100(1) 125.592(3) 2.393 1.07 (3) 0.82 (1) 0.74 (1) 0.0973(8) 0.1432(3) 0.3788(2) 0.5274(12) 0.0195(4) 0.2700(3) 1459 26 0.0692 0.0512 1.6797
Table 2. Selected Bond Lengths for Room Temperature LiMgN Bond Li-N(1) Li-N(2) 2 Li-N(3) Mg-N(1) Mg-N(2) 2 Mg-N(3) Li-Li 2 Li-Mg(1) Li-Mg(2) Li-Mg(3) 2 Mg-Mg 2
Bond Length/A˚ 2.391(6) 2.139(3) 2.159(6) 2.101(3) 2.158(1) 2.166(1) 2.255(7) 2.565(7) 2.488(7) 2.553(4) 2.703(3)
2.43(2) and 2.38(3) A˚, respectively.29,30 These latter two quaternary nitrides are isostructural (with one another) and consist of chains of alternating Li-N and MV-N edge-sharing tetrahedra, cross-linked into a 3-D structure by further Liþ ions. Bond valence calculations were performed using the results of the neutron refinement with the VALENCE software.31 Applying the parameters of Brese and O’Keeffe (Rij(Li-N)=1.61, Rij(Mg-N)=1.85; b=0.37) gives sums of 0.83 for Li, 1.80 for Mg and 2.63 for N.32 Omitting the “long” Li-N interaction reduces the sums at Li and N to 0.71 and 2.51, respectively. These values are below the expected ideal values and similar low bond valence sums for Mg and N were reported for Mg3N2 by Partin et al. who attributed the reduced values to Mg-Mg nonbonding interactions.17 In fact, the metal-metal bond distances we obtain from neutron data are similar to those observed in other sblock nitrides. The Li-Li distance (2.255(7) A˚) is similar to that observed in the ordered anti-fluorite superstructure of Li3AlN2 (2.21 A˚).33 The Mg-Mg distance (2.703(3) A˚) is slightly smaller than that observed in Mg3N2 (2.72 A˚) with the related anti-bixbyite structure (29) Chen, X. Z.; Ward, D. L.; Eick, H. A. J. Alloys Compd. 1994, 206, 129. (30) Chen, X. Z.; Eick, H. A. J. Solid State Chem. 1994, 113, 362. (31) Brown, I. D. J. Appl. Crystallogr. 1996, 29, 479. (32) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192. (33) Juza, R.; Hund, F. Z. Anorg. Allg. Chem. 1948, 257, 13.
Figure 3. Rietveld refinement profile for LiMgN at 873 K. Crosses indicate observed data, the upper continuous line shows calculated profile, and the lower continuous line the difference. Tick marks show reflection positions.
which can again rationalize the low bond valence sums observed. In potential hydrogen storage materials it is important to explore the structure under nonambient conditions. Hydrogen uptake in complex hydride systems occurs at elevated temperatures, so to understand the mechanisms involved one should assess the defect structure and ion mobility of the material under these conditions. To this end PND measurements were taken at elevated temperature. The scans at 473 and 673 K showed small shifts consistent with thermal expansion, and could be fitted to the same room temperature Pnma model. However, a phase change was observed to occur at a temperature between the 673 and 873 K data sets. The high temperature form could be modeled as a simple cubic disordered anti-fluorite-type structure (a = 5.0652(1) A˚ in Fm3m). This is the same structural model originally reported at room temperature for LiMgN by Juza and Hund.34 In our study, the orthorhombic form was always seen to reappear on cooling. The synthesis method of Juza and Hund was however quite different. In their preparation, a mixture of lithium metal and magnesium nitride was heated in a tube furnace under flowing nitrogen gas. It was first reacted at 1033 K and then furnace cooled. It was subsequently reground and then heated again, this time to 1323 K before furnace cooling. It is possible, therefore, that either the higher reaction temperature, and the cooling rate could account for the earlier observation of the simple cubic structure at room temperature. Figure 3 shows a refinement profile of the high temperature data for LiMgN at 873 K. Crystallographic parameters, refinement results and comparisons of bond lengths can be found in the Supporting Information, Tables SI2 and SI3. The metal-nitrogen bond distances show an increase in length with increasing temperature. This is commensurate with positive thermal expansion. It is interesting to note that the shortest Li-N bond length (Li-N(2)) shows very little variation with temperature (293-673 K) remaining at 2.139 A˚ until the phase change. This is mimicked by the (34) Juza, R.; Hund, F. Z. Anorg. Allg. Chem. 1948, 257, 1.
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shortest Mg-N bond length (Mg-N(1)), which shows a small change from 2.101 to 2.102 A˚ over the same temperature range. At 873 K in the cubic structure, lithium and magnesium share a single site, hence a unique Li/Mg-N distance of 2.1933 A˚. This represents a decrease from the average Li-N distance at 673 K (2.251 A˚) and an increase from the average Mg-N length (2.152 A˚). Further bond valence calculations (using Brown’s temperature correction35) show that the valence sums remain essentially the same for all temperatures, even after the phase change. Magnesium-Rich Phase (2). A magnesium-rich phase was synthesized with target stoichiometry Li0.51Mg2.49N1.83.
Figure 4. Crystal structure of Li0.24Mg2.76N1.92 at room temperature, viewed down the a direction. (Mg, Li)N4 tetrahedra are highlighted.
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Initial PXD data indexed to a cubic cell with a lattice parameter of a=9.965(1) A˚, which is marginally smaller than that reported in the literature by Yamane et al. and marginally larger than that in the parent Mg3N2 (a = 9.9621(3) A˚).17,20 While the parent phase, Mg3N2 adopts the space group Ia3 (cubic anti-bixbyite structure), the cubic unit cell becomes distorted by the substitution of Li atoms for Mg atoms resulting in a lower symmetry space group, I213 for the Mg-rich phase (Figure 4). Hence extra reflections are observed in the PXD pattern of the nonstoichiometric compound, most notably the [110] reflection at 12.525 2θ. The refinement against room temperature neutron data progressed smoothly but presented a more complicated picture than that from PXD alone. Extra phases were apparent in the PND profile. One of these is a small MgO impurity (
