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Crystal Chemistry of Perovskite-Type Hydride Namgh3: Implications for Hydrogen Storage

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Crystal Chemistry of Perovskite-Type Hydride Namgh3: Implications for Hydrogen Storage Chem. Mater. 2008, 20, 2335–2342 2335 Crystal Chemistry of Perovskite-Type Hydride NaMgH3: Implications for Hydrogen Storage Hui Wu,*,†,‡ Wei Zhou,†,‡ Terrence J. Udovic,† John J. Rush,†,‡ and Taner Yildirim†,§ NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau DriVe, MS 6102, Gaithersburg, Maryland 20899-6102, Department of Materials Science and Engineering, UniVersity of Maryland, College Park, Maryland 20742-2115, and Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104-6272 ReceiVed NoVember 26, 2007. ReVised Manuscript ReceiVed January 11, 2008 The crystal structure, lattice dynamics, and local metal-hydrogen bonding of the perovskite hydride NaMgH3 were investigated using combined neutron powder diffraction, neutron vibrational spectroscopy, and first-principles calculations. NaMgH3 crystallizes in the orthorhombic GdFeO3-type perovskite structure (Pnma) with a-b+a- octahedral tilting in the temperature range of 4 to 370 K. In contrast with previous structure studies, the refined Mg-H lengths and H-Mg-H angles indicate that the MgH6 octahedra maintain a near ideal configuration, which is corroborated by bond valence methods and our DFT calculations, and is consistent with perovskite oxides with similar tolerance factor values. The temperature dependences of the lattice distortion, octahedral tilting angle, and atomic displacement of H are also consistent with the recently observed high H mobility at elevated temperature. The stability and dynamics of NaMgH3 are discussed and rationalized in terms of the details of our observed perovskite structure. Further experiments reveal that its perovskite crystal structure and associated rapid hydrogen motion can be used to improve the slow hydrogenation kinetics of some strongly bound light-metal-hydride systems such as MgH2 and possibly to design new alloy hydrides with desirable hydrogen-storage properties. Introduction tunately, most of these hydrides show poor kinetics and irreversibility with respect to hydrogen absorption/desorption, The use of hydrogen as a primary fuel in the long term and suffer from the potential for toxic gas release (e.g., would greatly reduce CO emissions and pollutants, as well 2 ammonia, diborane) during dehydrogenation. Nonetheless, as our dependence on the world’s diminishing oil supplies. ternary complex hydrides containing only alkali and/or One of the major challenges in the quest to develop alkaline-earth metals may enhance kinetics and reversibility hydrogen-fueled vehicles is the development of lightweight and inhibit toxic-gas side products compared with the parent materials that will absorb and release hydrogen in the range borohydride, amide (imide) and Alanate compounds. of 1–10 bar and 298–473 K.1 Much effort has been focused Unlike the well-studied binary alkali and/or alkaline-earth on light-metal hydrides such as LiH and MgH because of 2 metal hydrides, structural information available for ternary their relatively high hydrogen densities (>5 wt %). However, or even quaternary hydrides is considerably less owing to most of them have rather slow absorption kinetics, relatively the complexity of their structural arrangements and difficul- high thermal stability, and/or problems with the reversibility ties involved in determining hydrogen positions by conven- of hydrogen absorption/desorption cycling. Recently, alloying tional X-ray diffraction (XRD) methods. Several attempts with Si or Ge has been shown to destabilize these hydrogen- were made to prepare ternary hydrides with the formula rich, yet strongly bound, light-metal hydrides at significantly 10–13 A AE H + (A ) Na, K, Rb, Cs, AE ) Mg, Ca), lower temperatures.2,3 Nonetheless, important challenges x y x 2y remain in improving their absorption kinetics and cycling (4) Bogdanovic, B.; Schwickardi, M. J. Alloys. Compd. 1997, 253 · 254, capability. Investigations have also been made to enhance 1–9. the storage properties of known complex hydrides4–7 as well (5) Lu, J.; Fang, Z. Z. J. Phys. Chem. B 2005, 109, 20830–20834. (6) Luo, W.; Sickafoose, S. J. Alloys Compd. 2006, 407, 274–281. as to develop new complex hydrides containing main group (7) Xiong, Z.; Wu, G.; Hu, J.; Liu, Y.; Chen, P.; Luo, W.; Wang, J. AdV. III and V elements with high hydrogen capacities.8,9 Unfor- Funct. Mater. 2007, 17, 1137–1142. (8) Cerny, R.; Filinchuk, Y.; Hagemann, H.; Yvon, K. Angew. Chem., Int. Ed. 2007, 46, 5765–5767. * To whom all correspondence should be addressed. Email: huiwu@ (9) Filinchuk, Y. E.; Yvon, K.; Meisner, G. P.; Pinkerton, F. E.; Balogh, nist.gov. M. Inorg. Chem. 2006, 45, 1433–1435. † National Institute of Standards and Technology. (10) Ronnebro, E.; Noreus, D.; Kadir, K.; Reiser, A.; Bogdanovic, B. J. ‡ University of Maryland. Alloys Compd. 2000, 299, 101–106. § University of Pennsylvania. (11) (a) Bouamrane, A.; Laval, J. P.; Soulie, J. -P.; Bastide, J. P. Mater. (1) http://www.eere.energy.gov/. Res. Bull. 2000, 35, 545–547. (b) Park, H. -H.; Pezat, M.; Darriet, B.; (2) Vajo, J. J.; Mertens, F.; Ahn, C. C., Jr.; Fultz, B. J. Phys. Chem. B Hagenmuller, P. ReV. Chim. Miner. 1987, 24, 525. (c) Maeland, A. J.; 2004, 108, 13977–13983. Lahar.; W, D. Z. Phys. Chem. 1993, 179, 181. (3) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J. Chem. Mater. 2007, 19, (12) Gingl, F.; Vogt, T.; Akiba, E.; Yvon, K. J. Alloys Compd. 1999, 282, 329–334. 125. 10.1021/cm703356v CCC: $40.75 2008 American Chemical Society Published on Web 02/16/2008 2336 Chem. Mater., Vol. 20, No. 6, 2008 Wu et al. 14,15 12 AExAyH2x+y (AE ) Sr, Ba, A ) Li), and AE′xAE′′yH2(x+y) MgH3 (t ) 1.041, 6H-BaTiO3-type) and the high-pressure 13,16 (AE ) Mg, Sr, and Ba). Some of these are able to form polymorph nine-layer (chh)3 CsMgH3 (t ) 1.093, BaRuO3- 20 a unique type of crystal structure---ABX3 perovskite with a type). large A cation surrounded by 12 X anions and a small B For perovskite hydrides, available crystallographic infor- cation coordinated with 6 X anions. It is well-known that mation is limited, with only eight reported perovskite-forming systems, i.e., LiSrH ,15 LiBaH ,15 NaMgH .10 KMgH ,11 ABO3 perovskites constitute one of the most important and 3 3 3 3 12 20 11 12 widely studied families of oxide compounds thanks to their RbMgH3, CsMgH3, RbCaH3, and CsCaH3. No neu- interesting dielectric/ferroelectric properties, magnetic be- tron diffraction data are available for KMgH3, RbCaH3, havior, electronic and ionic conductivity, and superconduc- LiSrH3, and LiBaH3, which were assigned a Pm-3m perovs- kite structure from XRD results, and the hydrogen sites were tivity. Their flexibility in accommodating a broad range of restrained by the symmetry operation. Some of the reported atomic substitutions provides a robust platform for probing structures are inconsistent with the general criteria for a stable correlations between their varied properties and the underly- perovskite structure because their tolerance factors were too ing structural chemistry.17 small to maintain a cubic perovskite structure with Pm-3m Generally, ABX3 perovskites can be described in terms symmetry. Some refined structures are questionable due to of the close packing of the AX3 layers, with B-site cations the unusually large distorted BX6 octahedra which make the in the centers of the BX6 octahedra. When the AX3 layers structure strained and crystallographically unstable. Further- are arranged via cubic close packing, the BX6 octahedra are more, few studies have been done concerning their properties connected exclusively through corner sharing, and the and applications. Because of the rich and well-established structure is termed “cubic” perovskite. The deviation from crystal chemistry of perovskites, we believe it is important the perfect cubic perovskite structure can be evaluated by to clarify and determine the structures of perovskite hydrides the Goldschmidt tolerance factor (t ) (r + r ) / √2(r + and rationalize their properties. Recently, researchers reported A X B reversible hydrogenation/dehydrogenation witha6wt% r ) ). In an “ideal” cubic perovskite, the A and B cations X hydrogen capacity in the perovskite hydride NaMgH .21 realize their equilibrium bond distances to the X anions 3 Moreover, a rapid hopping motion of H has been observed without inducing any distortion of the unit cell and in NaMgH by NMR,22 much faster than in MgH . It is clear ) 3 2 dA-X 2(dB-X). The resultant symmetry is cubic with space that more structural details for NaMgH are needed to ) - 3 group Pm-3m, and t 1. In many perovskites, the A X determine if there is some correlation between this high - and B X bond lengths are geometrically incompatible, and hydrogen mobility and hydrogenation/dehydrogenation re- the symmetry will be lowered. When the A cation is small versibility. Hence, in the present paper, we have investigated (“undersized”), t < 1. The A-X length can be shortened the crystal structure, lattice dynamics, and local metal- and the coordination number of the A cation lowered through hydrogen bonding configuration of NaMgH3, using combined a correlated tilting of the surrounding BX6 octahedra. neutron powder diffraction (NPD), neutron vibrational Beginning with the pioneering studies of Glazer18 and more spectroscopy (NVS), and first-principles calculations. We recently by Woodward,19 the variations in symmetry resulting have also studied the hydrogen-storage properties and from different types of octahedral tilting have been system- dynamical behavior of NaMgH3, as well as its potential to atized and rationalized for simple and complex perovskites. improve the hydrogenation thermodynamics and kinetics of When t > 1, the AX3 layers typically adopt mixed cubic (c) other hydride systems. and hexagonal (h), or pure hexagonal close packed stacking sequences. The resultant structure is called a “hexagonal” Experimental Section perovskite. The introduction of hexagonal stacking is ac- NaMgH3 was prepared using standard solid-state methods. companied by face-sharing of the adjacent BX6 octahedra Stoichiometric quantities (1:1 ratio) of the binary hydrides, NaH ° - - 23 and by the formation of 90 B X B bond angles.
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