Crystal Chemistry and Dehydrogenation ... - ACS Publications

Jul 22, 2009 - J. Rush,†,‡ and Taner Yildirim†,§. NIST Center for Neutron ... 3231 Walnut Street, Philadelphia, PennsylVania 19104-6272. ReceiV...
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J. Phys. Chem. C 2009, 113, 15091–15098

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Crystal Chemistry and Dehydrogenation/Rehydrogenation Properties of Perovskite Hydrides RbMgH3 and RbCaH3 Hui Wu,*,†,‡ Wei Zhou,†,‡ Terrence J. Udovic,† John. J. Rush,†,‡ and Taner Yildirim†,§ NIST Center for Neutron Research, National Institute of Standards and Technology, 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: June 4, 2009; ReVised Manuscript ReceiVed: July 6, 2009

Crystal structure, lattice dynamics, and bonding environments of RbMgH3 and RbCaH3 were investigated using neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. RbMgH3 exhibits a 6H-BaTiO3-type hexagonal perovskite structure, and RbCaH3 forms a Pm3m simple cubic perovskite. A very short Mg-Mg distance (∼2.77 Å) was found in RbMgH3, and its structural stability was ascribed to H- anion polarization and Mg-Mg bonding. Interestingly, RbCaH3 forms an ideal cubic perovskite despite its small tolerance factor. The inconsistency between the nominal tolerance factor and adopted structure types of RbCaH3 are discussed in terms of anion polarization and covalence effect on the cation-anion bond length deviations. Finally, we show that both RbMgH3 and RbCaH3 can be dehydrogenated and rehydrogenated at 300-400 °C under moderate pressures. Introduction Metal hydrides, including conventional (e.g., LaNi5H6 and Mg2NiH4) and complex hydrides (e.g., alanates, borohydrides, and amides), are an important family of materials that have great potential for hydrogen storage and have been extensively studied.1–4 In recent years, particular interest has been focused on the complex metal hydrides because of their high gravimetric and volumetric hydrogen densities.1,3,5–11 Complex metal hydrides that are interesting for hydrogen storage generally consist of alkali and/or alkaline earth cations and [AlH4]-, [NH2]-, and/ or [BH4]- anions. Hydrogen in these materials forms a directional covalent bond with the central atom in the anion unit. The transition states for atomic rearrangement often occur in an unfavorable bonding configuration. This increases the activation energy for hydrogen diffusion and thus leads to slow absorption kinetics and poor reversibility under moderate conditions.1 In addition, most of these hydrides suffer from toxic gas release (e.g., ammonia, diborane, etc.) during dehydrogenation.12 Therefore, development of reversible and kinetically favorable complex hydrides such as alkali and/or alkaline-earth metal complex hydrides without release of toxic gas products (involving elements such as B or N) is of great interest. Thus far, binary metal hydrides are relatively well studied. In contrast, ternary or quaternary alkali and/or alkaline-earth metal hydrides are less investigated because of their complex crystal structures and difficulties in determining the hydrogen positions by X-ray diffraction (XRD). With properly charged and sized cations, A and B, some of the ternary hydrides have been found to form ABH3 perovskite structures.13–18 While the structural characteristics of these perovskite hydrides are rarely investigated, the ABO3 perovskite oxides represent one of the most widely studied families of inorganic compounds due to * To whom correspondence should be addressed. E-mail: [email protected]. † National Institute of Standards and Technology. ‡ University of Maryland. § University of Pennsylvania.

their diverse electronic and magnetic properties accompanied with the large flexibility in accommodating a broad range of atomic substitution in both cation (A and B sites) and anion sites, and the resulting structural changes.19 The well-established crystal chemistry of ABO3 compounds certainly provides useful guidelines for analyzing the structural stabilities of perovskite hydrides. The structure type of a perovskite oxide is strongly affected by the compatibility of cation and anion sizes. In an ABO3 perovskite, if the BO6 octahedra share corners infinitely in all three dimensions, such a structure is termed a cubic perovskite. The A cation occupies the void created by 8 BO6-octahedra, giving the A cation a 12-fold anion coordination and the B-cation a 6-fold anion coordination. In an ideal case, the bond distances of A and B cations to the anions satisfy the geometric relationship dA-O ) (2(dB-O))1/2 and will not induce any distortion of the unit cell. The resultant symmetry is cubic with space group Pm-3m. In many other cases, the A-O and B-O bond lengths are geometrically incompatible, and the crystal symmetry will be lowered. The deviation from the perfect cubic perovskite structure can be evaluated by the Goldschmidt tolerance factor (t ) (rA + rO)/(21/2(rB + rO))), where the A-O and B-O bond lengths are estimated by the sum of cation and anion radii. Apparently, for an ideal cubic perovskite, t ) 1. When the A cation is small (undersized), t < 1; the surrounding BO6 can tilt while still maintaining its corner-sharing connectivity to shorten the A-O distance and lower the coordination number of the A cation. The first coordination sphere around the B cation remains unchanged, and only the soft B-O-B bond angle is disturbed. The symmetry will decrease to tetragonal, rhombohedral, orthorhombic, or monoclinic but without change of the cubic AO3 layer sequence. When the B cation is too small for its cage, t > 1. In some cases, the B cation will make an off-centered displacement, shifting the symmetric center of the unit cell, with the BO6-octahedra corner-sharing maintained, e.g., tetragonal BaTiO3. In some other cases, the perovskite compounds with t > 1 will involve mixed cubic (c) and hexagonal (h) or pure

10.1021/jp905255s CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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hexagonal close-packing of the AO3 layers. The introduction of hexagonal stacking is accompanied by face-sharing of adjacent BO6 octahedra. These structures are called hexagonal perovskites. One important issue about such structures is that the cations in the neighboring face-sharing octahedra pair are very close and thus have strong electrostatic repulsion; therefore, the stability of hexagonal perovskite strongly depends on the compensation of such repulsion. In contrast to the enormous number of known perovskite oxides, chalcogenides, and halogenides, there are only a few perovskite hydrides reported,13–18 and most of their structures were determined from XRD studies, without accurate data on the H positions. From the limited crystallographic information, some reported structures appear to be inconsistent with the geometric criteria for a stable perovskite. For example, RbCaH3 with t ) 0.928 was reported to form a cubic Pm-3m perovskite from a room temperature XRD measurement on a hydride sample.13 Since its tolerance factor is even smaller than that of NaMgH3 and NaMgH3 has been confirmed to form an orthorhombic Pnma perovskite,10 a high symmetry cubic structure seems rather unlikely for RbCaH3. Therefore, a neutron powder diffraction (NPD) study on a deuterided sample is needed to clarify such discrepancies because XRD cannot accurately determine the positions of H and consequently cannot distinguish the possible CaH6 octahedra tilting. Some theoretical efforts have also been reported to predict the formation of new perovskite hydrides.20,21 However, some of the predicted structures do not follow the general structural trend indicated by their tolerance factors. For some systems with t > 1, the theorists were not aware of the possible formation of hexagonal perovskite structure types and only considered the cubic Pm-3m phases. Experimentally, hexagonal perovskite structures have been found in RbMgH314 and CsMgH3 (high-pressure)16 systems, while the formation and stability of such hydrides have not been rationalized. Clearly, there is a lack of understanding on the stability of all possible perovskite structure types and awareness of the impact of the nature of H- anion and different cations. As for the properties and applications of perovskite hydrides, even fewer studies have been reported. NaMgH3 is the only system whose hydrogen storage properties have been well studied and consistently reported.17,22 We believe it is important to clarify the structures of perovskite hydrides and study their properties. Following our recent studies of the NaMgH3 system,17 two more compounds, RbMgH3 and RbCaH3, have been systematically studied in this work. We have used combined neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and firstprinciples calculations to elucidate their crystal structures, lattice dynamics, local bonding configurations, and dehydrogenation/ hydrogenation properties. The origin of the deviations of the actual crystal structures from the geometric criteria (i.e., tolerance factor) for certain perovskite hydride systems (RbCaH3, etc.) has been identified, and its implications were discussed. The stability of hexagonal perovskite RbMgH3 has also been rationalized. Experimental Section RbMgH3 and RbCaH3 were prepared using standard solidstate methods. Pure Rb metal and MgH2 (AlfaAesar,23 98%) or CaH2 (Aldrich, 99%) were mixed respectively via ball milling with a Fritsch Pulverisette 7 planetary mill at 400 rpm for 60 min. A small amount of extra hydrides (i.e., greater than a 1:1 stoichiometric ratio of hydride and Rb) were put in the mixtures to prevent the formation of Rb-rich impurities (i.e., Rb4Mg3H10,

Wu et al. Rb2MgH4 or Rb2CaH4) in the subsequent high-temperature synthesis of the target hydrides. The powder mixture was then wrapped in a Mo envelope and sealed in a stainless steel tube. The tube was connected to a hydrogen gas tank, as part of a hydrogenation system, and heated in a tube furnace. The powder mixture was heated overnight up to 400 °C under 20 bar H2 pressure. All sample handling was performed in a He-filled glovebox due to the extreme air-sensitivity of the hydrides. Phase identification and equilibrium were monitored on samples sealed in glass capillaries using a Rigaku diffractometer with a Cu KR source operated at 40 kV and 40 mA. All neutron scattering measurements were performed at the NIST Center for Neutron Research (NCNR). NPD studies were conducted on the high-resolution neutron powder diffractometer (BT-1) with the Cu(311) monochromator at a wavelength of 1.5403(2) Å and an in-pile collimation of 15 min of arc. Data were collected over the 2θ range of 3-168°. Rietveld structural refinements were done using the GSAS package.24 Because of the large incoherent neutron scattering cross-section of hydrogen, the sample utilized in the NPD measurements was deuterated by exposing it to gaseous deuterium at a pressure of 50 bar and a temperature of 673 K over a two-week period with several cycles of intermediate flushing and recharging of deuterium gas. The exchanged deuterium content was determined by gravimetric measurements, and the residual hydrogen content was checked using the neutron prompt-γ activation analysis (PGAA) facility, which is able to detect hydrogen as low as 2 µg.25 Pure MgH2 and CaH2 samples were used as standards to normalize γ-ray intensities. The final stoichiometry of H in RbMgD3 and RbCaD3 samples were found to be H:Mg ≈ 0.222 and H:Ca ≈ 0.354, indicating that deuterium exchange was successful. Neutron vibrational spectra were measured at 5 K using the BT-4 filter-analyzer neutron spectrometer (FANS) with the Cu(220) monochromator under conditions that provided energy resolutions of 2-4.5% over the vibrational energy range probed. Temperature programmed desorption (TPD) was conducted using a volumetric gas sorption Sieverts-type apparatus. Details of the Sieverts system characteristics and operation can be found in our previous publication.26 First-principles calculations were performed within the planewave implementation of the generalized gradient approximation to density functional theory (DFT) using the PWscf package.27 We used a Vanderbilt-type ultrasoft potential with PerdewBurke-Ernzerhof exchange correlation. A cutoff energy of 544 eV was found to be enough for the total energy and force to converge within 0.5 meV/atom and 0.005 eV/Å. Structure optimizations were performed with respect to atomic positions with lattice parameters set to the experimental values. The phonon calculations were conducted with the optimized structure using the supercell method with finite difference.28,29 Results Structure and Lattice Dynamics of RbMgH3. The XRD pattern of RbMgH3 can be indexed using the 6H-BaTiO3-type perovskite model with space group P63/mmc (No. 194), consistent with the previously reported structure at room temperature.14 The structure was then refined on this model using our NPD data collected on RbMgD3 at 300 K. Within the studied temperature range 5-300 K, RbMgD3 maintains the same structure without any phase transition. Rietveld refinement revealed a hexagonal perovskite structure with lattice parameters of a ) 5.8829(1) Å and c ) 14.2767(3) Å at 5 K and a ) 5.9097(1) Å and c ) 14.3327(3) Å at 300 K. No cation site (Rb/Mg) disordering was ascertained from the refinement, and

Perovskite Hydrides RbMgH3 and RbCaH3

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Figure 2. Crystal structure of (a) RbMgD3 and (b) RbCaD3 with refined thermal ellipsoids at 300 K. Rb atoms are indicated in pink, Mg in green, Ca in blue, and D in white. RbMgD3 crystallizes in a six-layer (hcc)2 hexagonal perovskite structure featured with face-sharing MgD6octahedra and a 90° Mg-D-Mg bond angle. RbCaD3 forms an ideal cubic perovskite with only corner-shared CaD6-octahedra.

Figure 1. (a) Experimental (circles), calculated (line), and difference (line below observed and calculated patterns) NPD profiles for RbMgD3 at 300 K. Vertical bars indicate the calculated positions of Bragg peaks for RbMgD3 and MgD2 (from the top), respectively. (b) Experimental (circles), calculated (line), and difference (line below observed and calculated patterns) NPD profiles for RbCaD3 at 300 K. Vertical bars indicate the calculated positions of Bragg peaks for RbCaD3 and CaD2 (from the top), respectively.

thus, the site occupancies of Rb and Mg atoms were fixed according to the chemical composition. The refined lattice parameters, fractional coordinates, D site occupancies, thermal parameters, and reliability factors for RbMgD3 measured at 5 and 300 K are summarized in Tables S1 and S2 (see Supporting Information). Figure 1a shows the NPD pattern for RbMgD3 at 300 K with an excellent quality of the fit. The refined structure (Figure 2a) is also in good agreement with the optimized structure from our DFT calculation. Compared to the cubic perovskites with only corner-sharing octahedra (e.g., NaMgH3), in RbMgD3 the shifting of some [RbD3] layers by (1/3, 2/3, 0) induces groups of face-sharing MgD6 octahedra (Mg1 in Table S2, Supporting Information); therefore, the structure is composed of cubic (c) and hexagonal (h) close-packing of [RbD3] layers and contains face- and cornersharing MgD6 octahedra (see Figure 2a). Therefore, although each D is still surrounded by six cations as in NaMgD3, there are two crystallographically different anion sites in RbMgD3, i.e., D1 in the cubic close packed layers between the cornersharing MgD6 octahedra and D2 in the hexagonally closed packed layers between two face-sharing MgD6 octahedra. Refinement of the NPD data suggests that D vacancies tend to

be present in the hexagonal close-packing layers (D2 in Table S2, Supporting Information). From the refined bond lengths (Table S3, Supporting Information), the corner-sharing MgD6 octahedron exhibits a nearly ideal configuration, with six equal Mg2-D bond lengths of 2.050(1) Å and D-Mg2-D angles ranging from 88.20(7)° to 91.8(7)°. The face-sharing MgD6 octahedra show some distortion with elongation between two neighboring Mg1 atoms so that the D2-Mg1-D2 angle decreases to 78.02(10)°. In RbMgD3, we also noticed that the Mg-Mg distance in the pair of face-sharing octahedra (2.773 Å) is much shorter than the closest approach distance in Mg metal (3.21 Å) and is comparable to the shortest Mg-Mg bonds recently reported in the R-Mg-Mg-R complex molecules, i.e., 2.766-2.889 Å for different R ligands. (Note: the oxidation state of Mg is +1 in these molecular compounds.)30,31 However, the separation between Mg in the face-sharing octahedra pair is larger than the average layer thickness (∼2.389 Å, 300 K) derived from RbMgD3 unit cell dimensions of the six-layer (hcc)2 structure due to the electrostatic repulsion between these two Mg2+ cations. The implications of the short Mg-Mg distance and the resulting Mg-Mg repulsion on the stability of hexagonal RbMgH3 will be discussed in the next section. In order to probe the chemical environment and local bonding states, and to ascertain any possible Mg-Mg bonding in RbMgH3, we performed NVS measurements and first-principles DFT calculations. NVS data directly reflect the vibrational density of states and are particularly sensitive to the hydrogen vibrational modes. Figure 3 shows the NV spectrum for RbMgH3 collected at 4 K. The calculated first-principles phonon spectrum based on the optimized RbMgH3 structure is also shown in Figure 3 and is in reasonably good agreement with

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Figure 3. Neutron vibrational spectrum of RbMgH3 at 5 K. Calculated spectra delineating both 1 phonon (dotted line) and 1 + 2 phonon (solid line) contributions are shown with the experimental data. The Mg-Mg stretching motion in the face-sharing octahedra pair is indicated by an arrow.

the observed NV spectrum. The phonon bands above 60 meV are dominated by the vibrations of hydrogen and can be assigned to the rocking, bending, and stretching modes of hydrogen in MgH6 octahedra. NV features below 30 meV are mainly attributed to metal-atom displacements. Also note that there is a small peak near 41 meV in both observed and calculated NV spectra, which corresponds to the Mg-Mg stretching motion in the face-sharing octahedra pair. The energy of such a Mg-Mg stretching mode in RbMgH3 aligns well with the mode at ν ∼325 cm-1 (i.e., ∼40.3 meV) observed recently in the Mg-complex molecules with direct Mg-Mg bonds.30 Structure and Lattice Dynamics of RbCaH3. The XRD pattern of RbCaH3 indicates a simple cubic perovskite structure, same as what was reported previously. This is surprising since its tolerance factor, 0.928, is even smaller than that of NaMgH3, and suggests a lower symmetry. Since XRD is much less sensitive to the positions of H and thus cannot distinguish the possible CaH6 octahedra tilting, Rietveld structural refinement was performed on NPD data collected from RbCaD3. Interestingly, all patterns in the temperature range of 5-300 K could only be satisfactorily fit using a Pm-3m simple cubic perovskite model (Figures 1b and 2b). Refinement using the Pnma NaMgH3 model (GdFeO3-type structure) or I4/mcm tetragonal CaTiO3 model did not properly converge and/or resulted in abnormal thermal factors for metal or D atoms. DFT structural optimization from various models was also performed, and the final minimum-energy structure always converged to a cubic structure without cell distortion and octahedral tilting (Figure 2b). Tables S4 and S5 (Supporting Information) list the refined crystallographic parameters of RbCaD3. All these observations confirm that RbCaD(H)3 indeed forms a simple cubic perovskite despite its unusually small tolerance factor. NVS data were also measured for RbCaH3 and compared to the phonon density of states calculated using the relaxed structure model to ensure the right local bonding environment

Wu et al.

Figure 4. Neutron vibrational spectrum of RbCaH3 at 5 K. Calculated spectra delineating both 1 phonon (dotted line) and 1 + 2 phonon (solid line) contributions are shown with the experimental data. Phonon modes contributed from extra CaH2 have been subtracted.

for H (see Figure 4). The calculated phonon modes are in overall good agreement with the observed NV. Inspection of the calculated phonon modes indicates that the vibrational bands in the range of 40-60 meV originate from hydrogen rocking and bending modes, the phonon band at about 137 meV is related to the H stretching mode, and the vibrational bands at about 68 and 90 meV are due to the dispersion of the 40-60 meV Γ phonons within the Brillouin Zone. Dehydrogenation/Rehydrogenation Properties. Temperature programmed dehydrogenation measurements were performed on RbMgH3 and RbCaH3 at a heating rate of 2 °C/min from room temperature to 400 °C to study their dehydrogenation properties. After dehydrogenation, all samples were rehydrogenated at the same conditions (under 20 atm H2 pressure at 300 °C). Figures 5 and 6 show these volumetric desorption results. RbMgH3 completely releases hydrogen, i.e., ∼1.5 equivalents of H2 per mol RbMgH3, at ∼392 °C. XRD after the TPD study indicates diffraction peaks from Mg and Rb metals. In contrast to a previously reported study,32 we did not observe a suggested composition of RbMgH4 from our neutron PGAA analysis, nor any new XRD peaks from an unknown hydride phase. The observed amount of desorbed hydrogen from our volumetric study is consistent with the decomposition reaction: RbMgH3 f Rb + Mg + 1.5 H2. According to the above-mentioned NPD structural analysis, our RbMgH3 sample contains a nearly single phase of RbMgH3, while the RbCaH3 sample contains a noticeable amount of unreacted CaH2 (14.56 wt %) using the current synthesis method. The very stable CaH2 would not dehydrogenate in the studied temperature range (RT to 450 °C) since its decomposition temperature is g600 °C even with destabilization additives such as Si.33 Therefore, the desorbed H2 is mainly from RbCaH3. The amount of released H2 shown in Figure 6 has been normalized to the actual amount of RbCaH3 contained in the sample. Compared to RbMgH3, RbCaH3 desorbs most of its hydrogen in a much more complicated fashion with three

Perovskite Hydrides RbMgH3 and RbCaH3

J. Phys. Chem. C, Vol. 113, No. 33, 2009 15095 different desorption stages would be necessary to more fully understand the dehydrogenation mechanism. Our current laboratory XRD study on the quenched samples shows that the dehydrogenated products were dominated by scattering from CaH2, we cannot differentiate the decomposed product CaH2 from the unreacted precursor. Also, the in situ study was limited by the strong evaporation of Rb in the sample tube or capillary. After desorption, both RbMgH3 and RbCaH3 can be rehydrogenated under 20 atm at a temperature of 300-400 °C for a couple of hours, and the subsequent dehydrogenation shows behavior similar to that of the first time desorption but with a lower hydrogen capacity (∼70% of that of the first cycle) presumably due to a small loss of volatile Rb during the prior desorption. Discussion

Figure 5. TPD results of hydrogen release for RbMgH3 with 2 °C/ min heating ramp. The desorption temperatures and rates are shown in the bottom panels. The amount of hydrogen gas released has been normalized as n(H2 gas)/mol RbMgH3.

Figure 6. TPD results of hydrogen release for RbCaH3 with 2 °C/min heating ramp. The desorption temperatures and rates are shown in the bottom panels. The amount of hydrogen gas released has been normalized as n(H2 gas)/mol RbCaH3.

noticeable desorption peaks at ∼170 °C, 280 °C, and 347 °C, which indicates various dehydrogenation steps. A future in situ structural study or diffraction on the quenched samples at

Our experimental results on the crystal structure and lattice dynamics for RbMgH3 and RbCaH3 are straightforward. The present determined structures of RbMgH3 and RbCaH3 as well as other previously reported ABH3 hydrides enable us to provide a detailed analysis on the general crystal chemistry of perovskite hydrides. In this section, we focus our discussion on the impact of cation/anion sizes on the actual perovskite type in hydrides and the stability of hexagonal perovskite hydrides. For all known perovskite hydrides, it seems that no strong correlation exists between the geometric criteria and the actual crystal symmetries, which is different from the perovskite oxides. One obvious example is RbCaH3, which adopts an ideal Pm-3m perovskite structure in spite of a small tolerance factor even less than NaMgH3, while the latter follows the general tolerance factor rule and exhibits an orthorhombic GdFeO3-type structure with octahedral tilting. These deviations need to be recognized since they are important for both theorists and experimentalists to explore new compounds with the right structures in a rational and targeted way. As mentioned earlier, the structure type of perovskite oxides is known to depend on the geometry constriction, tolerance factor. In the tolerance factor equation, the cation-anion (M-X) distance can essentially be approximated by the sum of effective ionic radii of cations and anions, assuming the radii are independent of structure type along with considerations of coordination number (CN), electronic spin, etc.34 In ABH3 hydrides, tolerance factor has also been used to predict their structure types assuming a constant size of H-, i.e., 1.3 Å or 1.4 Å.18,22 Although this assumption is generally valid for oxides, we found that it is problematic for hydrides. Figure 7 shows the H- radii derived from the M-H interatomic distances observed in alkali/alkaline earth hydrides (AH/AeH2) and AAeH3 perovskite hydrides as a function of cation radius34 and Pauling’s electronegativity. In contrast to O2- or halide anions, which usually assume nearly constant radii, a H- anion does not show a constant radius. Rather, it exhibits a large variation in different compounds. The H- anion radii derived from the M-H distances of alkali metal hydrides (rocksalt structure) and MgH2 show a strong dependence on the cation radius or electronegativity. Both cations and H- anions in these compounds have CN ) 6. H- radii with a different coordination numbers (i.e., CN ) 4) in the alkaline-earth metal hydrides AeH2 (Ae ) Ca, Sr, and Ba) also vary with the cation radius or electronegativity. Such wide variations in the radius of H- have been noted previously,34–36 and the differences in the observed M-H distances in binary hydrides were proposed to be caused by the large H- polarizability.36 In the simple AH and AeH2 hydrides, small and highly charged cations polarize H- strongly and cause

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Figure 7. H- sizes derived from the cation-anion bond distances in the AH (black circles), AeH2 (blue circles), and ABH3 (black diamonds from B-site cations and blue from A-site cations) as a function of the corresponding cation radius and Pauling electronegativity. The hydrogen coordination numbers in hydrides are indicated in red. The coordination numbers of cations, from which the H- radii were derived, are also indicated.

an increase in the hydrogen polarization and in the covalent character of the anion-cation bond. Therefore, smaller H- sizes are generally observed in hydrides with smaller and more electronegative cations. In the AAeH3 perovskite hydrides, H- anion radii (CN)6) can be derived from the M-H bonds either in B-site octahedra or in A-site cuboctahedra. Therefore, it is not obvious how to build a simple size relationship with the cation species. From Figure 7, in the complex AAeH3 perovskite hydrides with the same A-site cation (e.g., RbAeH3, Ae ) Mg and Ca), the Hanion radius derived from the A(XII)-H bond varies with the B-site cation size and electronegativity, regardless of the types of structures. Similarly, if the B-site cations remain the same (e.g., AMgH3, A ) Na, K, Rb, and Cs), the H- anion size derived from the B(VI)-H bond will then depend on the A-site cation species. However, such a trend with cation size or electronegativity is not observed if the H- radius is calculated from the M-H bond of the same site cation. For example, for RbAeH3 (Ae ) Mg and Ca), the H- anion radius derived from the Mg-H bond is actually larger than that from a Ca-H bond. For the A-site (e.g., ACaH3, A ) Rb and Cs), the H- radii derived from Rb-H is larger than that from Cs-H. The same deviation can be found in H- radii derived from Sr-H and Ba-H in ALiH3. Also, H- from Na-H in NaMgH3 is actually the largest in the

Wu et al. A(XII)MgH3 series (A ) Na, K, Rb, and Cs). Some of these deviations are unexplained, but some can be ascribed to the polyhedron distortion and covalence effect. For example, the larger H- anion derived from the Na-H bond compared to those derived from A-H bonds (A ) K, Rb, Cs) can presumably be explained by the polyhedral distortion. From the NPD structural study,17 NaMgH3 forms an orthorhombic perovskite structure with a-b+a- octahedral tilting. The MgH6 octahedra maintain a nearly ideal configuration with similar Mg-H bond lengths, and the tilting of MgH6 octahedra leads to a distorted cuboctahedral environment for the Na+ cation. From analysis of many compounds with polyhedron distortion, a larger mean cation-anion distance is usually observed in a compound with a higher degree of polyhedron distortion.34 Therefore, compared to the cubic KMgH3 perovskite, the distorted Na-cuboctahedra in NaMgH3 would result in a larger H- anion radius. It is difficult to explain the cubic Pm-3m perovskite structures for SrLiH3 and RbCaH3. Using a constant H- radius (e.g., ∼1.3 or 1.4 Å in previous reports18,22), we determined that the tolerance factors of SrLiH3 and RbCaH3 are much less than those of NaMgH3. Yet from structural refinement, Sr-H and Rb-H bonds are both exceptionally long, which might contribute to a larger tolerance factor based on a constant H anion radius. For SrLiH3, a short Li-H bond length caused by the covalence effect mentioned above will also lead to a larger tolerance factor. A similar covalence effect can also be applied to CaNiH3. Although the tolerance factor of CaNiH3 was calculated as 0.938 from a constant Hradius of 1.3 Å,18 CaNiH3 was found to form a cubic Pm-3m perovskite structure due to a very short Ni-H bond length.18 Because of such wide variations in H- size, the radius of Hcannot be assumed as a constant in the perovskite hydrides to derive the tolerance factor. The polarization of H- and the covalence of the M-H bond should be considered before the tolerance factor is used to predict the geometric stability of the perovskite hydrides. For example, LiMgH3 was calculated to form a cubic Pm-3m perovskite structure.20 Its nominal tolerance factor is 0.77 using a constant H- radius of 1.3 Å.18 When the covalence effect on shortening of the Li-H bond length is considered, the tolerance factor will be even smaller and not stable for a cubic perovskite. Indeed no perovskite structure was found in the Li-Mg-H system.37 NaBeH3 was predicted to form a Pm-3m cubic perovskite,21 and its tolerance factor was calculated as 1.087, assuming a constant H- size. Be2+ is much smaller and more electronegative than Mg2+, and the Be-H bond length is thus expected to be shorter than the simple sum of cation and anion radii, leading to a t > 1.087. Although NaBeH3 has not yet been identified experimentally, the large t value indicates that the predicted ideal cubic perovskite structure may not be stable. We now turn to the hexagonal perovskite structure of RbMgH3. As mentioned in the previous section, the Mg-Mg distance in the face-sharing octahedra pair is much larger than the average layer thickness in the six-layered RbMgD3 due to the repulsion between these two Mg. The stability of a hexagonal perovskite structure is strongly dependent on the compensation of such electrostatic repulsion. From the observations in perovskite oxides, the repulsion can be overcome by the formation of metal-metal bonds as is found in BaRuO3,38 or be reduced by the occupation of cations with smaller formal charges on the B-site. Instead of cation-cation bonding, the increased anion polarization that accompanies the formation of 90° B-O-B bonds was also proposed as the real reason for stabilization of the hexagonal BaTiO3 structure.39 RbMgD3 is isostructural with BaTiO3, and the Mg1-D2-Mg1 bond angle

Perovskite Hydrides RbMgH3 and RbCaH3

Figure 8. Charge-density map on the (110) plane of RbMgH3, showing the H-polarization in the 90° Mg-H-Mg bonds. Such anion polarization largely compromises the repulsion between Mg cations in the neighboring face-shared octahedra. The values of the contours are from 0 to 0.065 e/Å3. The atoms (H, small white; Rb, large pink; and Mg, small green spheres) are also shown on the corresponding crystallographic plane. Electron densities present at Rb and Mg sites are mainly contributed from their inner shell 4p and 2p electrons, respectively.

(i.e., 86.77°) is close to 90°. The small Mg2+ ion is able to strongly polarize H-. From the charge-density map projected on the (110) plane (Figure 8), Rb and Mg donate all of their outermost shell valence electrons to H. The highest charge density is situated at each H atom site, indicating the ionic bonding primarily formed in RbMgH3 (N.B.: in Figure 8, high electron densities present at Rb and Mg sites are contributed from their 4p and 2p electrons, which are considered in the calculations, respectively. These inner shell electron densities are not discussed here.) The charge density within a smaller scale clearly revealed the anion polarization for the H- in the 90° Mg-H-Mg bonds, with more charge density in the region between two Mg cations (see Figure 8). Such H- polarization will therefore largely compromise the repulsion between Mg cations in the neighboring face-sharing octahedra. Again, from our NPD data on RbMgH3, the Mg-Mg distance in the facesharing octahedral pair is observed to be much shorter than the Mg-Mg distance in Mg metal and comparable to the reported Mg-Mg bonds in Mg-complex molecules. We also observed a neutron vibrational peak originating from the Mg-Mg stretching mode in RbMgH3. The energy of such a Mg-Mg stretching vibration is relatively high compared to other metal phonon modes, indicating a rather strong Mg-Mg interaction. Therefore, the possible Mg-Mg bonding cannot be completely excluded from the stabilization mechanism of the RbMgH3 hexagonal perovskite structure. Summary Crystal structures, lattice dynamics, and bonding environments of RbMgH3 and RbCaH3 were studied using neutron diffraction, neutron vibrational spectroscopy, and first-principles calculations. RbMgH3 possesses a 6H-BaTiO3-type hexagonal perovskite structure in the temperature range of 5-300 K and

J. Phys. Chem. C, Vol. 113, No. 33, 2009 15097 is stabilized mostly by H- anion polarization in the 90° Mg-H-Mg bonds. The NVS data also suggests a strong Mg-Mg interaction as a result of a short Mg-Mg distance, which is comparable to the direct Mg-Mg bonds observed in the Mg-complex molecules. Unexpected from its small nominal tolerance factor, RbCaH3 forms a cubic Pm-3m perovskite in the 5-300 K range. The inconsistency between predicted geometry and actual structural types in all reported perovskite hydrides can be ascribed to the large deviations of cation-anion bond distances in terms of H- anion polarization and resulting cation-anion bond covalence. Our initial volumetric dehydrogenation studies showed that RbMgH3 and RbCaH3 can release hydrogen at lower temperatures (below 400 °C) with faster kinetics than the binary hydrides RbH, CaH2, and MgH2. Both hydrides can be rehydrogenated under moderate pressure and temperatures (e.g., 20 atm and 300 °C). Although the hydrogen capacities of RbMgH3 and RbCaH3 are inadequate to meet the requirements for practical fuel-cell vehicle applications, these compounds, as well as the previously studied NaMgH3, suggest that hydrides possessing a perovskite structure may still have potential to be used as additives17 to modify the thermodynamics of other hydrogen storage systems, considering the kinetics and reversibility associated with these special structure characteristics. Acknowledgment. This work was partially supported by DOE through EERE Grant No. DE-AI-01-05EE11104 (to T.J.U.) and BES Grant No. DE-FG02-08ER46522 (to T.Y.). Supporting Information Available: NPD patterns of RbMgH3 and RbCaH3 collected at 5 K and refined crystallographic parameters on NPD data for RbMgD3 and RbCaD3. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104 (3), 1283– 1315. (2) Schuth, F.; Bogdanovic, B.; Felderhof, M. Chem. Commun. 2004, 37, 2249–2258. (3) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111–4132. (4) Wu, H. ChemPhysChem 2008, 9, 2157–2162. (5) Bogdanovic, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1–9. (6) Lu, J.; Fang, Z. Z. J. Phys. Chem. B 2005, 109, 20830–20834. (7) Cerny, R.; Filinchuk, Y.; Hagemann, H.; Yvon, K. Angew. Chem., Int. Ed. 2007, 46, 1433–1435. (8) Filinchuk, Y. E.; Yvon, K.; Meisner, G. P.; Pinkerton, F. E.; Balogh, M. Inorg. Chem. 2006, 45, 1433–1435. (9) Chater, P. A.; David, W. I. F.; Johnson, S. R.; Edwards, P. P.; Anderson, P. A. Chem. Commun. 2006, 23, 2439. (10) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J.; Yildirim, T. Chem. Mater. 2008, 20, 1245–47. (11) Buchter, F.; Lodziana, Z.; Remhof, A.; Friedrichs, O.; Borgschulte, A.; Mauron, Ph.; Zut¨tel, A.; Sheptyakov, D.; Barkhordarian, G.; Bormann, R.; Chlopek, K.; Fichtner, M.; Sørby, M.; Riktor, M.; Hauback, B.; Orimo, S. J. Phys. Chem. B 2008, 112, 8042–8048. (12) (a) 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, 1594–1601. (b) Luo, W.; Sickafoose, S. J. Alloys Compd. 2006, 407, 274– 281. (c) Vajo, J. J.; Skeith, S. L.; Mertens, F. J. Phys. Chem. B 2005, 109, 3719. (d) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P.; Vajo, J. J. J. Phys. Chem. C 2007, 111, 12881–12885. (e) Meisner, G. P.; Scullin, M. L.; Balogh, M. P.; Pinkerton, F. E.; Meyer, M. S. J. Phys. Chem. B 2006, 110, 4186–4192. (13) (a) Bouamrane, A.; Laval, J. P.; Soulie, J.-P.; Bastide, J. P. Mater. Res. Bull. 2000, 35, 545–547. (b) Park, H.-H.; Pezat, M.; Darriet, B.; Hagenmuller, P. ReV. Chim. Min. 1987, 24, 525. (c) Maeland, A. J.; Lahar, W. D. Z. Phys. Chem. 1993, 179, 181. (14) Gingl, F.; Vogt, T.; Akiba, E.; Yvon, K. J. Alloys Compd. 1999, 282, 125.

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