crystal chemistry and dehydrogenation/rehydrogenation properties of perovskite hydrides rbmgh ...

Crystal Chemistry and Dehydrogenation/Rehydrogenation Properties of PerovskiteHydrides 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, Maryland20899-6102, 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 investigatedusing neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principlescalculations. RbMgH3 exhibits a 6H-BaTiO3-type hexagonal perovskite structure, and RbCaH3 forms a Pm-3m simple cubic perovskite. A very short Mg-Mg distance (∼2.77 Å) was found in RbMgH3, and its structuralstability was ascribed to H- anion polarization and Mg-Mg bonding. Interestingly, RbCaH3 forms an idealcubic perovskite despite its small tolerance factor. The inconsistency between the nominal tolerance factorand adopted structure types of RbCaH3 are discussed in terms of anion polarization and covalence effect onthe cation-anion bond length deviations. Finally, we show that both RbMgH3 and RbCaH3 can bedehydrogenated and rehydrogenated at 300-400 °C under moderate pressures.

Introduction

Metal hydrides, including conventional (e.g., LaNi5H6 andMg2NiH4) and complex hydrides (e.g., alanates, borohydrides,and amides), are an important family of materials that have greatpotential for hydrogen storage and have been extensivelystudied.1–4 In recent years, particular interest has been focusedon the complex metal hydrides because of their high gravimetricand volumetric hydrogen densities.1,3,5–11 Complex metal hy-drides that are interesting for hydrogen storage generally consistof alkali and/or alkaline earth cations and [AlH4]-, [NH2]-, and/or [BH4]- anions. Hydrogen in these materials forms adirectional covalent bond with the central atom in the anionunit. The transition states for atomic rearrangement often occurin an unfavorable bonding configuration. This increases theactivation energy for hydrogen diffusion and thus leads to slowabsorption kinetics and poor reversibility under moderateconditions.1 In addition, most of these hydrides suffer from toxicgas release (e.g., ammonia, diborane, etc.) during dehydroge-nation.12 Therefore, development of reversible and kineticallyfavorable complex hydrides such as alkali and/or alkaline-earthmetal 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-earthmetal hydrides are less investigated because of their complexcrystal structures and difficulties in determining the hydrogenpositions by X-ray diffraction (XRD). With properly chargedand sized cations, A and B, some of the ternary hydrides havebeen found to form ABH3 perovskite structures.13–18 While thestructural characteristics of these perovskite hydrides are rarelyinvestigated, the ABO3 perovskite oxides represent one of themost widely studied families of inorganic compounds due to

their diverse electronic and magnetic properties accompaniedwith the large flexibility in accommodating a broad range ofatomic substitution in both cation (A and B sites) and anionsites, and the resulting structural changes.19 The well-establishedcrystal chemistry of ABO3 compounds certainly provides usefulguidelines for analyzing the structural stabilities of perovskitehydrides.

The structure type of a perovskite oxide is strongly affectedby the compatibility of cation and anion sizes. In an ABO3

perovskite, if the BO6 octahedra share corners infinitely in allthree 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-cationa 6-fold anion coordination. In an ideal case, the bond distancesof A and B cations to the anions satisfy the geometricrelationship dA-O ) (2(dB-O))1/2 and will not induce any distortionof the unit cell. The resultant symmetry is cubic with spacegroup Pm-3m. In many other cases, the A-O and B-O bondlengths are geometrically incompatible, and the crystal symmetrywill be lowered. The deviation from the perfect cubic perovskitestructure can be evaluated by the Goldschmidt tolerance factor(t ) (rA + rO)/(21/2(rB + rO))), where the A-O and B-O bondlengths are estimated by the sum of cation and anion radii.Apparently, for an ideal cubic perovskite, t ) 1. When the Acation is small (undersized), t < 1; the surrounding BO6 can tiltwhile still maintaining its corner-sharing connectivity to shortenthe A-O distance and lower the coordination number of the Acation. The first coordination sphere around the B cation remainsunchanged, 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 cubicAO3 layer sequence. When the B cation is too small for its cage,t > 1. In some cases, the B cation will make an off-centereddisplacement, shifting the symmetric center of the unit cell, withthe BO6-octahedra corner-sharing maintained, e.g., tetragonalBaTiO3. In some other cases, the perovskite compounds with t> 1 will involve mixed cubic (c) and hexagonal (h) or pure

* To whom correspondence should be addressed. E-mail: [email protected].† National Institute of Standards and Technology.‡ University of Maryland.§ University of Pennsylvania.

J. Phys. Chem. C 2009, 113, 15091–15098 15091

10.1021/jp905255s CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/22/2009

hexagonal close-packing of the AO3 layers. The introductionof hexagonal stacking is accompanied by face-sharing ofadjacent BO6 octahedra. These structures are called hexagonalperovskites. One important issue about such structures is thatthe cations in the neighboring face-sharing octahedra pair arevery close and thus have strong electrostatic repulsion; therefore,the stability of hexagonal perovskite strongly depends on thecompensation of such repulsion.

In contrast to the enormous number of known perovskiteoxides, chalcogenides, and halogenides, there are only a fewperovskite hydrides reported,13–18 and most of their structureswere determined from XRD studies, without accurate data onthe H positions. From the limited crystallographic information,some reported structures appear to be inconsistent with thegeometric criteria for a stable perovskite. For example, RbCaH3

with t ) 0.928 was reported to form a cubic Pm-3m perovskitefrom a room temperature XRD measurement on a hydridesample.13 Since its tolerance factor is even smaller than that ofNaMgH3 and NaMgH3 has been confirmed to form an orthor-hombic Pnma perovskite,10 a high symmetry cubic structureseems rather unlikely for RbCaH3. Therefore, a neutron powderdiffraction (NPD) study on a deuterided sample is needed toclarify such discrepancies because XRD cannot accuratelydetermine the positions of H and consequently cannot distinguishthe possible CaH6 octahedra tilting. Some theoretical efforts havealso been reported to predict the formation of new perovskitehydrides.20,21 However, some of the predicted structures do notfollow the general structural trend indicated by their tolerancefactors. For some systems with t > 1, the theorists were notaware of the possible formation of hexagonal perovskitestructure types and only considered the cubic Pm-3m phases.Experimentally, hexagonal perovskite structures have been foundin RbMgH3

14 and CsMgH3 (high-pressure)16 systems, while theformation and stability of such hydrides have not been rational-ized. Clearly, there is a lack of understanding on the stabilityof all possible perovskite structure types and awareness of theimpact of the nature of H- anion and different cations. As forthe properties and applications of perovskite hydrides, evenfewer studies have been reported. NaMgH3 is the only systemwhose hydrogen storage properties have been well studied andconsistently reported.17,22

We believe it is important to clarify the structures ofperovskite hydrides and study their properties. Following ourrecent studies of the NaMgH3 system,17 two more compounds,RbMgH3 and RbCaH3, have been systematically studied in thiswork. We have used combined neutron powder diffraction(NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations to elucidate their crystal structures, latticedynamics, local bonding configurations, and dehydrogenation/hydrogenation properties. The origin of the deviations of theactual crystal structures from the geometric criteria (i.e.,tolerance factor) for certain perovskite hydride systems(RbCaH3, etc.) has been identified, and its implications werediscussed. The stability of hexagonal perovskite RbMgH3 hasalso been rationalized.

Experimental Section

RbMgH3 and RbCaH3 were prepared using standard solid-state methods. Pure Rb metal and MgH2 (AlfaAesar,23 98%) orCaH2 (Aldrich, 99%) were mixed respectively via ball millingwith a Fritsch Pulverisette 7 planetary mill at 400 rpm for 60min. A small amount of extra hydrides (i.e., greater than a 1:1stoichiometric ratio of hydride and Rb) were put in the mixturesto prevent the formation of Rb-rich impurities (i.e., Rb4Mg3H10,

Rb2MgH4 or Rb2CaH4) in the subsequent high-temperaturesynthesis of the target hydrides. The powder mixture was thenwrapped in a Mo envelope and sealed in a stainless steel tube.The tube was connected to a hydrogen gas tank, as part of ahydrogenation system, and heated in a tube furnace. The powdermixture was heated overnight up to 400 °C under 20 bar H2

pressure. All sample handling was performed in a He-filledglovebox due to the extreme air-sensitivity of the hydrides.Phase identification and equilibrium were monitored on samplessealed in glass capillaries using a Rigaku diffractometer with aCu KR source operated at 40 kV and 40 mA.

All neutron scattering measurements were performed at theNIST Center for Neutron Research (NCNR). NPD studies wereconducted on the high-resolution neutron powder diffractometer(BT-1) with the Cu(311) monochromator at a wavelength of1.5403(2) Å and an in-pile collimation of 15 min of arc. Datawere collected over the 2θ range of 3-168°. Rietveld structuralrefinements were done using the GSAS package.24 Because ofthe large incoherent neutron scattering cross-section of hydrogen,the sample utilized in the NPD measurements was deuteratedby exposing it to gaseous deuterium at a pressure of 50 bar anda temperature of 673 K over a two-week period with severalcycles of intermediate flushing and recharging of deuterium gas.The exchanged deuterium content was determined by gravi-metric measurements, and the residual hydrogen content waschecked using the neutron prompt-γ activation analysis (PGAA)facility, which is able to detect hydrogen as low as 2 µg.25 PureMgH2 and CaH2 samples were used as standards to normalizeγ-ray intensities. The final stoichiometry of H in RbMgD3 andRbCaD3 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 theBT-4 filter-analyzer neutron spectrometer (FANS) with theCu(220) monochromator under conditions that provided energyresolutions of 2-4.5% over the vibrational energy range probed.

Temperature programmed desorption (TPD) was conductedusing a volumetric gas sorption Sieverts-type apparatus. Detailsof the Sieverts system characteristics and operation can be foundin our previous publication.26

First-principles calculations were performed within the plane-wave implementation of the generalized gradient approximationto density functional theory (DFT) using the PWscf package.27

We used a Vanderbilt-type ultrasoft potential with Perdew-Burke-Ernzerhof exchange correlation. A cutoff energy of 544eV was found to be enough for the total energy and force toconverge within 0.5 meV/atom and 0.005 eV/Å. Structureoptimizations were performed with respect to atomic positionswith lattice parameters set to the experimental values. Thephonon calculations were conducted with the optimized structureusing the supercell method with finite difference.28,29

Results

Structure and Lattice Dynamics of RbMgH3. The XRDpattern of RbMgH3 can be indexed using the 6H-BaTiO3-typeperovskite model with space group P63/mmc (No. 194),consistent with the previously reported structure at roomtemperature.14 The structure was then refined on this model usingour NPD data collected on RbMgD3 at 300 K. Within the studiedtemperature range 5-300 K, RbMgD3 maintains the samestructure without any phase transition. Rietveld refinementrevealed a hexagonal perovskite structure with lattice parametersof 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

15092 J. Phys. Chem. C, Vol. 113, No. 33, 2009 Wu et al.

thus, the site occupancies of Rb and Mg atoms were fixedaccording to the chemical composition. The refined latticeparameters, fractional coordinates, D site occupancies, thermalparameters, and reliability factors for RbMgD3 measured at 5and 300 K are summarized in Tables S1 and S2 (see SupportingInformation). Figure 1a shows the NPD pattern for RbMgD3 at300 K with an excellent quality of the fit. The refined structure(Figure 2a) is also in good agreement with the optimizedstructure from our DFT calculation.

Compared to the cubic perovskites with only corner-sharingoctahedra (e.g., NaMgH3), in RbMgD3 the shifting of some[RbD3] layers by (1/3, 2/3, 0) induces groups of face-sharingMgD6 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 corner-sharing MgD6 octahedra (see Figure 2a). Therefore, althougheach D is still surrounded by six cations as in NaMgD3, thereare two crystallographically different anion sites in RbMgD3,i.e., D1 in the cubic close packed layers between the corner-sharing MgD6 octahedra and D2 in the hexagonally closedpacked 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 TableS2, Supporting Information). From the refined bond lengths(Table S3, Supporting Information), the corner-sharing MgD6

octahedron exhibits a nearly ideal configuration, with six equalMg2-D bond lengths of 2.050(1) Å and D-Mg2-D anglesranging from 88.20(7)° to 91.8(7)°. The face-sharing MgD6

octahedra show some distortion with elongation between twoneighboring Mg1 atoms so that the D2-Mg1-D2 angledecreases to 78.02(10)°. In RbMgD3, we also noticed that theMg-Mg distance in the pair of face-sharing octahedra (2.773Å) is much shorter than the closest approach distance in Mgmetal (3.21 Å) and is comparable to the shortest Mg-Mg bondsrecently reported in the R-Mg-Mg-R complex molecules, i.e.,2.766-2.889 Å for different R ligands. (Note: the oxidationstate of Mg is +1 in these molecular compounds.)30,31 However,the separation between Mg in the face-sharing octahedra pairis 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 thesetwo Mg2+ cations. The implications of the short Mg-Mgdistance and the resulting Mg-Mg repulsion on the stability ofhexagonal RbMgH3 will be discussed in the next section.

In order to probe the chemical environment and local bondingstates, and to ascertain any possible Mg-Mg bonding inRbMgH3, we performed NVS measurements and first-principlesDFT calculations. NVS data directly reflect the vibrationaldensity of states and are particularly sensitive to the hydrogenvibrational modes. Figure 3 shows the NV spectrum forRbMgH3 collected at 4 K. The calculated first-principles phononspectrum based on the optimized RbMgH3 structure is alsoshown in Figure 3 and is in reasonably good agreement with

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

(from the top), respectively.

Figure 2. Crystal structure of (a) RbMgD3 and (b) RbCaD3 with refinedthermal ellipsoids at 300 K. Rb atoms are indicated in pink, Mg ingreen, Ca in blue, and D in white. RbMgD3 crystallizes in a six-layer(hcc)2 hexagonal perovskite structure featured with face-sharing MgD6-octahedra and a 90° Mg-D-Mg bond angle. RbCaD3 forms an idealcubic perovskite with only corner-shared CaD6-octahedra.

Perovskite Hydrides RbMgH3 and RbCaH3 J. Phys. Chem. C, Vol. 113, No. 33, 2009 15093

the observed NV spectrum. The phonon bands above 60 meVare dominated by the vibrations of hydrogen and can be assignedto the rocking, bending, and stretching modes of hydrogen inMgH6 octahedra. NV features below 30 meV are mainlyattributed to metal-atom displacements. Also note that thereis a small peak near 41 meV in both observed and calculatedNV spectra, which corresponds to the Mg-Mg stretchingmotion in the face-sharing octahedra pair. The energy of sucha Mg-Mg stretching mode in RbMgH3 aligns well with themode at ν ∼325 cm-1 (i.e., ∼40.3 meV) observed recently inthe Mg-complex molecules with direct Mg-Mg bonds.30

Structure and Lattice Dynamics of RbCaH3. The XRDpattern of RbCaH3 indicates a simple cubic perovskite structure,same as what was reported previously. This is surprising sinceits tolerance factor, 0.928, is even smaller than that of NaMgH3,and suggests a lower symmetry. Since XRD is much lesssensitive to the positions of H and thus cannot distinguish thepossible CaH6 octahedra tilting, Rietveld structural refinementwas performed on NPD data collected from RbCaD3. Interest-ingly, all patterns in the temperature range of 5-300 K couldonly be satisfactorily fit using a Pm-3m simple cubic perov-skite model (Figures 1b and 2b). Refinement using the PnmaNaMgH3 model (GdFeO3-type structure) or I4/mcm tetragonalCaTiO3 model did not properly converge and/or resulted inabnormal thermal factors for metal or D atoms. DFT structuraloptimization from various models was also performed, and thefinal minimum-energy structure always converged to a cubicstructure without cell distortion and octahedral tilting (Figure2b). Tables S4 and S5 (Supporting Information) list the refinedcrystallographic parameters of RbCaD3. All these observationsconfirm that RbCaD(H)3 indeed forms a simple cubic perovskitedespite its unusually small tolerance factor.

NVS data were also measured for RbCaH3 and compared tothe phonon density of states calculated using the relaxedstructure model to ensure the right local bonding environment

for H (see Figure 4). The calculated phonon modes are in overallgood agreement with the observed NV. Inspection of thecalculated phonon modes indicates that the vibrational bandsin the range of 40-60 meV originate from hydrogen rockingand bending modes, the phonon band at about 137 meV isrelated to the H stretching mode, and the vibrational bands atabout 68 and 90 meV are due to the dispersion of the 40-60meV Γ phonons within the Brillouin Zone.

Dehydrogenation/Rehydrogenation Properties. Tempera-ture programmed dehydrogenation measurements were per-formed on RbMgH3 and RbCaH3 at a heating rate of 2 °C/minfrom room temperature to 400 °C to study their dehydrogenationproperties. After dehydrogenation, all samples were rehydro-genated at the same conditions (under 20 atm H2 pressure at300 °C). Figures 5 and 6 show these volumetric desorptionresults.

RbMgH3 completely releases hydrogen, i.e., ∼1.5 equivalentsof H2 per mol RbMgH3, at ∼392 °C. XRD after the TPD studyindicates diffraction peaks from Mg and Rb metals. In contrastto a previously reported study,32 we did not observe a suggestedcomposition of RbMgH4 from our neutron PGAA analysis, norany new XRD peaks from an unknown hydride phase. Theobserved amount of desorbed hydrogen from our volumetricstudy 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 ofunreacted CaH2 (14.56 wt %) using the current synthesismethod. The very stable CaH2 would not dehydrogenate in thestudied temperature range (RT to 450 °C) since its decomposi-tion temperature is g600 °C even with destabilization additivessuch as Si.33 Therefore, the desorbed H2 is mainly from RbCaH3.The amount of released H2 shown in Figure 6 has beennormalized to the actual amount of RbCaH3 contained in thesample. Compared to RbMgH3, RbCaH3 desorbs most of itshydrogen in a much more complicated fashion with three

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

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


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

different desorption stages would be necessary to more fullyunderstand the dehydrogenation mechanism. Our current labora-tory XRD study on the quenched samples shows that thedehydrogenated products were dominated by scattering fromCaH2, we cannot differentiate the decomposed product CaH2

from the unreacted precursor. Also, the in situ study was limitedby the strong evaporation of Rb in the sample tube or capillary.

After desorption, both RbMgH3 and RbCaH3 can be rehy-drogenated under 20 atm at a temperature of 300-400 °C fora couple of hours, and the subsequent dehydrogenation showsbehavior similar to that of the first time desorption but with alower hydrogen capacity (∼70% of that of the first cycle)presumably due to a small loss of volatile Rb during the priordesorption.

Discussion

Our experimental results on the crystal structure and latticedynamics for RbMgH3 and RbCaH3 are straightforward. Thepresent determined structures of RbMgH3 and RbCaH3 as wellas other previously reported ABH3 hydrides enable us to providea detailed analysis on the general crystal chemistry of perovskitehydrides. In this section, we focus our discussion on the impactof cation/anion sizes on the actual perovskite type in hydridesand the stability of hexagonal perovskite hydrides.

For all known perovskite hydrides, it seems that no strongcorrelation exists between the geometric criteria and the actualcrystal symmetries, which is different from the perovskiteoxides. One obvious example is RbCaH3, which adopts an idealPm-3m perovskite structure in spite of a small tolerance factoreven less than NaMgH3, while the latter follows the generaltolerance factor rule and exhibits an orthorhombic GdFeO3-typestructure with octahedral tilting. These deviations need to berecognized since they are important for both theorists andexperimentalists to explore new compounds with the rightstructures in a rational and targeted way.

As mentioned earlier, the structure type of perovskite oxidesis known to depend on the geometry constriction, tolerancefactor. In the tolerance factor equation, the cation-anion (M-X)distance can essentially be approximated by the sum of effectiveionic radii of cations and anions, assuming the radii areindependent of structure type along with considerations ofcoordination number (CN), electronic spin, etc.34 In ABH3

hydrides, tolerance factor has also been used to predict theirstructure types assuming a constant size of H-, i.e., 1.3 Å or1.4 Å.18,22 Although this assumption is generally valid for oxides,we found that it is problematic for hydrides. Figure 7 showsthe H- radii derived from the M-H interatomic distancesobserved in alkali/alkaline earth hydrides (AH/AeH2) and AAeH3

perovskite hydrides as a function of cation radius34 and Pauling’selectronegativity. In contrast to O2- or halide anions, whichusually assume nearly constant radii, a H- anion does not showa constant radius. Rather, it exhibits a large variation in differentcompounds. The H- anion radii derived from the M-Hdistances of alkali metal hydrides (rocksalt structure) and MgH2

show a strong dependence on the cation radius or electronega-tivity. Both cations and H- anions in these compounds haveCN ) 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 notedpreviously,34–36 and the differences in the observed M-Hdistances in binary hydrides were proposed to be caused by thelarge H- polarizability.36 In the simple AH and AeH2 hydrides,small and highly charged cations polarize H- strongly and cause

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

Figure 6. TPD results of hydrogen release for RbCaH3 with 2 °C/minheating ramp. The desorption temperatures and rates are shown in thebottom panels. The amount of hydrogen gas released has beennormalized as n(H2 gas)/mol RbCaH3.


an increase in the hydrogen polarization and in the covalentcharacter of the anion-cation bond. Therefore, smaller H- sizesare generally observed in hydrides with smaller and moreelectronegative cations.

In the AAeH3 perovskite hydrides, H- anion radii (CN)6)can be derived from the M-H bonds either in B-site octahedraor in A-site cuboctahedra. Therefore, it is not obvious how tobuild a simple size relationship with the cation species. FromFigure 7, in the complex AAeH3 perovskite hydrides with thesame A-site cation (e.g., RbAeH3, Ae ) Mg and Ca), the H-

anion radius derived from the A(XII)-H bond varies with theB-site cation size and electronegativity, regardless of the typesof structures. Similarly, if the B-site cations remain the same(e.g., AMgH3, A ) Na, K, Rb, and Cs), the H- anion size derivedfrom the B(VI)-H bond will then depend on the A-site cationspecies. However, such a trend with cation size or electrone-gativity is not observed if the H- radius is calculated from theM-H bond of the same site cation. For example, for RbAeH3

(Ae ) Mg and Ca), the H- anion radius derived from the Mg-Hbond is actually larger than that from a Ca-H bond. For theA-site (e.g., ACaH3, A ) Rb and Cs), the H- radii derived fromRb-H is larger than that from Cs-H. The same deviation canbe 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

A(XII)MgH3 series (A ) Na, K, Rb, and Cs). Some of thesedeviations are unexplained, but some can be ascribed to the poly-hedron distortion and covalence effect. For example, the largerH- anion derived from the Na-H bond compared to thosederived from A-H bonds (A ) K, Rb, Cs) can presumably beexplained by the polyhedral distortion. From the NPD structuralstudy,17 NaMgH3 forms an orthorhombic perovskite structurewith a-b+a- octahedral tilting. The MgH6 octahedra maintaina nearly ideal configuration with similar Mg-H bond lengths,and the tilting of MgH6 octahedra leads to a distorted cuboc-tahedral environment for the Na+ cation. From analysis of manycompoundswithpolyhedrondistortion,a largermeancation-aniondistance is usually observed in a compound with a higher degreeof polyhedron distortion.34 Therefore, compared to the cubicKMgH3 perovskite, the distorted Na-cuboctahedra in NaMgH3

would result in a larger H- anion radius. It is difficult to explainthe cubic Pm-3m perovskite structures for SrLiH3 and RbCaH3.Using a constant H- radius (e.g., ∼1.3 or 1.4 Å in previousreports18,22), we determined that the tolerance factors of SrLiH3

and RbCaH3 are much less than those of NaMgH3. Yet fromstructural refinement, Sr-H and Rb-H bonds are both excep-tionally long, which might contribute to a larger tolerance factorbased on a constant H anion radius. For SrLiH3, a short Li-Hbond length caused by the covalence effect mentioned abovewill also lead to a larger tolerance factor. A similar covalenceeffect can also be applied to CaNiH3. Although the tolerancefactor of CaNiH3 was calculated as 0.938 from a constant H-

radius of 1.3 Å,18 CaNiH3 was found to form a cubic Pm-3mperovskite structure due to a very short Ni-H bond length.18

Because of such wide variations in H- size, the radius of H-

cannot be assumed as a constant in the perovskite hydrides toderive the tolerance factor. The polarization of H- and thecovalence of the M-H bond should be considered beforethe tolerance factor is used to predict the geometric stability ofthe perovskite hydrides. For example, LiMgH3 was calculat-ed to form a cubic Pm-3m perovskite structure.20 Its nominaltolerance factor is 0.77 using a constant H- radius of 1.3 Å.18

When the covalence effect on shortening of the Li-H bondlength is considered, the tolerance factor will be even smallerand not stable for a cubic perovskite. Indeed no perovskitestructure was found in the Li-Mg-H system.37 NaBeH3 waspredicted to form a Pm-3m cubic perovskite,21 and its tolerancefactor was calculated as 1.087, assuming a constant H- size.Be2+ is much smaller and more electronegative than Mg2+, andthe Be-H bond length is thus expected to be shorter than thesimple 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 cubicperovskite structure may not be stable.

We now turn to the hexagonal perovskite structure ofRbMgH3. As mentioned in the previous section, the Mg-Mgdistance in the face-sharing octahedra pair is much larger thanthe average layer thickness in the six-layered RbMgD3 due tothe repulsion between these two Mg. The stability of a hexagonalperovskite structure is strongly dependent on the compensationof such electrostatic repulsion. From the observations inperovskite oxides, the repulsion can be overcome by theformation of metal-metal bonds as is found in BaRuO3,38 orbe reduced by the occupation of cations with smaller formalcharges on the B-site. Instead of cation-cation bonding, theincreased anion polarization that accompanies the formation of90° B-O-B bonds was also proposed as the real reason forstabilization of the hexagonal BaTiO3 structure.39 RbMgD3 isisostructural with BaTiO3, and the Mg1-D2-Mg1 bond angle

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


(i.e., 86.77°) is close to 90°. The small Mg2+ ion is able tostrongly polarize H-. From the charge-density map projectedon the (110) plane (Figure 8), Rb and Mg donate all of theiroutermost shell valence electrons to H. The highest chargedensity is situated at each H atom site, indicating the ionicbonding primarily formed in RbMgH3 (N.B.: in Figure 8, highelectron densities present at Rb and Mg sites are contributedfrom their 4p and 2p electrons, which are considered in thecalculations, respectively. These inner shell electron densitiesare not discussed here.) The charge density within a smallerscale clearly revealed the anion polarization for the H- in the90° Mg-H-Mg bonds, with more charge density in the regionbetween two Mg cations (see Figure 8). Such H- polarizationwill therefore largely compromise the repulsion between Mgcations in the neighboring face-sharing octahedra. Again, fromour NPD data on RbMgH3, the Mg-Mg distance in the face-sharing octahedral pair is observed to be much shorter than theMg-Mg distance in Mg metal and comparable to the reportedMg-Mg bonds in Mg-complex molecules. We also observed aneutron vibrational peak originating from the Mg-Mg stretchingmode in RbMgH3. The energy of such a Mg-Mg stretchingvibration is relatively high compared to other metal phononmodes, indicating a rather strong Mg-Mg interaction. Therefore,the possible Mg-Mg bonding cannot be completely excludedfrom the stabilization mechanism of the RbMgH3 hexagonalperovskite structure.

Summary

Crystal structures, lattice dynamics, and bonding environ-ments of RbMgH3 and RbCaH3 were studied using neutrondiffraction, neutron vibrational spectroscopy, and first-principlescalculations. RbMgH3 possesses a 6H-BaTiO3-type hexagonalperovskite structure in the temperature range of 5-300 K and

is stabilized mostly by H- anion polarization in the 90°Mg-H-Mg bonds. The NVS data also suggests a strongMg-Mg interaction as a result of a short Mg-Mg distance,which is comparable to the direct Mg-Mg bonds observed inthe Mg-complex molecules. Unexpected from its small nominaltolerance factor, RbCaH3 forms a cubic Pm-3m perovskite inthe 5-300 K range. The inconsistency between predictedgeometry and actual structural types in all reported perovskitehydrides can be ascribed to the large deviations of cation-anionbond distances in terms of H- anion polarization and resultingcation-anion bond covalence.

Our initial volumetric dehydrogenation studies showed thatRbMgH3 and RbCaH3 can release hydrogen at lower temper-atures (below 400 °C) with faster kinetics than the binaryhydrides RbH, CaH2, and MgH2. Both hydrides can be rehy-drogenated under moderate pressure and temperatures (e.g., 20atm and 300 °C). Although the hydrogen capacities of RbMgH3

and RbCaH3 are inadequate to meet the requirements forpractical fuel-cell vehicle applications, these compounds, as wellas the previously studied NaMgH3, suggest that hydridespossessing a perovskite structure may still have potential to beused as additives17 to modify the thermodynamics of otherhydrogen storage systems, considering the kinetics and revers-ibility associated with these special structure characteristics.

Acknowledgment. This work was partially supported byDOE through EERE Grant No. DE-AI-01-05EE11104 (toT.J.U.) and BES Grant No. DE-FG02-08ER46522 (to T.Y.).

Supporting Information Available: NPD patterns of Rb-MgH3 and RbCaH3 collected at 5 K and refined crystallographicparameters on NPD data for RbMgD3 and RbCaD3. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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