J. Phys. Chem. B 2006, 110, 25833-25837
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Mechanochemical Synthesis and Crystal Structure of r′-AlD3 and r-AlD3 Hendrik W. Brinks,* Andreas Istad-Lem, and Bjørn C. Hauback Institute for Energy Technology, Post Office Box 40, Kjeller, N-2027 Norway ReceiVed: May 19, 2006; In Final Form: August 27, 2006
AlD3 was synthesized by ball milling of 3LiAlD4 + AlCl3. Planetary ball milling at room temperature resulted in a mixture of AlD3 (R and R′) and Al in addition to LiCl, whereas cryomilling at 77 K resulted in only AlD3 and LiCl. The AlD3 obtained was a mixture of about 2/3R and 1/3R′. R′ was determined by powder neutron diffraction to take the β-AlF3 structure with space group Cmcm and a ) 6.470(3), b ) 11.117(5), and c ) 6.562(2) Å. It is built up of corner-sharing AlD6 octahedra in an open structure with hexagonal holes of radius 3.9 Å. R′ slowly decomposes during storage at 40 °C. R-AlD3 is also described by a corner-sharing AlD6 network but in a more dense ReO3-type arrangement. Both AlD3 modifications have slightly shorter Al-D distances compared to Na3AlD6, Na2LiAlD6, and K2NaAlH6.
1. Introduction AlH3 (alane) is one of the solid compounds with the largest hydrogen content (10.1 wt %) and is therefore of interest for hydrogen storage applications. AlH3 has been found to take at least six different crystal structures depending on the synthesis route:1 R, R′, β, γ, δ, and . R is the most stable,1 and a highprecision bomb calorimeter study determined the dehydrogenation (eq 1) enthalpy to be 7.61 ( 0.56 kJ/mol of H2 and total entropy of 30.04 ( 0.42 J/(K‚mol of R-AlH3).2
AlH3 ) Al + 3/2H2
(1)
From these values and the entropies of Al and H2, the rehydrogenation pressure at room temperature can be estimated to be above 100 kbar. DSC measurements determine the dehydrogenation enthalpy of R-AlH3 to be 5.7-6.6 kJ/mol of H2,3-5 and β-AlH3 and γ-AlH3 are found to be less stable.3-5 R-AlH3 is kinetically stable and can be stored for several years.6 This stabilization is probably caused by (hydr-)oxide layers at the particles, and the stability has also been reported to be dependent on the particle size of R-AlH3.1 As a consequence, AlH3 may be used as a chemical hydride. R-AlH3 releases hydrogen at g60 °C,1,7,8 whereas γ-AlH3 has been reported to release hydrogen during storage at room temperature1 and at a higher rate than R-AlH3 at 60 °C.8 With additives such as LiH, the dehydrogenation kinetics is enhanced.6 Powder X-ray diffraction (PXD) fingerprints for the different AlH3 phases were given by Brower et al.,1 but R-AlH3 is the only AlH3 phase with a complete crystal structure determination.9 R-AlD3 crystallizes in space group R3hc with AlD6 octahedra sharing all corners with one other octahedron and with three center Al‚‚‚D‚‚‚Al two-electron bonds. AlH3 has typically been synthesized from LiAlH4 and AlCl3 in diethyl ether, resulting in an adduct with 0.25-0.3 Et2O per AlH3. By adding LiAlH4 or LiBH4, in combination with heat treatment at about 60 °C, ether is removed and AlH3 crystallizes, usually in the R, β, or γ structure depending on the conditions.1 * Corresponding author: tel +47 63 80 64 99; fax +47 63 81 09 20; e-mail
[email protected].
Ethereal solutions of AlH3 containing excess LiAlH4, when heated under pressure to between 70 and 80 °C, yield the essential nonsolvated R′-AlH3 phase.1 An alternative and simpler synthesis route of R′-AlH310 and its crystal structure is here reported. 2. Experimental Section LiAlD4 (Sigma-Aldrich; >95% chemical purity, >98% isotope purity) and AlCl3 (Merck, 98%) were milled in a 3:1 ratio. All handling of the samples was carried out in an Ar atmosphere in a glovebox and transferred to sealed milling vials to prevent reaction with moisture and O2. Ball milling at room temperature was carried out with a Fritsch Pulverisette 6 with 2 g sample and 25-100 balls of 4 g each. A Spex 6750 freezer mill was used for cryomilling at 77 K with 1 g sample and a piston of 32 g. LAB PXD data were collected at an INEL XRG3000 diffractometer with monochromized Cu KR1 radiation, flat-plate geometry, and a CPS-120 curved, position-sensitive detector that continuously covers the 2θ range from 2° to 120° (∆2θ ) 0.029°). The sample was covered by a thin plastic film to prevent reaction with air. Synchrotron radiation (SR) PXD data at 22°C were collected at the Swiss-Norwegian beam line (station BM01B) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The sample was kept in rotating 0.8 mm boron-silica-glass capillaries. Intensities were measured in steps of ∆(2θ) ) 0.008°. The wavelength 0.49962 Å was obtained from a channel-cut Si(111) monochromator. Powder neutron diffraction (PND) data at 22 °C were collected with the PUS instrument at the JEEP II reactor at Kjeller (Norway).11 Neutrons with λ ) 1.5554 Å were obtained from a Ge(511) focusing monochromator. The detector unit consists of two banks of seven position-sensitive 3He detectors, each covering 20° in 2θ (binned in steps of 0.05°). The sample was placed in a rotating cylindrical vanadium sample holder with 6 mm diameter. The deuteride analogue was prepared to enable the study of deuterium positions by PND, to ascertain relationships between the hydrogen positions in the hydride. Rietveld refinements were carried out with the program Fullprof (version 2.8).12 The neutron scattering lengths were taken from the Fullprof library. Pseudo-Voigt profile functions
10.1021/jp0630774 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2006
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Figure 1. Observed intensities (circles) and calculated intensities from Rietveld refinements (upper line) of CM 3LiAlD4 + AlCl3 at 295 K for PND (PUS, Kjeller) data. Positions of Bragg reflections are shown with bars for LiCl, R-AlD3, and R′-AlD3 (from top). The difference between observed and calculated intensities is shown with the bottom line.
were used, and the background was modeled by interpolation between manually chosen points. 3. Results and Discussion Several complex hydrides have recently been synthesized by ball milling of hydrides and eventually in combination with chlorides, for example, Na2LiAlH6,13-15 K2NaAlH6,16 Li3AlH6,17 Mg(AlH4)2,18,19 Ca(AlH4)2,20,21 and LiMg(AlH4)3.22 In many cases this approach implies byproducts that would decrease the reversible hydrogen storage capacity in a storage system, but nevertheless this simplified procedure is useful for screening purposes in order to determine crystal structures and thermodynamic stabilities. High-symmetry byproducts can, for example, be included in the Rietveld refinements when the crystal structure is refined. The wet chemistry method with LiAlH4 and AlCl3 in diethyl ether is more complicated and not completely reproducible for synthesizing the pure metastable alane phases. Therefore, a simpler method by ball milling was searched for. In the present work, the following reaction was tried:
3LiAlD4 + AlCl3 ) 4AlD3 + 3LiCl
(2)
Starting at room temperature with a planetary ball mill with 100 balls running at 300 rpm for 5 min, small amounts of R-AlD3 and traces of R′-AlD3 were detected along with LiCl and Al. In situ measurements during milling reveal a sudden pressure increase after about 1 min. The pressure increase corresponds to 60% decomposition of the AlD3 formed in eq 2. The pressure is going through a maximum before it stabilizes in about 1.5 s. This may indicate a considerable heat released in the reaction that is gradually absorbed by the milling vial. Estimations based on the relative pressures and the ideal gas law show that the temporary temperature is at least 75 °C and this may induce decomposition of AlH3, which may also cool the sample. The enthalpy of eq 2 (with 1H) determined with the HSC program23 is -213 kJ/mol. Milder conditions down
to 100 rpm and 25 balls were tested with the same result. Mortaring did not give any reaction. Milling at 77 K has several advantages: (i) The heat released in the desired reaction may be absorbed by the vial before critical temperatures for thermal decomposition are reached, (ii) metastable compounds are less likely to decompose due to reduced mobility compared to room temperature, and (iii) increased brittleness at low temperature will lead to reduced particle sizes and shortened diffusion paths. LiAlD4 and AlCl3 were cryomilled at 77 K and only LiCl and AlD3 were detected in the product. No metallic aluminum was detected. In the PND data of the fresh sample prepared by cryomilling, two different phases of AlD3 were detected, both a phase corresponding to the model of Turley and Rinn9 for R-AlD39 and a phase with the same positions of the reflections corresponding to the PXD data of Brower et al.1 for R′-AlH3. The positions for these PXD data can be shown to be similar to the PXD data of β-AlF324 but shifted to higher angles due to smaller unit-cell dimensions. Recently, Ke et al.25 did use DFT calculations to refine AlH3 with β-AlF3 structure, and their values were used as a starting model for the present refinements. In Figure 1, the Rietveld refinement of PND data for the present sample is shown. Quantitative phase analysis indicate a mixture of 34% R′-AlD3 and 66% R-AlD3. Simulated PXD data of this R′-AlD3 structure model is in very good agreement with the fingerprint of Brower et al.1 SRPXD data of another fresh sample prepared by the same present technique was fitted with the same model by Rietveld refinements with good correspondence, cf. Figure 2. The improved sensitivity of SR-PXD compared to PND and LAB PXD resulted in detection of β-AlD3 in this sample. The content was estimated as 1.5 mol % of the total amount of alane. The structural parameters from Rietveld refinements of PND data for R′-AlD3 and R-AlD3 are given in Table 1, and the crystal structures are shown in Figures 3 and 4, respectively. The refined structure for R-AlD3 is in very good correspondence with that of Turley and Rinn.9 The R′-AlD3 structure consists
Synthesis and Crystal Structure of R′-AlD3 and R-AlD3
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Figure 2. Observed intensities (circles) and calculated intensities from Rietveld refinements (upper line) of CM 3LiAlD4 + AlCl3 at 295 K for SR-PXD (SNBL, ESRF) data. Positions of Bragg reflections are shown with bars for R′-AlD3, LiCl, R-AlD3, and β-AlD3 (from top). The difference between observed and calculated intensities is shown with the bottom line.
TABLE 1: Refined Structural Parameters for r′-AlD3 and r-AlD3a atom
x
y
z
B (Å2)
Al1 Al2 D1 D2 D3 D4
0 1/4 0 0.312(2) 0 0.298(4)
R′-AlD3 1/2 1/4 0.197(2) 0.1000(14) 0.465(3) 0.277(2)
0 0 0.451(4) 0.047(3) 1/4 1/4
1.1(3) 1.1(3) 2.5(1) 2.5(1) 2.5(1) 2.5(1)
Al D
0 0.6277(5)
0 0
0 1/4
1.0(2) 2.0(1)
R-AlD3
a Determined by PND data at 295 K. R′-AlD space group Cmcm, 3: Z ) 12, a ) 6.470(3) Å, b ) 11.117(5) Å, and c ) 6.562(2) Å. R-AlD3: space group R3hc, Z ) 6, a ) 4.4364(3) Å, and c ) 11.7963(10) Å. Reliability factors are Rwp ) 2.91% and χ2 ) 1.40. Estimated standard deviations are shown in parentheses.
of AlD6 octahedra where all deuterium atoms are shared between two octahedra. This corner-sharing network is more open than in R-AlD3, giving rise to hexagonal shaped pores with a diameter of 3.9 Å. The volume per AlD3 unit is also increased from 33.5 Å3 for R to 39.3 Å3 for R′. In R′-AlD3 there are two different types of Al with slightly different sizes, giving average Al-D distances of 1.68(1) and 1.73(1) Å, respectively. For comparison, the Al-D distance in octahedral coordination is on average 1.744 Å for Li3AlD6, 1.760 Å for Na2LiAlD6, and 1.759 Å for K2NaAlH6. The shortest D-D distance is found within the octahedra: 2.31(2) Å. The Al-D-Al angles in the ab plane are 134.93(1)° and 140.3(9)°, and the Al-D-Al angles in the c direction are 150.50(2)° and 153.31(2)°. The shortest Al-Al distance is 3.216(1) Å. R′-AlD3 and R-AlD3 do both have corner-sharing AlD6 octahedra, but the connectivity of the octahedra is significantly different. The R-AlD3 structure may be described as a ReO3type structure with rotated octahedra. This arrangement is similar to, for example, Na2LiAlD6,14 for which Li, Al, and D are basically forming the same network with AlD6 and LiD6
octahedra as AlD6 octahedra are forming in R-AlD3. The major difference is that the octahedra in Na2LiAlD6 are not rotated, and in addition the size of the LiD6 octahedra are different from the AlD6 octahedra. Na2LiAlD6 may hence be regarded as stabilized R-AlD3 with half the Al substituted by Li and with Na in interstitial positions to compensate for the charge difference. K2NaAlH616 with the same structure as Na2LiAlD6 shows the same feature, and also Na3AlD6,26 with a related structure but with a different mode of rotations of the octahedra compared to R-AlD3. For R-AlD3 (shown in Figure 4) all Al-D distances are 1.711 Å with estimated standard deviation 0.001 and 0.002 Å (Turley and Rinn:9 1.709 Å with estimated standard deviations of 0.003 and 0.008 Å), which is similar to the average for R′-AlD3 but shorter than Na2LiAlD6, K2NaAlH6, and Na3AlD6. This may indicate a difference in the chemical bond in corner-sharing networks of AlH6 compared to isolated AlH63- octahedra separated by electropositive metals. This is in line with DFT calculations by Ke et al.,25 where the bond overlap population for both modifications were found to be closer to NaH than Na3AlH6; that is, the Al-H bonds in AlH3 are polar-covalent with more ionic contribution than for the Al-H bonds in Na3AlH6. The expansion per D atom compared to metallic Al is 5.6 Å3 for R-AlD3 and 7.6 Å3 for R′-AlD3. This is more than typical values for interstitial metal hydrides of 2-3 Å3.27 Without the rotation of the octahedra (maintaining the same Al-D distances), the volume per AlD3 unit would have been 39.9 Å3 for the R-AlD3 phase, whereas further rotation of the octahedra may give too-short distances between deuterium in different octahedra. The measured shortest D-D distance in R-AlD3 is 2.414(1) Å within the octahedra and 2.585(1) Å between different octahedra. In the imagined case of no rotation, the shortest D-D distance between the octahedra would have been 3.422 Å. The rotations clearly shorten the D-D distance between the octahedra.
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Figure 3. Crystal structure of R′-AlD3. The unit cell is shown to the left, and the pores through the whole structure are highlighted to the right.
Figure 4. Crystal structure of R-AlD3. The unit cell is shown to the left, and the connectivity of the octahedra is illustrated to the right. Each octahedron is sharing one corner with one other octahedron, building a distorted primitive Al sublattice.
Figure 5. Connectivity of (a) R-AlD3 and (b) R′-AlD3. For R′-AlD3, four of the six octahedra in the first coordination sphere are interconnected and this gives rise to pores.
The rotation is also evident from the Al-D-Al angles that are 141.33(8)° compared to 180° without rotations. The observed rotation gives a shortest Al-Al distance for R-AlD3 of 3.229(1) Å. The Al sublattice is a slightly deformed primitive packing of Al (in each corner of stacked cubes) with a six-coordination of Al. Hence, Al does not form any close packed layers. The connectivity of the AlD6 octahedra for R-AlD3 and R′AlD3 is further illustrated in Figure 5. For R-AlD3, none of the octahedra in the first coordination sphere are connected with each other, whereas for R′-AlD3 four of the six octahedra in the first coordination sphere of both Al1 and Al2 are interconnected in pairs. This arrangement gives rise to layers with
hexagonal holes. The last two of the six neighboring octahedra connect this layer with layers above and below, resulting in pores through the structure. Elemental analysis of R′-AlD3 prepared in Et2O by Brower et al.1 did indicate a carbon content corresponding to about 0.02 Et2O per AlD3 unit. In the present procedure no solvents were used. Unless there are traces of Et2O left in the sample from the preparation of LiAlD4, R′-AlD3 herein prepared are not stabilized by solvent molecules. This is supported by the fact that cryomilling experiments with AlCl3 and NaAlH4, which is prepared in a different way than LiAlD4, also yields a large amount of R′-AlH3.
Synthesis and Crystal Structure of R′-AlD3 and R-AlD3 R′-AlD3 is regarded as the least stable of these two modifications.1 Four separate PND measurements of R′-AlD3 were carried out during four subsequent days at room temperature, and no difference in R′-AlD3 content was detected. The thermal stability was further tested by measuring the pressure developed in an 11.90 cm3 sample holder at 40.00 ( 0.04 °C for 2 weeks. The pressure increased corresponding to 30% decomposition of the AlD3 formed in eq 2, indicating that R′-AlD3 prepared in this way may not be stable during long-time storage at room temperature. Acknowledgment. We gratefully acknowledge Professor Craig M. Jensen, University of Hawaii, for his insight and his encouragement to investigate aluminum hydride. The skilful assistance from the project team at the Swiss-Norwegian Beam Line, ESRF, is gratefully acknowledged References and Notes (1) Brower, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L.; Snover, J. A.; Terada, K. J. Am. Chem. Soc. 1976, 98, 2450-2453. (2) Sinke, G. C.; Walker, L. C.; Oetting, F. L.; Stull, D. R. J. Phys. Chem. 1967, 47, 2759. (3) Claudy, P.; Bonnetot, B.; Letoffe, J. M. Thermochim. Acta 1978, 27, 205-211. (4) Orimo, S.; Nakamori, Y.; Kato, T.; Brown, C.; Jensen, C. M. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 5-8. (5) Graetz, J.; Reilly, J. J. J. Alloys Compd. 2006, 424, 262. (6) Sandrock, G.; Reilly, J. J.; Graetz, J.; Zhou, W. M.; Johnson, J.; Wegrzyn, J. J. Appl. Phys. A 2005, 80, 687. (7) Herley, P. J.; Christofferson, O. J. Phys. Chem. 1981, 85, 18821888. (8) Graetz, J.; Reilly, J. J. J. Phys. Chem. B 2005, 109, 22181-22185.
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