Low-pressure Structural Modification 

of Aluminum Hydride 

by Jennifer A. Ciezak-Jenkins 

ARL-TR-5463 February 2011 

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Army Research Laboratory 

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ARL-TR-5463 February 2011 

Low-pressure Structural Modification 

of Aluminum Hydride 

Jennifer A. Ciezak-Jenkins 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution unlimited. 
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Low-pressure Structural Modification of Aluminum Hydride 

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14. ABSTRACT 

In-situ isothermal high-pressure synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the 

structural properties, equation of state, and vibrational dynamics of alpha-aluminum hydride (AlH3). The x-ray measurements 

show that the pressure-volume relations remain smooth to near 50 GPa. Although there is no evidence for a first-order phase 

transition, the unit cell axial ratio (c/a) is anomalous near 2.5 GPa, which correspond to the onset of a monoclinic structural

distortion. Infrared measurements conducted within a similar pressure range show a distinct discontinuity in the frequencies of 

the Al-H lattice modes. The combined data provide evidence for a monoclinic structural distortion beginning near 2.5 GPa. 

The results are consistent with recent synchrotron x-ray diffraction measurements and theoretical calculations. All structural

changes are reversible upon pressure release. 

15. SUBJECT TERMS

High-pressure, Diamond anvil cell, aluminum hydride 

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Jennifer Ciezak-Jenkins 

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iii

Contents 

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Experimental Methodology 2

3. Results and Discussion 3

3.1 X-ray Diffraction .............................................................................................................3

3.2 Infrared Spectroscopy......................................................................................................5

4. Discussion and Conclusions 8

5. References 10

List of Symbols, Abbreviations, and Acronyms 12

Distribution List 13
iv 

List of Figures 

Figure 1. The crystal structure of α-AlH3. The larger and smaller atoms correspond to Al and 

hydrogen, respectively. Reproduced from reference 17. ..........................................................2

Figure 2. Select in-situ x-ray diffraction patterns of AlH3 measured upon isothermal 

compression at ambient temperature. The diffraction patterns are offset for clarity. ...............3

Figure 3. Variation for the c/a ratio with pressure. The solid line is drawn to guide the eye. .......4

Figure 4. Volume per formula unit of AlH3 as a function of pressure from this study 

(triangles). The red line is a fit of the experimental data using a third-order Birch 

Murnaghan equation (20). Circles indicate the high-pressure data of Goncharenko et al. 

(14). ............................................................................................................................................5

Figure 5. IR spectra of AlH3 as a function of pressure in the spectral range between 500 and 

2500 cm–1. Spectra are offset for clarity. ..................................................................................6

Figure 6. Pressure dependence of the frequencies of the four observed IR active modes. The 

dashed line indicates the onset of the structural discontinuity. Linear fits are shown for 

the frequency shifts. ...................................................................................................................7
v 

Acknowledgments 

Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL) 

was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy 

Sciences, under contract no. DE-AC02-98CH10886. The Geophysical Laboratory of the 

Carnegie Institution of Washington is thanked for providing access to their high-pressure gas 

loading facility. The Advanced Light Source is supported by the Director, Office of Science, 

Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DEAC02-05CH11231. 

 
vi 

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1 

1. Introduction 

Since its synthesis was first reported in the 1950s (1), the physical and chemical properties of 

aluminum hydride (AlH3) have been assessed for a number of applications, ranging from 

hydrogen storage to rocket propulsion. Although early thermodynamic calculations suggested 

the material would be an ideal propellant, the stability and sensitivity issues of the material could 

not be overcome until the development of a new synthesis process by SRI International in the 

late 90s (2). The SRI synthesis method considerably improved the purity of the yield of the most 

thermodynamically stable polymorph, α-aluminum hydride, and subsequent analysis in solid 

propellant applications conducted by the Navy at China Lake suggested significant 

improvements in propulsion properties over the material synthesized in earlier attempts (3). 

Computational simulations have predicted that replacement of aluminum (Al) with AlH3 would 

result in a 10% gain in specific impulse for over 20 s; such an improvement over Al propellants 

is possible because of the low ignition (1450 K) and combustion (3391 K) temperatures of AlH3 

(4, 5). However, its widespread implementation into propellant formulations has been 

complicated by reproducibility in performance characteristics from seemingly similar synthesis 

conditions, as well as its sensitivity towards oxidation, hydration, and friction, although some 

progress has been made in the stabilization by using a coating or additives (6). 

Six polymorphs of AlH3 have been identified (7), four of which have been reproducibly 

synthesized with well-characterized crystal structures (8–11), with the α-phase being the 

thermodynamically stable form at ambient conditions. As is shown in figure 1, α -aluminum 

hydride has AlH6 octahedrons linked by Al-H-Al bridges arranged in a rhombohedral lattice. 

The crystal structure of the α-aluminum hydride has been examined by x-ray diffraction up to 

pressures of 160 GPa (12–14) and by neutron diffraction to 7.2 GPa (15). Although several 

x-ray studies as well as the neutron diffraction have shown the rhombohedral lattice structure 

persists to pressures approaching 75 GPa (14), Graetz et al. (13) suggested a monoclinic 

distortion of the rhombohedral unit cell occurs under very modest pressures between 1 and 

7 GPa, as a result of the tilting of the AlH6 octahedral units and shrinking of the Al-H bond 

distance. Since subsequent high-pressure x-ray diffraction and Raman spectroscopic (16, 17) 

studies have not observed convincing evidence of the monoclinic distortion, considerable 

ambiguity exists as to the exact phase behavior of AlH3 at pressures below 10 GPa. It is well 

known that crystal structure and bonding strongly influence the physical and chemical 

characteristics of a material―diamond, for example, has considerably different properties than 

graphite, and conclusive evidence regarding the monoclinic distortion may help to explain the 

discrepancies in performance between two batches synthesized under seemingly similar 

conditions. As such, the phase behavior of AlH3 under moderate pressure conditions warrants 

further examination. 
2 

Figure 1. The crystal structure of α-AlH3. The larger and 

smaller atoms correspond to Al and hydrogen, 

respectively. Reproduced from reference 17. 

2. Experimental Methodology 

Aluminum hydride was obtained from ATK Thiokol and used without further purification; the 

sample was stored in a dry atmosphere between experimental runs. A flake of approximately 

50 microns in diameter and a thickness of 20 microns was positioned in a piston-cylinder type 

diamond anvil cell equipped with 300-micron diamond culets and a stainless steel gasket. For 

both the infrared (IR) spectroscopic and x-ray diffraction measurements, a neon pressure 

medium was loaded into the diamond anvil cell using a specialized high-pressure gas loading 

system (18). The in-situ pressure within the diamond anvil cell was determined from the 

frequency shift of the ruby R1 luminescence of a micrometer-sized ruby sphere (19). 

Room temperature isothermal angle dispersive x-ray powder microdiffraction experiments were 

conducted at Beamline 12.2.2 of the Advanced Light Source by using focused monochromatic xrays (λ = 0.4959 Å) and a high-resolution image plate detector. The recorded two-dimensional 

diffraction images were then integrated to produce high-quality angle dispersive x-ray diffraction 

patterns using FIT2D. Synchrotron IR absorption measurements to near 20 GPa were obtained 

at beamline U2A of the National Synchrotron Light Source (NSLS) of Brookhaven National 

Laboratory (BNL). The spectral resolution of ±4 cm–1 was used for all IR measurements. 
3 

3. Results and Discussion 

3.1 X-ray Diffraction 

The alpha phase of AlH3 has a rhombohedral lattice of space group R-3c (8) and consists of 

structures built from AlH6 octahedrons linked by Al-H-Al bridges. The lattice parameters for the 

rhombohedral cell are a = 4.492 Å and c = 11.821 Å at 0.0 GPa and 295 K (8). Good agreement 

with previously reported results (14) for the rhombohedral unit cell was observed with eight 

characteristic diffraction peaks below 25° at 0.3 GPa, as is shown in figure 2. To better 

understand the phase behavior of α-aluminum hydride subjected to moderate pressures, x-ray 

diffraction was measured upon isothermal compression at room temperature to pressures near 

50 GPa. The diffraction peaks identified at near ambient conditions remain traceable over the 

entire pressure range studied and no new reflections are noted that could signify a phase 

transition. A generalized loss of diffraction intensity is observed, which may be attributed to 

thinning of the sample in response to compression. Additionally, broadening of the diffraction 

peaks are noted, which was previously used to support the proposed monoclinic distortion. 

Although conclusive evidence cannot be drawn from the x-ray diffraction results of this study 

alone, when coupled with the detailed analysis of the infrared spectroscopic trends discussed 

below, the broadening does appear to support the distortion. 

Figure 2. Select in-situ x-ray diffraction patterns of AlH3 measured upon isothermal 

compression at ambient temperature. The diffraction patterns are offset for clarity. 
4 

The lattice parameters a and c for a rhombohedral cell in a hexagonal framework give the 

pressure dependence of the axial ratio, c/a, for the unit cell, also referred to as the deformation 

parameter. The results are shown in figure 3. There is a distinct discontinuity near 2.3 GPa, 

which may reflect the onset of the monoclinic structural modification. Figure 4 shows the 

pressure-volume (P-V) equation of state (EOS) data determined from the diffraction data fit to 

the rhombohedral unit cell, coupled with the results of an earlier high-pressure experiment (14). 

The experimental data from this study was fit using a third-order Birch-Murnaghan EOS (20). 

No sharp discontinuities, which could indicate a phase transition, are observed in the EOS data 

obtained from this study. A bulk modulus of K0 = 45.3 ± 0.4 GPa and a pressure derivative K0’ 

= 3.6 ± 0.5 GPa was derived from the fit. These values are slightly larger than the previous 

values of K0 = 42.3 GPa and K0’ = 3.5 reported by Goncharenko et al. (14), but well within the 

error limits. The lack of discontinuities in the EOS at low pressures suggests the monoclinic 

distortion to the rhombohedral unit cell is very subtle. 

Figure 3. Variation for the c/a ratio with pressure. The 

solid line is drawn to guide the eye. 
5 

Figure 4. Volume per formula unit of AlH3 as a function of 

pressure from this study (triangles). The red line is 

a fit of the experimental data using a third-order 

Birch Murnaghan equation (20). Circles indicate 

the high-pressure data of Goncharenko et al. (14). 

3.2 Infrared Spectroscopy 

Select IR spectra of AlH3 on isothermal compression at room temperature up to 25 GPa are 

shown in figure 5. The frequencies of the vibrational bands are plotted as a function of pressure 

in figure 6. All of the fundamental vibrations were identified by Roszinski et al. (21) and 

Kolesnikov et al. (22). AlH3 has six IR active modes (2Au+ 4Eu), but the spectral resolution 

permitted only identification of four IR-active bands: Al-H bending modes near 650 and 

850 cm–1 and Al-H asymmetric stretching modes near 1665 and 1775 cm–1. At pressures below 

2.7 GPa, a weak band is observed near 2200 cm–1, which can be attributed to the diamond anvils. 
6 

Figure 5. IR spectra of AlH3 as a function of pressure in the spectral range 

between 500 and 2500 cm–1. Spectra are offset for clarity. 
7 

Figure 6. Pressure dependence of the frequencies of the four 

observed IR active modes. The dashed line indicates 

the onset of the structural discontinuity. Linear fits 

are shown for the frequency shifts. 

The following are immediately noticeable in the spectra over the pressure range studied: 

1. The spectral intensities of the 1665 cm–1 and its shoulder vibration at 1775 cm–1 diminish 

gradually with increasing pressure. Near 25 GPa, the broad doublet splits into two welldefined bands. 

2. At ambient pressure, the 650 and 850 cm–1 Al-H bending modes appear as a single broad 

band with a well-resolved shoulder. Slightly above 2.5 GPa, the band splits into two 

distinct features due to an abrupt shift in the frequencies of the two modes. The two modes 

persist as singlet bands to 25.4 GPa. 

3. The 650 cm–1 band splits into a weak doublet near 12 GPa. It can be speculated that this 

splitting is the result of the Davydov effect, where the molecular symmetry of the unit cell 

is distorted resulting in slightly differing vibrational frequencies. 

4. All spectroscopic changes are reversible upon pressure release. 

As mentioned previously, slightly above 2.5 GPa, nonlinearities as a function of pressure are 

observed for the four IR-active vibrational modes. IR frequencies measured at 3 GPa and above 

do not extrapolate down to the ambient pressure values observed from the decompressed 
8 

recovered samples. A monoclinic phase distortion resulting from the increasing tilt of the AlH6

octahedral units was proposed on the basis of high-resolution x-ray diffraction measurements 

between 1 and 7 GPa (13). The observation suggested that the transition may be second or 

higher order and result in a mixture of phases, as in iron titanium hydride (23). 

4. Discussion and Conclusions 

Goncharenko et al. (14) investigated the phase behavior of AlH3 by x-ray diffraction to 160 GPa. 

The results confirmed the slight variation in the experimentally observed positions of the Bragg 

peaks from the calculated positions of the R-3c space group, which was earlier attributed to a 

monoclinic distortion of the alpha phase between 1 and 7 GPa (13). In both studies, the 

structural deviation was attributed to slight pressure gradients within the diamond anvil cell. The 

pressure dependence of the volume derived from this study was plotted and compared to the 

Goncharenko et al. (14) study with good agreement between the two within experimental error. 

However, it is interesting to point out the c/a ratio derived from the x-ray diffraction in the 

current experiments has a distinct discontinuity near 2.7 GPa, which coincides with a similar 

discontinuity in the IR frequencies near this pressure. While the origin of this phenomenon 

remains speculative, several independent x-ray diffraction patterns obtained near this pressure 

also demonstrated the discontinuity. 

The combined high-pressure IR and x-ray results suggest that the monoclinic distortion occurs 

sluggishly near 2.5 GPa, but the distortion does not result in a complete phase transition over the 

entire pressure range studied. Under increasing pressures at room temperatures, displacements 

and the dipole-dipole interactions between the neighboring AlH6 octahedral units occur. The 

Al-H bending modes involves the relative displacement of the octahedral units along the a lattice 

vector. Initially, the hydrogen is symmetrically located between two neighboring AlH6 octahedra 

at a distance of 1.703 Å (13), but upon compression over the threshold value of 2.5 GPa, 

destabilization of the octahedral units occurs as a result of the shrinking Al-H bond. Further, the 

large split of the Al-H bending modes suggests the effect is not symmetric across the cell. Given 

that this effect was reproduced in several independent experiments, it seems highly unlikely that 

deviatoric stresses from pressure media could be the cause. The origin of the monoclinic 

distortion will be examined in future work using high-pressure radial diffraction techniques. 

The present study has provided new information on the monoclinic distortion of the 

rhombohedral unit cell of AlH3. As was previously observed, the alpha phase has a large region 

of thermodynamic stability at ambient pressure persisting to near 60 GPa (14). Under relatively 

modest pressures, structural changes were documented between 2 and 4 GPa by x-ray diffraction 

and IR spectroscopy. On compression at room temperature, an abrupt frequency discontinuity is 

observed in the Al-H bending and Al-H stretching modes, which implies a structural 

transformation. Concordant with the IR results, the pressure dependence of the c/a axial ratio 
9 

anomalous near 2.7 GPa. All changes observed upon compression are fully reversible. Further 

experiments are underway to determine the cause of the structural instability. 

 
10 

5. References 

1. Siegel, B.; Scheiler, L. Energetics of Propellant Chemistry; N.Y.: John Wiley and Sons, Inc. 

1964, 101–102, 139–140, 150–151. 

2. Petrie, M. A.; Bottaro, J. C.; Penwell, P. E.; Bomberger, D. C.; Schmitt, R. J. Patent 

WO2000076913. 

3. Johnson, C.; Chan, M. Evaluation of AlH3 for Propellant Application, NAWCWD TP-8524, 

May 2002. 

4. DeLuca, L.; Galfetti, L.; Severini, F.; Rosettini, L.; Meda, L.; Marra, G.; D’Andrea, B.; 

Weiser, V.; Calabro, M.; Vrozhtsov, A.; Glazunov, A.; Pavlovets, G. Physical and Ballistic 

Characterization of AlH3-based Space Propellants. Aerosp. Sci. Technol. 2007, 11, 18. 

5. Bazyn, T.; Eyer, R.; Krier, H.; Glumack, N. Combustion Characteristics of Aluminum 

Hydride at Elevated Pressure and Temperature. J. Propul. Power 2004, 20, 427. 

6. Thome, V.; Kempa, P. B.; Herrmann, M. Structure, Chemical and Physical Behavior of 

Aluminum Hydride. Proc. International Annual Conference ICT (34th) 2003, 104, 1. 

7. Brower, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L.; 

Snover, J. A.; Terada, K. Preparation and Properties of Aluminum Hydride. J. Am. Chem. 

Soc. 1976, 98, 2450. 

8. Turley, J. W.; Rinn, H. W. Crystal Structure of Aluminum Hydride. Inorg. Chem. 1969, 8, 

18. 

9. Brinks, H. W.; Lem, A. I.; Hauback, B. C. Mechanochemical Synthesis and Crystal 

Structure of α’-AlD3 and α-AlD3. J. Phys. Chem. B 2006, 110, 25833. 

10. Yartus, V. A.; Denys, R. V.; Maehlen, J. P.; Frommen, C.; Fichtner, M. ; Bulychev, B. M., 

Emerich, H. Double-Bridge Bonding of Aluminum and Hydrogen in the Crystal Structure of 

γ-AlH3. Inorg. Chem. 2007, 46, 1051. 

11. Brinks, H. W.; Brown, C.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. The 

Crystal Structure of γ-AlD3. J. Alloys Compd. 2007, 441, 364. 

12. Baranowski, B.; Hochheimer, H. D.; Strossner, K.; Honle, W. High-Pressure X-ray 

Investigation of AlH3 and Al at Room Temperature. J. Less-Common Met. 1985, 113, 341. 

13. Graetz, J.; Chaudhuri, S.; Lee, Y.; Vogt, T. Pressure Induced Structural and Electronic 

Changes in α-AlH3. Phys. Rev. B 2006, 74, 214114. 
11 

14. Goncharenko, I. N.; Eremets, M. I.; Hanfland, M.; Tse, J. S.; Amboage, M.; Yao, Y.; Trojan, 

I. A. Pressure-Induced Hydrogen-Dominant Metallic State in Aluminum Hydride. Phys. 

Rev. Lett. 2008, 100, 045504. 

15. Goncharenko, I. N.; Glazkov, V. P.; Irodova, A. V.; Somenkov, V. A. Neutron Diffraction 

Study of Crystal Sructure and Equation of State AlD3 up to the Pressure of 7.2 GPa. Physica 

B: Condensed Matter 1991, 174, 117. 

16. Wong, C.-P.; Miller, P. J. Vibrational Spectroscopic Studies of Alane. J. Ener. Mater.

2005, 23, 169. 

17. Shimura, N.; Takeichi, T.; Kume, T.; Sasaki, S.; Shimizu, H.; Ohmura, A.; Ikeda, K.; 

Nakamori, Y.; Orimo, S. High Pressure Raman and Visible Absorption Study of AlH3. 

Journal of Physics: Conference Series 2010, 215, 012047. 

18. Jayaraman, A. Diamond Anvil Cell and High-pressure Physical Investigations. Rev. Mod. 

Phys. 1983, 55, 65. 

19. Zha, C. S.; Mao, H. K.; Hemley, R. J. Elasticity of MgO and a Primary Pressure Scale to 

55 GPa. Proc. Nat. Acad. Sci. 2000, 97, 13494. 

20 Birch, F. Finite Strain Isotherm and Velocities for Single-Crystal and Polycrystalline NaCl 

at High Pressures and 300 °K. J. Geophys. Res. 1978, 83, 1257. 

21. Roszinski, H.; Dautel, R.; Zeil, W. Infrared Spectroscopic Investigations on the 

Coordination Compounds of AlH3 and AlD3 in the Solid and Gaseous States. VI. 

Communication on Molecular Physical Investigations on the Knowledge of Coordination 

Bonds. Zeitschrift fur physikalische Chemie Neue Folge 1963, 36, 26. 

22. Kolesnikov, A. I.; Adams, M.; Antonov, V. E.; Chirin, N. A.; Goremychkin, E. A.; Inikhova, 

G. G.; Marushkin, Y. E.; Prager, M.; Sashin, I. L. Neutron Spectroscopy of Aluminum 

Trihydride. J. Phys. Condens. Matter 1996, 8, 2529. 

23. Reilly, J. J.; Wiswall, R. H. Formation and Properties of Iron Titanium Hydride. Inorganic 

Chemistry 1974, 13, 218. 

 
12 

List of Symbols, Abbreviations, and Acronyms 

Al aluminum 

AlH3 aluminum hydride 

BNL Brookhaven National Laboratory 

EOS equation of state 

IR infrared 

NSLS National Synchrotron Light Source 

P-V pressure-volume 
13 

No. of 

Copies Organization 

 1 ADMNSTR 

 ELEC DEFNS TECHL INFO CTR 

 ATTN DTIC OCP 

 8725 JOHN J KINGMAN RD STE 0944 

 FT BELVOIR VA 22060-6218 

 1 HC US ARMY RSRCH LAB 

 ATTN RDRL CIM G T LANDFRIED 

 BLDG 4600 

 ABERDEEN PROVING GROUND MD 

 21005-5066 

 3 HCS US ARMY RSRCH LAB 

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 MAIL & RECORDS MGMT 

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 ATTN RDRL WML B J CIEZAK-JENKINS 

 BLDG. 390 

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 21005-5069 

TOTAL: 6 (2 ELEC, 4 HCS) 

 
14 

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