A2BH6 Complex Hydrides: Structure & Storage
- A2BH6 complex hydrides are perovskite-type materials with an A2BH6 stoichiometry, exhibiting vacancy-ordered structures and diverse bonding characteristics.
- They demonstrate excellent thermodynamic and mechanical stability with exothermic formation energies, with some alkali-based variants meeting DOE hydrogen storage targets.
- First-principles studies reveal tunable electronic properties—from metallic to semiconducting and half-metallic regimes—making these hydrides promising for hydrogen storage and optical applications.
Searching arXiv for the specified hydride papers and closely related A2BH6-type work. arXiv_search(query="A2BH6 complex hydrides (Zosiamliana et al., 26 Jul 2025, Fatouaki et al., 26 Oct 2025)", max_results=10) Searching arXiv for exact identifiers and topic keywords. ABH complex hydrides are hydride perovskites with composition ABH in which the A-site cation and the B-site metal jointly determine structural stability, hydrogen-storage capacity, dehydrogenation thermodynamics, and electronic response. First-principles studies on AAlH and ASiH with A = Li, Na, and K identify a vacancy-ordered double perovskite family with negative formation energies, mechanical stability, and hydrogen-storage metrics that in several cases exceed U.S. Department of Energy targets (Zosiamliana et al., 26 Jul 2025). A related study by El Fatouaki et al. on ACrH with A = Ca, Sr, and Ba extends the A0BH1-type landscape to compounds that are thermodynamically, mechanically, dynamically, and thermally stable, while exhibiting lower gravimetric capacities and a distinct spin-polarized electronic structure (Fatouaki et al., 26 Oct 2025).
1. Structural archetype and compositional scope
For A2AlH3 and A4SiH5, all studied hydrides adopt the vacancy-ordered double perovskite structure with space group Fm–3m, where the A-site cations occupy 8c 6, the B-site atom occupies 4a 7, and hydrogen occupies 24e 8, forming isolated BH9 octahedra (Zosiamliana et al., 26 Jul 2025). This structural motif defines the local coordination environment used throughout the reported thermodynamic, elastic, electronic, optical, and hydrogen-storage analysis.
For A0CrH1, El Fatouaki et al. report an ideal cubic perovskite structure, also in space group Fm={3}m (No. 225), with A atoms on 8c 2, Cr on 4a 3, and H on 24e 4 sites (Fatouaki et al., 26 Oct 2025). Geometry optimization gives lattice constants from 5 to 6 and equilibrium volumes from 7 to 8. The reported coordinate conventions differ between the two studies, but both datasets place the B atom at 4a and H at 24e within the same cubic space-group setting.
The available compositions therefore span alkali-metal A sites with B = Al or Si, and alkaline-earth A sites with B = Cr. This suggests that the A9BH0 designation covers a chemically diverse family in which the same gross stoichiometry supports markedly different bonding and physical behavior.
2. Thermodynamic, mechanical, and dynamic stability
For A1AlH2 and A3SiH4, thermodynamic stability is quantified through the formation energy per formula unit,
5
computed with both GGA and hybrid-HSE06 (Zosiamliana et al., 26 Jul 2025). All reported 6 values are negative. For A7AlH8, the GGA and HSE06 values are 9 and 0 eV for Li1AlH2, 3 and 4 eV for Na5AlH6, and 7 and 8 eV for K9AlH0. For A1SiH2, the corresponding values are 3 and 4 eV for Li5SiH6, 7 and 8 eV for Na9SiH0, and 1 and 2 eV for K3SiH4. The negative values indicate exothermic formation.
Mechanical stability for the cubic A5AlH6 and A7SiH8 phases is assessed using Born’s criteria,
9
Representative GGA elastic constants are 0 GPa, 1 GPa, and 2 GPa for Li3AlH4, and 5 GPa, 6 GPa, and 7 GPa for Li8SiH9; all studied combinations satisfy the required inequalities (Zosiamliana et al., 26 Jul 2025).
For A0CrH1, the formation energy is defined per atom as
2
with calculated values of 3, 4, and 5 eV/atom for Ca, Sr, and Ba, respectively (Fatouaki et al., 26 Oct 2025). Elastic constants from stress–strain calculations are 6, 7, and 8 GPa; 9, 0, and 1 GPa; and 2, 3, and 4 GPa for Ca5CrH6, Sr7CrH8, and Ba9CrH00, respectively. All satisfy the cubic Born criteria.
The A01CrH02 study also reports dynamic and thermal stability beyond static elasticity. Linear-response phonon calculations show no imaginary frequencies over the full Brillouin-zone in the 03–04 THz range, and ab initio molecular dynamics at 05 K for approximately 06 ps in the NVT ensemble shows no drift in temperature, total energy, kinetic energy, or potential energy, with the cubic framework remaining intact (Fatouaki et al., 26 Oct 2025). A plausible implication is that, within the examined conditions, the A07CrH08 phases are stable against both harmonic lattice instabilities and short-timescale thermal disorder.
3. Thermal response and dehydrogenation thermodynamics
For the Li-containing A09AlH10 and A11SiH12 hydrides, the dehydrogenation temperature is estimated from
13
with 14 for H15 gas (Zosiamliana et al., 26 Jul 2025). The computed values are 16 K (GGA) and 17 K (HSE06) for Li18AlH19, and 20 K (GGA) and 21 K (HSE06) for Li22SiH23. These values fall within the reported favorable hydrogen desorption temperature range of 24 to 25 K for the leading candidates.
The same study evaluates heat capacity and vibrational entropy within the quasi-harmonic Debye model. At low temperature,
26
while at high temperature 27, the Dulong–Petit limit (Zosiamliana et al., 26 Jul 2025). The computed 28 curves rise as 29 at low 30 and level off to 31 at high 32, while 33 increases monotonically. This places the thermal response of the A34AlH35 and A36SiH37 series within the expected Debye-law regime.
For A38CrH39, the desorption temperature at 40 bar is estimated through the van’t Hoff relation,
41
using 42 for H43 (Fatouaki et al., 26 Oct 2025). The reported values are approximately 44 K for Ca45CrH46, 47 K for Sr48CrH49, and 50 K for Ba51CrH52. Among these, Sr53CrH54 has the lowest applicable hydrogen desorption temperature.
4. Hydrogen-storage metrics
For A55AlH56 and A57SiH58, the gravimetric and volumetric hydrogen-storage descriptors are defined as
59
and
60
with 61, 62, 63 the formula-unit mass, 64 the cell volume, and 65 Avogadro’s number (Zosiamliana et al., 26 Jul 2025). The U.S. DOE targets are 66 and 67.
| Compound | 68 | 69 |
|---|---|---|
| Li70AlH71 | 12.9 % | 142.9 g H72/L |
| Na73AlH74 | 7.65 % | 109.5 g H75/L |
| K76AlH77 | 5.44 % | 80.5 g H78/L |
| Li79SiH80 | 12.6 % | 144.3 g H81/L |
| Na82SiH83 | 7.55 % | 111.8 g H84/L |
| K85SiH86 | 5.38 % | 85.6 g H87/L |
All A = Li, Na compounds exceed both DOE benchmarks (Zosiamliana et al., 26 Jul 2025). Li88AlH89 and Li90SiH91 stand out with gravimetric capacities of approximately 92 and volumetric densities of approximately 93.
For A94CrH95, the gravimetric storage capacity is written as
96
with reported values of 97 wt\% for Ca98CrH99, 00 wt\% for Sr01CrH02, and 03 wt\% for Ba04CrH05 (Fatouaki et al., 26 Oct 2025). The volumetric capacity follows the same order, with Ca06CrH07 reaching approximately 08. These values show that A09CrH10 compounds remain relevant to hydrogen storage, but they do not match the gravimetric performance reported for the Li- and Na-based A11AlH12 and A13SiH14 materials.
5. Electronic structure and optical response
Electronic-structure calculations distinguish sharply between Al-, Si-, and Cr-based members of the A15BH16 family. In A17AlH18, all compounds are metallic and exhibit no band gap, whereas A19SiH20 compounds are indirect semiconductors of the X21 type, with GGA and HSE06 gaps of 22 and 23 eV for Li24SiH25, 26 and 27 eV for Na28SiH29, and 30 and 31 eV for K32SiH33 (Zosiamliana et al., 26 Jul 2025). Density-of-states analysis attributes the metallicity of A34AlH35 to Al–3p/H–1s hybridization crossing 36, whereas in A37SiH38 the Si–3p/H–1s covalent states are separated, producing a gap.
For the semiconducting A39SiH40 hydrides, the optical absorption coefficient 41 rises sharply in the ultraviolet between 42 and 43 eV, with peak values exceeding 44 (Zosiamliana et al., 26 Jul 2025). The reported strong UV absorption indicates potential UV-optical applications, although the same study explicitly notes that, for hydrogen storage, the key electronic distinction is the semiconducting versus metallic character.
The A45CrH46 series introduces spin-polarized behavior absent from the Al/Si systems. Ca47CrH48 and Sr49CrH50 are reported as half-metals, with a spin-up band gap of approximately 51 eV for Ca and 52 eV for Sr, while the spin-down channel is metallic; Ba53CrH54 is metallic in both channels (Fatouaki et al., 26 Oct 2025). The spin polarization at the Fermi level,
55
is essentially 56 for Ca57CrH58 and Sr59CrH60, and 61 for Ba62CrH63. Partial DOS shows Cr d states dominating near 64, with strong Cr–H hybridization deeper in the valence band.
Optically, A65CrH66 compounds have static dielectric constants 67, 68, and 69 for Ca, Sr, and Ba, respectively, and 70, 71, and 72 (Fatouaki et al., 26 Oct 2025). In the visible range from 73 to 74 eV, 75 is nearly zero, indicating high transparency. Strong UV absorption occurs between 76 and 77 eV, with peaks in 78 of approximately 79 at 80 eV for Ca81CrH82, 83 at 84 eV for Sr85CrH86, and 87 at 88 eV for Ba89CrH90. This suggests that A91BH92 hydrides can support semiconducting, metallic, half-metallic, and strongly UV-absorbing regimes within the same stoichiometric class.
6. Comparative trends and leading candidates
Within the A93AlH94 and A95SiH96 series, the A-site trend from Li to Na to K is explicit: 97 and 98 decrease as A becomes heavier and larger, which dilutes the H99 fraction and expands the lattice (Zosiamliana et al., 26 Jul 2025). The B-site trend is also explicit: Al-based hydrides give slightly higher 00 because Al is lighter than Si, whereas Si-based hydrides are marginally denser because of smaller octahedra. Both Al- and Si-based compounds meet DOE targets when A = Li or Na.
Among these alkali-based compounds, Li01AlH02 and Li03SiH04 emerge as the best candidates. The reported basis for this ranking is their exceptionally high 05 of roughly 06, volumetric density of roughly 07, moderate 08 of approximately 09–10 K, negative formation energies between about 11 and 12 eV, and mechanical stability (Zosiamliana et al., 26 Jul 2025). The same study concludes that these two compounds are the most promising solid-state hydrogen-storage materials within the examined A13BH14 series.
Within the A15CrH16 family, Ca17CrH18 is the stiffest material, with the highest bulk, shear, and Young’s moduli, and also the highest gravimetric and volumetric hydrogen-storage performance of that series (Fatouaki et al., 26 Oct 2025). Sr19CrH20 provides the lowest desorption temperature, 21 K, and retains essentially 22 spin polarization. Ba23CrH24 is fully metallic and has the lowest hydrogen-storage capacity of the three.
A recurrent misconception is that all A25BH26 hydrides should be expected to satisfy the same hydrogen-storage benchmarks. The reported results do not support that simplification. In the available data, all A = Li and Na members of the A27AlH28 and A29SiH30 families exceed both DOE benchmarks, whereas Ca31CrH32, Sr33CrH34, and Ba35CrH36 have gravimetric capacities of 37, 38, and 39 wt\%, respectively (Zosiamliana et al., 26 Jul 2025, Fatouaki et al., 26 Oct 2025). A plausible implication is that A-site mass and size, together with B-site electronic chemistry, govern a trade-off space linking storage capacity, desorption thermodynamics, mechanical response, and electronic functionality across A40BH41 complex hydrides.