---
title: 'A2BH6 Complex Hydrides: Structure & Storage'
url: https://www.emergentmind.com/topics/a2bh6-complex-hydrides
type: topic
---

# A2BH6 Complex Hydrides: Structure & Storage

Searching arXiv for the specified hydride papers and closely related A2BH6-type work.
arXiv_search(query="A2BH6 complex hydrides 2507.19810 2510.22749", max_results=10)
Searching arXiv for exact identifiers and topic keywords.
A$_2$BH$_6$ complex hydrides are hydride perovskites with composition A$_2$BH$_6$ 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 A$_2$AlH$_6$ and A$_2$SiH$_6$ 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 [2507.19810]. A related study by El Fatouaki et al. on A$_2$CrH$_6$ with A = Ca, Sr, and Ba extends the A$_2$BH$_6$-type landscape to compounds that are thermodynamically, mechanically, dynamically, and thermally stable, while exhibiting lower gravimetric capacities and a distinct spin-polarized electronic structure [2510.22749].

## 1. Structural archetype and compositional scope

For A$_2$AlH$_6$ and A$_2$SiH$_6$, all studied hydrides adopt the vacancy-ordered double perovskite structure with space group Fm–3m, where the A-site cations occupy 8c $(\tfrac14,\tfrac14,\tfrac14)$, the B-site atom occupies 4a $(0,0,0)$, and hydrogen occupies 24e $(x,\tfrac12,\tfrac12)$, forming isolated BH$_6$ octahedra [2507.19810]. This structural motif defines the local coordination environment used throughout the reported thermodynamic, elastic, electronic, optical, and hydrogen-storage analysis.

For A$_2$CrH$_6$, El Fatouaki et al. report an ideal cubic perovskite structure, also in space group Fm\={3}m (No. 225), with A atoms on 8c $(\tfrac14,\tfrac34,\tfrac34)$, Cr on 4a $(0,0,0)$, and H on 24e $(x,\tfrac12,\tfrac12)$ sites [2510.22749]. Geometry optimization gives lattice constants from $7.220\ \text{\AA}$ to $8.082\ \text{\AA}$ and equilibrium volumes from $376.30\ \text{\AA}^3$ to $527.85\ \text{\AA}^3$. 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 A$_2$BH$_6$ 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 A$_2$AlH$_6$ and A$_2$SiH$_6$, thermodynamic stability is quantified through the formation energy per formula unit,
$$
E^f = \frac{E_T - (2E_A + E_B + 6E_H)}{7},
$$
computed with both GGA and hybrid-HSE06 [2507.19810]. All reported $\Delta H_f$ values are negative. For A$_2$AlH$_6$, the GGA and HSE06 values are $-0.66$ and $-0.71$ eV for Li$_2$AlH$_6$, $-0.74$ and $-0.75$ eV for Na$_2$AlH$_6$, and $-0.91$ and $-0.79$ eV for K$_2$AlH$_6$. For A$_2$SiH$_6$, the corresponding values are $-0.77$ and $-0.87$ eV for Li$_2$SiH$_6$, $-0.88$ and $-0.95$ eV for Na$_2$SiH$_6$, and $-1.09$ and $-1.04$ eV for K$_2$SiH$_6$. The negative values indicate exothermic formation.

Mechanical stability for the cubic A$_2$AlH$_6$ and A$_2$SiH$_6$ phases is assessed using Born’s criteria,
$$
C_{11}>0,\qquad C_{44}>0,\qquad C_{11}-C_{12}>0,\qquad C_{11}+2C_{12}>0.
$$
Representative GGA elastic constants are $C_{11}=31.8$ GPa, $C_{12}=25.7$ GPa, and $C_{44}=39.3$ GPa for Li$_2$AlH$_6$, and $C_{11}=46.8$ GPa, $C_{12}=18.7$ GPa, and $C_{44}=37.9$ GPa for Li$_2$SiH$_6$; all studied combinations satisfy the required inequalities [2507.19810].

For A$_2$CrH$_6$, the formation energy is defined per atom as
$$
E_{\rm form} = E({\rm A_2CrH_6}) - 2E({\rm A}) - E({\rm Cr}) - 3E({\rm H_2}),
$$
with calculated values of $-0.630$, $-0.628$, and $-0.631$ eV/atom for Ca, Sr, and Ba, respectively [2510.22749]. Elastic constants from stress–strain calculations are $C_{11}=116.87$, $56.39$, and $64.86$ GPa; $C_{12}=30.42$, $0.78$, and $9.91$ GPa; and $C_{44}=40.14$, $9.86$, and $19.02$ GPa for Ca$_2$CrH$_6$, Sr$_2$CrH$_6$, and Ba$_2$CrH$_6$, respectively. All satisfy the cubic Born criteria.

The A$_2$CrH$_6$ 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 $0$–$50$ THz range, and ab initio molecular dynamics at $T=300$ K for approximately $10$ ps in the NVT ensemble shows no drift in temperature, total energy, kinetic energy, or potential energy, with the cubic framework remaining intact [2510.22749]. A plausible implication is that, within the examined conditions, the A$_2$CrH$_6$ phases are stable against both harmonic lattice instabilities and short-timescale thermal disorder.

## 3. Thermal response and dehydrogenation thermodynamics

For the Li-containing A$_2$AlH$_6$ and A$_2$SiH$_6$ hydrides, the dehydrogenation temperature is estimated from
$$
T_D = \frac{\Delta H}{\Delta S},
$$
with $\Delta S \approx 130.7\ \text{J K}^{-1}\text{mol}^{-1}$ for H$_2$ gas [2507.19810]. The computed values are $487$ K (GGA) and $524$ K (HSE06) for Li$_2$AlH$_6$, and $568$ K (GGA) and $642$ K (HSE06) for Li$_2$SiH$_6$. These values fall within the reported favorable hydrogen desorption temperature range of $450$ to $650$ K for the leading candidates.

The same study evaluates heat capacity and vibrational entropy within the quasi-harmonic Debye model. At low temperature,
$$
C_V \simeq \frac{12}{5}\pi^4 nR \left(\frac{T}{\Theta_D}\right)^3,
$$
while at high temperature $C_V \to 3nR$, the Dulong–Petit limit [2507.19810]. The computed $C_V$ curves rise as $T^3$ at low $T$ and level off to $3nR$ at high $T$, while $S_{\rm vib}(T)$ increases monotonically. This places the thermal response of the A$_2$AlH$_6$ and A$_2$SiH$_6$ series within the expected Debye-law regime.

For A$_2$CrH$_6$, the desorption temperature at $1$ bar is estimated through the van’t Hoff relation,
$$
\ln P = -\frac{\Delta H}{RT} + \frac{\Delta S}{R}
\quad\Longrightarrow\quad
T_d \approx \frac{\Delta H}{\Delta S},
$$
using $\Delta S \approx -130.7\ \text{J mol}^{-1}\text{K}^{-1}$ for H$_2$ [2510.22749]. The reported values are approximately $465.2$ K for Ca$_2$CrH$_6$, $463.7$ K for Sr$_2$CrH$_6$, and $466.1$ K for Ba$_2$CrH$_6$. Among these, Sr$_2$CrH$_6$ has the lowest applicable hydrogen desorption temperature.

## 4. Hydrogen-storage metrics

For A$_2$AlH$_6$ and A$_2$SiH$_6$, the gravimetric and volumetric hydrogen-storage descriptors are defined as
$$
C_{\rm wt}\% = \left[\frac{(H/M)\cdot m_H}{m_{\rm Host} + (H/M)\cdot m_H}\right]\cdot 100,
$$
and
$$
\rho_V = \frac{N_H \cdot m_H}{V\cdot N_A},
$$
with $H/M = 6$, $m_H = 1.008\ \text{g mol}^{-1}$, $m_{\rm Host}$ the formula-unit mass, $V$ the cell volume, and $N_A$ Avogadro’s number [2507.19810]. The U.S. DOE targets are $C_{\rm wt} > 5.5\%$ and $\rho_V > 40\ \text{g H}_2\text{L}^{-1}$.

| Compound | $C_{\rm wt}$ | $\rho_V$ |
|---|---:|---:|
| Li$_2$AlH$_6$ | 12.9 % | 142.9 g H$_2$/L |
| Na$_2$AlH$_6$ | 7.65 % | 109.5 g H$_2$/L |
| K$_2$AlH$_6$ | 5.44 % | 80.5 g H$_2$/L |
| Li$_2$SiH$_6$ | 12.6 % | 144.3 g H$_2$/L |
| Na$_2$SiH$_6$ | 7.55 % | 111.8 g H$_2$/L |
| K$_2$SiH$_6$ | 5.38 % | 85.6 g H$_2$/L |

All A = Li, Na compounds exceed both DOE benchmarks [2507.19810]. Li$_2$AlH$_6$ and Li$_2$SiH$_6$ stand out with gravimetric capacities of approximately $12\%$ and volumetric densities of approximately $140\ \text{g L}^{-1}$.

For A$_2$CrH$_6$, the gravimetric storage capacity is written as
$$
C_{\rm wt}=\frac{6M_H}{2M_A+M_{\rm Cr}+6M_H}\times 100\%,
$$
with reported values of $4.37$ wt\% for Ca$_2$CrH$_6$, $2.69$ wt\% for Sr$_2$CrH$_6$, and $1.82$ wt\% for Ba$_2$CrH$_6$ [2510.22749]. The volumetric capacity follows the same order, with Ca$_2$CrH$_6$ reaching approximately $106.6\ \text{gH}_2/\text{L}$. These values show that A$_2$CrH$_6$ compounds remain relevant to hydrogen storage, but they do not match the gravimetric performance reported for the Li- and Na-based A$_2$AlH$_6$ and A$_2$SiH$_6$ materials.

## 5. Electronic structure and optical response

Electronic-structure calculations distinguish sharply between Al-, Si-, and Cr-based members of the A$_2$BH$_6$ family. In A$_2$AlH$_6$, all compounds are metallic and exhibit no band gap, whereas A$_2$SiH$_6$ compounds are indirect semiconductors of the X$\rightarrow\Gamma$ type, with GGA and HSE06 gaps of $1.03$ and $2.08$ eV for Li$_2$SiH$_6$, $1.29$ and $2.37$ eV for Na$_2$SiH$_6$, and $2.23$ and $3.38$ eV for K$_2$SiH$_6$ [2507.19810]. Density-of-states analysis attributes the metallicity of A$_2$AlH$_6$ to Al–3p/H–1s hybridization crossing $E_F$, whereas in A$_2$SiH$_6$ the Si–3p/H–1s covalent states are separated, producing a gap.

For the semiconducting A$_2$SiH$_6$ hydrides, the optical absorption coefficient $\alpha(\omega)$ rises sharply in the ultraviolet between $6$ and $12$ eV, with peak values exceeding $10^6\ \text{cm}^{-1}$ [2507.19810]. 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 A$_2$CrH$_6$ series introduces spin-polarized behavior absent from the Al/Si systems. Ca$_2$CrH$_6$ and Sr$_2$CrH$_6$ are reported as half-metals, with a spin-up band gap of approximately $2.005$ eV for Ca and $1.862$ eV for Sr, while the spin-down channel is metallic; Ba$_2$CrH$_6$ is metallic in both channels [2510.22749]. The spin polarization at the Fermi level,
$$
P=\frac{N_\uparrow(E_F)-N_\downarrow(E_F)}{N_\uparrow(E_F)+N_\downarrow(E_F)},
$$
is essentially $100\%$ for Ca$_2$CrH$_6$ and Sr$_2$CrH$_6$, and $0\%$ for Ba$_2$CrH$_6$. Partial DOS shows Cr d states dominating near $E_F$, with strong Cr–H hybridization deeper in the valence band.

Optically, A$_2$CrH$_6$ compounds have static dielectric constants $\varepsilon_1(0)=61.29$, $32.58$, and $25.84$ for Ca, Sr, and Ba, respectively, and $\varepsilon_2(0)=20.75$, $9.52$, and $4.47$ [2510.22749]. In the visible range from $1.5$ to $3$ eV, $\varepsilon_2$ is nearly zero, indicating high transparency. Strong UV absorption occurs between $4$ and $8$ eV, with peaks in $\alpha(\omega)$ of approximately $1.91\times 10^5\ \text{cm}^{-1}$ at $7.99$ eV for Ca$_2$CrH$_6$, $2.06\times 10^5\ \text{cm}^{-1}$ at $8.30$ eV for Sr$_2$CrH$_6$, and $1.62\times 10^5\ \text{cm}^{-1}$ at $7.53$ eV for Ba$_2$CrH$_6$. This suggests that A$_2$BH$_6$ 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 A$_2$AlH$_6$ and A$_2$SiH$_6$ series, the A-site trend from Li to Na to K is explicit: $C_{\rm wt}\%$ and $\rho_V$ decrease as A becomes heavier and larger, which dilutes the H$_2$ fraction and expands the lattice [2507.19810]. The B-site trend is also explicit: Al-based hydrides give slightly higher $C_{\rm wt}\%$ 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, Li$_2$AlH$_6$ and Li$_2$SiH$_6$ emerge as the best candidates. The reported basis for this ranking is their exceptionally high $C_{\rm wt}$ of roughly $12\%$, volumetric density of roughly $144\ \text{g L}^{-1}$, moderate $T_D$ of approximately $450$–$650$ K, negative formation energies between about $-0.7$ and $-0.9$ eV, and mechanical stability [2507.19810]. The same study concludes that these two compounds are the most promising solid-state hydrogen-storage materials within the examined A$_2$BH$_6$ series.

Within the A$_2$CrH$_6$ family, Ca$_2$CrH$_6$ 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 [2510.22749]. Sr$_2$CrH$_6$ provides the lowest desorption temperature, $463.7$ K, and retains essentially $100\%$ spin polarization. Ba$_2$CrH$_6$ is fully metallic and has the lowest hydrogen-storage capacity of the three.

A recurrent misconception is that all A$_2$BH$_6$ 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 A$_2$AlH$_6$ and A$_2$SiH$_6$ families exceed both DOE benchmarks, whereas Ca$_2$CrH$_6$, Sr$_2$CrH$_6$, and Ba$_2$CrH$_6$ have gravimetric capacities of $4.37$, $2.69$, and $1.82$ wt\%, respectively [2507.19810; 2510.22749]. 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 A$_2$BH$_6$ complex hydrides.

Source: https://www.emergentmind.com/topics/a2bh6-complex-hydrides