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# The effect of Mn content on structural and sorption properties of the combined AB3 and A2B7-type phases in the La-Y-Ni-Mn system

G Fedlouk, J Monnier, L. Laversenne, J Zhang

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G Fedlouk, J Monnier, L. Laversenne, J Zhang. The effect of Mn content on structural and sorption properties of the combined AB3 and A2B7-type phases in the La-Y-Ni-Mn system. 18th International Symposium On Metal-Hydrogen Systems, Fermin Cuevas - ICMPE, May 2024, Saint-Malo, France, France. hal-04567421

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https://hal.science/hal-04567421

Submitted on 3 May 2024

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# The effect of Mn content on structural and sorption properties of the combined AB₃ and A₂B₇-type phases in the La-Y-Ni-Mn system.

G. Fedlouk<fnref n="1" />, J. Monnier<fnref n="1" />, L. Laversenne<fnref n="2" /> and J. Zhang<fnref n="1" />

In recent years, new intermetallic compounds AB<sub>y</sub> (2 ≤ y ≤ 5, A being a rare earth or Mg and B a transition metal) have emerged in the field of solid hydrogen storage [1]. These compounds have inter-growth structures, which corresponds to the stacking of two basic units [AB<sub>5</sub>] and [A<sub>2</sub>B<sub>4</sub>] along the crystallographic c-axis [2]. According to the literature, AB<sub>3</sub>-type compounds provide higher hydrogen storage capacity than A<sub>2</sub>B<sub>7</sub>-type compounds because they contain more [A<sub>2</sub>B<sub>4</sub>] units. For the same reason, their cyclability is worse than that of A<sub>2</sub>B<sub>7</sub>-type compounds. Therefore, uniting both A<sub>2</sub>B<sub>7</sub> and AB<sub>3</sub> structures in the same alloy could be interesting for hydrogen storage.

Apart from the crystallographic structure, playing on the chemistry is also a key point to optimize properties. For example, the rare earth-based binary AB<sub>y</sub> alloys exhibit multi-plateaus behavior and poor reversibility which makes them unsuitable for application. The substitutions, notably of A by Mg, but also of B by Mn or Co, reduce the number of plateaus to a single plateau [3, 4]. However, due to magnesium's high evaporation pressure and high reactivity to air, the synthesis can be difficult especially at an industrial scale. Therefore, we are interested in magnesium-free compounds with stacking structure AB<sub>y</sub> (3 ≤ y ≤ 3.6), and particularly the (La,Y)(Ni,Mn)<sub>y</sub> system.

The present work shows the study of the substitution of Ni by Mn in LaY<sub>2.3</sub>Ni<sub>11.5-x</sub>Mn<sub>x</sub> (0.3 ≤ x ≤ 2.5) compounds. 8 alloys containing 3 different phases (A<sub>2</sub>B<sub>7</sub> H, A<sub>2</sub>B<sub>7</sub> R and AB<sub>3</sub> R) were synthesized and studied. We noticed that the lattice parameters of the phase AB<sub>3</sub> R do not vary linearly with Mn content which indicates a non-random distribution of Mn atoms over different sites (Figure 1). Therefore, a neutron analysis at ILL was conducted on these compounds. Their sorption properties were also investigated, and an increase of the maximum hydrogen storage capacity with increasing of Mn content was observed; up to 2 wt% for LaY<sub>2.3</sub>Ni<sub>9.4</sub>Mn<sub>2.1</sub>. The correlation between the structure and hydrogenation properties of these compounds will be presented.

<figure><img src="image_1.png" /><figcaption>Figure 1 Evolution of lattice parameters of AB₃ R phase in LaY₂.₃Ni₁₁.₅-xMnₓ (0.3 ≤ x ≤ 2.5) alloys as a function of Mn content</figcaption></figure>

## References

[1] Z. Chen et al, Chinese Journal of Chemical Engineering, vol. 29 (2021), 1-12.

[2] J.-C. Crivello et al, The Journal of Physical Chemistry C, vol 115 (2011), 25470-25478.

[3] J. Zhang et al, Journal of Alloys and Compounds, vol 852 (2021), 157008.

[4] Y. Shi et al, Electrochimica Acta, vol 296 (2019), 18-26.

<footnote n="1">¹ Univ Paris Est Creteil, CNRS, ICMPE, UMR 7182, 2 rue Henri Dunant, 94320 Thiais, France</footnote>

<footnote n="2">² Institut Néel, CNRS, 25 Av. des Martyrs, 38042 Grenoble, France
ghofrane.fedlouk@cnrs.fr</footnote>
The effect of Mn content on structural and sorption properties of the combined AB₃ and A₂B₇-type phases in the La-Y-Ni-Mn system.

G. Fedlouk<fnref n="1" />, J. Monnier<fnref n="1" />, L. Laversenne<fnref n="2" /> and J. Zhang<fnref n="1" />

In recent years, new intermetallic compounds AB_y (2 ≤ y ≤ 5, A being a rare earth or Mg and B a transition metal) have emerged in the field of solid hydrogen storage [1]. These compounds have inter-growth structures, which corresponds to the stacking of two basic units [AB_5] and [A_2B_4] along the crystallographic c-axis [2]. According to the literature, AB_3-type compounds provide higher hydrogen storage capacity than A_2B_7-type compounds because they contain more [A_2B_4] units. For the same reason, their cyclability is worse than that of A_2B_7-type compounds. Therefore, uniting both A_2B_7 and AB_3 structures in the same alloy could be interesting for hydrogen storage.

Apart from the crystallographic structure, playing on the chemistry is also a key point to optimize properties. For example, the rare earth-based binary AB_y alloys exhibit multi-plateaus behavior and poor reversibility which makes them unsuitable for application. The substitutions, notably of A by Mg, but also of B by Mn or Co, reduce the number of plateaus to a single plateau [3, 4]. However, due to magnesium's high evaporation pressure and high reactivity to air, the synthesis can be difficult especially at an industrial scale. Therefore, we are interested in magnesium-free compounds with stacking structure AB_y (3 ≤ y ≤ 3.6), and particularly the (La,Y)(Ni,Mn)_y system.

The present work shows the study of the substitution of Ni by Mn in LaY_{2.3}Ni_{11.5-x}Mn_x (0.3 ≤ x ≤ 2.5) compounds. 8 alloys containing 3 different phases (A_2B_7 H, A_2B_7 R and AB_3 R) were synthetized and studied. We noticed that the lattice parameters of the phase AB_3 R do not vary linearly with Mn content which indicates a non-random distribution of Mn atoms over different sites (Figure 1). Therefore, a neutron analysis at ILL was conducted on these compounds. Their sorption properties were also investigated, and an increase of the maximum hydrogen storage capacity with increasing of Mn content was observed; up to 2 wt% for LaY_{2.3}Ni_{9.4}Mn_{2.1}. The correlation between the structure and hydrogenation properties of these compounds will be presented.

<figure><img src="image_3.png" /><figcaption>Figure 1 Evolution of lattice parameters of AB₃ R phase in LaY_{2.3}Ni_{11.5-x}Mn_x (0.3 ≤ x ≤ 2.5) alloys as a function of Mn content</figcaption></figure>

References

[1] Z. Chen et al, Chinese Journal of Chemical Engineering, vol. 29 (2021), 1-12.

[2] J.-C. Crivello et al, The Journal of Physical Chemistry C, vol 115 (2011), 25470-25478.

[3] J. Zhang et al, Journal of Alloys and Compounds, vol 852 (2021), 157008.

[4] Y. Shi et al, Electrochimica Acta, vol 296 (2019), 18-26.

<footnote n="1">¹ Univ Paris Est Creteil, CNRS, ICMPE, UMR 7182, 2 rue Henri Dunant, 94320 Thiais, France</footnote>

<footnote n="2">² Institut Néel, CNRS, 25 Av. des Martyrs, 38042 Grenoble, France
ghofrane.fedlouk@cnrs.fr</footnote>