Thermodynamic stability of alkali metal/zinc double-cation borohydrides at low temperatures

Abstract

We study the thermodynamic stability at low temperatures of a series of alkali metal/zinc double-cation borohydrides, including LiZn(BH${}_4$)${}_3$, LiZn${}_2$(BH${}_4$)${}_5$, NaZn(BH${}_4$)${}_3$, NaZn${}_2$(BH${}_4$)${}_5$, KZn(BH${}_4$)${}_3$, and KZn${}_2$(BH${}_4$)${}_5$. While LiZn${}_2$(BH${}_4$)${}_5$, NaZn(BH${}_4$)${}_3$, NaZn${}_2$(BH${}_4$)${}_5$ and KZn(BH${}_4$)${}_3$ were recently synthesized, LiZn(BH${}_4$)${}_3$ and KZn${}_2$(BH${}_4$)${}_5$ are hypothetical compounds. Using the minima-hopping method, we discover two new lowest-energy structures for NaZn(BH${}_4$)${}_3$ and KZn${}_2$(BH${}_4$)${}_5$ which belong to the $C2/c$ and $P2$ space groups, respectively. These structures are predicted to be both thermodynamically stable and dynamically stable, implying that their existence may be possible. On the other hand, the lowest-energy $P1$ structure of LiZn(BH${}_4$)${}_3$ is predicted to be unstable, suggesting a possible reason elucidating why this compound has not been experimentally identified. In exploring the low-energy structures of these compounds, we find that their energetic ordering is sensitive to the inclusion of the van der Waals interactions. We also find that a proper treatment of these interactions, e.g., as given by a non-local density functional such as vdW-DF2, is necessary to address the stability of the low-energy structures of these compounds.

Final version

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• Ternary mixed-anion semiconductors with tunable band gaps from machine-learning and crystal structure prediction
• Pressure-stabilized binary compounds of magnesium and silicon
• Dense superconducting phases of copper-bismuth at high pressure
• Tuning the hydrogen desorption of Mg(BH${}_4$)${}_2$ through Zn alloying