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

TL;DR

This study investigates the low-temperature thermodynamic stability of various alkali metal/zinc borohydrides, discovering new stable structures and explaining the instability of some compounds through advanced computational methods.

Contribution

The paper introduces new predicted stable structures for NaZn(BH$_4$)$_3$ and KZn$_2$(BH$_4$)$_5$ using minima-hopping, and highlights the importance of van der Waals interactions in stability predictions.

Findings

NaZn(BH$_4$)$_3$ and KZn$_2$(BH$_4$)$_5$ are predicted to be thermodynamically and dynamically stable.

The lowest-energy LiZn(BH$_4$)$_3$ structure is unstable, explaining experimental absence.

Van der Waals interactions significantly influence the energetic ordering of these compounds.

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.