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

摘要

We study the thermodynamic stability at low temperatures of a series ofalkali 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 theminima-hopping method, we discover two new lowest-energy structures forNaZn(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 boththermodynamically stable and dynamically stable, implying that their existencemay be possible. On the other hand, the lowest-energy $P1$ structure ofLiZn(BH$_4$)$_3$ is predicted to be unstable, suggesting a possible reasonelucidating why this compound has not been experimentally identified. Inexploring the low-energy structures of these compounds, we find that theirenergetic ordering is sensitive to the inclusion of the van der Waalsinteractions. 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 toaddress the stability of the low-energy structures of these compounds.