Van der Waals interactions in the ground state of Mg(BH4)2 from density functional theory

TL;DR

This study uses advanced density functional theory with van der Waals corrections to accurately determine the ground-state structure of Mg(BH4)2, resolving previous discrepancies between experimental and theoretical results.

Contribution

The paper demonstrates that incorporating nonlocal van der Waals interactions in density functional theory is essential for correctly predicting the structure of Mg(BH4)2.

Findings

vdW-DF functional agrees with experimental data

α-Mg(BH4)2 phase is thermodynamically favored

Long-range dispersive interactions are crucial for accurate modeling

Abstract

In order to resolve an outstanding discrepancy between experiment and theory regarding the ground-state structure of Mg(BH4)2, we examine the importance of long-range dispersive interactions on the compound's thermodynamic stability. Careful treatment of the correlation effects within a recently developed nonlocal van der Waals density functional (vdW-DF) leads to a good agreement with experiment, favoring the {\alpha}-Mg(BH4)2 phase (P6122) and a closely related Mn(BH4)2-prototype phase (P3112) over a large set of polymorphs at low temperatures. Our study demonstrates the need to go beyond (semi)local density functional approximations for a reliable description of crystalline high-valent metal borohydrides.