Theoretical description of alkali metal closo-boranes – towards the crystal structure of MgB12H12

Abstract

Solid state closo-borane salts of alkali metals have very high ionic conductivity. This makes them interesting for practical applications as solid state electrolytes, and has triggered extensive research efforts. Improvement and understanding of their properties require accurate theoretical description of their static and dynamical properties. In this work, we report accuracy assessment of density functional theory in the description of solids with B₁₂H₁₂²⁻ anions. We show that these aromatic anions interact via weak dispersive forces. For that reason, non-local exchange–correlation functionals give better description of structural properties and phonons in Li₂B₁₂H₁₂ and Na₂B₁₂H₁₂. Numerically efficient semi-local methods provide satisfactory results when applied in structure volumes obtained in a non-local method. An extensive structural search for stable crystalline phases of MgB₁₂H₁₂ predicts a new denser lattice with C2/c symmetry that is stabilized by van der Waals interactions. These structures might be discovered as anhydrous MgB₁₂H₁₂ in high pressure experiments, avoiding the amorphous state at ambient pressures.