A model for the formation energies of alanates and boranates

TL;DR

This paper presents a simple ionic bonding model to predict formation energies of alkali and alkaline earth alanates and boranates, aligning well with computational and experimental data, and highlighting stability factors.

Contribution

The model simplifies formation energy predictions by focusing on ionic bonding, showing structure details are less critical, and explaining stability differences between boranates and alanates.

Findings

Model agrees with first-principles calculations and experiments.

Boranates are more stable than alanates due to stronger bonding.

Cation size has minimal impact on formation energy.

Abstract

We develop a simple model for the formation energies (FEs) of alkali and lkaline earth alanates and boranates, based upon ionic bonding between metal cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained from first principles calculations and with experimental FEs. The model shows that details of the crystal structure are relatively unimportant. The small size of the (BH4)- anion causes a strong bonding in the crystal, which makes boranates more stable than alanates. Smaller alkali or alkaline earth cations do not give an increased FE. They involve a larger ionization potential that compensates for the increased crystal bonding.