Orientational Disorder of NH$_3$ in Hexammine Magnesium Borohydride

TL;DR

This study reveals that ammonia molecules in Mg(NH3)6(BH4)2 exhibit orientational disorder at room temperature, which freezes upon cooling, impacting its potential for hydrogen storage and solid-state battery applications.

Contribution

The paper provides the first structural solution showing orientational disorder of NH3 in Mg(NH3)6(BH4)2 using synchrotron X-ray diffraction, clarifying its crystal structure.

Findings

Ammonia molecules are orientationally disordered at room temperature.

Cooling to 120 K causes a unit cell expansion and freezing of ammonia orientation.

Vibrational modes in IR spectrum are fully assigned based on the structure.

Abstract

Hexammine magnesium borohydride, Mg(NH$_3$)$_6$(BH$_4$)$_2$, consists of adducted NH$_3$ molecules locked in a matrix of Mg cations and borohydride anions. It is a candidate material for hydrogen storage, with 16.8wt\% hydrogen stored in both the NH$_3$ and borohydride anions. It also may be of interest as a Mg$^{2+}$ conducting electrolyte in solid state batteries. Its crystal structure has, until now, eluded a proper structural solution due to ambiguity regarding the NH$_3$ position and behaviour. In this work, we show using synchrotron X-ray diffraction that the room-temperature structure can be solved only with a model assuming orientational disorder of ammonia molecules within the crystal structure. Cooling the sample to 120\,K yields additional Bragg peaks, which can only be solved with a unit cell expansion consistent with a freezing of the orientational freedom of ammonia molecules. Using this insight from the structure solution, we perform a full assignment of the vibrational modes in the room-temperature IR spectrum.