In situ Synchrotron X-ray Thermodiffraction of Boranes

Boranes of low molecular weight are crystalline materials that have been much investigated over the past decade in the field of chemical hydrogen storage. In the present work, six of them have been selected to be studied by in situ synchrotron X-ray thermodiffraction. The selected boranes are ammonia borane NH3BH3 (AB), hydrazine borane N2H4BH3 (HB), hydrazine bisborane N2H4(BH3)2 (HBB), lithium LiN2H3BH3 (LiHB) and sodium NaN2H3BH3 (NaHB) hydrazinidoboranes, and sodium triborane NaB3H8 (STB). They are first investigated separately over a wide range of temperature (80–300 K), and subsequently compared. Differences in crystal structures, the existence of phase transition, evolutions of unit cell parameters and volumes, and variation of coefficients of thermal expansion can be observed. With respect to AB, HB and HBB, the differences are mainly explained in terms of molecule size, conformation and motion (degree of freedom) of the chemical groups (NH3, N2H4, BH3). With respect to LiHB, NaHB and STB, the differences are explained by a stabilization effect favored by the alkali cations via M···H interactions with four to five borane anions. The main results are presented and discussed herein.