Experimental evidence of Na2[B12H12] and Na formation in the desorption pathway of the 2NaBH4 + MgH2 system

In the present work we report the desorption pathway of the 2NaBH4 + MgH2 system.
Ex-situ X-ray powder diffraction (XRPD) and solid state magic angle spinning (MAS) nuclear magnetic
resonance (NMR) measurements have been performed on samples heat-treated up to 450 °C for
different times. Ex-situ X-ray powder diffraction experiments conducted on fully desorbed samples
allowed to identify nanocrystalline MgB2 and metallic Na as dehydrogenation products. 11B and 23Na
NMR analyses have been also carried out in order to evaluate the structural evolution of decomposed
materials. Our measurements show that the local structure of MgB2 is influenced by the replacement of
Mg with Na atoms in the Mg sites. Moreover, amorphous Na2[B12H12] was detected in the partially
desorbed sample and in the final products of the decomposition reaction. The presence of the [B12H12]2-
anion was confirmed by both direct comparison with the 11B{1H} NMR spectrum of pure Na2[B12H12]
and dynamic Cross Polarization experiments.