NMR investigation of nanoporous γ-Mg(BH 4) 2 and its thermally induced phase changes

The nanoporous polymorph γ-Mg(BH 4) 2 with 33% void space has a much lower density than the α and β forms, suggesting that reorientation and diffusion dynamics of BH 4 groups may be enhanced. We report hydrogen NMR line shapes and T 1 and T 1D measurements from -125 to 225 °C, along with some 11B results. The main component of the hydrogen NMR line does not show motional narrowing from translational diffusion up to 175 °C, although a small mobile fraction appears at 175 °C. T 1D measurements show no evidence for "slow" motions (faster than 10 2 s -1) up to 175 °C, ruling out enhanced overall diffusivity. The hydrogen T 1 is sensitive to reorientations of the BH 4 units. In the γ-phase, a T 1 minimum at 85 MHz is observed at 50 °C, similar to α-phase; the high barrier to reorientation in γ (compared to β-phase where the minimum appears near-135 °C) is confirmed by broadening of the γ-phase hydrogen spectrum at -50°C and lower temperatures. A sharp increase in T 1 at or above 175 °C signals a transformation of γ-phase to another structure of higher density. T 1 of the transformation product is similar but not equal to that of the β-phase, suggesting the transformation product is primarily β-phase. This is confirmed by powder X-ray diffraction of the transformed material. In situ X-ray powder diffraction studies reveal that γ-Mg(BH 4) 2 transforms upon heating in vacuum into the β-phase, and undergoes an amorphization in the presence of gases that can be adsorbed into the pores. © 2012 American Chemical Society.