NMR Investigation of Nanoporous γ-Mg(BH4)2 and Its Thermally Induced Phase Changes

The nanoporous polymorph gamma-Mg(BH4)(2) with 33% void space has a much lower density than the alpha and beta forms, suggesting that reorientation and diffusion dynamics of BH4 groups may be enhanced. We report hydrogen NMR line shapes and T-1 and T-ID measurements from -125 to 225 degrees C, along with some B-11 results. The main component of the hydrogen NMR line does not show motional narrowing from translational diffusion up to 175 degrees C, although a small mobile fraction appears at 175 degrees C. T-ID measurements show no evidence for "slow" motions (faster than 10(2) s(-1)) up to 175 degrees C, ruling out enhanced overall diffusivity. The hydrogen T-1 is sensitive to reorientations of the BH4 units. In the gamma-phase, a T-1 minimum at 85 MHz is observed at 50 degrees C, similar to alpha-phase; the high barrier to reorientation in gamma (compared to beta-phase where the minimum appears near -135 degrees C) is confirmed by broadening of the gamma-phase hydrogen spectrum at -50 degrees C and lower temperatures. A sharp increase in T-1 at or above 175 degrees C signals a transformation of gamma-phase to another structure of higher density. T-1 of the transformation product is similar but not equal to that of the beta-phase, suggesting the transformation product is primarily beta-phase. This is confirmed by powder X-ray diffraction of the transformed material. In situ X-ray powder diffraction studies reveal that gamma-Mg(BH4)(2) transforms upon heating in vacuum into the beta-phase, and undergoes an amorphization in the presence of gases that can be adsorbed into the pores.