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

Abstract

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 11 B 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.

Nuclear magnetic relaxation studies of BH4 in metal Borohydrides

“…Recently, nuclear magnetic resonance (NMR) studies of atomic motion in metal borohydrides have received growing attention. Reorientational motions of BH 4 anion of light metal borohydrides (metal = Li, Na, Mg, Ca, and so on) crystallites, in particular, have been actively investigated via spin–lattice relaxation time ( T 1 ) measurements of 1 H and 11 B nuclei over wide temperature ranges and multiple magnetic fields 7–14 . Dynamic NMR studies allowed the measurement of atomic jump rates and activation energies of borohydrides (pure form or with halide additives) in various crystallographic phases (throughout multiple phase transitions) 15–16 .…”

Anion and Cation Dynamics in Polyhydroborate Salts: NMR Studies

“…Nuclear magnetic resonance (NMR) has proved to be an efficient technique for studies of atomic motion in solids at the microscopic level. This technique was widely used to investigate the atomic jump motion in alkali–metal borohydrides [ 12 , 13 , 14 , 15 , 22 , 23 , 24 , 25 , 26 , 27 ] and alkaline–earth borohydrides [ 28 , 29 , 30 , 31 ]. However, systematic studies of the anion and cation dynamics in more complex systems, such as mixed BH 4 -based compounds and closo -hydroborates and the related compounds, have just begun.…”

Kinetic Barriers and Microscopic Mechanisms of Noble Gas Adsorption by Nanoporous γ‐Mg(BH4)2 Obtained by Means of Sub‐Second X‐Ray Diffraction

“…The barriers for [BH 4 ] À orientational jumps in g-Mg(BH 4 ) 2 are significantly higher and include at least two motions. [41,42] Taking into account these considerations together with the crystal structure of g-Mg(BH 4 ) 2 , we propose two concerted mechanisms for Kr adsorption: the first is caused by Kr diffusion along 1D channels running in c direction and it dominates at high temperatures (> 175 K), while at low temperatures (< 175 K) the second mechanism, namely the diffusion through the windows between channels, governs the kinetics, see Figure 5 d. The first kinetic mechanism involves a larger ( % 5.8 ) aperture of the channels, which does not affect the diffusion of Kr (kinetic diameter of 3.6 ) to such extent as a smaller interchannel aperture with % 5.0 in size.…”