Nuclear Magnetic Relaxation Studies of Internal Rotations and Phase Transitions in Borohydrides of Lithium, Sodium, and Potassium

Abstract

Proton spin–lattice relaxation times, $T_1$ , have been measured as a function of temperature for KBH₄, NaBH₄, and LiBH₄. For NaBH₄ and KBH₄, ²³Na and ¹¹B relaxation measurements were also made. In all cases, the magnetization recovery is approximately exponential. Correlation times, ${\tau }_c$ , derived from the $T_1$ data were used to calculate activation energies, $V$ , for BH₄⁻ ion reorientations. For the cubic phase of KBH₄, $\text{V\hspace{0.17em}=\hspace{0.17em}14.8\hspace{0.17em}±\hspace{0.17em}0.4}\mathrm{kJ}/\mathrm{mole}$ (3.55 ± 0.1 kcal/mole) (± always refers to rms error) from measurements on proton and ¹¹B. For NaBH₄, $V$ was found to be 11.2 ± 0.5 and 14.8 ± 0.7 kJ/mole (2.7 ± 0.1 and 3.5 ± 0.2 kcal/mole) for the high‐ (cubic) and low‐temperature (tetragonal) phases; an anomaly in ${\tau }_c$ was observed at temperatures slightly below the phase transition, and may be interpreted as a relatively sudden change in $V$ associated with the phase transition. In LiBH₄, a rather broad minimum was observed for the proton $T_1$ vs temperature; this has been interpreted as due to two inequivalent BH₄⁻ tetrahedra with activation energies of 20 ± 1 and 16 ± 1 kJ/mole (4.7 ± 0.3 and 3.8 ± 0.3 kcal/mole). The proton and ¹¹B nuclei are relaxed by magnetic dipolar interactions, but quadrupolar fluctuations are the dominating relaxation mechanism for ²³Na in the cubic phase of NaBH₄.