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

Proton spin–lattice relaxation times, T1, 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, τc, derived from the T1 data were used to calculate activation energies, V, for BH4− ion reorientations. For the cubic phase of KBH4, V = 14.8 ± 0.4 kJ/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 τc 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 T1 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.