To study the reorientational motion of the BH 4 groups and the translational diffusion of Li + ions in LiBH 4 −LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, we have measured the 1 H, 11 B, and 7 Li NMR spectra and spin−lattice relaxation rates in these compounds over the temperature range 18−520 K. It is found that, at low temperatures, the reorientational motion of the BH 4 groups in LiBH 4 −LiI solid solutions is considerably faster than in all other borohydridebased systems studied so far. Our results are consistent with a coexistence of at least two types of reorientational processes with different characteristic rates. For the faster reorientational process, the average activation energies derived from our data are 53 ± 4, 39 ± 4, and 33 ± 4 meV for the LiBH 4 −LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, respectively. In the studied range of iodine concentrations, the Li + jump rates are found to decrease with increasing I − content. The activation energies for Li diffusion obtained from our data are 0.63 ± 0.01, 0.65 ± 0.01, and 0.68 ± 0.01 eV for the samples with 2:1, 1:1, and 1:2 molar ratios, respectively.
■ INTRODUCTIONLithium borohydride, LiBH 4 , containing 18.4 mass % of hydrogen is considered as a promising material for hydrogen storage 1 and a prospective superionic conductor. 2 This compound is an ionic crystal consisting of Li + cations and tetrahedral [BH 4 ] − anions. At low temperatures LiBH 4 has the orthorhombic structure (space group Pnma).3−5 At T 0 ≈ 380 K it undergoes a first-order phase transition to the hexagonal structure (space group P6 3 mc).3−5 The transition from the lowtemperature (LT) orthorhombic to the high-temperature (HT) hexagonal phase is accompanied by the 3 orders of magnitude increase in the electrical conductivity, 2 so that the HT phase of LiBH 4 can be considered as a lithium superionic conductor. The HT phase of LiBH 4 is also characterized by the fast reorientational motion of BH 4 tetrahedra 6,7 with the jump rates exceeding 10 12 s −1 . Recently, it has been found 8−11 that the HT phase of LiBH 4 can be stabilized down to low temperatures by a partial halide ion substitution of [BH 4 ] − anions. Such a substitution results in the formation of Li(BH 4 ) 1−y X y solid solutions (X = Cl, Br, or I) with the hexagonal structure. The stabilizing effect increases with increasing size of the halide ion; thus, the strongest effect is observed for I − substitution. The temperature of the hexagonal-to-orthorhombic phase transition for Li(BH 4 ) 1−y I y solid solutions is found to decrease with increasing iodine content, from 351 K for y = 0.067 to 213 K for y = 0.25. 12 For y = 0.33, no signs of the phase transition have been found down to 173 K.12 Thus, the hexagonal Li(BH 4 ) 1−y I y solid solutions with y ≥ 0.33 appear to be stable down to low temperatures. In practice, these solid solutions are prepared by ball milling the LiBH 4 −LiI mixtures with subsequent annealing. It is interesting to note that while the room-temperature p...