Nuclear Magnetic Resonance Study of Atomic Motion in Bimetallic Perovskite-Type Borohydrides ACa(BH4)3(A = K, Rb, or Cs)

To study the dynamical properties of the novel series of bimetallic perovskite-type borohydrides ACa(BH4)3(A = K, Rb, or Cs), we have measured the 1H and 11B nuclear magnetic resonance spectra and spin–lattice relaxation rates in these compounds over broad temperature ranges (5–560 K) and resonance frequency ranges (14–90 MHz). Our measurements have revealed several jump processes with different characteristic rates. For all the compounds studied, the spin–lattice relaxation rates at low temperatures (T< 340 K) are governed by fast reorientations of BH4groups. However, the experimental data in this range cannot be described in terms of a single reorientational process; this suggests a coexistence of at least two types of BH4reorientations. Taking into account the linear coordination of each BH4group in ACa(BH4)3by two Ca atoms, we can attribute different reorientational processes to BH4rotations around inequivalent symmetry axes. The parameters of reorientational motion in ACa(BH4)3have been evaluated using the model with a two-peak distribution of activation energies. It has been found that the transitions from the low-temperature phases to the high-temperature phases of ACa(BH4)3are accompanied by the onset of translational diffusion of intact BH4groups. However, the jump rates for these diffusive processes remain below 108s–1up to the high-Tlimits of our experimental ranges.