Structural and Dynamical Properties of NaBH₄ and KBH₄: NMR and Synchrotron X-ray Diffraction Studies

Abstract

The temperature dependence of the structural parameters of NaBH 4 has been investigated by synchrotron X-ray diffraction over the range of 80-500 K. On the basis of the diffraction data, we revise the structure of the low-temperature (tetragonal) phase of NaBH 4 and discuss the changes in the lattice parameters and the patterns of H • • • H contacts in the low-T and high-T phases. To study the atomic motion in NaBH 4 and KBH 4 , we have performed nuclear magnetic resonance (NMR) measurements of the 1 H and 11 B spin-lattice relaxation rates in these compounds over wide ranges of temperature (82-424 K) and resonance frequency (14-90 MHz). For both compounds, the NMR results are governed by the thermally activated reorientational motion of the BH 4 groups, and the changes in the jump rates of reorientations are traced over the range of 8 orders of magnitude. The activation energies of the reorientational motion are found to be equal to 0.151 ( 0.002 eV (low-T phase of NaBH 4 ), 0.126 ( 0.003 eV (high-T phase of NaBH 4 ), and 0.161 ( 0.002 eV (high-T phase of KBH 4 ). Structural changes in the series of alkali metal borohydrides are analyzed in connection with the changes in the phase-transition temperature, the compressibility, and the barrier of the reorientational BH 4 motion. On the basis of this analysis, we discuss the role of different interatomic interactions in determining the physical properties of alkali metal borohydrides.

Effect of chloride substitution on the order–disorder transition in NaBH4 and Na11BD4

“…Refinements from PND data confirm the structural models of Fischer and Züttel [12] [17]. Together with the observed two-phase presence within the transition range of temperatures, these facts suggest this transition to be of the first order, in conformity with the lambda shape for the heat capacity in Ref.…”

Nuclear Magnetic Resonance Studies of Reorientational Motion and Li Diffusion in LiBH4–LiI Solid Solutions

“…In this section, we shall discuss the parameters of BH 4 reorientations which govern the behavior of the spin−lattice relaxation rates in the region of the low-temperature peak for LiBH 4 H dipole−dipole interactions. 15,18 As noted above, the frequency dependence of R 1 H in this region (see Figure 1) suggests a certain distribution of the reorientation jump rates τ −1…”

“…Since the H−B and H−H terms in eq 4 show nearly the same temperature and frequency dependences, it is practically impossible to determine the amplitude parameters ΔM HBi and ΔM HHi independently from the fits. The estimates for alkali-metal borohydrides 15,18 indicate that ΔM HBi and ΔM HHi are close to each other. Therefore, for parametrization of the R 1 H data we shall assume that ΔM HBi = ΔM HHi ≡ ΔM i .…”

“…NMR measurements of nuclear spin−lattice relaxation rates can, in principle, trace the changes in τ −1 over much broader dynamic ranges. For example, the BH 4 reorientation rates in NaBH 4 18 Another advantage of NMR is that the spin−lattice relaxation measurements at different resonance frequencies allow one to detect the presence of a distribution of atomic jump rates. 30 Such a distribution is expected to exist in LiBH 4 −LiI solid solutions where the local environment changes from one BH 4 group to another.…”

Complex inorganic structures from powder diffraction:case of tetrahydroborates of light metals

“…On the other hand the dynamic rotation disorder of the BH 4 groups studied by various spectroscopic methods seems to be quite common feature of the alkali metal borohydrides as observed in HT-hexagonal LiBH 4 (Skripov et al, 2008), RThexagonal a-Mg(BH 4 ) 2 and RT-cubic phases of other alkali metal borohydrides (Verdal et al, 2010;Babanova et al, 2010). Freely rotating protons in the imide group NH have been proposed by molecular dynamics calculations in Li 2 NH (Ludueña and Sebastiani, 2010).…”