In the current research field of batteries, there are strong demands for high-performance hydrogen storage materials and ionic (H⁺, Li⁺, etc.) conductors. Recently, a series of novel materials Li₅MoH₁₁, Li₅WH₁₁, Li₆NbH₁₁, Li₆TaH₁₁ which contain unusual ninefold hydrogen-coordinated clusters MH₉ (M= Mo, W, Nb, Ta), whose structure is shown in the inset of Fig. 1, were synthesized by Orimo group in Tohoku University [1]. These are of interest since they have high hydrogen density and possible high Li ionic conductivity. The dynamics of MH₉ cluster may play important roles in determining the physical properties of the materials. We have performed the neutron powder diffraction (NPD) and quasielastic neutron scattering (QENS) experiments of Li₅MoH₁₁ and Li₆NbH₁₁.

Both samples were synthesized from solid-state reactions under high pressure (ca. 5 GPa) and high temperature (ca. 700 K). Owing to these difficulties, the sample amounts of Li₅MoH₁₁ and Li₆NbH₁₁ were 33 mg and 20 mg, respectively. The NPD and QENS measurements were conducted using high intensity total diffractometer NOVA (BL21) and biomolecular dynamic spectrometer DNA (BL02) in MLF, J-PARC, respectively. The DNA data were collected with both high and low energy resolution modes.

From the Rietveld analyses for the NPD data at room temperature, we have checked the purity of the samples (Li₅MoH₁₁: 75 %, Li₆NbH₁₁: 57 %) and confirmed that the MH₉ clusters are orientationally disordered. Figure 1 shows the intermediate scattering functions of Li₅MoH₁₁. Similar data were obtained also in Li₆NbH₁₁. Relaxation phenomena were observed in a wide temperatures range between 150 K and 300 K. It is quite unusual that the relaxation occurs in a wide time range over 3 orders of magnitude as shown in the data at 300 K. Therefore, the data were fitted with not an exponential but Kohlraush-Williams-Watts (KWW) function

I(Q,t) = a(Q) + b(Q)exp[–(t/τ_KWW)^β],

where Q is magnitude of a scattering vector, t is a time, τKWW is a relaxation time, β is a non-exponential parameter, and a(Q) and b(Q) are elastic and relaxation components, respectively. The β parameter (0 < β < 1) reflects the width of the relaxation time distribution; no distribution at β = 1. The fittings were satisfactory as shown by solid curves in Fig. 1. The determined β were 0.38 for Li₅MoH₁₁ and 0.33 for Li₆NbH₁₁, indicating that the distribution of the relaxation time is very wide; cf. β is around 0.5 for glass forming liquids. The activation energies obtained from the temperature dependence of the relaxation times were 15 kJmol^–1 for Li₅MoH₁₁ and 27 kJmol^–1 for Li₆NbH₁₁. The magnitude of the activation energy may depend on the ionic valence and size (M-H distance) of the hydrogen cluster (MoH₉^3-: 1.76 Å, NbH₉^4-: 1.86 Å) .

Fig. 1. Intermediate scattering functions (Q = 1.53 Å^-1) of Li₅MoH₁₁ measured at several temperatures between 150 K and 300 K. The solid curves are fitting curves to the KWW functions with β = 0.38. The inset shows the structure of the MH₉ cluster.

Figure 2 shows the Q dependence of EISF (Elastic Incoherent Scattering Factor) which is defined by EISF = a(Q)/[a(Q) + b(Q)]. As shown by the solid curves in Fig. 2, EISF was well fitted by

EISF = 1 – A + Aj₀²(QR),

where _j0(QR) is a 0th order sphere Bessel function with a radius R. This indicates that MH₉ clusters are spherically rotating at 300 K, which is consistent with the structural information. The rotational states of the MH₉ clusters are now investigated with a first-principle MD simulation technique. We guess that the wide distribution of the relaxation time is associated with the positional disorder of Li ions conducting in the crystals.

Fig. 2. Elastic Incoherent Structure Factor (EISF) obtained at 300 K for Li₅MoH₁₁ and Li₆NbH₁₁. The solid curves are the fitting curves to the equation in the figure.

This work is financially supported by JSPS KAKENHI No. JP 18H05513 and JP18H05518.

References

• [1] S. Takagi, Y. Iijima, T. Sato, H. Saitoh, K. Ikeda, T. Otomo, K. Miwa, T. Ikeshoji, and S. Orimo, Sci. Rep. 7, 44253 (2017).

Authors

• Y. Ohmasa, H. Akiba, S. Takagi^a, S. Orimo^a, H. Saito^b, T. Yamada^c, Y. Kawakita^d, K. Ikeda^e, T. Otomo^e and O. Yamamuro
• ^aTohoku University
• ^bQST
• ^cCROSS
• ^dJAEA
• ^eKEK