Formation of novel transition metal hydride complexes with ninefold hydrogen coordination

Shigeyuki Takagi,Toshiya Otomo,Kazutoshi Miwa,Tamio Ikeshoji,Shin-Ichi Orimo,Toyoto Sato,Kazutaka Ikeda,Yuki Iijima,Hiroyuki Saitoh

doi:10.1038/srep44253

Abstract

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH9]3−), tungsten ([WH9]3−), niobium ([NbH9]4−) and tantalum ([TaH9]4−) in novel complex transition-metal hydrides, Li5MoH11, Li5WH11, Li6NbH11 and Li6TaH11, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

Highlights

Obtained samples were investigated using a combination of experimental and ab initio approaches
We examined the dynamics of the room-temperature phase of Li5MoH11 as an example using both first-principles molecular dynamics (FPMD) calculations in the isothermal– isobaric (NPT) ensemble at 298 K and neutron diffraction (ND) measurements at room temperature
We have examined the possibility of the formation of novel transition metal hydride complexes with high H-coordination using a combined theoretical/experimental approach

Summary

Introduction

Obtained samples were investigated using a combination of experimental and ab initio approaches. Li6NbH11 has a structure similar to that of Li5MoH11; it has an additional Li site between the NbH9 tricapped trigonal prisms along the c-axis, which accommodates the 6:1 stoichiometry between Li and Nb. The calculated standard heats of formation

Results

Conclusion