Stability of Zirconium-Substituted Face-Centered Cubic Yttrium Hydride.

The stability of a zirconium (Zr)-substituted face-centered cubic (FCC) yttrium (Y) hydride (Y1-xZrx hydride) phase was investigated experimentally and theoretically. Two possible sites for hydrogen atoms exist in the FCC structure, namely, T- and O-sites, where hydrogen is present at the center of the tetrahedron and the octahedron composed of Y and/or Zr metals. The P-C isotherms revealed that the hydrogen content per metal (H/M) with 33% Zr-substituted YH3-δ was 2.2-2.3, which was lower than the expected value calculated from the starting composition of YH3-33% ZrH2 (Y0.67Zr0.33H2.67, H/M = 2.67). Hydrogen at the O-site in Y1-xZrx hydride mainly reacted during hydrogen desorption/absorption. On the basis of theoretical analyses, the hydrogen atoms do not occupy the center of the octahedron, when at least two of the six vertices of the octahedron were composed of Zr. The O-sites, where more than two Zr atoms coordinate, nonlinearly increased with the Zr content, and when the Zr content was >50%, almost no hydrogen atoms occupy the O-sites. The theoretical discussion supported the experimental results, and the Zr substitution was confirmed to reduce the occupancy of H at the O-site in the FCC YH3 significantly.

Facile Synthesis of LiH-Stabilized Face-Centered-Cubic YH3 High-Pressure Phase by Ball Milling Process.

A face-centered-cubic (FCC) YH3 phase is known to be stable only under high pressure (HP) of more than gigapascal order, and it reverts to the hexagonal YH3 ambient-pressure phase when the pressure is released. We previously found that the FCC YH3 can be stabilized even at ambient pressure by substituting Y for 10 mol % Li (LiH-stabilized YH3, LSY). The LSY was synthesized by heat treatment under gigapascal HP, but this process is unfavorable for mass production; that is, only a few tens of milligrams of a sample can be obtained in a single batch. In this study, we overcame this problem by applying a ball milling (BM) process for synthesizing the LSY phase, and the yield by the BM process reached on the order of grams. We confirmed that the structure of the BM sample was the same as that of the HP sample by X-ray diffractometry, Raman spectroscopy, and neutron total scattering pair distribution function analyses. The crystallinity of the BM sample, however, was lower than that of the HP sample. The difference in the crystallinity affects the thermal stability of the LSY. The BM sample with a lower crystallinity released hydrogen at a lower temperature. The BM sample was found to reversibly desorb/absorb hydrogen maintaining its initial FCC structure when the rehydrogenation temperature was at 423 K. However, when the rehydrogenation temperature of BM sample was more than 573 K, the FCC structure changed to the hexagonal ambient pressure phase due to thermal instability of FCC phase for the BM sample.

Stabilization of Face-Centered Cubic High-Pressure Phase of REH3 (RE = Y, Gd, Dy) at Ambient Pressure by Alkali or Alkaline-Earth Substitution.

Heavy rare-earth trihydrides such as GdH3, DyH3, and yttrium trihydide (YH3) usually show a hexagonal crystal structure under ambient pressure. This structure is known to transform to a face-centered cubic (FCC) one at higher pressures (at the order of GPa), and the FCC one returns to the hexagonal structure when the applied pressure is released. In this study, we investigated the structure of alkaline or alkaline-earth (A)-substituted REH3 (RE = Y, Gd, Dy; A = Li, K, or Mg) using X-ray diffraction, and measured the phase transition pressure. We found that this FCC high-pressure phase can be stabilized by 10-33 mol % A substitution for RE in the REH3. The mechanism of phase stabilization is simply explained by the ionic radius ratio between the cation and anion ( rcat/ rani), as well as the stabilities of other ionic crystals such as perovskite materials. For all considered REH3 samples, the FCC phase becomes stable when rcat/ rani > 0.856, such as in the case of 10 mol % Li-substituted YH3.

Li-Ion Conductivity and Phase Stability of Ca-Doped LiBH4 under High Pressure.

The effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type LiBH4 phase emerging under high pressures in the range of gigapascals has been investigated. In situ electrochemical measurements under high pressure were performed using a cubic-anvil-type apparatus. Ca doping drastically enhanced the ionic conductivity of the rock-salt-type phase: the ionic conductivity of undoped and 5 mol %Ca-doped LiBH4 was 2.2 × 10-4 and 1.4 × 10-2 S·cm-1 under 4.0 GPa at 220 °C, respectively. The activation volume of LiBH4-5 mol %Ca(BH4)2, at 3.2 cm3·mol-1, was comparable to that of other fast ionic conductors, such as lithium titanate and NASICONs. Moreover, Ca-doped LiBH4 showed lithium plating-stripping behavior in a cyclic voltammogram. These results indicate that the conductivity enhancement by Ca doping can be attributed to the formation of a LiBH4-Ca(BH4)2 solid solution; however, the solid solution decomposed into the orthorhombic LiBH4 phase and the orthorhombic Ca(BH4)2 phase after unloading the high pressure.