Abstract
High-pressure hydrogenation behaviors of pure metals have not been investigated extensively, although intense research of hydrogenation reactions under high pressure has been conducted to find novel functional hydrides. The former provides us with valuable information for the high-pressure synthesis of novel functional hydrides. A pressure–temperature phase diagram of the Ta–H system has been determined using the in situ synchrotron radiation X-ray diffraction technique below 9 GPa and 600 °C in this study. At room temperature, the phase boundary obtained between distorted bcc TaH~1 and hcp TaH~2 was consistent with the previously reported transition pressure. The experimentally obtained Clapeyron slope can be explained via the entropy change caused by hydrogen evolution from TaH~2.
Highlights
Hydrides have a variety of functionalities, including high-temperature superconductivity [1,2,3], fast ionic conductivity [4,5], and H− conduction [6,7]
Applying high pressure is an essential technique for investigating functional hydrides, because novel hydrides can be synthesized under high pressure due to the extremely high chemical potential of hydrogen
Hydrogenation reactions of pure metals have received little attention. Such studies provide us with useful information for the high-pressure synthesis of novel functional hydrides
Summary
Hydrides have a variety of functionalities, including high-temperature superconductivity [1,2,3], fast ionic conductivity [4,5], and H− conduction [6,7]. Such studies provide us with useful information for the high-pressure synthesis of novel functional hydrides. Kuzovnikov et al confirmed that hydrogenating Ta above 5 GPa at room temperature could synthesize theoretically predicted hcp TaH~2. During depressurization at room temperature, the formed dihydride decomposed into monohydride at 2.2 GPa. The crystal structure of the monohydride has been reported to be a face-centered orthorhombic one in which lattice constant a is comparable to b and c is nearly equal to a/ √2 .
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