The origin of the highly crystallized face-centered cubic YH[formula omitted] high-pressure phase when quenched to ambient condition

Abstract

The origin of the face-centered cubic (FCC) structure in yttrium trihydride stabilized (or quenched) at ambient pressure was investigated using transmission electron microscopy (TEM) and in situ synchrotron x-ray diffraction (XRD) measurements under high pressures in the gigapascal range. Although the FCC-YH3 high-pressure phase is normally unstable at ambient pressure, in this study, we found that the FCC-YH3 phase, with high crystallinity, could be obtained via sintering at 1373 K for 24 h under 6 GPa, and subsequent quenching at a rate greater than 1000 K/min. The XRD profile of the FCC-YH3 phase showed sharp peak patterns, i.e.,it had a high crystallinity; however, the TEM observations revealed that the surfaces of the particles were significantly disordered. It was observed that these surface defects played an important role in keeping the FCC-YH3 high-pressure phase present at the ambient pressure. In situ XRD measurements during high-pressure and temperature processing confirmed that the YH3 phase structure had completely transformed from HCP to FCC at 1173 K and 9 GPa; however, the amount of HCP-YH3 ambient pressure phase increased continuously during cooling and depressurization. The sample cooling rate during in situ measurements was 90 K/min, which was more than ten times slower than that in the ex situ experiments. Since the defects introduced into the surface of the particles during the slow cooling process were insufficient, a reversal of the HCP-FCC reversible structural change was observed during the depressurization after sintering.