Quantum Beam Science Research

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Observation of Curious Long-Period Structures in
Metal Hydride–A Clue to Understand the Hydrogen-Metal Interactions and Their Role–hexagonal

Intensity (a.u.)

experimentintermediate

cubicsimulation

Rietveld refinement°
Fig.4-19 X-ray diffraction patterns of YH3(a) X-ray diffraction pattern of YH3 measured at 14.0 GPa.
(b) Diffraction pattern simulated with a 27-layered longperiod structural model.
(c) Result of pattern fitting by Rietveld method.

Fig.4-20 Pressure variation of metal lattices of YH3
From right, low-pressure hexagonal structure, long-period
stacking structures at 14.0 and 17.9 GPa, and high-pressure
cubic structure.

Hydrogen (H) is the smallest, lightest and chemically active
element. Hydrogen reacts with almost all metals and form
hydrides, in which hydrogen atoms occupy the interstices ofmetal lattices. The interstitial H atoms form chemical bonds
such as ionic or covalent-like bonds with the surrounding metal
atoms and change the lattice structure and electronic state.
Crystal structure and electronic state are changed by applying
pressure as well. We have investigated the structure of yttrium
trihydride (YH3) under high pressure. YH3 is an insulator, but
was found to become a metal again upon compression beyond
20 GPa. The high-pressure structural study of YH3 would
provide a clue to elucidate the insulator-metal transition interms of the metal-H and/or H-H interactions.
The process of the pressure-induced hexagonal-cubictransition of YH3 was investigated by synchrotron radiation
X-ray diffraction experiments at “SPring-8”. The hexagonal
structure gradually transformed into a cubic structure in awide pressure span from 11 to 20 GPa, while the electronic
state changed from insulating to metal. The intermediate state
while in the above pressure range has been interpreted as thecoexistence of the low-pressure hexagonal and high-pressure
cubic phases. However, we have found that the intermediate
state was a single phase and their diffraction patterns were well
reproduced by models with long-period stacking structures ofY metal layers. Fig.4-19 shows (a) a measured high-resolution
diffraction pattern, (b) a pattern simulated with a long-period
model, and (c) the result of profile fitting analysis. The longperiod structures consisted of Y-metal layers periodicallyarranged in ABA-type (H-type) and ABC-type (K-type) stacking
sequence. The 14.0-GPa structure of a unit cell containing
27 layers was characterized as consisting of three sub unit
cells, each having a 4-H and 5-K layer sequence, which werestacked up along the 3-fold axis. The17.9-GPa structure wasdescribed with the 27 layers as well but here the sub unit cells
consisted of a 2-H and 7-K layer sequence (Fig.4-20). The
K-type component increased in number gradually withincreasing pressure, and eventually became the only type. Suchstructural transition does not occur in Y metal containing no H
atoms. The interstitial H atoms likely played a dominant role in
the formation of the long-period structures.

Reference
Machida, A. et al., Long-period Stacking Structures in Yttrium Trihydride at High Pressure, Physical Review B, vol.76, no.5, 2007, p.052101-1-052101-4.
JAEA R&D Review 2008

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