Metastable Hexagonal Close-Packed Palladium Hydride in Liquid Cell Transmission Electron Microscopy
Authors: Chun Dong Won , Hong Jaeyoung , Bae Jee-Hwan , Jo Hyesung , Park Hee-Young , Lee Sehyun , Hong Sung Jun , Chun Hoje , Cho Min Kyung , Kim Juyoung , Kim Joodeok , Son Yongju , Jin Haneul , Suh Jin-yoo , Kim Sung-Chul , Roh Ha-Kyung , Lee Kyu Hyoung , Kim Hyung-Seok , Chung Kyung Yoon , Yoon Chang Won , Lee Kiryeong , Kim Seo Hee , Ahn Jae-Pyoung , Baik Hionsuck , Kim Gyeung Ho , Han Byungchan , Jin Sungho , Hyeon Taeghwan , Park Jungwon , Son Chang Yun , Yang Yongsoo , Lee Young-Su , Yoo Sung Jong 
Published: December 2020

Abstract
Metastable phases—kinetically favored structures—are ubiquitous in nature. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions such as temperature, pressure, or crystal size. As the crystals grow further, they typically undergo a series of transformations from metastable to lower-energy phases and ultimately energetically stable phases, as described by Wilhelm Ostwald. Metastable phases, however, sometimes provide superior chemical and physical properties, and hence, discovering and synthesizing novel metastable phases are promising avenues for achieving innovations in materials science. The most common strategy for synthesizing a metastable material involves manipulating thermodynamic conditions such as temperature and pressure during the course of synthesis. However, the search for metastable materials has mainly been heuristic, on the basis of experiences, intuitions, or even speculative predictions. This limitation necessitates the advent of a new paradigm to discover novel metastable phases, i.e., by “rational design and synthesis” instead of a “rule of thumb,” and based on ab initio methods, i.e., the calculation of thermodynamic and kinetic properties of materials with various compositions, crystal structures, and crystal sizes. The design rule is embodied in the discovery of a metastable hexagonal close-packed (HCP) palladium hydride (PdHx), synthesized in a liquid cell environment using a transmission electron microscope (TEM). The metastable HCP structure is stabilized through a unique interplay among the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favors the HCP structure on the sub-nanometer scale, and an insufficient supply of Pd inhibits further growth and subsequent transition toward the thermodynamically stable face-centered cubic (FCC) structure. The crystal structure was modulated (HCP or FCC) by adjusting the H concentration inside the TEM liquid cell, providing strong evidence for the crucial role of the H concentration. Monte Carlo simulations reveal that an unexpected inhomogeneous distribution of interstitial H atoms, distinct from the predominant occupation at the octahedral interstitial sites, is key to stabilizing nanoscale HCP PdHx. Furthermore, insufficient Pd brings a multi-step nucleation and growth pathway, deduced from in situ liquid cell TEM combined with atomic electron tomography, which maintains the metastable phase intact. These findings provide new thermodynamic insights into metastability-engineering strategy to be deployed in discovering new metastable phases.