Real space imaging of hydrogen at a metal - metal hydride interface

Hydrogen as a prospective fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. For a better understanding of these metal-metal hydride systems, and in particular their interfaces, real-space imaging of hydrogen with atomic resolution is required. However, hydrogen has not been imaged before at an interface. Moreover, to date, a robust technique that is capable to do such light-element imaging has not been demonstrated. Here, we show that integrated Differential Phase Contrast (iDPC), a recently developed imaging technique performed in an aberration corrected scanning transmission electron microscope, has this capability. Atomically sharp interfaces between hexagonal close-packed titanium and face-centered tetragonal titanium monohydride have been imaged, unambiguously resolving the hydrogen columns. Exploiting the fact that this monohydride has two types of columns with identical surrounding of the host Ti atom we have, 30 years after they were first proposed, finally resolved which one of the proposed structural models holds for the interface. Using both experimental and simulated images, we compare the iDPC technique with the currently more common annular bright field (ABF) technique, showing that iDPC is superior regarding complicating wave interference effects that may lead to erroneous detection of light element columns