Towards observing interstitial hydrogen in solution in metal lattices

Hydrogen-bearing materials are central to low-carbon energy technologies¹, but hydrogen itself is difficult to detect and localize at the nanoscale^2-6. This hinders the development of advanced materials with better efficiency. Atom probe tomography (APT) can map hydrogen in three dimensions and at high resolution, yet characterization of dilute, mobile, interstitial hydrogen in metals has remained challenging because of low solubility, the ambiguity owing to hydrogen trapping by structural defects, as well as the artefacts introduced during hydrogen introduction^4-6. Here we address whether interstitial hydrogen in solution in materials can be distinguished from defect-trapped hydrogen in APT. Using deuterium as a proxy for hydrogen, together with face-centred cubic (FCC) multi-principal-element alloys (MPEAs) containing coherent FCC precipitates, we show that deuterium atoms reproducibly concentrate in hydrogen-affinitive phases in a manner consistent with theoretical predictions. We further resolve differences in deuterium content between phases with distinct local chemistries and crystal structures, consistent with their different calculated hydrogen solution energies. These findings demonstrate that APT can move beyond identifying hydrides and solute-decorated defects to examine hydrogen in solid solution, providing new insights into hydrogen partitioning in structural alloys and informing materials design for hydrogen-related applications.