Defect Engineering and Hydrogen Bronze Formation in α‐MoO ₃ Crystals by Hydrogen Implantation

ABSTRACT Ion implantation is introduced as a novel and controllable approach to incorporate hydrogen into α‐MoO ₃ crystals and induce the formation of hydrogen molybdenum bronze phases (H _x MoO ₃ ) below the implanted layer. By tuning the ion fluence, this technique enables precise defect engineering and phase transformation, offering a versatile and reproducible strategy for tailoring the material functional properties. High‐resolution X‐ray diffraction (HRXRD) reveals a gradual expansion of the b lattice parameter for fluences up to 1 × 10 ¹⁷ cm ⁻² , attributed to defect‐induced lattice distortion. For higher fluences, instead of typical strain saturation, a new diffraction peak emerges, revealing the formation of type‐I H _x MoO ₃ , as also confirmed by Raman spectroscopy. This phase forms below the implanted layer and extends up to ∼1 µm into the material, as confirmed by transmission electron microscopy (TEM). This phase transformation demonstrates reversibility, with the H _x MoO ₃ signature disappearing upon air annealing at 300°C. HRXRD curve fitting enables assessment of strain/damage profiles evolution with fluence, providing insight into the defect creation and accumulation mechanisms. These structural and compositional changes are accompanied by a quasi‐linear increase of electrical conductivity with fluence, by several orders of magnitude, attributed to the presence of H _x MoO ₃ phases, extended defects, and new suboxide minority phases.