Defect Engineering and Hydrogen Bronze Formation in α-MoO₃ Crystals by Hydrogen Implantation

Ion implantation is introduced as a novel and controllable approach to incorporate hydrogen into α-MoO₃ crystals and induce the formation of hydrogen molybdenum bronze phases (H_xMoO₃) 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 × 10¹⁷ 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_xMoO₃, 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_xMoO₃ 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_xMoO₃ phases, extended defects, and new suboxide minority phases.