Nanocrystalline Photochromic LaHO Films with Rapid Optical Recovery for Passive Light-Control Coatings

Over the past decade, oxygen-containing rare-earth hydrides (REHOs) have emerged as promising inorganic photochromic materials for passive light-control coatings. However, the photochromic behavior of oxygen-containing lanthanum hydride (LaHO) has remained unexplored. Here, we report the synthesis of LaHO thin films by reactive magnetron sputtering, followed by controlled oxidation and encapsulation with aluminum nitride and oxide layers to improve their stability. The resulting transparent films (average transmittance of 68–72% between 500 and 700 nm) exhibited reversible photodarkening with a relative contrast of 6–17% after 10 min of illumination under a solar simulator and fast bleaching characterized by effective time constants τb,eff of 0.6–1.4 min. Structural and compositional analyses revealed textured polycrystalline films of the cubic LaHO phase (a = 5.65–5.76 Å) with an average crystallite size of ∼12 nm, indicative of a nanocrystalline microstructure, and an oxygen-to-lanthanum atomic ratio of ∼1, as confirmed by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. This study demonstrates photochromism in LaHO, introducing a member of the photochromic REHO family, while highlighting the necessity of effective encapsulation to ensure environmental stability for light-modulating coating applications.