Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu₃ S₄ and EuSm₂ S₄ . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu₃ S₄ , EuSm₂ S₄ , SmS_1.9 , and Sm₃ S₄ . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl₂ formed EuS while EuI₂ formed Eu₃ S₄ , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI₂ initially forms EuS, and converts over time to Eu₃ S₄ .