Response of Lanthanide Sesquioxides to High-Energy Ball Milling

Sesquioxides (M₂O₃) exhibit rich polymorphism with distinct phases that form over broad compositional, pressure, and temperature ranges. This makes these materials an ideal model system for studying the effects of high-energy ball milling and the far-from-equilibrium conditions induced by complex mechanical interactions. Polycrystalline bixbyite-structured binary sesquioxides (M₂O₃, M = Gd, Dy, Ho, Er, Yb, and Y) were processed by high-energy ball milling and the resulting structural modifications were characterized by synchrotron X-ray diffraction. Ball milling drives the initial cubic structure (“C-type”) in each oxide to the monoclinic, “B-type” structure, with the rate of formation and maximum attainable phase fraction dependent on the cation size. The B-type phase fraction increases with milling time for each sesquioxide, but reaches steady-state behavior below unity, which contrasts with previous studies that induced a complete transformation by exposure to temperature, pressure, or ion radiation. This behavior suggests a complex interaction regime within a planetary ball mill characterized by transient processes, which exert simultaneous 1) driving forces to form the B-type phase and 2) kinetic pathways to partially recover the C-type phase. We show that these two processes are correlated with the effects of pressure and temperature during mechanical interactions between the sample and milling tools.