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.