X-ray diffraction study of the polymorphism in Er2O3 driven by ball milling

Abstract

Polymorphic transformations in rare-earth sesquioxides (RE-SOs) are accessible via ball-milling. Prolonged mechanical grinding induces the formation of denser polymorphic structures, replicating the phenomena typically observed under high-pressure conditions. In this study, Er₂O₃ cubic phase with bixbyite structure (C-type) was subjected to different milling sessions with diverse vibrational milling frequencies. X-ray diffraction experiments combined with Williamson–Hall (W–H) and Rietveld analysis were adopted to study the microstructural aspects encompassing crystal size, microstrain, and unit cell distortion. As the frequency increased, the transition from the Er₂O₃ cubic to the monoclinic denser structure (B-type) was partially activated. When the milling frequency reached the critical condition, the polymorphic transformation was completely realized. The analysis of the microstructural changes triggered by high frequency milling discloses the key-role of the microstrain, rather than the reduction of the crystal size, in driving the structural transition from cubic to monoclinic polymorphs. Furthermore, ball milling conducted at the highest frequency promotes, rather than amorphization, the formation of the polymorphic form exhibiting a fluorite (CaF₂)-type structure, indicating the presence of point lattice defects involving oxygen vacancies. As a result, the structural analysis revealed the distinct role of microstructural features in promoting polymorphism within RE-SOs.