Order-disorder phase transformation in ion-irradiated rare earth sesquioxides

An order-to-disorder (OD) transformation induced by ion irradiation in rare earth (RE) sesquioxides, $Dy2O3$⁠, $Er2O3$⁠, and $Lu2O3$⁠, was studied using grazing incidence x-ray diffraction and transmission electron microscopy. These sesquioxides are characterized by a cubic $C$-type RE structure known as bixbyite. They were irradiated with heavy $Kr++$ ions $(300keV)$ and light $Ne+$ ions $(150keV)$ at cryogenic temperature $(∼120K)$⁠. In each oxide, following a relatively low ion irradiation dose of $∼2.5$ displacements per atom, the ordered bixbyite structure was transformed to a disordered, anion-deficient fluorite structure. This OD transformation was found in all three RE sesquioxides (⁠$RE=Dy$⁠, Er, and Lu) regardless of the ion type used in the irradiation. The authors discuss the irradiation-induced OD transformation process in terms of anion disordering, i.e., destruction of the oxygen order associated with the bixbyite structure.