Swift-heavy ion irradiation response and annealing behavior of A(2)TiO(5) (A = Nd, Gd, and Yb)

The structural responses of A(2)BO(5) (A = Nd, Gd, and Yb; B = Ti) compositions irradiated by high-energy Au ions (2.2 GeV) were investigated using transmission electron microscopy, synchrotron X-ray diffraction and Raman spectroscopy. The extent of irradiation-induced amorphization depends on the size of the A-site cation, with smaller lanthanides having less susceptibility to the accumulation of radiation damage. In the track-overlapping regime, complete amorphization is observed in all three compounds, despite the ability of Yb2TiO5 to incorporate a great deal of structural disorder into its initial defect-fluorite structure (Fm-3m). This is attributed to the high cation radius ratio (A:B = 2:1), which reduces the stability of the structure upon ion irradiation. The fully-amorphized samples were subsequently isochronally heated at temperature intervals from 100 degrees C to 850 degrees C. X-ray diffraction analysis indicated a similar damage recovery process in Nd2TiO5 and Gd2TiO5, where both compositions recover their original structures (Pnma) at 850 degrees C. In contrast, Yb2TiO5 exhibited recrystallization of a metastable, non-equilibrium orthorhombic phase at similar to 550 degrees C, prior to a transformation to the stable defect-fluorite phase (Fm-3m) at 625 degrees C. These compositional variations in radiation tolerance and thermal recovery processes are described in terms of the energetics of disordering during the damage and recrystallization processes.