Effect of structure and composition on the electronic excitation-induced amorphization of La₂Ti_2-xZr_xO₇ ceramics

An understanding of the response of ceramics (A₂B₂O₇) operating in extreme environments is of interest for a broad range of applications, including potential nuclear wasteforms and in-core electronics. Here, ab initio molecular dynamic simulations have been used to investigate the effect of structure and B-site (=Ti, Zr) cation composition of lanthanum-based oxides on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La₂Ti₂O₇ occurs for a lower degree of electronic excitation than for cubic pyrochlore La₂Zr₂O₇. While in each case the formation of O₂-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO₆ octahedra in the monoclinic perovskite phase facilitates the formation of O₂-like molecules, while such octahedral rotation is not possible in the cubic pyrochlore phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La₂TiZrO₇) is found to be more resistant in the monoclinic perovskite than in the cubic pyrochlore phase, which may be related to the lower bandgap of the cubic phase.