  Understanding the response of ceramics operating in extreme environments is
of interest for a variety of applications. 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 (La$_2$$B_2$O$_7$) on
electronic-excitation-induced amorphization. We find that the amorphous
transition in monoclinic layered perovskite La$_2$Ti$_2$O$_7$ occurs for a
lower degree of electronic excitation than for cubic pyrochlore
La$_2$Zr$_2$O$_7$. While in each case the formation of O$_2$-like molecules
drives the structure to an amorphous state, an analysis of the polyhedral
connection network reveals that the rotation of TiO$_6$ octahedra in the
monoclinic phase can promote such molecule formation, while such octahedral
rotation is not possible in the cubic 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 (La$_2$TiZrO$_7$) is
found to be more resistant in the monoclinic than in the cubic phase, which may
be related to the lower bandgap of the cubic phase. These results illustrate
the complex interplay of structure and composition that give rise to the
radiation resistance of these important functional materials.
