TOPOLOGY AND GEOMETRY IN THE IRRADIATION-INDUCED AMORPHIZATION OF INSULATORS

The irradiation-induced loss of crystallinity - the metamict transformation, or more loosely amorphization - is a common response of many insulating solids to accumulated atomic displacements on the order of 1 dpa. Readily-amorphizable solids are typically more complex ionic compounds or covalently-bonded solids in which lattice energy is less critically influenced by atomic correlations more remote than near-neighbor, and in which the structural alternatives for atomic rearrangement, which may thus differ little in energy, become an important factor in the ease of amorphization. Topology is the parameter most commonly invoked in considering the possible alternative arrangements, one aspect of which concerns connectivity of structural units such as cation coordination polyhedra. Connectivity considerations are shown to provide a criterion for predicting ease of amorphization at least as reliable, and more mechanistically significant, than the more usual criteria of melting point, homologous crystallization temperature or ionicity. The range of structural possibilities is more properly enumerated by combinatorial geometry; the role played by combinatorial geometric parameters in aperiodic structures is most easily appreciated for network solids, in which covalently-bonded coordination polyhedra are connected to each other in modes which do not overconstrain the structural freedoms available. An illustration is drawn from fully-connected silica networks whose structures, whether periodic or aperiodic, can be uniquely described by a characteristic unit of local structure (the local cluster) based on one-dimensional (ring) connectivity directly related to density. Energy-filtered electron diffraction is shown to be a useful experimental tool for characterizing amorphized structure and to provide information on atomic structural arrangements inaccessible by high-resolution imaging techniques. Use of this technique has demonstrated that aperiodic structures derived from quartz amorphized by electron-, neutron- and ion-irradiation all differ; that the aperiodic silica structures derived from electron-induced amorphization of cristobalite and tridymite are similar, but differ from that derived from quartz; and that the structures of electron-amorphized silicas are similar to that of silica glass.