Amorphization of Complex Ceramics by Heavy-Particle Irradiations

Abstract

Abstract“Complex” ceramics, for the purpose of this paper, include materials that are generally strongly bonded (mixed ionic and covalent), refractory and frequently good insulators. They are distinguished from simple, compact ceramics (e.g., MgO and UO2) by structural features which include: 1.) open network structures, best characterized by a consideration of the shape, size and connectivity of coordination polyhedra; 2.) generally, complex compositions which characteristically lead to multiple cation sites and lower symmetry; 3.) directional bonding; 4.) bond-type variations, from bond-to-bond, within the structure. The heavy particle irradiations not only include ion-beam irradiations, but also recoil-nucleus damage resulting from a-decay events from constituent actinides. The latter effects are responsible for the radiation-induced transformation to the metamict state in minerals. The responses of these materials to irradiation are complex, as energy may be dissipated ballistically by transfer of kinetic energy from an incident projectile or radiolytically by conversion of radiation-induced electronic excitations into atomic motion. This results in isolated Frenkel defect pairs, defect aggregates, isolated collision cascades or bulk amorphization; all may occur concurrently. Thus, the amorphization process is heterogeneous. Only recently have there been systematic studies of heavy particle irradiations of “complex” ceramics on a wide variety of structure-types and compositions as a function of dose and temperature. In this paper, we review the conditions for amorphization for the tetragonal orthosilicate, zircon [ZrSiO4]; the hexagonal orthosilicate/phosphate apatite structure-type [X10(ZO4)6(F,Cl,O)2]; the isometric pyrochlores [A1-2B2O6(O,OH,F)o-1pH2O] and its monoclinic derivative zirconolite [CaZrTi2O7]; the olivine (derivative - hcp) structure types, α-VIA2IVBO4, and spinet (ccp,) γ-VIA2IVBO4.