Amorphization of complex ceramics by heavy-particle irradiations

摘要

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 UO(sub 2)) by structural features which include: (1) open network structures, best characterized by a consideration of the shape, size and connectivity of coordination polyhedra; (2) complex compositions which characteristically lead to multiple cation sites and lower symmetry; (3) directional bonding; (4) bond-type variations within the structure. The heavy particle irradiations include ion-beam irradiations and 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. 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 (ZrSiO(sub 4)); the hexagonal orthosilicate/phosphate apatite structure-type (X(sub 10)(ZO(sub 4))(sub 6)(F,Cl,O)(sub 2)); the isometric pyrochlores (A(sub 1-2)B(sub 2)O(sub 6)(O,OH,F)(sub 0-1p)H(sub 2)O) and its monoclinic derivative zirconotite (CaZrTi(sub 2)O(sub 7)); the olivine (derivative - hcp) structure types, (alpha)-(sup VI)A(sub 2)(sup IV)BO(sub 4), and spinel (ccp), (gamma)-(sup VI)A(sub 2)(sup IV)BO(sub 4).