This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

The structure of sodium borate glass was investigated by molecular dynamics simulation using coordination dependent potential model. The simulated alkali borate glass consists of basic units, BO3, triangle, BO4 tetrahedra and structural groups such as boroxol ring and triborate units. The coordination of boron is converted from 3 to 4 by adding alkali oxide.

Simulation of soda-lime-silicate (sis) glass has been performed using molecular dynamics (MD). The local structure of each component is analyzed extensively in terms of total radial distribution function and coordination number and found to be insensitive to the composition change. Because of its big size, Na+/Ca2+ shows a behavior rather like O2− instead of Si4+. It is evident that the CN and local structure of Na+ with O2− are similar to those in crystalline a-Na2Si2O5. Finally, the Na+/Ca2+ cluster phenomenon is discussed.

Structure of β"-alumina has been studied through atomistic simulation. Distribution of Mg2+ in the spinel blocks, Na+ as well as Nd3+ in the conduction planes have been calculated and compared with the structure models derived experimentally. Mg2+ ions, as the stabilizer of β"-alumina structure, prefer to occupy the tetrahedral Al(2) sites in the spinel block. Two configurations for Mg2+ distribution are proposed. Due to the presence of Mg2+, oxygen ions in or adjacent to the conduction plane, O(5), O(4) and O(3), are slightly displaced from their original sites, leading to the removal of the local symmetry at the mO site. Strongly affected by the Mg2+ distribution, Nd3+ions, originally introduced into the BR site in the conduction plane, are found to move towards the distorted mid-oxygen sites, which is in excellent agreement with the previous estimate from spectroscopie study.

This paper extends the concepts that were developed to explain the structural rearrangement of the wurtzite AlN lattice due to incorporation of small amounts of oxygen, and to directly use them to assist in understanding the polytypoid structures. Conventional and high-resolution transmission electron microscopy, specific electron diffraction experiments, and atomistic computer simulations have been used to investigate the structural nature of the polytypoids. The experimental observations provide compelling evidence that polytypoid structures are not arrays of stacking faults, but are rather arrays of inversion domain boundaries (IDB's). A new model for the polytypoid structure is proposed with the basic repeat structural unit consisting of a planar IDB-P and a corrugated IDB. This model shares common structural elements with the model proposed by Thompson, even though in his model the polytypoids were described as consisting of stacking faults. Small additions (≃ 1000 ppm) of silicon were observed to have a dramatic effect on the polytypoid structure. First, it appears that the addition of Si causes the creation of a new variant of the planar IDB (termed IDB-P'), different from the IDB-P defect observed in the AlN-Al2O3 polytypoids; second, the addition of Si influences the structure of the corrugated IDB, such that it appears to become planar.

The model proposed by Harris et al. [J. Mater. Res. 5, 1763–1773 (1990)], describing planar inversion domain boundaries in aluminum nitride, consists of a basal plane of aluminum atoms octahedrally coordinated with respect to oxygen, and with a translation of R = 1/3〈1011〉. This thin sandwich is inserted onto the basal plane of the wurtzite structure of aluminum nitride. This model does not take into consideration any interfacial relaxation phenomena, and is arguably electrically unstable. Therefore, this paper presents a refinement of the model of Harris et al., by incorporating the structural relaxations arising from modifications in local chemistry. The interfacial structure was investigated through the use of conventional transmission electron microscopy, convergent electron diffraction, high resolution transmission electron microscopy, analytical electron microscopy, and atomistic computer simulations. The refined planar inversion domain boundary model is closely based on the original model of Harris et al.; however, the local chemistry is changed, with every fourth oxygen being replaced by a nitrogen. Atomistic computer simulation of these defects, using a classical Born model of ionic solids, verified the stability of these defects as arising from the adjustment in the local chemistry. The resulting structural relaxations take the form of a 0.3 mrad twist parallel to the interface, a contraction of the basal planes adjacent to the planar inversion domain boundary, and an expansion of the c-axis component of the displacement vector; the new displacement vector across the interface is R = 1.3〈1010〉 + ∊〈0001〉, where ∊meas = 0.387 and ∊calc = 0.394.

Three distinct morphologies of curved (curved, facetted, and corrugated) inversion domain boundaries (IDB's), observed in aluminum nitride, have been investigated using conventional transmission electron microscopy, convergent beam electron diffraction, high-resolution transmission electron microscopy, analytical electron microscopy, and atomistic computer simulations. The interfacial structure and chemistry of the curved and facetted defects have been studied, and based upon the experimental evidence, a single model has been proposed for the curved IDB which is consistent with all three observed morphologies. The interface model comprises a continuous nitrogen sublattice, with the aluminum sublattice being displaced across a {1011} plane, and having a displacement vector R = 0.23〈0001〉. This displacement translates the aluminum sublattice from upwardly pointing to downwardly pointing tetrahedral sites, or vice versa, in the wurtzite structure. The measured value of the displacement vector is between 0.05〈0001〉 and 0.43〈0001〉; the variation is believed to be due to local changes in chemistry. This is supported by atomistic calculations which indicate that the interface is most stable when both aluminum vacancies and oxygen ions are present at the interface, and that the interface energy is independent of displacement vector in the range of 0.05〈0001〉 to 0.35〈0001〉. The curved IDB's form as a result of nonstoichiometry within the crystal. The choice of curved IDB morphology is believed to be controlled by local changes in chemistry, nonstoichiometry at the interface, and proximity to other planar IDB's (the last reason is explained in Part III). A number of possible formation mechanisms are discussed for both planar and curved IDB's. The Burgers vector for the dislocation present at the intersection of the planar and curved IDB's was determined to be b = 1/3〈1010〉 + t〈0001〉, where tmeas = 0.157 and tcalc = 0.164.

Computer-based atomistic simulation techniques have been used to investigate the crystal chemistry and phase relationships in alkaline earth hexa-aluminates. The lattice energies for hexa-aluminate-related structural models are calculated and successfully predict that barium prefers the β-alumina-type structures with charge-balancing defects, whilst calcium or strontium prefers the ideal magnetoplumbite structure. In the case of magnesium, it is predicted that it does not prefer a hexa-aluminate structure.

Atomistic computer simulation is performed to investigate the extent and nature of nonstoichiometry in La2CuO4, and La2NiO4. Results suggest anion Frenkel to be the dominant intrinsic defect in both compounds. Spontaneous oxidation is predicted for La2NiO4 but not for La2CuO4; however, both compounds are more easily oxidised than reduced, as opposed to Nd2CuO4, a superconducting compound with same generic formula. Oxidation takes place via accommodation of oxygen interstitials; however, the possibility of metal vacancies can not be completely ignored in La2CuO4. There is only one favorable oxygen interstitial site which is same in both the compounds La2MO4 (M=Cu, Ni). We found a favorable oxygen interstitial site in T′ structure too. Relaxed structures around an oxygen interstitial in these compounds are reported.

We describe computer-based atomistic simulation studies of defect behaviour in oxides. In illustrating the value of the technique, contact with experiment is discussed through comparison with measured energies of ion migration and other energies. Since the success of these methods is dependent on the extent to which the interatomic potentials used provide a realistic model of the material, consideration is given to transferability of potentials, especially to tenary compounds where the widely adopted practice of fitting potential parameters to available crystal data is not feasible. Our principal examples will include ionic conductivity in gadolinium zirconate, electronic defect properties of BaTiO3 and grossly nonstoichiometric rutile.