Tilt boundaries, atomic structures

The structural unit model has been used successfully to predict the structures of grain boundaries in perovskite structured SrTiOs bicrystals [11.31-11.34]. The structural units observed for symmetric SrTiOs [001] tilt boundaries are shown in Fig. 11.8. In a similar manner to the isolated dislocation cores in YBCO, the structural units also appear to contain atomic positions where the cations are too close together. Again, depending on the structural unit, the close separation of the atomic columns can occur for either of the sub-lattice sites,... 

Matsunaga, K., Nishimura, H., Saito, T., Yamamoto, T, and Ikuhara, Y. (2003) High-resolution transmission electron microscopy and computational analyses of atomic structures of [0001] symmetric tilt grain boundaries of AI2O3 with equivalent grain-boundary planes. Philos. Mag., 83 (36), 4071 082. 

Oba, F., Ohta, H., Sato, Y, Hosono, H., Yamamoto, T, and Ikuhara, Y (2004) Atomic structure of [OOOlj-tilt grain boundaries in ZnO A high-resolution TEM study of fiber- textured thin films. Phys. Rev. B, 70, 125415-125420. 

A further insight in this rather simple nanostructure was given by a numeric approach based on the Embedded Atom Method this new route was developed to model the structure of grain boundaries in metallic nanostructured powders. Numeric aspects are detailed in [160]. As it is illustrated in Fig. 4.20, realistic atomic structures of twisted and tilted double and triple grain boundaries as a function of the relative disorientation of the grains were successfully modelled by... 

An alternative, but to some extent complementaty approach to the structure of grain boundaries notes that as the tilt angle between the crystals forming the grain boundary increases, planes of lower atomic concentrations, the high index planes, such as (221), (331) and (115) in the face-centred strucmre, become parallel to the grain boundary. There is therefore a decrease in the number of metal-metal bonds at the boundary as the tilt angle increases. 

Recent investigations of 45°/[001] tilt grain boundaries have found considerably higher values than had previously been reported for grain boundaries with similar macroscopic geometry (see Fig. 10.1). The former were produced by the sputter-induced epitaxy modification technique and were extremely well structured atomically (see Fig. 10.4(d)). Thus, the particular processing condi-... 

Fig. 11.8. The structural units for symmetric SrTi03 [001] tilt grain boundaries. Each of the units contains a reconstructed atomic column projecting as two closely spaced sites, either a Ti—O column or a Sr column. Asymmetric [001] tilt grain boundaries use only the units with the Ti—O reconstruction.

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