Large lattice parameters

The character of the pattern does not alter when the angle of inclination of the specimen to the beam is changed. Thus the geometry of a polycrystalline pattern is a set of lengths, Hhki, i.e. set of inter-planar distances dhki characteristic of the crystal lattice. Polycrystal-type electron diffraction patterns provided a complete three dimensional set of diffraction reflections, however, two or more reflections, with different hkl can overlap in one ring of the pattern, especially in the cases where the material studied has large lattice parameters. 

A striking feature of the YB66 cubic structure is that the unit cell contains more than 1600 atoms due to the large lattice parameter. Related to this, a novel application for YB66 was discovered and will be described in the next section. 

A cubic phase of space group Fdim is usually observed in type II systems based on a variety of hydrated lipid mixtures [163,166]. A type I Fdim phase with a very large lattice parameter has been claimed to form in a brine-butanol-SDS-toluene system [167]. The unit cell of the Fdim phase contains two quasi-spherical, differently sized types of inverse micelles [160]. There are eight of the larger and 16 of the smaller micelles per unit cell. 

The micrograph shown in Fig. 1(a) was taken in 1998 in the framework of a transmission electron microscopy (TEM) study on the S28-phase in the system Al-Pd-Mn [1]. The 28-phase has very large lattice parameters and contains about 1500 atoms in its unit cell. According to the high degree of complexity of the structure, the material is referred to as a complex metallic alloy (CMA). 

Defects as shown in Fig. 1(a) are the subject of the present chapter. They are referred to as metadislocations, and occur in numerous structurally complex metallic materials. The concept of metadislocations addresses a central problem in the plasticity of materials with large lattice parameters In these materials, conventional dislocation-based deformation mechanisms are prone to failure. This is a direct consequence of the elastic strain energy, which, per unit length of dislocation, is given by... 

In recent years, CMAs emerged as a novel field in materials science [8]. These materials are intermetallic phases, which possess a highly complex atomic structure. The most prominent feature of CMA phases is that they have large lattice parameters, typically ranging from one up to tens of nanometers. With the advent of this research field, new interest in defects and plasticity of large lattice parameter materials emerged. Indeed, deformation processes involving novel mechanisms and defect types were discovered in CMAs. 

As for the Tc saturation in A3C60 with large lattice parameters, it has been repeatedly pointed out that electron correlation effects cannot be ignored in systems with very narrow bands. Such effects become progressively more important as the inter-fullerene spacing increases. As a result, localization of the conduction electrons should occur at a certain lattice constant, being accompanied by metal to Mott-insulator and to molecular semiconductor/insulator transitions [85,131], and Tc will saturate. 

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