Aristotype

Inspection of the atomic positions reveals how the symmetry is being reduced step by step. In the aristotype usually all atoms are situated in special positions, i.e. they have positions on certain symmetry elements, fixed values of the coordinates, and specific site symmetries. From group to subgroup at least one of the following changes occurs for each atomic position ... 

The aristotype can be considered to be either the packing of spheres itself, or the NiAs type which corresponds to the packing in which all octahedral interstices are occupied by Ni atoms (Wyckoff position 2a). In the aristotype these interstices are symmetry equivalent subgroups result if the interstices are occupied only partially or by different kinds of atoms (or if the Ni atoms of NiAs are partially removed or substituted). By this procedure the sites of the interstices become non-equivalent. 

Collodio-Chloride Printing Out Paper Also known as Collodion Aristotype Paper... 

While all these stmctnres derive from the elpasolite aristotype, the Li3M F6 componnds (M = Al, Ga, Ti, V, Cr, Fe) exist at room temperatnre in two modifications. The a-form (e.g. a-LisAlFe ) is still a strongly distorted variant of the elpasolite/cryolite family in which the coordination of all lithium ions is reduced to six. The /3-form, determined at /S-LisVFe and confirmed by recent single-crystal work on the Al, Ga, Ti, and Cr compounds (Ref 82 and literatnre cited... 

Figure 13. Stracture of the cubic aristotype for perovskite compounds.

The idealised or aristotype perovskite stmcture is cubic and is adopted by SrTiOj at room temperature (but not at all temperatures). There are two general ways of listing the atoms in the cubic unit cell. The standard crystallographic description places the choice of origin at the Sr atom ... 

Table 1. List of structures derived from the rutile aristotype ordered by increasing values of a (see above figure). The coefficient f is a measure of the deformation of the octahedra.

Fig 7. Possible structures obtained from deformations of the perowskite aristotype by rotating the octahedra about the [111] direction. The patterns of the octahedra are characteristic of their respective height. 

Table 2. List of structures derived from the perowskite aristotype ordered by increasing values of(o.

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