Kikuchi patterns are used for the orientation determination of diffracting crystals. For this purpose, the expected phases for the investigated material must be known.
However, what happens in case of unexpected phases?
Kikuchi pattern of triclinic arsenopyrite (FeAsS)
This phase is pseudo-monoclinic:
a=5.74Å, b= 5.67Å, c= 5.79Å, α=90°, β= 112.3°, γ=90°
This leads to the following negative scenarios:
- phases are overseen and falsely described by others,
- phases have to be considered subsequently, and – the worst case –
- phases are unknown and cannot be described at all.
Especially the third case is very disappointing, when you have high quality patterns but cannot assign them to a phase and orientation.
What is a phase? What does this term actually stand for?
A crystalline phase describes a volume with homogeneous chemical composition, in which the arrangement of atoms as well as the configuration of their environment is identical. This periodic structure is called the crystal structure of the phase.
The chemical compositions are usually easy to determine. A differentiation of crystal structures is more complicated. In such cases, one uses a trick:
Replace a difficult problem with one that is easier to solve!
The structurally diverse configuration using different elements leads to the fact that the periodicity of the structure, described as a purely mathematical translational lattice and visualized as points, is practically different for each phase.
Thus, one very likely does not need any structural data at all to recognize a phase.
For a successful separation of phases, the so-called lattice parameters are sufficient in almost all cases.
The probability that two different phases are chemically identical and also described by identical lattice parameters is practically zero.