Abstract
The main distortion taking place in a structural phase transition is a result of a freezing of the so-called primary mode. This corresponds to the degree of freedom of the system that becomes unstable at the phase transition. However, secondary modes are also triggered at the phase transition and can also have non-zero amplitudes in the distorted structure, so symmetry-mode analysis of the global distortion is necessary for determining the ‘weight’ of primary and secondary modes in a structural phase transition. The conventional scheme for this type of analysis is based on the representation theory of space groups. The aim of this contribution is to present a summary and some examples of an alternative method for the computation of primary and secondary modes necessary. It only requires the systematic use of International Tables of Crystallography. From these tables and for any space group the fully symmetric modes, i.e. those transforming according to the identity representation of the group can be derived for any orbit of atoms. This property, systematically used for all intermediate subgroups between the space groups of the two phases, results in a direct determination of the relevant symmetry-modes. The distinction of the symmetry-modes present in the primary mode from the rest comes out directly in the process of calculation.
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