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RESEARCH PRODUCT

Mechanisms at structural phase transitions

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Abstract Structural phase transitions in crystalline solids are accompanied with the change of symmetry. After EHRENFEST [Ehrenfest, P., Phasenumwandlungen im ublichen und erweiterten Sinn, klassifiziert nach den entsprechenden Singularitaten des thermodynamischen Potentials. Proc. Kon., Amsterdam Acad., 1933, 36(Suppl 75b), 153–157] they can be distinguished between first and second order transitions. Two examples for first and second order transitions will be discussed in order two elaborate the characteristic features. The statements are verified by diffraction experiments; the second order transitions will be discussed on the basis of the LANDAU [Landau, L. and Lifshitz, E.M., Lehrbuch der Theoretischen Physik Band 3, Quantenmechanik. Akademie Verlag, Berlin, 1986] theory. The first example is the superstructure of Bi4Ti4O11, and the second example describes the sequence of modulated and non-modulated phases in melilite compounds e.g. Ca2CoSi2O7. The sequence of phases in this compound exhibits both first and second order transitions. For both examples it will be shown, to what extent the characteristic criteria of first or second order transitions are valid. It will be pointed out that close to the transition point of second order transitions pronounced deviations are observed. From these features new concepts for the mechanism of the transition can be concluded. Precursors of the low symmetry structure are formed beyond the transition point in the high symmetry phase. For these precursors there is a continuous change from short range order to long range order, which is attained at the transition point. If the precursor domains adopt long range order an intermediate, incommensurately modulated structure may occur between the high symmetric structure and the structure/superstructure of low symmetry.