COUPLED SOFT MODES AND DOUBLE TRIPLE POINTS IN FERROELECTRICS

P-T diagrams for ferroelectrica of the displacement type are construct ed based on the coupling of two or more soft modes of the lattice. The y are compared with experimental phase diagrams. Typical are in partic ular triple points and sequences of triple points. This one also finds in purely elastically instable systems, suggesting that the construct ion principle offered has actually a broader basis. Also, a generalisa tion of the method such as to include phase transitions of the first k ind is possible.