Abstract
AbstractRaman scattering measurements are made in the tetragonal phase of KNbO3. The results are in good agreement with the previous corresponding neutron data, and can be assigned to symmetry species. Comparison of these results to those available in the other phases enables to observe the effect of the phase transformations on the LO–TO splitting and on the degeneracy lifting of each initial cubic mode. It is shown that the lowest frequency F1u cubic mode is the most affected one by the different cell distortions; at each transition, it splits into “soft” and “stiffened” components; it is the so‐called “ferroelectric” mode which drives the successive phase changes. On the other hand, it is remarkable that the phonons derived from the F2u “silent” mode are nearly degenerate, even in the ferroelectric phases; it is also the case for the phonons issued from the second F1u cubic mode. Finally, it is emphasized that the phase transitions in KNbO3 can be described by a displacive‐type mechanism.
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