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Mechanism of the Phase Change in PbK2LiNb5O15: Dielectric, structural, and Raman scattering studies

Experiments reveal that PbK2LiNb5O15 which belongs to the tetragonal tungsten bronze family presents paraelectric and ferroelectric phases and a complex structural change between them. High and low temperature phases are of symmetry P4/mbm and Pba2 respectively, so that this change is also of ferroelastic type. As presented here, crystallographic results hint at a displacive character of the ferroelectric ordering but show a more complex behaviour, with a clear order-disorder mechanism which accompanies the appearance of ferroelasticity. To complete our knowledge of this material, we have performed Raman experiments which exhibit a low frequency mode, but no clear soft mode is observed.