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. 
Yaovi Gagou (LPMC), Mimoun Elmarssi (LPMC), Marie Ang\`ele Fr\'emy   (L2MP), Nadir Aliouane (HMI), Daoud Mezzane (F.S.T.G.),...