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