Phonons and Relaxations in Unfilled Tetragonal Tungsten-Bronzes

The lead-free unfilled tetragonal tungsten-bronzes SBN and CBN are investigated by Raman, infrared and high-frequency dielectric spectroscopies. The substitution in the different channels affects phonons as well as relaxations. Relaxations in SBN show similar qualitative behaviour on increasing Sr content and relaxor behaviour, but for the extreme relaxor samples all characteristic frequencies are higher with stronger contribution to the permittivity in the GHz-THz range is stronger. The presence of a soft anharmonic central mode in the THz range together with the slowing down of a relaxation from GHz to MHz ranges reveals the coexistence of displacive and order-disorder scenarios for the ferroelectric phase transition in this family