Ferroelectric Transition and Low-Temperature Dielectric Relaxations in Filled Tungsten Bronzes

A comprehensive review on the latest development of the ferroelectric transition and low-temperature dielectric relaxations of filled tungsten bronze ceramics are presented together with some new issues. In the filled tungsten bronze ceramics M6-pRpTi2+pNb8-pO30 (p=1, 2; M=Ba or Sr; R=rare earth or Bi), a ferroelectric transition is generally indicated together with up to three low-temperature dielectric relaxations. The ferroelectric transition is determined as 4/mmm 4mm, and the low-temperature dielectric relaxations are deeply concerned with the structure modulations due to the order/disorder of ions in A1 and A2 sites, their random cross occupancy, and the order/disorder of B-site ions. Both the ferroelectric transition and low-temperature dielectric relaxations are dominated by the composition and radius difference between A1- and A2-site ions, r. The normal ferroelectric transition might be expected if the ratio of the biggest ion and second big ion is 2:1, otherwise the diffuse or relaxor ferroelectric is expected. Meanwhile, the larger r generally results in the normal ferroelectric, and the smaller r will lead to the diffuse or relaxor ferroelectric. Moreover, the effects of A sites order/disorder and the random cross occupancy of A-site ions are primary, and the effects of B-site ordering/disordering are secondary. The right ratio of 2:1 for A2- and A1-site ions and the large r should be the guidelines for designing the possible multiferroic tungsten bronzes.