  A novel lead-free tungsten bronze type ceramic Sr5BiTi3Nb7O30, was prepared
by a conventional solid-state reaction route. The room-temperature crystal
structure shows an average structure with centro-symmetric space group P4/mbm
identified by synchrotron XRD. Temperature dependence of dielectric
permittivity indicates that Sr5BiTi3Nb7O30 is a ferroelectric relaxor with Tm
near 260 K. The ceramic displays stronger frequency dispersion and lower
phase-transition temperature compared with Sr6Ti2Nb8O30. A macroscopic and
phenomenological statistical model was employed to describe the temperature
dependence of their dielectric responses. The calculated size of polar
nanoregions (PNRs) of Sr5BiTi3Nb7O30 compared with Sr6Ti2Nb8O30 implies that
the stronger diffusion phase transition for the former is related to the
disorder emerged in both A and B sites. The smaller PNRs can be activated at
lower temperature but have smaller electrical dipole moment. This is the origin
of relaxor behavior of Sr5BiTi3Nb7O30 with lower Tm and dielectric
permittivity. The PNRs is related to a local structure with a polar space group
P4bm, which contributes to the dielectric frequency dispersion of relaxor
behavior. This work opens up a promising feasible route to the development of
relaxor ferroelectrics in tungsten bronze type oxides.
