Crossover from normal to relaxor ferroelectric in Sr0.25Ba0.75(Nb1−xTax)2O6 ceramics with tungsten bronze structure

The crossover from normal to relaxor ferroelectrics in Sr0.25Ba0.75(Nb1−xTax)2O6 (x = 0, 0.1, 0.25, and 0.5) ceramics was determined together with the structure evolution. A single-phase tungsten bronze structure in space group P4bm (x = 0 and 0.1) or P4/mbm (x = 0.25 and 0.5) was obtained in the entire composition range. The lattice parameter a increased, while c, tetragonality c/a, and cell volume decreased with increasing x. Meanwhile, the dielectric maximum temperature Tm went down and the dielectric peak exhibited an enhanced diffused nature with a stronger frequency dispersion, where the saturated P–E loop (x = 0 and 0.1) transformed to a slim relaxor loop (x = 0.25) and finally to the linear one (x = 0.5). Raman spectra results revealed the reduced covalence between the B-site cation and the oxygen ion as a result of Ta-substitution. The coexistence of Nb5+ and Ta5+, which possessed the distinct covalence with the oxygen ion, played the key role in the structural variation and crossover from normal to relaxor ferroelectrics in the present system.