Relaxor Nature and Energy Storage Properties of Sr2-xMxNaNb5-xTixO15 (M = La3+ and Ho3+) Tungsten Bronze Ceramics

A series of lead- and bismuth-free tetragonal tungsten bronze ceramics with chemical formula given as Sr(2-x)M(x)NaNb(5-x)TixO(15) (M = La3+ and Ho3+) were prepared. Through codoping of A- and B-sites, switching from ferroelectric to relaxor at room temperature was achieved, and different kinds of ion doping showed different degrees of polarity loss and relaxor behavior. The mechanism of polarity loss and relaxor behavior was investigated. The relaxor behavior exhibited by current systems does not seem to satisfy the previous crystal chemistry framework and structural modulation theory. The results of XRD refinements and polar response show that the relaxation of different intensities can be reasonably explained by the degree of orthogonal distortion and the change of tetragonality. The final results demonstrate that the relaxor nature of the current systems is determined by the disorder of charge and the structural disorder, and the latter is the dominant factor. Finally, energy storage density of 1.7 J/cm(3) and energy storage efficiency of 89% were obtained in the Sr1.88Ho0.12NaNb4.88Ti0.12O15. The charging-discharging performance is also evaluated, and the discharge energy storage density of 1.42 J/cm(3) and power density of 54.33 MW/cm(3) are obtained.