In this paper, the effect of BiNbO 4 doping on the structural characteristics and dielectric properties were investigated systematically. With the increase in the BiNbO 4 content, a systematic structural characteristics change from ferroelectric tetragonal to cubic phase. Dielectric properties changed from classical ferroelectric behavior to dispersive relaxor-like behavior, further to linear behavior. Because of the created defect dipoles, the long-range dipolar interaction was interrupted and the weak couplings were formed. Energy storage density reached the maximum of 0.797 J/cm 3 with energy efficiency of 92.5% in 0.9BT-0.1BN. The nonlinearity was suppressed obviously, which could enhance the energy storage density in lead-free relaxor materials.capacitors, co-doping, dielectric properties, energy density, nonlinearity | INTRODUCTIONThe pulse power capacitors were used in many applications, such as laser weapons, high-power microwave systems and other power systems, 1-6 which needed to release all the stored energy in microseconds. High permittivity and high breakdown strength (BDS) were the key problems of enhancing the energy density. The BaTiO 3 -based ceramics had been studied for decades in the pulsed-power capacitors applications. was well known as the high permittivity-and temperaturestability. Nb was used to adjust the Curie temperature near the room temperature. Besides, BiNbO 4 was a good microwave dielectric material with low dielectric loss. 20,21 However, few people study the mechanism of simultaneous increase in dielectric constant and linearity.As we all know, larger polarization, smaller P r , relative low nonlinearity, higher energy efficiency, and higher BDS were the determining factors in lead-free relaxor materials for the energy storage applications. However, few paid attention to the dielectric properties beyond the solid solubility. In this paper, the properties of BaTiO 3 with Bi 2 O 3 and Nb 2 O 5 co-doped in a wide range were investigated systematically. The idea was to form weakly coupled relaxor polarization and adjust the Curie temperature with Bi 2 O 3 -Nb 2 O 5 co-doped for reducing the P r.9-11,13-15 And beyond the solid solubility, the formed second phase led to suppress the nonlinearity for improving the energy density. The mechanisms of maintaining high permittivity and suppressing the nonlinearity were investigated systematically.

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The structures of TaO6 and NbO6 oxygen octahedra in xBa(Mg1∕3Ta2∕3)O3+(1−x)Ba(Mg1∕3Nb2∕3)O3 perovskite ceramics with x=0, 0.25, 0.50, 0.75, and 1.0 were investigated by the extended x-ray absorption fine structure method. The decline in the microwave dielectric constant as x increases is caused mainly by the decrease of the mean volume of the oxygen octahedra, regardless of the 1:2 ordered structure and the distortion of the oxygen octahedron. The low Qf values of the TaO6 and NbO6 mixed samples are caused by not only the degrading of the 1:2 ordered structure but also the distortion of oxygen octahedral cages.

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