Critical role of lattice vacancies in A1-site induced by Ca2+-doping on relaxor behavior and electric properties in tungsten bronze Ba2NaNb5O15 ferroelectrics

Abstract

Constructing relaxor ferroelectric for tetragonal tungsten bronze (TTB) compounds to server the urgent demand of energy storage for an high-power discharge capacitors is important, but the huge challenge of achieving significant breakthrough still lies in the unclear nature of relaxation compared with perovskite structure. In present work, the TTB-type Ba2CaxNa1-2xNb5O15 (BCNN, 0.1 ≤ x ≤ 0.5) ceramics were thoroughly investigated the variation of dielectric and ferroelectric properties accompanying with A1-site crystal vacancies induced by introducing Ca2+. With increasing of Ca2+ concentration, the distortion of NbO6 polar unit was alleviated due to the lattice shrinks arising from the higher lattice vacancies in A1 sites and smaller radius of Ca2+, as confirmed by XRD and Raman spectra. Analyses of I–V characteristics for BCNN ceramics indicate that conductive behavior is dominated by the space charge limited conduction, which associated with the oxygen vacancies arising from the evaporate of sodium element during high-temperature sintering. It is found that the space charge plays a crucial role in the unique dielectric and ferroelectric characteristics. Besides the typical phase transitions from ferroelectric (4 mm) to paraelectric (4/mmm), low-temperature dielectric anomaly derived from the freeze-out of polarizability caused by distorted polar octahedron. The apparent dielectric dispersion and extremely higher dielectric constant at lower frequencies arise from the pace charge polarization confirmed the rationality of the heterostructure by electric impedance spectroscopy (EIS). The ferroelectricity is associated with the insulation leakage from the space charge, besides the ferroelectric domain switching, that further confirmed by the conduction mechanism. Moreover, the sintered BCNN ceramics displayed the homogeneous equiaxed grain together with uniform distribution, which endows an excellent optical transmittance of BCNN ceramics at x = 0.5 and far exceeds that of iso-structural SCNN ceramics. The above results not only confirm the critical role of modulation in A1-site for constructing relaxor, but also delivers a new path into the design dielectric materials with ultrahigh transparent performance.