A-site disorder induced ferroelectric to relaxor phase transition of Na(Nb0.75Ti0.25)O3 ceramics by Bi3+ for ultra-high energy storage properties

Abstract

The present work introduces a new insight into effect of A-site disorder to develop the energy storage properties of Na(Nb0.75Ti0.25)O3 ceramic. Lead-free (Na1-xBix)(Nb0.75Ti0.25)O3 (x = 0.0, 0.05 and 0.1) (abbreviated as NNTBx) ceramics were synthesized using solid-state reaction technique. The introduction of Bi3+ shown a significant effect into the crystal structure, dielectric, ferroelectric and energy storage properties of NNT ceramics. The addition of Bi2O3 induced ferroelectric to relaxor phase transition with psedu-cubic crystal structure at 0.1. Significant inhibits of grain size to sub-micrometer has been achieved at 0.1 sample due to lower ionic radius of Bi+3 (1.34 Å, CN=12) to the ionic radius of Na1+ (1.39 Å, CN=12). Meanwhile, the dielectric constant increased from 300 to 1500 when the Bi3+-content increased from 0.0 to 0.1. Effectively increasing in maximum polarization (Pmax) and significant reducing in remnant polarization (Pr) resulting in increasing ΔP (Pmax – Pr) has been obtained at 0.1 content of Bi2O3. This enhancement is attributed to hybridization between Bi3+ 6Pand O2− 2P instead of Na1+ 2S and O2− 3S orbitals. On the other hand, the substitution of monovalent (Na1+) by trivalent (Bi3+) lead to create sodium vacancies into the A-sites of NNT lattice subsequently reducing the Pr value. The tolerance factor (τ) decreased from 0.955 to 0.953 when Bi2O3 increases from 0.0 to 0.1 indicate increasing the cations disorder, charge misfit and relaxor degree of NNT ceramics. The sample with 0.1 of Bi2O3 demonstrate remarkably high energy storage density (Wrec = 17.5 J/cm3), and high relatively energy storage efficiency (n = 80 %) at ultra-high dielectric breakdown strength (Eb = 700 kV/cm). Partially, (NNTB0.1) exhibit an outstanding stability of energy storage properties in terms of temperature (25 to 150 °C) and frequency (2–50 Hz). The variation of Wrec and n were observed about 1 % within the whole range of applied temperature and frequency. These results imply the high potential of (Na0.9Bi0.1)(Nb0.75Ti0.25)O3 ceramic in temperature-stable advanced pulsed power capacitor applications.