Dramatic Influence of A-Site Nonstoichiometry on the Electrical Conductivity and Conduction Mechanisms in the Perovskite Oxide Na_0.5Bi_0.5TiO₃

Abstract

Recently, there has been considerable interest in the perovskite phase Na 0.5 Bi 0.5 TiO 3 (NBT) as a promising lead-free piezoelectric material. Here we report low levels of Na nonstoichiometry (±2 atom % on the A-site) in the nominal starting composition of NBT ceramics can lead to dramatic changes in the magnitude of the bulk (grain) conductivity ( b ) and the conduction mechanism(s). Nominal starting compositions with Na-excess exhibit high levels of oxide-ion conduction with  b ~ 2.2 mScm -1 (at 600 °C) and an activation energy (E a ) < 1 eV whereas those with Na-deficiency are dielectrics based on intrinsic electronic conduction across the band gap with  b ~ 1.6 Scm -1 (at 600 °C) and E a ~ 1.7 eV.Drying of reagents especially Na 2 CO 3 changes the starting stoichiometry slightly due to small amount of adsorbed moisture in the raw materials but influences significantly the electrical properties. This demonstrates the bulk electrical properties of NBT to be highly sensitive to low levels of A-site nonstoichiometry and reveals how to fine-tune the nominal starting composition of NBT ceramics to suppress leakage conductivity for piezoelectric and high temperature capacitor applications.

Large pyroelectric properties at reduced depolarization temperature in A-site nonstoichiometry composition of lead-free 0.94Na x Bi y TiO3–0.06Ba z TiO3 ceramics

“…CN 6). The excess in the elements content will compensate the loss of elements due to evaporation at the calcining and sintering stages and may also substitute other cations in the composition [24]. For instance, the excess in the barium content in Composition D may compensate the deficiency in the Ba 2?…”

“…The relative permittivity (e r ) of all the compositions increases with temperature, which behaves like a typical ferroelectric material [31]. The relative permittivity (e r ) shows a significant increase with the increase in the x, y, and z contents, with values of 396, 597, 893, and 933, respectively, for the Compositions A to D at RT and 1 kHz; therefore, the small variation in start compositions can be responsible for the significant increase in the relative permittivity value [24].…”

“…Changing the stoichiometric state of the composition can also make a large effect on the depolarization temperature (T d ) of the N x B y T-0.06B z T compositions, where the Na 0.49 Bi 0.50 TiO 3 shows lower T d than Na 0.50 Bi 0.50 TiO 3 [24]. Although depolarization temperature is an important parameter in pyroelectric study, its origin has not been fully understood for NBT-xBT.…”

Effects of A-site nonstoichiometry on oxide ion conduction in 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 ceramics

“…A recent study by Li et al focused on the effect of drying the NaCO 3 starting reagent and found that the transport properties are sensitive to the control of stoichiometry. 29 In addition, a recent study by Li et al 30 explicitly showed that oxygen vacancies were responsible for the high leakage current in Bi-deficient BNT, and consequently revealed that BNT has great potential for oxide ion conductor applications.…”

Dramatic influence of Dy3+ doping on strain and domain structure in lead-free piezoelectric 0.935(Na1/2Bi1/2)TiO3−0.065BaTiO3 ceramics

“…C Recently, enhanced piezoelectricity and large electric-field (E-field) induced strains have been successfully found in the Na 1/2 Bi 1/2 TiO 3 (NBT)-based ferroelectrics, which are considered the most promising environmental friendly alternatives to toxic Pb-based counterparts. [1][2][3][4] Around the morphotropic phase boundary (MPB), the Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 (NBBT), Na 1/2 Bi 1/2 TiO 3 -K 1/2 Bi 1/2 TiO 3 -SrTiO 3 (NBT-KBT-ST), as well as Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 -K 1/2 Na 1/2 NbO 3 (NBT-BT-KNN) are nonpolar and cubic above the Curie temperature (T c ), yet become spontaneously polarized and mechanically distorted below T c . [5][6][7] Indeed, typical ferroelectrics consist of complex domain patterns with different polarization directions, which can be switched under sufficiently large E-field.…”