Article pubs.acs.org/cm
Dramatic Influence of A‑Site Nonstoichiometry on the Electrical Conductivity and Conduction Mechanisms in the Perovskite Oxide Na0.5Bi0.5TiO3 Ming Li,† Huairuo Zhang,† Stuart N. Cook,‡ Linhao Li,† John A. Kilner,‡,§ Ian M. Reaney,† and Derek C. Sinclair*,† †
Department of Materials Science and Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, United Kingdom ‡ Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdom § International Institute for Carbon-Neutral Energy Research (I2CNER), 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: Recently, there has been considerable interest in the perovskite phase Na0.5Bi0.5TiO3 (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 (σb) and the conduction mechanism(s). Nominal starting compositions with Na excess exhibit high levels of oxide-ion conduction with σb ∼ 2.2 mS cm−1 at 600 °C and an activation energy (Ea) < 1 eV whereas those with Na deficiency are dielectrics based on intrinsic electronic conduction across the band gap with σb ∼ 1.6 μS cm−1 at 600 °C and Ea ∼ 1.7 eV. Drying of reagents, especially Na2CO3, changes the starting stoichiometry slightly due to a 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.
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INTRODUCTION Lead zirconate titanate (Pb(Zr1−xTix)O3, PZT) has been the most widely used piezoelectric material in applications of actuators, sensors, and transducers since the 1950s. The toxicity of lead has raised environmental concerns for many years and consequently has led to extensive studies on developing leadfree piezoelectric materials over the past decade.1−4 The perovskite oxide Na0.5Bi0.5TiO3 (NBT) is one of the favored candidates to replace PZT, and various solid solutions with other ferroelectric materials such as BaTiO3 and K1−xNaxNbO3 have been studied both for piezoelectric and high temperature capacitor (dielectric) applications.5−17 NBT has a complex crystal structure.18−25 Early neutron powder diffraction studies proposed NBT to exhibit a rhombohedral (space group R3c) structure at room temperature (RT).18 Recent high resolution synchrotron powder X-ray diffraction data suggest a monoclinic structure with space group Cc is a better fit.19 Furthermore, the average structure determined by neutron powder diffraction and synchrotron powder X-ray diffraction is different from the local structure, as revealed by X-ray and neutron total scattering experiments and pair distribution function (PDF) analysis.20,21 The local deviations from the average structure occur because of distinctly different displacements of Na and Bi ions20 and complex in-phase and out-of-phase octahedral tilting.22 The © 2014 American Chemical Society
complexity of the local structure has been directly demonstrated by transmission electron microscopy,22 which confirms an essentially random distribution of Bi and Na and reveals significant disorder of the octahedral rotations and cation displacements. It has been proposed that NBT consists of nanoscale domains which exhibit a−a−c+ in-phase tilting at a length limited to only a few unit cells. Assemblages of such nanodomains exhibit an average a−a−c− antiphase tilting system, and the authors of ref 22 refer to the structure as being best described by a “continuous tilt” model. Such a model is consistent with the “average” monoclinic Cc symmetry from synchrotron powder X-ray diffraction data.19 Apart from its complex crystal structure, NBT also exhibits unexpected, interesting electrical and piezoelectric properties.26−29 Small deviations in nominal A-site cation stoichiometry change the RT dc resistivity of NBT by 3 orders of magnitude and influence the piezoelectric and dielectric properties.26 In particular, Na excess (e.g., Na0.51Bi0.50TiO3.005) or Bi deficiency (e.g., Na0.50Bi0.49TiO2.985) in the starting composition lowers dc resistivity and the piezoelectric coefficient (d33) but enhances the depolarization temperature Received: December 5, 2014 Revised: December 23, 2014 Published: December 26, 2014 629
DOI: 10.1021/cm504475k Chem. Mater. 2015, 27, 629−634
Article
Chemistry of Materials
calcined at 800 °C for 2 h. The resultant powders were ball milled for 4 h followed by a second calcination at 850 °C for 2 h and ball milled for 6 h. Pellets were sintered at 1125−1150 °C for 2 h in air and covered using powders of the same composition during sintering. The moisture content in raw materials was examined using a PerkinElmer Pyris 1 and a Setaram SETSYS Evolution for thermogravimetric analysis (TGA). A combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) were employed to examine the phase purity and ceramic microstructure. Impedance spectroscopy measurements were performed using an Agilent E4980A, an HP 4192A impedance analyzer, and a Solartron 1260 system. Au or Pt paste electrodes were used. IS data were corrected for sample geometry (thickness/area of pellet). High frequency instrumental-related (impedance analyzer, lead, and sample jig) inductance effects were corrected by performing a short circuit measurement. For 18O tracer diffusion measurements, the standard procedure for introducing an 18O penetration profile into a solid from a large volume of gas was employed.32,33 Dense samples (relative density > 95%) were annealed in highly enriched 18O2 gas. The oxygen isotope profiles were measured by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS, ION-TOF GmbH, Münster, Germany). More detailed experimental information can be found in the Supporting Information in ref 30.
(Td). In contrast, Na deficiency (e.g., Na0.49Bi0.50TiO2.995) or Bi excess (e.g., Na0.50Bi0.51TiO3.015) in the starting composition enhances the dc resistivity and d33 but lowers Td.26−28 Of particular interest is the origin of the change in NBT resistivity by small variations in the nominal starting Na and Bi concentrations. In a previous study,30 we used a combination of impedance spectroscopy, 18O tracer diffusion studies, and transport number measurements for oxide-ion conductivity to reveal the effect of Bi nonstoichiometry on the electrical properties of Na0.50Bi0.50+xTiO3+1.50x where x = −0.01 (Na0.50Bi0.49TiO2.985; NBi 0 . 4 9 T), 0 (Na 0 . 5 0 Bi 0 . 5 0 TiO 3 ; NBT), and 0.01 (Na0.50Bi0.51TiO3.015; NBi0.51T). The defect chemistry of NBT is rather different from other well-known titanates such as (Ba,Sr,Ca)TiO3 in that the electrical conductivity is dominated by oxide-ion conduction in NBT and NB0.49T rather than electronic conduction. We have attributed the differences in behavior and the high oxide-ion conductivity in these compositions to high oxygen ion mobility associated with highly polarized Bi3+ cations and weak Bi−O bonds31 as well as oxygen vacancies generated through loss of a small amount of Bi2O3 during ceramic processing according to the Kroger-Vink equation, × 2Bi ×Bi + 3OO → 2V‴Bi + 3V •• O + Bi 2O3
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RESULTS Phase Purity and Compositional Analysis. XRD patterns of Na0.49BT, NBT, and Na0.51BT after double calcination at 800 and 850 °C for 2 h showed no evidence of any secondary phase(s), Supporting Information Figure S1. The cell volume extracted using space group R3c with data collected using Si as an internal standard is similar for all three samples: 351.48 (14) Å3 (Na0.49BT); 351.49(9) Å3 (NBT); and V = 351.72 (10) Å3 (Na0.51BT). In contrast, SEM and EDS revealed a Na-rich secondary phase in Na0.51BT, Supporting Information Figure S2d, and STEM high angle annular dark field (HAADF) Z-contrast images revealed a small amount of TiO2 secondary phase in Na0.49BT, Supporting Information Figure S2e. The analyzed main phase compositions of Na0.49BT, NBT, and Na0.51BT by EDS, Supporting Information Table S1, were close to the cation ratios in NBT and were not unambiguously distinguishable from each other within instrument resolution and standard errors. These results demonstrate the A-site nonstoichiometry to be very limited in NBT. Dielectric Properties. The small differences in starting composition have a modest influence on the magnitude and temperature of the relative permittivity maximum (εr,max and Tm, respectively) but have a dramatic effect on the dielectric loss (tan δ). The temperature dependence of εr, Figure 1a, reveals Na0.51BT to exhibit the highest εr,max of ∼3200 with Tm ∼315 °C. With decreasing Na-starting content, εr,max decreases to ∼3000 for NBT and ∼2700 for Na0.49BT; however, Tm increases to ∼320 °C for NBT and ∼335 °C for Na0.49BT. Na0.51BT and NBT exhibit high levels of dielectric loss (>0.05) above ∼300 °C, whereas Na0.49BT exhibits very low tan δ (
