Differences in nature of electrical conductions among Bi4Ti3O12-based ferroelectric polycrystalline ceramics

Bismuth titanate Bi4Ti3O12 (BiT), was one of the most promising lead-free high-temperature piezoelectric materials, due to high Curie temperature (675 °C) and large spontaneous polarization (50 µC/cm2); however, extensive studies had revealed that high leakage conductivity interferes with the poling process, hindering its practical applications. In this paper, an electrically insulating property was achieved by a low level Nb donor substitution to suppress a high level of holes associated with high oxygen vacancy concentration. Bi4Ti2.97Nb0.03O12 ceramic showed significant enhancements of electrical resistivity by more than three order of magnitude and activity energy with value >1.2 eV, which are significant for piezoelectric applications of BiT-based materials. However, pure and A2O3-excess (A = Bi, La and Nd; 3 at %) BiT ceramics, were mixed hole and oxygen ion conductors. Schottky barriers were both formed at grain boundary region and the sample-electrode interface, because of the existence of semiconducting bulk. Interestingly, the electron conduction could be suppressed in N2, as a consequence, they became oxide ion conductors with conductivity of about 4 × 10−4 S cm−1 at 600 °C.


Results
Structure and compositional analysis. Powder diffraction refinement with a Le-Bail fit (GSAS-EXPGUI program) 44,45 was carried out to characterize crystal structures of the prepared oxides, as shown in Fig. 1. The orthorhombic Aba2 (ICSD #240210) transformed from B2cb (Aba2: abc = B2cb: b′c′a′) was used as an initial structural model. For each composition, the calculated data well agrees with the experimental ones, and the reliability factors (R wp , R p and reduced χ 2 ; Table 1) are reasonable. However, the refined results reveal small amounts   Table 1. Crystal data, lattice parameters (a, b, c) and unit cell volume (V) of BiT, BiT-A and BiT-Nb, whose orthorhombicity is defined as 2(c − a)/(c + a).
Scientific RepoRts | 7: 4193 | DOI: 10.1038/s41598-017-03266-y of Bi 2 O 3 in BiT-A (e.g., BiT-Bi and BiT-La), corresponding to the small diffraction peak at 2θ = ∼28° (insets of Fig. 1b and c). In addition, the (0l0) diffraction peaks in BiT, BiT-Bi and BiT-La, e.g., (060), (080) and (0140), are abnormally intensive, while the major peak (171) is severely supressed (Fig. 1a-c). Absolutely, they were highly textured in the c axis direction. However, the texture is not pronounced in BiT-Nb (Fig. 1d). The degree of texture can be expressed by using Lotgering orientation factor (LOF), f 46 . For (0l0) preferred orientation, f is defined as following equations: BiT and BiT-A, highly c-oriented structure results in high f value, being around 0.9, much higher than that for BiT-Nb (0.54). Table 1 lists refined lattice parameters of BiT, BiT-A and BiT-Nb. BiT-Nb shows larger lattice parameters (a, b and c) and unit cell volume (V) than BiT, which should be the results of incorporation of Nb 5+ with larger size at B site (Nb 5+ : N0.64 Å, Ti 4+ : 0.605 Å; 6 CN) 47 . BiT-Bi and BiT-La show increased V, however, this value for BiT-Nd decreases.
As indicated by surface SEM (scanning electron microscope) images (Fig. S1a-c; Supplementary Information), the great majority of grains in BiT-A are overlarge and laminated. The aspect ratio L/t (length/thickness) of the laminated grains ranges from 10 to 30. Probably, that stacking and laminating a few such grains leads to high degree of texture in them. In contrast, BiT-Nb shows size-smaller grains, which is consistent with the results of the Ta/Nb substituted BiT ceramics 25 . And no pores are found within grains, which are visible in BiT-A (cross-section SEM images; insets of Fig. S1a-d, Supplementary Information). It is presumed that the incorporation of Nb 5+ into the lattices leads to a lower grain-growth rate in the a-b plane due to low V O ) in the perovskite layers. Figure 2a and b show microstructure of the grain boundary region of the polished BiT-Bi and BiT-La samples, respectively. It is clear that heterogeneous structure is observed at the grain boundaries. As indicated by EDS (energy-dispersive x-ray spectroscopy) line scans, BiT-Bi and BiT-La show no obvious variation in Bi and Ti intensities along the line 1 → 3, whereas a composition deviation with respect to oxygen is observed at the position 2 of grain boundary ( Fig. 2c and d). This position may correspond to the grain-boundary surface layer, which is possibly created by the accumulation of space charges. In addition, grain composition for them is analyzed by EDS surface scans of five entire grains, and the average atomic ratios are normalized and listed in  Fig. 3a. The relevant EDS line scan reveals the absence of element Ti in Bi-rich phase (Fig. 3b), which is likely ascribed to be Bi 2 O 3 .  Table 2. Composition analyses for BiT-Bi and BiT-La. EDS data for grain composition were obtained by surface scans on five entire grains, and the mean value and standard deviation were listed. Point 2 at grain boundary was shown in Fig. 2a   Impedance spectra and component response. Figure 4 demonstrates variable-temperature impedance diagrams of the air-, O 2 -and N 2 -processed BiT-Bi samples. For the three samples, two complete arcs at high and medium frequencies and, an incomplete one at low frequencies (0.1-1 Hz) due to the limited measurement range of the instrument, are observed in the 250 °C diagrams (Fig. 4a). The 250 °C capacitance (C′) plots of BiT-Bi show a plateau with capacitance values of ∼5 nF/cm at low frequencies (Fig. 5). Therefore, the incomplete arc can be ascribed to a grain boundary (GB) response 48 . Previously, BiT single crystals showed two resolved impedance semicircles, which were considered as the results of the effects of crystalline plate (CP) and plate boundary (PB), respectively 43 . In this study, BiT-Bi ceramic also presents mica-like grains, where CP and PB are visible (inset of Fig. S1a; Supplementary Information). Therefore, the two complete arcs as mentioned above are likely to be associated with the responses of CP and PB, respectively. As indicated by inset of Fig. 4a and the frame inside Fig. 4e, two component parts are observed at the CP region. In BiT-Bi, the electrical properties of the (Bi 2 O 2 ) 2+ layers should be pronounced due to high degree of texture in the c axis direction. Therefore, it is likely that they correspond to the AC responses of the (Bi 2 O 2 ) 2+ layers (higher frequency) and the pseudo-perovskite blocks (lower frequency), respectively 49 . It is reasonable that the former has a lower electrical capacitance than the latter, because of the polarization vector of BiT along the crystallographic a-axis (within the pseudo-perovskite blocks). When temperature is elevated, the grain boundary arc becomes complete, and another arc (the 4th arc; inset of Fig. 4b) is visible in the 400 °C air/O 2 impedance diagrams, at low frequencies (Fig. 4b). The same data presented as Z″ plots (Fig. 4f) also exhibit an additional peak at low frequencies. The 400 °C C′ plot of the O 2 sample shows a high capacitance plateau (10 −7 -10 −6 F/cm) in the same frequency range (Fig. 5a). These experiments suggest that the most likely origin of the 4th arc is ascribed to the electrode effect (discussed later) 38,42 . However, a clear Warburg impedance contribution can be observed below 1 Hz in the 400 °C Z * plot of the N 2 sample (Fig. 4b).
To further increasing temperature, such a Warburg response presents an intensive inclined spike at 550 °C and 700 °C ( Fig. 4c and d), due to limited ionic diffusion into a partially blocking electrodes 42,50 . This, combined with steeply increased capacitance at low frequencies ( Fig. 5b), indicates that the N 2 sample shows prominent ionic conduction, and the principal conducting species could be O 2− ions 51 . In contrast, the air/O 2 samples only show a tail associated with a weak trace of Warburg impedance at the lowest frequencies, due to significantly enhanced electronic conduction 53,54 . The ionic transference number (t ion ) can be evaluated from these Warburg impedances in the case of ion blocking electrode condition, which was described in the literature [52][53][54] . t ion in N 2 is roughly evaluated to be about 0.85 at 550 °C, which are much higher than those in air and O 2 (<0.1). High t ion in N 2 is apparently associated with high [V O

••
] in the sample. In general, the time constant difference for relaxation of each component is large at low temperature, so one can see separated semicircles for each component (see Fig. 4a and b) 55 . However, at high temperature, the time constants for different component are very close and/or relatively small for some of the components, so we normally observe two component semicircles at 700 °C. Like electrolytes and solid-state ionic conducting materials 56, 57 , a modified Randles equivalent circuit was used to calculate the impedance data of the N 2 sample, as shown in Fig. 6a An equivalent circuit including several (R//CPE)s in series (Fig. 6b) was used to calculate the impedance data of the air/O 2 samples. Here, the bulk is integrated and considered as a (R//CPE) component. The parameters and errors for the equivalent circuits are reasonable, and the calculated data well agree with the experiment ones. Similar impedance behaviors as functions of atmosphere and temperature are found for BiT-La and BiT-Nd, which are not shown here. For BiT-La with Pt and Ag electrodes, there are both four overlapping arcs in the 450 °C Z* plots (Fig. S2, Supplementary Information). The resistance (the 4th arc) and the capacitance associated with the electrode effect in the low frequency range are clearly affected by the electrode materials, because of different work functions of them. However, the bulk response that dominates the high frequency data remains unchanged. These experiment results are consistent with those for CaCu 3 Ti 4 O 12 ceramics 38 . Slightly affected boundary responses may be ascribed to two different preparation conditions for Ag and Pt electrodes.
In contrast, BLT and BiT-Nb show dissimilar AC responses, as shown in Fig. 7. In Fig. 7a, the N 2 -processed BLT sample shows a small inclined spike at 400 °C, which turns into a distorted arc at 750 °C (inset of Fig. 7a).    Fig. 7b and d) plots show two capacitance plateaus and two Z″ peaks, respectively. It means that the resistances and the capacitances of BLT and BiT-Nb are principally derived from two regions, i.e., grain (high frequency) and GB (low frequency), respectively. pO 2 dependences of resistivity(ρ)and conductivity(σ). Figure 8 shows ρ bulk (bulk resistivity) and ρ GB (GB resistivity) Arrhenius plots for BiT-Bi, BLT and BiT-Nb measured under different atmospheres. As shown in Fig. 8a, BLT shows slightly higher ρ bulk than BiT-Bi. While its ρ GB increases obviously, by about two orders of magnitude (air-processed) (Fig. 8b). By comparison, BiT-Nb shows excellent electrical insulating property, whose ρ bulk and ρ GB are both much higher than those of BiT-Bi by approximately three orders of magnitude. In addition, the ρ GB of BiT-Bi and BLT is strongly atmosphere-dependent, whereas the ρ bulk of them is nearly independent of pO 2 . By and large, both ρ bulk and ρ GB of the samples increase with decreasing pO 2 , indicating that p-type conduction is predominant in them. However, n-type conduction dominates BiT-Nb above T c , and therefore the ρ bulk and ρ GB of the N 2 sample are much lower than those of the air/O 2 samples above 650 °C. BiT-Bi and BLT show comparable activation energy (E a ) for the bulk and the grain boundary barrier, being 0.5-0.6 eV and 1.0-1.2 eV, respectively. The two values of BiT-Nb become much higher, being 1.37-1.71 and 1.28-1.83 eV, respectively. Figure 9 shows σ bulk (bulk conductivity) and σ GB (GB conductivity) as a function of pO 2 for BiT-Bi and BiT-Nb at different temperatures. In Fig. 9a, the σ bulk and σ GB of BiT-Bi both increase with increasing pO 2 , and logσ bulk vs log pO 2 and logσ GB vs log pO 2 both present nonlinear variations. It is clear that ionic conductivity is approximately independent of pO 2 , while hole conductivity linearly increases with increasing pO 2 31, 32 . These nonlinear variations suggest that BiT-Bi is an oxide ionic and p-type mixed conductor. By comparison, σ bulk showed a very smaller dependence of pO 2 , indicating that ion conduction dominates the bulk. As indicated by Fig. 9b, the σ bulk and σ GB of BiT-Nb show strong dependence of oxygen activity, which is in response to a prominent electronic conduction. In the present oxides, oxygen vacancies mainly arise from the evaporation of Bi 2 O 3 during sintering, and the incorporation of oxygen into the V O •• sites follows after cooling, expressed by the following reaction.  However at 750 °C, the two conductivities both increase with decreasing pO 2 , which is directly opposite to the experiments exhibited at 500 °C. This change is consistent with the results of YMnO 3 ceramics 58 . It is presumed that BiT-Nb above T c is an oxide ionic and n-type mixed conductor. Under reducing conditions the required electrons (e′) are created through the release of oxygen from the lattices, expressed by the following equation.
The reduction of Ti 4+ to Ti 3+ is likely to be the source of conduction. And the electrons are probably trapped by the donor defects such V O •• and Nb Ti • , leading to high E a (>2.0 eV) for the N 2 sample in the high temperature range (see Fig. 8). In addition, good fittings for the σ bulk and σ GB of BiT-Nb at 750 °C can be obtained by an equation of σ bulk (σ GB ) = σ ion + σ electronic (pO 2 ) m (m = −1/6). In the case of both 500 °C (BiT-Nb) and 600 °C (BiT-Bi), the exponent m is equal to 1/6. For the O 2-processed BiT-Bi sample at 600 °C (pO 2 = 1 atm), the calculated σ ion and σ electronic (hole) in the bulk are 3.02 × 10 −4 and 2.21 × 10 −4 S cm −1 , respectively, and they are estimated to be 0.68 × 10 −3 and 1.64 × 10 −3 S cm −1 at the GB region, respectively. Figure 10a shows temperature dependence of dielectric permittivity (ε′) at several frequencies for the air-, O 2 -and N 2 -processed BiT samples. High-and-sharp dielectric permittivity peak corresponds to ferroelectric-to-paraelectric phase transition. It is observed that the atmosphere has no influence on T c of BiT, which is around 673 °C and the same as those reported in the literature 6,40 . Notably, an anomaly associated with two primarily high permittivity regions is detected below T c , which agrees with the results of Shulman et al. 12 . Also, there are several peaks observed in the tan δ-T plots (Fig. 10b), corresponding to the steep increases of dielectric permittivities in the ε′ vs. T plots (Fig. 10a), which all move toward higher temperature with increasing frequency. The dielectric relaxation and the loss peak in the low temperature range (region I) are not almost affected by altering pO 2 , indicating an intrinsically physical nature, i.e., the bulk response. In contrast, the medium-and high-temperature dielectric relaxations in regions II and III, respectively, are strongly affected. They are likely in response to the boundary capacitance and the electrode capacitance, respectively, being referred to the results of Fig. 4 and ref. 38. The notable peak between 450 and 550 °C in the dielectric loss tangent (inset of Fig. 10b) for the air/O 2 samples seems to be attributed to the conduction loss.

Phase transition and dielectric relaxation.
As shown in Fig. 10c and d, the Nb donor substitution suppresses the loss and the relaxation process dramatically, and a sharp transition peak is observed at T c . However, the A 2 O 3 additions are not critical to the dielectric data. In addition, the T c of BiT-Bi and BiT-Nb is close to that of BiT, while the T c of BiT-La and BiT-Nd decreases to 638 °C and 647 °C, respectively (Fig. 10c). For A-site bismuth-containing Aurivillius compounds, the polarity Bi 3+ with 6 s 2 lone pair electrons causes the deformation from the prototype structure 59 . Therefore, it is presumable that non-polarity La 3+ and Nd 3+ were incorporated into the lattices by replace the A-site Bi 3+ , and thus the T c of BiT-La and BiT-Nd decreases significantly. Newnham et al. proposed that T c vs. x in Bi 4−x RE x Ti 3 O 12 follows a linear relationship 60 . In this study, the substitution content x in BiT-La and BiT-Nd is roughly estimated to be about 0.1 ( Fig. S3; Supplementary Information), close to nominal A-site excess level (Bi 4 A 0.12 Ti 3 O 12. 18 ). Furthermore, the change trend of T c is in accord with that of structural orthorhombicity (see Table 1), and higher T c corresponds to higher orthorhombicity value.

Discussion
Commonly, Schottky barriers can form in electrically heterogeneous ceramics at insulating grain boundaries between semiconducting grains, which is referred to as an internal barrier layer capacitor (IBLC) effect 61 . Also, Schottky barriers can occur between "leaky" or semiconducting ceramics and metal electrodes, acting as a non-ohmic electrode effect 62 . The formation of these barriers can be ascribed to compositional variations and/ or a mismatch between Fermi energy levels between the two materials that meet at the interface 38,48 . Polarization effects at these barrier layers can generate nonintrinsic and colossal dielectric permittivities in ceramics such as CaCu 3 Ti 4 O 12 (CCTO) 38 . In this study, the bulks of BiT-A clearly are semiconducting, whose resistivity are lower than 10 5 Ω cm above 300 °C (Fig. 8a). Thus the Schottky barriers are possibly generated at the grain boundary regions in them, which is consistent with the results of CCTO and Pb(Fe 1/2 Nb 1/2 )O 3 ceramics 37, 38 . The results of Figs 8 and 9 indicate that BiT-Bi is an oxygen ion and hole mixed conductor. It is presumed that the bulk of BiT-Bi is dominated by positive defects (h • and V O •• ), which can be charge-compensated by grain boundary acceptor surface charges. Such a barrier layer may be associated with the depletion of V O •• at the grain boundary region during oxidative cooling 63 , as sketched in Fig. 11a.
It is possible that the electrodes nominally "blocking" for ions, allow for a certain ionic leakage, while the electrodes nominally "reversible" for electrons, still represent a certain interfacial resistance 52,64 . In the case of the air/O 2 BiT-Bi samples, it is believed that weak Warburg impedance at the lowest frequencies corresponds to the limited ionic diffusion into a partially blocking electrode (see Fig. 4). The additional arc between the boundary impedance and the Warburg one is presumably attributed to the electrode effect due to deviation from electronic reversibility. Two types of electrode effects together with the boundary barrier effects that dominate the intermediate frequency data, induce two high dielectric permittivity regions for BiT-A. Therefore, the following successions of layers starting from the inside are possibly: bulk/plate boundary/grain boundary/electrode.
One can speculate that oxide ion conduction may be a common feature of perovskite materials with a high [V O •• ]. Furthermore, high oxygen ion conductivity has been recorded in the literature for intergrowths of Aurivillius with Brownmillerite structure, and cubic δ-Bi 2 O 3 is known as a fast ion oxygen conductor 11,65,66 . For BiT-A, it is reasonable that predominant ionic conduction in the bulk is associated with the textured structure along the c-axis direction, in addition to high [V O

••
] in the perovskite blocks. However, hole conduction is dominant at the boundaries, and thus the ρ GB or σ GB is apparently dependent of pO 2 (see Figs 8b and 9a). A detailed structure analysis for pure BiT showed that some Bi ions in the perovskite layers are overbonded with a valence state of >3+ 67 . A Pb 2+ → Pb 3+ hopping conduction has been proposed in PZT perovskite 68 . Presumably in BiT-Bi, hole conduction is associated with oxidation of Bi 3+ to Bi 4+ , and the relevant E a is probably close to that for the trapping of holes by Pb 2+   The E a of about 0.6 eV for the bulk of BiT and BiT-A can be attributed to a compromise of oxide ion conduction and hole conduction. Higher E a (1.0-1.2 eV) for the boundary barriers may be attributed to trapping of holes (h • ) by the grain-boundary acceptor surface charges (A′) 52 .
In order to achieve predominant oxygen ion conduction, it is necessary to avoid samples picking up oxygen from high pO 2 environments. Otherwise, the rich-oxygen atmosphere encourages the incorporation of foreign oxygen into the lattices (Eq. 1); as a consequence, the samples transform into p-type electronic conductors with higher conductivity and lower activation energy. As indicated by the XPS (X-ray photoelectron spectroscopy) results (Fig. 12a), the increased pO 2 leads to the increases in the nominal valence states of Bi and Ti, which is related to both a decrease in [V O •• ] and an increase in [h • ] 72 . Therefore, BiT-A show a transition from the dominant oxide ion conduction to the dominant p-type semiconduction with increasing pO 2 .
As shown in Fig. S4 (Supplementary Information), BiT shows similar conduction behaviors as functions of atmosphere and temperature. BiT-A relative to BiT shows slightly increased ρ bulk , and the variations in ρ PB (PB resistivity) and ρ GB are slight and irregular ( Fig. S5; Supplementary Information). This, combined with the evolutions in V and T c (see Table 1 and Fig. 10c), indicates that A 3+ could be mainly diffused into the lattices by substituting the A-site Bi 3+ 33,73 . Thus in BLT, hole conduction and oxygen ion conduction are both suppressed. As sketched in Fig. 11b, BiT-Nb relative to BiT-Bi becomes rather more insulating. Two situations can be considered by the Nb substitution for the Ti site (B site), as expressed by the following equations.  ] in the perovskite blocks, which supports significantly increased ρ bulk of BiT-Nb (see Fig. 9). As indicted by the results of Figs 8 and 9, for BiT-Bi (>350 °C) and BiT-Nb, the grain resistivity of is lower than the grain resistivity. However, it does not mean that grain boundaries are electrically more conduction than bulk grains. In fact, grain boundaries in this study are electrically more insulating as shown in Fig. 11b, because the thicknesses of them are far smaller than the grain sizes of samples. This is consistent with the results of other electroceramics 73 . The XPS results of Fig. 12b Fig. 13. Therefore, the oxygen ion conduction is weak in BiT-Nb, of which the electrical conduction is primarily attributed to an electronic mechanism. Previous researches revealed that additional polarization associated with the electrode-sample interface was readily observed for CCTO ceramics with relatively low ρ GB 38 . If ρ GB was large, the electrode polarization was obscured by sample-related effects. Thus, the absence of the electrode effect in BLT and BiT-Nb may be associated with high ρ GB of them, which are much higher than those of BiT and BiT-A (see Fig. 8b).

Conclusions
Structure and electrical properties of modified BiT-based ceramics could be tailored readily by changing chemical compositions, including the perovskite A and B sites. As a consequence, different electrical conductions and dielectric properties among them were found. Excess A 2 O 3 had no obvious effects on electrical resistivity and conduction mechanism. BiT and BiT-A all showed prominent oxide ion conduction under N 2 , which was likely associated with high [V O •• ] in the perovskite blocks and the pronounced texture in the c-axis direction. However, the hole conduction was prominent in the air/O 2 samples, because of the absorption of oxygen by them.
In addition to the semiconducting bulk, impedance data revealed that Schottky barriers had been formed both at grain boundary and at the sample-electrode interface in heterogeneous BiT and BiT-A, which contributed to two abnormally high ε′ regions below T c . BLT with 0.75 La substitution at A site and BiT-Nb with 0.03 Nb substitution at B site, both showed a suppression in electrical conduction including ionic and electronic. Especially, a low level of Nb donor substitution led to the significant decreases in [V O •• ] and [h • ]; thus BiT-Nb showed much higher ρ and E a than BiT and BiT-A. The results in this study are significant for BiT-base high-temperature piezoelectric sensors in solving the origin of high leakage conductivity. Also, they provided some probability for the future work that BiT-based ceramics are considered as new oxide ion conductors by adjusting appropriately chemical compositions. Characterization and measurements. XRD data were collected by using an automated diffractometer (X'Pert PRO MPD, Philips, Eindhoven, The Netherlands) with a nickel filter (Cu Kα radiation) at room temperature. SEM images were observed by using a field-emission scanning electron microscopy (JEOL-6700F, Japan Electron Co., Tokyo, Japan) instrument equipped with an EDS system, at room temperature. TEM image and compositional analysis were performed by using a transmission electron microscopy (Tecnai F30, FEI, Hillsboro, OR, USA) instrument equipped with a high angle annular dark-field (HAADF) detector and an EDS system, at room temperature. XPS data were performed with a spectrometer (VG ESCALAB220i-XL, Thermo Scientific, Surrey, UK) with Al Kα (E = 1486.6 eV) radiation, at room temperature. Temperature dependences of dielectric permittivity and dielectric loss were measured by using an LCR meter (4284 A, Agilent, CA, USA). Impedance data were performed by using an Impedance Analyzer (Solartron, SI 1260, Hampshire, U.K.), with a frequency range of 0.1-1 M Hz and an AC measuring voltage of 0.3 V. The same sample was used for electrical property measurements firstly in air, subsequently in N 2 , and finally in O 2 , at a slow cooling rate. pO 2 dependence of conductivity was measured in the range of 10 −6 -1 atm, which was controlled by mixing O 2 and N 2 gases and monitored by a zirconia oxygen sensor. Ag and Pt electrodes for the measurements of the electrical properties were made of fired-on silver paste at 850 °C and sputtered at room temperature (after removing Ag), respectively.