Electrical Properties of Ca-Doped BiFeO3 Ceramics

Article pubs.acs.org/cm

Electrical Properties of Ca-Doped BiFeO3 Ceramics: From p-Type Semiconduction to Oxide-Ion Conduction Nahum Masó*,† and Anthony R. West*,† †

Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, U.K. ABSTRACT: The conductivity of Ca-doped BiFeO3 ceramics varies by many orders of magnitude, depending on the oxygen partial pressure during processing. Bi1−xCaxFeO3−(x/2)+δ ceramics are mixed oxide ion/electron conductors at 800 °C, but the electron conduction can be suppressed; when sintered and cooled in N2 from 800 °C, they are oxide ion conductors with activation energy ∼0.82−1.04 eV and conductivity ∼1 × 10−5 S cm−1 at 300 °C, comparable to that of 8 mol % yttria-stabilized zirconia. When heated in O2 at 125 bar, however, they are mainly p-type semiconductors with conductivity ∼1 × 10−3−4 × 10−5 S cm−1 at room temperature and activation energy ∼0.27−0.40 eV. The oxygen stoichiometry varies over the range 0 < δ < ∼0.016, depending on processing conditions. The semiconductivity is attributed to mixed valence Fe with < ∼3.2% Fe4+. KEYWORDS: Ca-doped BiFeO3, mixed oxide ion/electron conduction

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INTRODUCTION Bismuth ferrite (BiFeO3) has attracted considerable attention in recent years since it exhibits both G-type antiferromagnetic order with a long-periodicity spiral below the Néel temperature (TN) at ∼640 K and ferroelectricity below the Curie temperature (TC) at ∼1100 K, which may allow fabrication of novel functional materials involving coupling between magnetic and electrical order. We show here that it can also be either a good oxide ion conductor or a p-type semiconductor, depending on doping and processing conditions. Several studies into A-site substitution of isovalent (La3+, Nd3+, and Sm3+) or acceptor (Ca2+, Sr2+, Ba2+, and Pb2+) cations for Bi and B-site substitution of donors (V5+, Nb5+, Mn4+, or Ti4+) for Fe have been carried out in order to improve the magnetic and ferroelectric properties. With A-site doping of Ca2+, the phase diagram of the resulting Bi1−xCaxFeO3−δ solid solution contains several phase fields but disagreements over composition limits.1−6 The most recent studies5,6 show rhombohedral (R3c) and tetragonal, pseudocubic phase fields for 0 ≤ x < ∼0.07 and 0.2 ≤ x < 0.5, respectively, with a twophase mixture for intermediate compositions. It appears to be well-established1−3,6 that the oxidation state of Fe is Fe3+, and, consequently, charge compensation in Cadoped BiFeO3 is by oxygen vacancies. However, there is disagreement over the nature of the electrical properties. Bi1−xCaxFeO3−δ ceramics are reported to be “leaky” dielectrics at room temperature,5 with activation energies in the range 0.27 to 0.5 eV attributed to hole conduction arising from oxidation of Fe3+ to Fe4+. Other reports concluded that oxygen vacancies are responsible for conduction3,6,7 although the evidence on which this is based was not given. At high temperatures, above ∼600 °C, ceramics act as permeable membranes to oxygen8 with the implication that the materials are either oxide ion conductors or mixed oxide ion/electronic conductors; this is further supported by similar results on Sr-doped BiFeO3 (ref 9) © 2012 American Chemical Society

and by rapid oxygen exchange rates at higher temperatures, ∼850 °C, with small associated resistance, in Bi1−xSrxFeO3−δ thin films.10 Here we show that the electrical properties of Ca-doped BiFeO3 ceramics are very dependent on the oxygen partial pressure during sintering and subsequent cooling. In particular, the properties change from p-type semiconduction to a high level of oxide ion conduction, comparable to that of 8 mol % yttria-stabilized zirconia (8-YSZ), with decreasing PO2

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EXPERIMENTAL SECTION

Sample s of three compositions, Bi 0 . 9 7 Ca 0 . 0 3 FeO 2 . 9 8 5 + δ , Bi0.95Ca0.05FeO2.975+δ, and Bi0.70Ca0.30FeO2.85+δ, were prepared by solid state reaction using powders, and drying temperatures, of Bi2O3 (99.99% pure, Acros Chemicals, 180 °C), CaCO3 (99% pure, Sigma-Aldrich, 180 °C), and Fe2O3 (99% pure, Sigma-Aldrich, 400 °C). These were mixed in an agate mortar and pestle for ∼30 min, with acetone added periodically to form a paste. Pellets were pressed and placed on sacrificial powder of the same composition in alumina boats. Initial firing was at 850 °C for 20 min to eliminate CO2 after which the pellets were ground, repressed, fired again at 850 °C for 20 min, ground, repressed isostatically at 200 MPa, given a final firing at ∼860−945 °C for 2 h in air, and then cooled slowly by switching off the furnace. For selected experiments, samples were heated at ∼800 °C for 1 h in N2 at ∼1.01 bar in a tube furnace and in O2 at ∼125 bar in a Morris high pressure furnace, followed by slow cooling. For comparison purposes, pellets of 8 mol % yttria-stabilized zirconia (Sigma−Aldrich) were pressed isostatically at 200 MPa, fired at 1600 °C for 6 h in air, and then cooled naturally by switching off the furnace. During firing, pellets were placed on a bed of sacrificial powder of the same composition in Pt foil boats. The phases present were analyzed by X-ray Powder Diffraction (XRD) using a Stoe StadiP Diffractometer, CuKα1 radiation with a Received: March 2, 2012 Revised: May 10, 2012 Published: May 22, 2012 2127

dx.doi.org/10.1021/cm300683e | Chem. Mater. 2012, 24, 2127−2132

Chemistry of Materials

Article

Table 1. Lattice Parameters, Volume (V), Volume Per Formula Unit (V/Z), and Pellet Density for a Range of Compositionsa lattice parameters (Å)

a

composition

reference

a

c

V (Å3)

V/Z (Å3)

pellet density (%)

Bi0.97Ca0.03FeO2.985+δ Bi0.95Ca0.05FeO2.975+δ Bi0.70Ca0.30FeO2.85+δ

BCF03 BCF05 BCF30

5.578(1) 5.578(1) 3.9203(2)

13.853(3) 13.850(3) −

373.2(1) 373.15(12) 60.250(6)

62.20 62.19 60.25

86 95 95

The rhombohedral unit cell is given on hexagonal axes.

linear position-sensitive detector; lattice parameters were determined by least-squares refinement for reflections in the range 15 < 2θ < 80°, using the software WinXPow version 1.06, and an external Si standard. For electrical property measurements, pellets (∼0.5 g, ∼1.5 mm thick, ∼6.5 mm diameter, and pellet density as shown in Table 1) were coated with electrodes made from Pt paste that was decomposed and hardened by heating to 800 °C for 1 h. Impedance measurements used a combination of Agilent 4294A, E4980A, and Solartron SI 1260 impedance analyzers over the frequency range 10 mHz to 2 MHz, with an ac measuring voltage of 0.1 V and over the temperature range 10 to 725 K. For subambient measurements, an Oxford Cryostat with Intelligent Temperature Controller (ITC 503S) was used. Impedance data were corrected for overall pellet geometry and for blank capacitance of the conductivity jig. Resistance values were obtained from intercepts on the real, Z′ axis. Conductivity and capacitance data are reported in units of S cm−1 and F cm−1, respectively, that refer to correction for only the overall sample geometry.

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RESULTS AND DISCUSSION Samples of Bi1−xCaxFeO3−x/2, x = 0.03 (BCF03), x = 0.05 (BCF05), and x = 0.30 (BCF30), appeared to be single phase by XRD after firing at 860, 925, and 945 °C, respectively. The patterns of BCF03 and BCF05 closely resembled that of rhombohedral BiFeO3 and were indexed accordingly with refined lattice parameters summarized in Table 1. These showed a slightly decrease of c and volume with x. By contrast, the pattern of BCF30 was indexed and refined on a cubic unit cell, Table 1. Lattice parameters are in agreement with those reported previously.5,6 Results are presented for three sets of processing conditions for each composition, with samples heated and cooled in atmospheres of N2, air, and high-pressure O2. The same pellets were used throughout for each composition, and electrodes were not removed between subsequent heat treatments. The N2-processed samples were insulating and their impedance was measured over the temperature range 440−700 K, whereas the air- and O2-processed samples were much more conducting and were measured over the range 150−320 K. Impedance data for the air- and O2-processed samples, measured in air, showed a single, almost ideal, semicircle, Figure 1(a,b) for BCF30; representation of the same data in Z″/M″ spectroscopic plots (c) showed a single peak in both cases with maxima at similar frequencies, indicating that the sample was electrically homogeneous and could be represented, ideally, by a single parallel RC element (inset in a) in which R represents the bulk resistance and C represents its capacitance or permittivity. Capacitance, C′, data as a function of frequency (d) showed a frequency-independent plateau that represented the bulk response of the sample with associated permittivity values that increased from ∼65 to ∼135 with increasing temperature; these permittivity values are comparable to those reported previously.3,5−7 By contrast, BCF03 and BCF05 showed a similar impedance response that contained two main resistances (R1 and R2) which together dominated the total resistance (R1+R2), Figure 2(a,b). The capacitance values of the

Figure 1. Impedance complex plane plots (a,b), Z″/M″ spectroscopic plots (c), and C′ spectroscopic plot (d) at several temperatures for BCF30 sintered in air followed by slow cooling. Angular frequency, ω = 2πf. Noisy data at low frequencies in (d) have been omitted for clarity.

Figure 2. Impedance complex plane plots (a,b) and C′ spectroscopic plots (c,d) at several temperatures for BCF05 sintered in air followed by slow cooling.

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high frequency component, C1, e.g. ∼6 pF cm−1 at 200 K, represented the bulk capacitance of the sample with a permittivity of ∼70, Figure 2(c). The capacitance values of the low frequency component, C2, were of the order of nanofarads, Figure 2(d), and were attributed to a conventional grain boundary. Critically, there was no evidence of any additional impedance associated with charge transfer processes at the sample− electrode interface in the air-processed samples. Thus, the low frequency impedance data terminated with the low frequency intercept of the arc on the Z′ axis (inset in Figures 1 and 2 b) and the C′ values did not increase beyond ∼2 × 10−11 F for BCF30 and ∼1 × 10−9 F for BCF03 and BCF05. Similar data were seen for both the air- and O2-processed samples, but the conductivities of the O2 samples were somewhat higher, Figure 3, indicating a hole conduction

used in sample preparation for impedance measurements. Results are summarized in Table 2. A small increase in sample weight was seen on heating in air and especially O2. Assuming that the N2 sample contained Fe exclusively in the +3 oxidation state, then the amount of oxygen in the O2 sample in the formula Bi0.95Ca0.05FeO2.975+δ is estimated as δ = 0.011(2), whereas in Bi0.70Ca0.30FeO2.85+δ it is estimated as δ = 0.016(3). The N2-processed samples were insulating at room temperature with σ ≪ 10−6 S cm−1, Figure 4. Impedance data for the

Figure 4. Comparison of impedance complex plane plot at 310 K for BCF30 sintered in either air or nitrogen followed by slow cooling.

Figure 3. Comparison of impedance complex plane plots (a,b) and Z″/M″ spectroscopic plots (c,d) at several temperatures for BCF30 sintered in either air or oxygen followed by slow cooling.

mechanism; the hole concentration was controlled by the schematic reaction 1 V •• O2 → OOx + 2h• O + 2 and therefore, increased with increasing PO2. The holes are believed to be associated with Fe, as Fe4+ ions. In order to estimate the concentration of Fe4+ ions, a series of experiments was conducted in which pellets were weighed at every stage after the same sequence of heat treatments and cooling rates

Figure 5. Impedance complex plane plots (a,b) Z″/M″ spectroscopic plots (c) and C′ spectroscopic plot (d) at several temperatures for BCF30 sintered in nitrogen followed by slow cooling. Noisy data in (d) have been omitted.

Table 2. Average Pellet Weight of Five Measurements (in grams) and Weight Change (in grams) for Bi1−xCaxFeO3−(x/2)+δ Ceramics As a Function of Heat Treatment Conditions Followed by Slow Cooling Bi0.95Ca0.05FeO2.975+δ nitrogen air oxygen

Bi0.70Ca0.30FeO2.85+δ

pellet weight

weight change

δ

pellet weight

weight change

δ

0.49086(10) 0.49108(8) 0.4914(1)

− +0.0002(1) +0.0005(1)

− 0.005(3) 0.011(2)

0.49519(9) 0.49540(12) 0.49572(4)

− +0.0002(2) +0.00053(9)

− 0.010(5) 0.016(3)

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samples processed in N2 are summarized in Figures 5 and 6. The impedance data on heating and cooling were recorded in an atmosphere of N2 (estimated PO2 level ∼2 × 10−3 atm) since samples gradually reoxidized in air. Again, BCF30 showed a single arc in the Z* complex plane, Figure 5(a,b), and single peaks in the Z″/M″ spectroscopic plots, Figure 5(c), whereas BCF03 and BCF05 showed two semicircles, Figure 6(a,b). In addition, the impedance data for all three compositions showed an additional low frequency feature, which had the character of an inclined Warburg spike (inset in Figures 5,6 a and b) and the C′ data increased significantly at lower frequencies (Figures 5,6 d), reaching a value of 10−4 F at 10−2 Hz and ∼615 K. The combination of the high C′ values and the Warburg spike indicated that in this sample the principal conducting species were either O2− or H+ ions. In order to identify the principal conducting species, impedance measurements were carried out first in an atmosphere of dry N2 and next in an atmosphere of wet N2. Dry and wet atmospheres were obtained by passing the gas through Dreschel bottles containing silica gel and water, respectively. Bulk resistance (R1) data, obtained as shown in Figures 5,6(a,b), are presented as conductivity Arrhenius plots for the dry and wet atmospheres in Figure 7. Similar conductivity values were observed in both dry and wet atmospheres of N2 but the activation energy of BCF30 decreased on increasing the humidity of the atmosphere. The

Figure 7. Bulk Arrhenius plot for Ca-doped BiFeO3 ceramics measured in either a dry or wet atmosphere of N2. Activation energies, in eV, are noted beside each data set.

Figure 6. Impedance complex plane plots (a,b) and C′ spectroscopic plots (c,d) at several temperatures for BCF05 sintered in nitrogen followed by slow cooling.

Figure 8. Bulk Arrhenius plot for Ca-doped BiFeO3 ceramics heated in air, nitrogen, and oxygen followed by slow cooling. Activation energies, in eV, are noted beside each data set.

similarity in conductivities in dry and wet atmosphere indicated that the principal conducting species were O2− ions rather than H+ ions. Bulk resistance (R1) data for the 3 sets of processing conditions (O2, air, and N2), are presented as conductivity Arrhenius plots in Figure 8. The conductivities of the air- and, especially, O2-processed samples were several orders of magnitude greater than that of the N2-processed samples with correspondingly smaller activation energies. Although the

conductivity of the N2-processed samples was much less, these materials are oxide ion conductors with conductivity comparable to that of 8 mol % YSZ, the well-known oxide ion conductor, Figure 9. The conductivity showed a small decrease, accompanied by an increase in activation energy with increasing x. This may be attributed to the trapping of oxide ion vacancies 2+ (V•• ions (CaBi ′ ), similar to the O ) by the substituted Ca trapping phenomena observed in stabilized zirconias. 2130



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electrolytic domain, the crossover region is displaced to lower PO2 values with decreasing temperature, but nevertheless, for the measurements reported here for the N2 processed samples, conduction is still electrolytic. For isothermal studies, the dependence of conductivity on PO2 can be rationalized in terms of Figure 10; this simplified model requires modification however if, as shown in Figure 8, the activation energy changes with positive hole concentration, either as a consequence of processing in different atmospheres or because the carrier concentration is temperature dependent. It is assumed that the samples were in equilibrium with the surrounding atmosphere at 800 °C but equilibrium may not have been fully maintained during cooling and, to a certain extent, the samples therefore represent a frozen-in high temperature state. The relatively small Fe4+ concentration, up to 3.2%, leading 4+ to the expanded formula e.g. Bi0.70Ca0.30Fe3+ 0.968Fe0.032O2.866, is sufficient to dramatically increase the level of electronic conductivity, as shown in Figure 8, but is probably too small to be detected readily by Mössbauer spectroscopy, which accounts for the observation from several studies1−3,6 that samples prepared in air contain Fe in the trivalent oxidation state. These results also account for the discrepancies in electrical properties reported in the literature since, as shown here, the electrical properties change dramatically simply by changing the atmosphere from N2 to air/O2 during sintering and subsequent cooling, as shown by subsequent impedance measurements over a wide range of lower temperatures. Oxide ion conductivity comparable to that of 8 mol % YSZ is achieved by a combination of two factors: (i) acceptor doping to create oxygen vacancies which are mobile and (ii) avoidance of partial reoxidation which leads to hole conduction via Fe4+ species. There is much scope for seeking higher levels of oxide ion conduction in related, acceptor-doped materials. These results also point to possible cathode applications in solid oxide fuel cells where both high electronic and high ionic conductivity are necessary. However, an evaluation of the chemical stability in environments encountered during cell fabrication and operation is required.

Figure 9. Bulk Arrhenius plot for Ca-doped BiFeO3 ceramics heated in nitrogen followed by slow cooling. Activation energies as shown in Figure 8. Data for 8 mol % yttria-stabilized zirconia are shown for comparison.

In the air- and O2-processed samples, the conductivity is much lower and the activation energy higher for BCF30 than for BCF03 and BCF05, Figure 8. Although the concentration of holes and, therefore, Fe4+ increased with x, Table 2, the decrease in conductivity and increased activation energy is attributed to trapping of holes (h•) by CaBi ′. At 800 °C, extrapolation of the data in Figure 8 shows that the conductivities of the samples processed in air/O2 and N2 are likely to be comparable, but because of the very different activation energies, the conductivities at lower temperatures differ greatly. We conclude that in the absence of electronic conductivity, Ca-doped BiFeO3 ceramics are good oxide ion conductors comparable to 8 mol % YSZ. When, however, the samples contain some Fe4+ ions, arising from heating in air/O2, the level of hole conductivity greatly exceeds that of the oxide ion conductivity. The changes in conductivity are fully reversible on decreasing and subsequently increasing the oxygen partial pressure of the processing/cooling atmosphere. The dependence of conductivity on sample processing conditions, atmosphere and temperature shown in Figure 8, can be understood using the schematic conductivity profiles shown in Figure 10. Oxide ion conductors have an electrolytic

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CONCLUSIONS The electrical properties of Ca-doped BiFeO3 ceramics are very dependent on the oxygen partial pressure during sintering and subsequent cooling. The conductivity varies by many orders of magnitude, and the properties change from p-type semiconduction to a high level of oxide ion conduction with decreasing PO2.

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Figure 10. Schematic dependence of conductivity on temperature and atmosphere.

AUTHOR INFORMATION

Corresponding Author

*A.R.W.: e-mail: a.r.west@sheffield.ac.uk. N.M.: e-mail: n. maso@sheffield.ac.uk.

domain where oxide ions are the principal current carriers, but above, and below, certain values of PO2, conduction becomes predominantly electronic with p-type and n-type behavior, respectively. Samples processed in N2 fall within the electrolytic domain with activation energy in the range ∼0.82 to 1.04 eV, whereas samples processed in air/O2 fall within the p-type domain with lower activation energy, in the range 0.27 to 0.45 eV. The processing temperature of 800 °C represents the approximate crossover between electrolytic and p-type domains. Because of the higher activation energy in the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EPSRC for financial support. REFERENCES

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Electrical Properties of Ca-Doped BiFeO3 Ceramics

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