Dielectric response and electric conductivity of ceramics obtained from

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Feb 16, 2016 - Dielectric response and electric conductivity of ceramics obtained .... dielectric permittivity of BFO ceramics with grain sizes of ~200 nm.
Journal of Alloys and Compounds 671 (2016) 493e501

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Dielectric response and electric conductivity of ceramics obtained from BiFeO3 synthesized by microwave hydrothermal method  ska, E. Markiewicz*, M. Błaszyk, B. Hilczer, B. Andrzejewski K. Chybczyn  , Poland Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 October 2015 Received in revised form 11 February 2016 Accepted 12 February 2016 Available online 16 February 2016

BiFeO3 powder which formed ball-like structures resembling flowers was obtained by microwave hydrothermal synthesis. The flowers were of a dozen or so mm in diameter and the thickness of the crystallites forming petals could be controlled. The material was characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. Dielectric response of ceramics obtained from the powder contained three extrinsic contributions, which could be correlated with the differences in temperature variation of the ac conductivity. The dielectric relaxation between 150 K and 300 K was related to reorientations of Fe3þeFe2þ dipoles and characterized by an activation energy of 0.4 eV, which was independent of the petal thickness. The dielectric and electric response in the range 300 K ÷ 450 K usually ascribed to the grain boundary and interfacial polarization effect was diffused and could not be characterized. Above 450 K the activation energy of dc conductivity was 1.73 eV and 1.52 eV for ceramics consisting of crystallites of mean thickness of 160 nm and 260 nm, respectively. The energies, which are considerably higher than those reported earlier for BFO nanoceramics, were discussed considering the interactions between oxygen vacancies and size scaled ferroelectric domain walls, which in BiFeO3 are associated with electrostatic potential steps. © 2016 Elsevier B.V. All rights reserved.

Keywords: Bismuth ferrite Microwave hydrothermal synthesis Dielectric response Electric conductivity Ferroelectric domain walls

1. Introduction Bismuth ferrite BiFeO3 (BFO) is hitherto the only known singlephase multiferroic exhibiting long-range magnetic and electric el temperature ordering at room temperature with high Ne TN ¼ 640 K and ferroelectric Curie temperature TC ¼ 1100 K [1]. At room temperature the rhombohedrally distorted cubic perovskite structure of BFO belongs to the trigonal R3c space group and the unit cell contains two chemical units. A covalent bonding due to hybridization of the Bi6s with the O2p orbitals results in a shift of the Bi3þ ions from their centrosymmetric position in the perovskite lattice. The stereochemical activity leading to an off-centering of the A-type Bi3þ ions is responsible for the ferroelectric properties, whereas magnetic properties of BFO are carried by the Fe3þ ions with partially filled d orbitals located in the B-sites of the perovskite lattice. The G-type antiferromagnetic ordering is accompanied by a weak canting of the magnetic moments resulting in a cycloidally modulated superstructure with the period of 62 nm.

* Corresponding author. E-mail address: [email protected] (E. Markiewicz). http://dx.doi.org/10.1016/j.jallcom.2016.02.104 0925-8388/© 2016 Elsevier B.V. All rights reserved.

Complexity of the structure, with coupled magnetic and ferroelectric order parameters, as well as difficulties in processing conditions, responsible for impurities and vacancies [1,2], are considered as the origin of unusual properties of BFO. The problem of intrinsic dielectric properties of BFO single crystals has been recently studied theoretically by Graf et al. using the first principle method and molecular dynamic simulations [3]. The results show that the intrinsic dielectric permittivity is low (~30 at room temperature) and at the ferroelectric Curie point reaches the value of ~180. Rather complex dielectric response with various extrinsic contributions has been reported for BFO ceramics and related mainly to the processing technology [1,2,4e9]. The long coherence length of cycloidal spin modulation in BFO is the most important reason of the enhancement of ferromagnetic properties in the nanoscale because rotation of the spins along the wave vector direction can be incomplete [10e22]. A contribution to ferromagnetic ordering may also be due to enhanced strain in nanograins/nanocrystals [11,14] and to uncompensated surface spins [18]. Moreover, considerable off-center displacements of the Bi3þ and Fe3þ ions with respect to the neighboring oxygen network [14] as well as an increase in dielectric polarization in the vicinity of TN pointing to strong multiferroic coupling have been reported in

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BFO nanoparticles with sizes below 28 nm [15]. As the size effect was found to be the way of improving the multiferroic properties further enhancement of multiferroic coupling was achieved by doping or cooping BFO with trivalent/divalent metal ions [19e22]. The doping resulted in further reduction of the average particle size and considerable increase in the saturation magnetization and the dielectric response. In Co-doped BFO the main increase in the magnetization has been ascribed to the superexchange Fe3þeO2-eCo3þ interaction, whereas the off-centered Fe3þ ions in the oxygen octahedron were found to be responsible for higher dielectric polarization [20]. As it has been stated above, the microstructure, i.e. the crystallite size and shape, has an impact on the dielectric response and the electric conductivity of BFO ceramics. Room temperature dielectric permittivity of BFO ceramics with grain sizes of ~200 nm was found to decrease from a few hundred to a few dozen in the frequency window 100 Hz ÷ 10 kHz and to remain constant at higher frequencies [4e6]. Temperature variation of the dielectric response of BFO nanoceramics [5,6] as well as conventional BFO ceramics [7] was found to be rather complex. One can distinguish the well separated low-temperature dielectric relaxation (LTDR) related to the effect of mixed valence of the Fe ions, the midtemperature dielectric relaxation (MTDR) ascribed to grain boundary effect and the high-temperature dielectric relaxation (HTDR) due to the ordering of oxygen vacancies. Here we report on the interactions of the oxygen vacancies with the ferroelectric domain walls within the HTDR range in BFO ceramics obtained from powder synthesized by microwave hydrothermal method. The powder grains consisted of flowerelike agglomerates of a dozen mm in diameter and 100e300 nm thick petals arranged perpendicularly to the flower center. 2. Experimental 2.1. Sample preparation Microwave assisted hydrothermal method was used to obtain the BFO powder samples [17]. Bismuth and iron nitrates: Bi(NO3)3$5H2O (0.81 g) and Fe(NO3)3$9H2O (0.67 g) were used as precursors. At the first step the precursors together with Na2CO3 (3.53 g) were added into a 6 M KOH aqueous solution (25 ml distilled water and 8.4 g KOH). In order to control the crystallite sizes the polyethylene glycol PEG2000 of 0.01% or 1% weight contents was added into the mixture, which was next transferred into a Teflon reactor and placed into a CEM MARS 5 microwave oven for ts ¼ 20 min at Ts ¼ 200  C. After the synthesis, the product of the reaction was cooled down to 20  C, collected by a filtration kit, rinsed with nitric acid and distilled water and dried for 2 h. The synthesis resulted in a powder consisting of resembling flowers ball-like porous grains of a dozen mm in diameter. Samples obtained from the solution containing 0.01 wt. % of PEG2000 were denoted as BFO-0.01%PEG and those synthesized with 1 wt. % of PEG2000 denoted as BFO-1%PEG. For dielectric measurements the samples were pressed at room temperature into pellets ~1 mm thick and 8 mm in diameter under pressure of ~1 GPa. The main faces of the samples were covered with gold sputtered electrodes in Baltec SCD050 sputter coater. 2.2. Sample characterization The crystallographic structure of the BiFeO3 flowers was studied by X-ray diffraction method (XRD) using a diffractometer equipped with Co lamp (l ¼ 0.17,928 nm) and an HZG4 goniometer operating in Bragg-Brentano geometry within the 2Q range from 25 to 100 . Analysis of the room temperature XRD patterns revealed that the

samples had contained only the BFO rhombohedral phase with R3c space group without any impurities. The parameters of the hexagonal unit cell obtained by the Rietveld refinement are summarized in Table 1. The morphology of BFO powder obtained by microwave hydrothermal synthesis and the microstructure of the ceramics were studied using field emission scanning electron microscope (SEM) FEI NovaNanoSEM 650. The SEM micrographs of the samples with various content of PEG2000 are shown in Fig. 1a and b. In Fig. 1a one can observe that the synthesized BFO powder contains ball-like agglomerates of ~15 mm in diameter resembling roses. The flowers are composed of many crystalline petals arranged perpendicularly to the flower center and the thickness of the crystallites is strongly correlated with the PEG2000 content in the solution the flowers were grown from. The higher content of PEG2000 resulted in thicker petals (Fig. 1b). The histograms of the petals thicknesses for particular samples shown in Fig. 1c were obtained from SEM images with use of Digimizer software. The petal thicknesses distribution can be well fitted with a log-normal distribution function:

   1 1 1 D f ðDÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi exp  2 ln2 2 D D 2s m 2ps

(1)

where Dm denotes the median thickness of the crystallites and s is the distribution width [23]. The distributions of the crystallites/ petals thickness for BFO samples synthesized with the addition of 0.01 wt. % and 1 wt. % of PEG2000 are shown in Fig. 1 c. For the lower content of PEG2000 (sample BFO-0.01%PEG) the histogram can be fitted with a single log-normal function. However, the histogram of the samples BFO-1%PEG reveals two separated maxima which are fitted using a superposition of two log-normal functions. Values of the mean thickness of the petals , the median thickness obtained for both Dm1, Dm2 as well as the distribution width s1, s2 are summarized in Table 2. It is apparent that the thickness of the petals and the distribution width increase with the PEG2000 content. The wider size distribution is an evidence of weaker dipole interaction between the grains separated by thicker layer of PEG2000 [24]. The morphology of the BFO-0.01%PEG and BFO-1%PEG ceramic samples is shown in Fig. 2. One can observe that the porosity of the ceramics depends on the thickness of the crystallites: ceramics prepared from the BFO powder with thicker petals exhibits higher porosity in comparison with that of the ceramics obtained from powder containing thinner petals. The density of the grain boundaries of the BFO-0.01%PEG ceramics is of course higher than that of the ceramics prepared from BFO-1%PEG powder. X-ray photoelectron spectroscopy was used to verify the oxidation states of Fe ions. The spectra were collected with an UHV (standard pressure of 5$1010 mbar) VG Scienta R3000 spectrometer and AlKa radiation (1486.6 eV). The Fe2p core level spectra of BFO-1%PEG powder spilled onto a conducting adhesive carbon tape fixed to a molybdenum sample holder were collected at room temperature. The binding energy was determined by reference to the C1s component at the energy of 285 eV and the GaussianeLorentzian functions were used to deconvolute the line shape. The presence of iron ions with mixed valence was confirmed by the

Table 1 Lattice parameters of the hexagonal unit cell refined for BFO synthesized with various PEG 2000 contents. Samples BFO-0.01%PEG BFO-1%PEG

a ¼ b [Å] 5.576 5.575

c [Å] 13.856 13.856

a, b, g [deg] 

V [Å]3 

a ¼ b¼ 90 , g ¼ 120 a ¼ b¼ 90 , g ¼ 120

373.028 372.966

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Fig. 3. Fe2p core level spectrum of BFO-1%PEG sample.

Fig. 1. SEM micrographs of BFO-0.01%PEG and BFO-1%PEG samples: (a) ball-like flowers and individual petals (b). The histograms (c) show the thickness distribution of the petals. The solid lines are the best fits to the logenormal distribution function.

Table 2 Mean thickness , median thickness Dm1, Dm2 and distribution width s1, s2 of the petals in BFO synthesized with various PEG 2000 contents. Sample

[nm]

Dm1 [nm]

s1

Dm2 [nm]

s2

BFO-0.01%PEG BFO-1%PEG

159 260

120 138

0.3 0.6

371

0.08

Fe3þ ions, Fe2p1/2 at 724.9 eV and Fe2p3/2 at 711.5 eV due to Fe2þ ions, and the signal shifted towards lower energies ascribed to the presence of metallic iron Fe02p3/2 at 708.8 eV. The 2p core level spectra of Fe3þ and Fe2þ ions overlap due to strong multiple splitting and shake up phenomena resulting in satellites marked as „dash-dot” lines in Fig. 3. One can distinguish the satellites of Fe2þ2p3/2 and Fe3þ2p3/2 signals at 719.5 eV and 721.9 eV, and overlapping satellites of Fe2þ2p1/2 and Fe3þ2p1/2 signals at 733 eV [25,26]. Comparison of the integrated intensities of the Fe2þ and Fe3þ contributions to the Fe2p core level spectra yields information that only about one half of the iron ions on the surface of the BFO1%PEG crystallites appear in the Fe3þ oxidation state. It should be however noted, that inelastic scattering of electrons excited by a laboratory X-ray source (energy of ~1.5 keV) limits the probing depth to about 3 nm [27]. Thus in the case of BFO one obtains information on chemical and electronic structure of 6 lattice constants thick surface layer [1]. 2.3. Dielectric response and electric conductivity measurements

Fig. 2. SEM images of the surfaces of ceramics obtained from BFO synthesized with various contents of PEG2000.

Fe 2p core level spectrum shown in Fig. 3. The line shape deconvolution yielded the following set of peaks: Fe2p1/2 at 727.0 eV and Fe2p3/2 at 713.7 eV with spineorbit splitting of 13.3 eV related to

Dielectric response ε*(f, T) ¼ ε0 (f, T)  iε00 (f, T) and the ac conductivity s0 (f. T) of the ceramic samples were studied using an Alpha-A High Performance Frequency Analyzer (Novocontrol

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GmbH) combined with a Quatro Cryosystem temperature controller. The measurements were performed in the temperature range 125 K  T  525 K on heating at a rate of 1 K/min and the frequency f varied from 1 Hz to 1 MHz at the oscillation voltage of 1 V. The samples were fixed between two additional external electrodes in a sample holder and placed into a cryostat. The dielectric permittivity and electric conductivity data were collected and evaluated by WinDETA impedance analysis software.

3. Results and discussion Temperature variation of the real ε0 and imaginary ε00 parts of the dielectric permittivity at the frequency f ¼ 1 kHz obtained on the first and the consecutive heating runs of a pellet obtained by pressing the BFO-1%PEG powder is shown in Fig. 4. One can observe that the first dielectric measurement reveals sharp peaks in the temperature dependences of ε0 and ε00 in the vicinity of 490 K. The both peaks disappear after heating to 500 K and are not observed during the second and subsequent heating runs. Looking for the reason of this behavior we performed differential scanning calorimetry (DSC) measurements for assynthesized BFO powder sample using a NETZSCH DSC 200F3 equipment at a heating rate of 5 K/min in the temperature range 300e600 K. As shown in Fig. 5 a broad exothermal anomaly is apparent in the temperature range ~500e560 K for BFO-1%PEG powder. We relate the anomaly to a recrystallization process of individual petals in the powdered samples and consider this process to be responsible for the dielectric anomalies in the BFO-1% PEG ceramics (solid lines in Fig. 4). The endothermal anomalies below 480 K are most likely due to water evaporation from the powdered sample. The difference in the temperature range of the DSC exothermal anomaly and the dielectric anomalies results from different experimental conditions: the DSC experiment was carried out at a heating rate of 5 K/min using powder samples, whereas the dielectric measurements were performed on pellets at a rate of 1 K/ min. As the second and the third heating do not reveal any DSC and dielectric anomaly in the vicinity of 490 K further results of the dielectric response and electric conductivity will be presented for the samples after recrystallization. Temperature dependences of the real ε0 and imaginary ε00 parts of the dielectric permittivity measured in the temperature range 125e525 K at various frequencies for BFO-0.01%PEG and BFO-1%

Fig. 4. Temperature dependence of real ε0 and imaginary ε00 parts of dielectric permittivity of BFO-1%PEG ceramic sample at 1 kHz obtained on the 1-st and the 2-nd heating at a rate of 1 K/min.

Fig. 5. DSC spectrum obtained for BFO-1%PEG powder on the 1-st and the 2-nd heating at a rate of 5 K/min.

PEG ceramic samples are shown in Fig. 6. The ε0 values below 150 K are rather low (ε’ z 15 ÷ 17) for all frequencies, whereas at higher temperatures one can observe a dispersion which increases considerably with the temperature: at 525 K ε0 (1 Hz)z10,000 and ε0 (1 MHz)z30. Similar behavior is observed for the ε00 (f, T) dependences but the absorption is significant even at low temperatures. The dielectric response ε*(f, T) of BFO-0.01%PEG and BFO-1% PEG shown in Fig. 6 is very similar to that observed for hotepressed nanoceramics obtained from BFO powder synthesized by high energy mechanical activation [6] as well as for ceramics with welldefined grain and grain boundary structure obtained by precipitation-synthesized BFO heated at 973 K [5]. One can distinguish three temperature ranges characterized by different relaxation processes: 150 Ke300 K, 300 Ke450 K and above 450 K (separated by white vertical lines in Fig. 6) and we will denote the processes as low-temperature dielectric relaxation (LTDR), middleetemperature dielectric relaxation (MTDR) and high-temperature relaxation (HTDR) after Hunpratub et al. [5]. The peculiar dielectric response has been related to the intrinsic heterogeneity of the polycrystalline materials the microstructure of which is determined by the size and the interconnection of grains and grain boundaries [5,6]. The grain interiors and the grain boundaries exhibit different electric conductivity: the conductivity of grain/ crystallite boundaries in ferrites is related to electron hopping Fe2þe/Fe3þ between localized ferric and ferrous ions and is considerably lower than that of the interior of the grains/crystallites [28]. In external electric field the difference in the conductivity is responsible for interfacial polarization and its frequency and temperature behavior. The role of the interfacial polarization in ACu3Ti4O12 (A ¼ Ca) type materials with giant dielectric permittivity has been discussed repeatedly [29e31], whereas the role of electric conductivity in the dielectric behavior has been stressed by Gheorghiu et al. [7]. To study the physical nature of the dielectric response we measured temperature dependences of s0ac conductivity in the frequency range 1 Hze1 MHz for BFO-0.01%PEG and BFO-1%PEG ceramic samples. As follows from Fig. 7, one can distinguish three temperature ranges of different s0ac (f, T) behavior (separated by white vertical lines) which correspond to the LTDR, MTDR and HTDR ranges in analogy with the dielectric response (Fig. 6). The s0ac conductivity shows also a considerable dispersion which vanishes at higher temperatures. In the low-temperature region (below ~300 K in Fig. 6) one can observe local maxima in ε00 (T) dependences, which become shifted towards higher temperature

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Fig. 6. Temperature dependences of real ε0 and imaginary ε00 parts of dielectric permittivity at frequencies of 1 Hz, 1.58 Hz, 2.51 Hz, 3.98 Hz, 6.31 Hz, 10 Hz, 15.8 Hz, 25.1 Hz . . 100 kHz, 158 kHz, 251 kHz, 398 kHz, 631 kHz, 1 MHz for BFO-0.01%PEG and BFO-1%PEG ceramics.

Fig. 7. Temperature dependences of the ac conductivity at frequencies of 1 Hz, 1.58 Hz, 2.51 Hz, 3.98 Hz, 6.31 Hz, 10 Hz, 15.8 Hz, 25.1 Hz . . 100 kHz, 158 kHz, 251 kHz, 398 kHz, 631 kHz, 1 MHz for BFO-0.01%PEG and BFO-1%PEG ceramics.

with increasing frequency f of the measuring electric field. Similar frequency behavior is apparent in a step-wise increase in s0ac (T) dependences shown in Fig. 7. The dielectric absorption maxima are well separated as shown in Fig. 8 so one can determine temperature dependence of the relaxation time t (Fig. 9). The relaxation times t of the low-temperature relaxation obey Arrhenius law pointing to the thermally activated process:

  E tðTÞ ¼ t0 exp a1 kB T

(2)

where Ea1 denotes the activation energy, t0 is the preexponential factor and kB is the Boltzmann constant. One can find that the activation energies calculated from Eq. (2) are the same for BFO-0.01%PEG and BFO-1%PEG ceramic samples, Ea1 ¼ 0.4 eV. The value of Ea1 of the low-temperature relaxation

does not depend on the thickness of the petals forming the flowers. Moreover, the obtained value is consistent with that published in our previous work concerning BFO nanoceramics prepared by mechanochemical synthesis [6]. The activation energy is most likely related to the reorientations of Fe3þFe2þ dipoles, which in the case of mixed valence can be described by electron transfer between localized ferric and ferrous ions: Fe3þþe/ Fe2þ. The XPS experiments (Fig. 3) have shown that about 50% of the iron ions appears in the ferrous state so the reorientations of the Fe3þFe2þ dipoles in the surface layer of the crystallites are responsible for the behavior of LTDR. One cannot however exclude the presence of the dipoles in the interior of the crystallites. The Fe3þFe2þ dipoles may develop polar clusters which grow in size with decreasing temperature and do not freeze contributing to the relaxation process.

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Fig. 8. Temperature dependences of imaginary parts of dielectric permittivityε00 in the LTDR range at frequencies of 1 Hz, 1.58 Hz, 2.51 Hz, 3.98 Hz, 6.31 Hz, 10 Hz, 15.8 Hz, 25.1 Hz… 100 kHz, 158 kHz, 251 kHz, 398 kHz, 631 kHz, 1 MHz for BFO-0.01%PEG and BFO-1%PEG ceramics.

Fig. 9. Relaxation times t of the LTDR versus reciprocal temperature T for BFO-0.01% PEG ceramics (open circles) and for BFO-1%PEG ceramics (squares).

The dielectric anomalies are very weakly outlined within the temperature range of MTDR due to high electric conductivity. The maxima of dielectric absorption are diffused making the calculation of the activation energy of the process impossible. Taking into account similarity to the behavior reported for BFO samples prepared by other methods we can ascribe MTDR to the effect of grain boundaries resulting in MaxwelleWagner polarization. Hunpratub at al [5]. observed a giant low frequency dielectric response in the temperature range 300e400 K for polycrystalline BiFeO3 ceramics obtained from pure precipitation-synthesized BiFeO3 with welldeveloped grain boundaries heated at 973 K, whereas the same material heated at 873 K characterized by poor grain boundary structure exhibited relatively low dielectric permittivity. In our previous work [6] we also reported the MTDR process in hotepressed BFO ceramics obtained from mechanochemically synthesized nanopowder. Moreover, our suggestions were based on the results for CaCu3Ti4O12 ceramics [29,31] for which a close relationship between the low frequency dielectric permittivity at room temperature and the grain sizes as well as the shaping of grain boundaries was found. The effect of grain boundary (Fig. 2) is more pronounced in samples containing 0.01% PEG characterized by thinner petals and larger effective surface of the grains.

In the range of HTDR one can observe a strong increase in dielectric permittivity ε0 and dielectric losses ε00 with rising temperature. To study the physical nature of HTDR, we analyzed the frequency dependences of s0ac conductivity at various temperatures (Fig. 10). At temperatures higher than ~450 K one can observe a low-frequency plateau of the conductivity, which allows to extract the dc conductivity contribution. The s0ac conductivity values at 1 Hz at temperatures 450 K  T  500 K enabled us to obtain the Arrhenius plot and determine the activation energies of the dc conductivity Ea2 (Fig. 11). The calculated activation energies Ea2 ¼ 1.73 eV for the BFO-0.01%PEG and 1.52 eV for BFO-1%PEG ceramics are much higher compared to ~1 eV typical for the movement of doubly ionized oxygen vacancies VO0 in perovskites [6,32e34]. The reason for increased activation energy can be perceived as a result of interactions between the oxygen vacancies and the ferroelectric/ferroelastic domain walls (DWs) in BFO. Our BFO0.01%PEG and BFO-1%PEG ceramics can be considered as consisting of crystallites with the mean thickness of 160 nm and 260 nm, respectively (Fig. 2; Table 2) and one can use the size scaling of the domain structure. The scaling law has been proposed by Kittel for magnetic domains [35,36] and has been extended to the ferroelectric and ferroelastic domain structures [37,38]. Recently it has been shown that the distance w between the DWs in ferroics with shapes of parallel-sided slabs can be scaled with the slab thickness d within six orders of magnitude according to the Kittel's scaling relation [39]:

w2 fd

(3)

The ferroelectric/ferroelastic domain structure of BFO appears in three variants: 71 DWs, 109 DWs and 180 DWs [40]. The topological walls separating ferroelectric domains in BFO were reported to have unusual properties due to significant distortion of the rhombohedral symmetry of the unit cells inside the wall [41e47]. Piezoelectric force microscope and high resolution transmission electron microscope studies supported by density functional theory calculations for 100 nm thick BFO films revealed electrostatic potential steps of 0.02 eV, 0.15 eV and 0.18 eV across the 71, 109 and 180 domain walls, respectively [41]. The results have been confirmed by the first principles studies which used the bulk symmetry to determine the DW orientations in BFO [42]. Further detailed studies combining conductive and piezoresponse microscopies clarified the problem of electric conductivity at the DWs in BFO [44,46,47]. The experiments have shown that the measuring electric field induces local distortions of the polarization profile of

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Fig. 10. Frequency dependences of ac conductivity s0ac at constant temperatures for BFO-0.01%PEG and BFO-1%PEG ceramics.

calculated as:

pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi   jwBFO0:01  wBFO1 j  160  260 pffiffiffiffiffiffiffiffiffi ¼ wBFO0:01 160

(4)

Thus the activation energy of high-temperature conductivity in BFO-0.01%PEG ceramics is expected to be higher than that in BFO1%PEG samples because the ionized oxygen vacancies are pinned by higher density of obstacles in form of DWs with electrostatic potential steps. It should be however observed, that the explanation is only qualitative since we assumed that the dielectric and elastic properties of the crystallites with ¼ 160 nm and 260 nm are the same. Moreover, we did not consider the thickness of the walls, which in the case of 109 is of the order of 1 nm and is suggested to be wider for the walls between antiparallel orientations of dielectric polarization [41]. 4. Conclusions

Fig. 11. High-temperature sdc conductivity as a function of reciprocal temperature for BFO-0.01%PEG ceramics (open circles) and BFO-1%PEG ceramics (squares).

the DW, which results in a dynamic electronic conductance of the walls. The above mentioned electronic properties of 109 and 180 topological boundaries in BFO and the size scaling of the domain widths in the BFO-0.01%PEG and BFO-1%PEG petals (in shape of parallel-sided slabs) seem to be responsible for the increase in activation energy of the high-temperature dc conductivity. Using the mean petal thickness as an approximation of thicknesses of the crystallites in the ceramics we assessed that the distance between neighboring DWs in BFO-0.01%PEG petals is of about 28% shorter than that in BFO-1%PEG polycrystalline samples. The relative distance between the DWs in BFO-0.01%PEG petals was

Powder BiFeO3 samples in form of flower-like porous grains of a dozen of mm in diameter were obtained by microwave hydrothermal synthesis. The thickness of the crystallites/petals was controlled by PEG2000 addition: SEM studies showed that addition of 0.01 wt. % of PEG resulted in the petals of mean thickness of ~160 nm, whereas flowers with mean petal thickness of ~260 nm were grown from solution containing 1 wt. % of PEG. XRD characterization of the samples revealed rhombohedral phase with R3c space group without any impurities, however about one half of the iron ions on the crystallite surfaces was found in XPS studies to exhibit 2 þ valence. The dielectric response of ceramics obtained by consolidation of BFO-0.01%PEG and BFO-1%PEG powders was found to exhibit three extrinsic contributions to the ε*(f, T) dependences. The similar result was reported earlier for BFO nanoceramics [5,6] as well as conventional BFO ceramics [7]. In the same temperature ranges we observed also different behavior of the ac conductivity s0ac (f, T). The

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low-temperature dielectric relaxation in the range 150 K  T  300 K was found to be characterized by the activation energy Ea1 ¼ 0.4 eV for BFO-0.01%PEG and BFO-1%PEG ceramics and related to the reorientation processes of Fe3þFe2þ dipoles. We reported the same value of the activation energy of the lowtemperature relaxation process for hot-pressed ceramics obtained from mechanochemically synthesized BFO nanopowder [6]. The dielectric response of our samples in the temperature range 300 K  T  450 K was found to be diffused and we did not characterize the relaxation process ascribed by Hunpratub et al. to the effect of grain boundaries resulting in MaxwelleWagner polarization [5]. The most interesting effect was found in the high temperature range, above 450 K. Extracting the dc conductivity contribution to s0ac (f, T) dependences from the low-frequency plateau we determined the activation energy of the electric transport Ea2 ¼ 1.73 eV and 1.52 eV for BFO-0.01%PEG and BFO-1%PEG ceramics, respectively. The obtained Ea2 values are considerably higher than the activation energies of double ionized oxygen vacancies in nanoceramics produced from mechanochemically synthesized BFO [6] and reported also for other perovskites [32,33]. The increase in the activation energy was discussed from the point of view of interactions between oxygen vacancies and size scaled ferroelectric/ferroelastic domain walls which in BFO are associated with the steps of the electrostatic potential [42,43].

[8]

[9]

[10]

[11] [12]

[13]

[14]

[15] [16]

[17]

[18]

Prime novelty statement Ball-like flowers of BiFeO3 with petals the thickness of which can be controlled within 100 ÷ 300 nm were obtained by microwave assisted hydrothermal synthesis. The dielectric response of ceramics with mean crystallite thickness ¼ 160 nm and ¼ 260 nm was found to be correlated with temperature variation of the ac conductivity and below 450 K to be similar to that of nanoceramics prepared from mechanoechemically synthesized BiFeO3. Above 450 K the dc conductivity was found to be characterized by activation energy of 1.73 eV for ceramics with ¼ 160 nm and 1.52 eV for ceramics with ¼ 260 nm considerably higher than that for other BiFeO3 nanoceramics. The increase in the activation energy we relate to interactions between oxygen vacancies and size scaled ferroelectric/ferroelastic domain walls, which in BiFeO3 are associated with electrostatic potential steps. Acknowledgments We would like to thanks to M. Sc. Michal Matczak from Institute of Molecular Physics Polish Academy of Sciences and NanoBiomedicalCentre, Adam Mickiewicz University in Poznan, for SEM micrographs. References [1] G. Catalan, J.F. Scott, Physics and applications of bismuth ferrite, Adv. Mater. 21 (2009) 2463e2485 (and references therein). [2] T. Rojac, A. Bencan, B. Malic, G. Tutuncu, J.L. Jones, J.E. Daniels, D. Damjanovic, BiFeO3 ceramics: processing, electrical, and electromechanical properties, J. Am. Ceram. Soc. 97 (2014) 1993e2011 (and references therein). [3] M. Graf, M. Sepliarsky, R. Machado, M.G. Stachiotti, Dielectric and piezoelectric properties of BiFeO3 from molecular dynamics simulations, Solid State Commun. 218 (2015) 10e13. [4] M. Kumar, K.L. Yadav, G.D. Varma, Large magnetization and weak polarization in solegel derived BiFeO3 ceramics, Mater. Lett. 62 (2008) 1159e1161. [5] S. Hunpratub, P. Thongbai, T. Yamwong, R. Yimnirun, S. Maensiri, Dielectric relaxations and dielectric response in multiferroic BiFeO3 ceramics, Appl. Phys. Lett. 94 (2009) 062904, 1e3. [6] E. Markiewicz, B. Hilczer, M. Błaszyk, A. Pietraszko, E. Talik, Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders, J. Electroceram. 27 (2011) 154e161. [7] F. Gheorghiu, M. Calugaru, A. Ianculescu, V. Musteata, L. Mitoseriu, Preparation

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