DIELECTRIC CHARACTERIZATION OF THE BiFe_0.5CR_0.5O₃ CERAMICS

1. Introduction

Oxide multiferroic materials that contain lone-pair cation(s) and transition metal cation(s) attract scientific interest because they may exhibit polar and magnetic ordering in a single phase. Bismuth ferrite, BiFeO₃ (BFO), is one of the  best-known type-I multiferroic materials. Moreover, BFO is the  only Bi-containing simple perovskite, which can be manufactured in the bulk form via conventional techniques at ambient pressure. BFO shows a  ferroelectric ordering and a  complex magnetic ordering with a long-period cycloid, both remaining well above room temperature [1]. Its Curie temperature is as high as T_C = 1103 K and the Néel temperature is T_N = 643 K [1]. In the ferroelectric state, the  perovskite structure of BFO is rhombohedral with the space group R3c.

Synthesis of single-phase perovskite BiCrO₃ using a  high-pressure high-temperature method was first reported by Sugawara et al. in 1968 [2]. The first measurements showed that the material is an antiferromagnet with a weak parasitic ferromagnetic contribution below 123 K [2]. Forty years ago, BiCrO₃ was studied in more detail using neutron diffraction in the temperature range between 7 and 490 K [3]. It was found that BiCrO₃ is monoclinic (space group C2/c) below 420 K, while above this temperature it exhibits a non-polar orthorhombic (Pnma) structure. Four magnetic anomalies were revealed at temperatures of 40, 75, 109 and 111 K, respectively [4].

BiFe_0.5Cr_0.5O₃ (Bi₂FeCrO₆) is an equimolar solid solution between the  end members of the  binary BiFeO₃-BiCrO₃ system. Based on the ab initio calculation reported in 2005, it was predicted that this perovskite composition would demonstrate the properties that would far surpass any known multiferroic: a polarization of 80 μC/cm² along the [111] axis and magnetization of 160 emu/cm³ [5, 6]. Polarization arises from a single bismuth atom (as in BiFeO₃ or BiCrO₃) and ferrimagnetism from the  exchange of iron and chromium atoms. The structure of this material was considered to be similar to BFO with iron cation replaced by chromium cation in every second place of the [111] axis [5, 6].

In 2007, Suchomel et al. reported the successful preparation of BiFe_0.5Cr_0.5O₃ ceramics using a high-pressure synthesis method [7]. Indeed, the crystal structure of the  obtained BiFe_0.5Cr_0.5O₃ perovskite was found to be rhombohedral (R3c) at room temperature (Scheme 1). The material turned out to be paramagnetic at room temperature due to the structural disorder of the Fe³⁺ and Cr³⁺ cations. No dielectric properties have been reported.

Still, in 2007, an epitaxial thin layer of BiFe_0.5Cr_0.5O₃ deposited on SrTiO₃ substrate using a pulsed laser deposition technique was reported [8, 9]. The material was shown to be ferroelectric with a  high polarization (60  ±  1  μC/cm²) at 77  K and with antiferromagnetic ordering. It was also found that the magnetic hysteresis in BiFe_0.5Cr_0.5O₃ occurs at room temperature and its magnetization value is one order of magnitude higher than that of the BFO film of the same thickness [8, 9]. Two years later, the same group of scientists observed a  magnetic phase transition in the  BiFe_0.5Cr_0.5O₃ film between 600 and 800 K, but the exact critical temperature has not been determined [10].

In summary, the  perovskite composition BiFe_0.5Cr_0.5O₃ has not been extensively studied, but the available results lead to the conclusion that it is a ferroelectric with a probable phase transition above room temperature.

Here we report the results of dielectric characterization of chromium-substituted bismuth ferrite (BiFe_0.5Cr_0.5O₃) ceramics in a wide temperature range using impedance spectroscopy.

2. Experiment

The BiFe_0.5Cr_0.5O₃ ceramics were prepared under high-pressure and high-temperature conditions as described in Ref [11]. The thickness of the samples was 0.6–0.7 mm and the area was between 1.2 and 3.4 mm². The samples were electrodes with silver paint. Dielectric measurements were performed in the wide frequency range from 20 Hz to 1 GHz over the  temperature interval of 200–500  K. In the 20 Hz – 1 MHz range, a Hewlett Packard 4284 LCR-meter was used to measure the capacitance and the loss tangent of samples, and a model of the flat dielectric capacitor was used to calculate the  complex dielectric permittivity. Measurements of complex transmission and reflection coefficients were performed using an Agilent 8714ET network analyzer in the 1 MHz – 1 GHz range. In this case, a multimode capacitor model was used to obtain complex dielectric permittivity.

3. Results and discussion

The temperature dependence of the real and imaginary parts of the  dielectric permittivity of BiFe_0.5Cr_0.5O₃ ceramics is shown in Fig. 1. In the region of relatively high temperatures, a growth of both parts with temperature is observed, which is caused by the conductivity. The rise of the real part of the dielectric permittivity at the lowest frequency can be influenced by Maxwell–Wagner polarization, that is a phenomenon where the charges in the material accumulate near the electrodes until they reach a critical total value and new charges are injected from the electrode layer.

Figure 2 shows the  dependence of the  real and imaginary parts of the  dielectric constant on the  frequency at different temperatures. As the frequency is increased, both ε' and ε'' decrease, which is a property of ferroelectrics [12]. At low temperatures, 300 K and below, there is a visible dielectric dispersion that can be associated with ferroelectric domains [13]. The dispersion maximum shifts to higher frequencies with increasing temperature. At low frequencies (less than 1 MHz), a conductivity pot is visible.

From the  approximation of the  frequency dependences by the Cole–Cole equation [14], the relaxation times were obtained, which are shown in Fig. 3. Based on the obtained data, the activation energy was calculated using the  Arrhenius law τ  =  τ₀ exp(E_A/kT). The  value of activation energy for BiFe_0.5Cr_0.5O₃ is E_A = 0.181 ± 0.002 eV and τ₀ = 22.5 ± 0.1 ps.

In the considered BiFe_0.5Cr_0.5O₃ ceramics, electrical conductivity is visible at high (over 300 K) temperatures. The  dependence of the  electrical conductivity on the frequency at different temperatures is shown in Fig. 4. One can see from the obtained dependences that the static part of the conductivity is observed from room temperature.

The direct current conductivity values were obtained using the  Amond–West law  [15]. Figure 5 shows the dependence of σ_DC on the inverse temperature. The  value of activation energy for BiFe_0.5Cr_0.5O₃ calculated according to the  Arrhenius law [16] is E_A = 0.302 ± 0.006 eV.

Figure 6 shows the  frequency dependence of the real and imaginary parts of the complex resistivity of BiFe_0.5Cr_0.5O₃ ceramics at 400 and 500 K.

One can see that the peak of the real part shifts to higher frequencies with increasing temperature. Besides, the  lower the  temperature, the  higher the real and imaginary parts of the resistance are observed. A  high value of specific resistance can also be influenced by electrical contacts. The measured resistivity was fitted using the  modified Cole–Cole equation for resistivity processes, that is represented by the solid curve in Fig. 6. The relaxation time values were also obtained from the fit. Figure  7 shows the  dependence of the  relaxation time of the BiFe_0.5Cr_0.5O₃ ceramics on the inverse temperature. The value of activation energy for BiFe_0.5Cr_0.5O₃ calculated using the  Arrhenius law [16] is E_A = 0.319 ± 0.005 eV.

4. Conclusions

Dielectric properties of the  BiFe_0.5Cr_0.5O₃ ceramics synthesized under high pressure have been studied using broad-band dielectric spectroscopy over the  temperature range of 200–500  K. It was revealed that the  BiFe_0.5Cr_0.5O₃ ceramics exhibit electrical conductivity above about 300  K. Below 300 K, a dielectric dispersion caused by ferroelectric domains is observed. The  measured complex dielectric permittivity and the complex resistivity were fitted using the Cole–Cole equation. The electrical conductivity was approximated with the Almond–West law and a DC contribution was estimated. Activation energy values were found via the Arrhenius law.

Acknowledgements

The research done at the University of Aveiro was supported by the Project of CICECO – Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC).

References

DIELEKTRINĖ BiFe_0,5Cr_0,5O₃ KERAMIKOS CHARAKTERISTIKA

^a Vilniaus universiteto Taikomosios elektrodinamikos ir telekomunikacijų institutas, Vilnius, Lietuva

^b Nepriklausomi nuo afiliacijos tyrėjai

^c Aveiro universiteto Medžiagų institutas CICECO, Aveiras, Portugalija

Santrauka

Multiferoikas – medžiaga, turinti ir feroelektrinių, ir feromagnetinių savybių. Viena tokių medžiagų yra bismuto ferito keramika (BFO), kuri turi feroelektrinių bei feromagnetinių savybių kambario temperatūroje, todėl sulaukė didelio tyrinėtojų dėmesio. Dėl paprastos gardelės struktūros ir savybių kitimo kambario temperatūroje dauguma naujų keramikų yra sintetinamos su šios medžiagos elementais: bistumu, geležimi ir deguonimi. Viena tokių medžiagų – bismuto ferumo chromo oksidas (BiFe_1–xCr_xO₃), apie kurią žinoma mažai, tačiau tikimasi, kad tai bus aukštos temperatūros multiferoikas. Dėl feroelektrinių ir feromagnetinių savybių tikėtina, kad multiferoikai bus plačiai pritaikomi elektronikoje, pavyzdžiui, magnetinei atminčiai kontroliuoti elektriniu lauku arba atvirkščiai.

Šiame darbe publikuojami BiFe_0,5Cr_0,5O₃ keramikos dielektriniai tyrimai. Tyrimai atlikti plačiajuostės dielektrinės spektroskopijos metodu 20  Hz  –  1  GHz dažnių diapazone esant 200–500  K temperatūrai. BiFe_0,5Cr_0.5O₃ keramikos dielektriniams spektrams daugiausia įt,kos turi elektrinis laidumas, kurio temperatūra yra daugiau nei 400 K ir dažnis žemesnis nei 100 Hz. BiFe_0,5Cr_0,5O₃ keramikoje žemose temperatūrose (žemiau 300 K) matoma dielektrinė dispersija, kurią nulemia feroelektriniai domenai. BiFe_0,5Cr_0,5O₃ keramikoje žemuose dažniuose stebimi elektrinio laidumo indukuoti procesai, aukštuose dažniuose – dipolinė relaksacija.