Materials with high dielectric permittivity values, so-called giant permittivity, reaching ε¢-10⁵, have been widely investigated because of their various applications in the microelectronic industry [1] .

Multiferroic materials exhibit ferroic order parameters simultaneously, namely ferroelectricity, ferromagnetism and ferroelasticity, in the same phase [2] [3] . These materials have been widely studied in recent years because of their potential applications in new generations of memory devices, magnetic field sensors, etc. [4] [5] . However, the possibilities for room temperature single phase multiferroic materials are limited. Most of these materials have high ferroelectric Curie temperature T_c and high magnetic transition temperature (Neél temperature T_N) [6] [7] .

The main multiferroic oxide compounds are BiFeO₃, BiMnO₃ and ReMnO₃ (Re = Y, Ho, Lu). Among them, BiFeO₃ is one of the most investigated multiferroic materials, which belongs to the rhombohedral space group R3c, and transforms to the orthorhombic space group Pbnm at a ferroelectric Curie temperature T_c-1103 K. In addition, it shows typical G-type antiferromagnetic properties below its Neél temperature T_N-643 K [8] [9] .

Although BiFeO₃ (BFO) materials show potential in several applications, some of their drawbacks still need to be resolved, such as large dielectric loss and high leakage current, which are induced by oxygen vacancy, non-stoichiometry and valence fluctuation of Fe ions at room temperature [2] . However, because of the higher leakage current in the bulk BFO, it was difficult to measure the ferroelectric properties of BFO at room temperature. The problem of higher leakage hinders not only the studies of the electrical properties of BFO but also the application of BFO in electrical devices. Oxygen vacancies in the BFO material are the major origin of the formation of Fe²⁺, according to

V O X + 2Fe 3 + ⇔ V O • • + 2Fe 2 +

where V O • • is the oxygen vacancy and V O X is the oxygen position [10] . Following the Le Chatelier principle [11] , the concentration of Fe²⁺ is reduced if the content of Fe³⁺ is decreased. Therefore, substitution of rare-earth ions (La, Nd and Gd) at Bi³⁺ sites in BFO or transition-metal-ions (Mn, Cu and Co) at Fe³⁺ sites would control the formation of oxygen vacancies and reduce the concentration of Fe²⁺ in BFO [12] [13] . Among such ions, Cr doping BFO could improve the electrical properties and give lower leakage current than pure BFO [14] .

In this work, we study the synthesis of BFO ceramics doped with Cr. The effect of Cr²⁺ substitution on structural and dielectric properties, and leakage current of BFO namely BiFe_1−xCr_xO₃ (x = 0.2 and 0.4) ceramics is outlined in detail.

Polycrystalline samples of BiFe_1−xCr_xO₃ (x = 0.2 and 0.4) were synthesized by a solid-state reaction at a high temperature of 1273 K and a pressure = 7 GPa. A mixed powder of Bi₂O₃ (99.9%), Fe₂O₃ (99.9%) and Cr₂O₃ (99.9%) with stoichiometric proportions in a 1:1 mole ratio was mixed in an agar mortar for half an hour, then packed into a gold capsule (≈4 × 6 mm²). It was then heated in a cubic anvil-type apparatus under 7 GPa at 1273 K for 1 hour [15] . X-ray diffractions (XRD) were carried out using a diffractometer with Cu-Kα source. The X-ray patterns were collected at an interval of 0.01˚. The structural parameters were refined by employing the Rietveld analysis of diffraction data in the 2θ range of 20˚ - 70˚. Dielectric permittivity measurements were carried out using an RLC meter in conjunction with a laboratory-made sample holder and a heating arrangement. Dielectric data were collected while heating in the temperature range from room temperature up to 773 K and using a frequency = 10 kHz. Leakage currents were measured by using a 4200-SCS semiconductor characterization system (Keithly).

Figure 1(a) shows the XRD patterns of calcined powder of BiFe_1−xCr_xO₃ with x = 0.2 and 0.4 at room temperature.

All the peaks were indexed in single phase without any formation of a second phase with perovskite structure. Although calcination was performed, there were some impurity phases of Bi₂Fe₄O₉ and Bi₁₂O₃ as shown at 2θ = 24.5˚ - 30.1˚ for the samples, as reported by several authors [16] . The samples showed a rhomohedral structure with R3C spacing group and have been indexed as a hexagonal unit cell.

To further investigate the effect of increasing Cr content on the structure of BiFe_1−xCr_xO₃, subtle XRD patterns with the strongest diffraction peaks of 2θ around 22˚ - 23˚ and 45.5˚ - 46.5˚ were obtained, as shown in Figure 1(b) and Figure 1(c). The figure shows that when the Cr³⁺ substitution is increased, the position of peaks shifted towards the higher angle side as x increased, leading to a decrease in lattice parameters for x = 0.4. This was expected because the ionic radius of Cr³⁺ (0.615 Å) is smaller than that of Fe³⁺ (0.645 Å) [2] [15] . Thus, a distortion and deformation of the structure occurred when Fe³⁺ and Cr³⁺ ions were substituted at the B-site. The lattice parameter data are given in Table 1.

Figure 2 and Figure 3 displays the temperature-dependent variation in dielectric constant ε¢ and dielectric loss tanδ for BiFe_1−xCr_xO₃ ceramics (x= 0.2 and 0.4) performed at 10 kHz. The dielectric constant for the two samples shows a continuous increase with elevating temperatures. A significant anomaly (plateau) was observed around 220˚C - 240˚C in the dielectric constant of the samples, which is consistent with other reported studies [17] [18] [19] . This anomaly (plateau) is attributed to the transient interaction between oxygen ion vacancies and Fe²⁺ and Fe²⁺ redox being responsible for the conductivity of the samples. The plateau range becomes narrower with the Cr³⁺ substitution, showing that the Cr³⁺ substitution decreases the oxygen vacancies, which in turn reduces in the concentration of Fe²⁺ in BiFe_1−xCr_xO₃.

Thereafter, as shown in Figure 2, the dielectric constant increases rapidly with increasing temperature. The dielectric peak is observed in the samples (x = 0.2 and 0.4) at around 430˚C and 360˚C, respectively, which is above the Neél temperature. The Neél temperatures observed for x = 0.2 and for x = 0.4 are 242˚C and 170˚C, respectively [inset of Figure 2(a) and Figure 2(b)] as confirmed from our earlier magnetic study report [15] .

This peak in the dielectric constant is attributed to a transformation from the antiferromagnetic order to the paramagnetic order, indicating an effect of disappearing magnetic order as compared to electric order, and attests to magneto-electric