Dielectric relaxations in pure, La-doped, and (La, Co)-codoped BiFeO₃: Post-sintering annealing studies

Highlights

• All the ceramics were prepared via solid-state reaction method.

• Up to three thermally activated relaxations were found in the samples.

• The relaxations can be eliminated by annealing in N2 and recreated by annealing in O2.

• Our results firmly evidence that holes are the relaxing species.

Abstract

Pure, La-doped, and (La,Co)-codoped BiFeO₃ ceramic samples were prepared by solid-state reaction. By means of dielectric permittivity, electric modulus spectroscopy, and impedance analysis, the dielectric properties of the samples were systematically investigated in the temperature range of 140–330 K and frequency range of 20–10⁷ Hz. Three, two, and one dielectric relaxations were found in pure, La-doped, and (La,Co)-codoped samples, respectively, in the investigated temperature window. Post-sintering annealing studies reveal that both p- and n-types carriers coexist in the samples with the p-type carriers (holes) being the major carriers acting as relaxing species. The dielectric properties are determined by the competition between the n-type carriers (electrons) and the holes. La-doping and (La,Co) dual doping enhance the role played by the holes. Impedance analysis shows that the low-temperature (high-frequency) relaxation is a polaronic relaxation resulting from the hoping motion of holes inside grains. The middle-temperature (frequency) and high-temperature (low-frequency) relaxations are Maxwell-Wagner relaxations caused by the hoping motion of holes blocked by grain boundaries and sample-electrode contacts, respectively. These results underscore the role of sample preparing conditions in the dielectric properties of BiFeO₃ and suggest that optimizing the preparing procedures would be a promising strategy to achieve superior properties of BiFeO₃.

Introduction

Materials exhibiting two or all three coupled ferroic orders of ferro/antiferromagnetics, ferroelectrics, and ferroelastics are termed as multiferroics [1,2]. Multiferroic behavior endows materials with the “product” properties, leading to various types of new physical phenomena in the materials and offering the potential for multifunctional devices which are unachievable by conventional ferroic materials [3,4]. The great potential for practical applications as well as the rich physics, have led to a tremendous flurry of research interest in multiferroic materials in recent years. Unfortunately, because the ferroelectric ordering (which requires the cations to have an empty d orbital) is incompatible with the ferromagnetic ordering (non-empty d orbital is needed), there are enumerable multiferroic materials exist in nature [5]. Additionally, the observed multiferroic behavior of most multiferroics frequently occurs in a low temperature range of $T$ < 50 K [6], which strongly limits the applications of these materials. BiFeO₃ is so far the only known room-temperature multiferroic material. It, therefore, goes to the fore front of research in the multiferroic realm.

A formidable problem limiting the practical application of BiFeO₃, is the relatively high leakage current. Although, many strategies, including for example doping [7], sample processing [[8], [9], [10], [11]], film strain [[12], [13], [14], [15]], etc. have been adopted to solve the problem, it is still difficult to achieve better ferroelectric and magnetic properties simultaneously in single-phase BiFeO₃. A prerequisite for solving the problem is to completely understand the mechanism of the leakage current. Physically, the current can be contributed by weakly localized electrons/holes and/or ions. The hoping motions of these carriers not only yield conduction but also create polarization [16]. Therefore, dielectric spectroscopy can provide useful insight into the microscopic features of the conducting/relaxing species.

So far, the dielectric measurements were performed extensively on BiFeO₃ thin films [12,[17], [18], [19]] and ceramics [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. The results are considerable discrepancies, providing strong motivation for further studies. Two main features of the dielectric properties of BiFeO₃ can be extracted: (1) only one relaxation with an activation energy of ∼1.0 eV was found in thin film system [12,17,19], and (2) up to four relaxations were reported in ceramic samples. For clarity, these relaxations are termed as L1-L4 in the order of ascending temperature. Details of them are summarized in Table 1. Different mechanisms were proposed for L2-L4. For example, charge carriers hopping between Fe²⁺ and Fe³⁺ ions [26,27], reorientations of Fe²⁺ - Fe³⁺ dipoles [33], the migration of singly charged oxygen vacancies [28], etc. were argued to be putative mechanism for L2; Maxwell-Wagner relaxation due to grain boundaries [26] and migrations of oxygen vacancies [27] were proposed as possible mechanism for L3; defect ordering [26], short-range motion of doubly charged oxygen vacancies [12], and interfacial polarization [33] were considered to be the mechanism for L4.

It is noteworthy that, although the reported mechanisms for these relaxations are inconsistent, there is a consensus that these relaxations are strongly related to oxygen vacancies. It is well-known that oxygen vacancies and their migration play a crucial role in determining the (di)electric, magnetic, and optical properties of BiFeO₃ [[34], [35], [36]]. Therefore, detailed investigations of the influence of oxygen vacancy on the dielectric behavior can not only deepen the understanding of the mechanism of the leakage current, but also be helpful for optimizing the preparation conditions for future potential applications. In the present work, the dielectric properties of the nominally pure, La-, and (La, Co)-doped BiFeO₃ samples after different annealing treatments were studied.