Elsevier

Materials Letters

Volume 264, 1 April 2020, 127314
Materials Letters

Unusual combination of conduction and dielectric relaxations in nanocrystalline α-Fe2O3

https://doi.org/10.1016/j.matlet.2020.127314Get rights and content

Highlights

  • Nanocrystalline α-Fe2O3 shows a dielectric loss peak at higher frequencies.

  • Combined Cole-Cole conduction and dielectric relaxation function is used to study the electrical properties of the material.

  • The material possesses pinned and free dipoles.

  • Hopping of charge carriers and lattice disorder create a combined relaxation in the material.

Abstract

The present study elucidates the temperature and frequency-dependent electrical properties of α-Fe2O3 synthesised by co-precipitation method. XRD and Raman spectroscopy are employed to study the formation and crystal structure of the material. The material shows two relaxation peaks in the frequency range 1 Hz–1 MHz. The ac electrical data of the material are analysed with the novel relaxation function combining conduction and dielectric relaxations. The results reveal that the material possesses two types of dipoles, one corresponds to the free dipoles of FeO6 molecules, and the other corresponds to the pinned dipoles created by charge carriers and ion lattice. The dc conductivity, conduction relaxation time and dielectric relaxation time obey Arrhenius law.

Introduction

The research in the fields of dielectric, ferroelectric and piezoelectric materials has got recent attention due to the wide applicability of these materials in different areas such as microwave tuners, electro-optic modulators, capacitors, sensors, micro-electromechanical systems, transducers etc. Corundum structured magnetic oxides like α-Fe2O3 (Hematite) are considered to be promising materials due to their applications in data storage, photocatalysis, sensors, spintronics, energy storage etc. [1], [2], [3], [4]. Hematite is one of the stable iron oxides with low cost, environmental friendliness, biocompatibility and non-toxicity. It has a rhombohedral structure with n-type semiconducting properties. The detailed study on the electrical properties of semiconducting materials is important not only due to the fundamental interest but also for various applications. As far as we know, it is very hard to locate a detailed qualitative analysis of the temperature-dependent ac electrical properties of hematite nano-crystallites in the literature.

Impedance or dielectric spectroscopy is a powerful tool to investigate the charge carrier dynamics in different types of materials. It measures the impedance of a system as a function of frequency and provides information about energy storage and dissipation. The fundamental understanding of dipole relaxations and dipole–dipole interactions from conduction and dielectric dispersion data is still challenging [5]. Properly combined relaxation functions are needed to analyse the materials having both conduction and dielectric relaxations. Kumar et al. proposed a novel relaxation model combining Cole-Cole type conduction and dielectric relaxation functions and studied the ac electrical properties of nano-crystalline Mn0.5Zn0.5Fe2O4 exhibiting two different relaxations. Later, the model was extended to other systems and the results were reported [5], [6], [7], [8]. In the present study, we have investigated the ac electrical properties of nano-crystalline α-Fe2O3 synthesised using co-precipitation method. The material shows two different relaxation processes, and they are shown to be combined conduction and dielectric relaxations. These interesting results are discussed and presented.

Section snippets

Experimental

The material α-Fe2O3 is prepared by co-precipitation method. Firstly, the appropriate amount of iron chloride hexahydrate (FeCl3·6H2O) is dissolved in 150 ml of double distilled water with constant stirring at room temperature. At a temperature of 80 °C, NH4OH is added dropwise to the above solution to trigger precipitation, and the pH is maintained at 11. The obtained precipitate is collected by centrifugation and washed with water several times to remove chloride ions. The product is further

Results and discussion

The XRD pattern and JCPDS reference (033-0664) of the material are shown in Fig. 1a. It confirms the crystallisation of materials in the form of a rhombohedral structure of space group R-3c. The average crystallite size of the material is calculated using the Williamson-Hall (W-H) method and found to be 49 nm. W-H plot depicted in Fig. s1 shows a positive slope. The plot indicates the lattice strain present in the material. Lattice defects or oxygen vacancies present in the material may produce

Conclusions

The material α-Fe2O3 is prepared successfully by co-precipitation method. XRD and Raman analyses confirmed the formation and rhombohedral structure of the material. AC electrical data of the material show good agreement with Cole-Coe type combined conduction and dielectric relaxation model. Analysis of dielectric spectroscopic data revealed the presence of pinned dipoles and free dipoles in the material. The dc conductivity, conduction and dielectric relaxations follow Arrhenius behaviour. The

CRediT authorship contribution statement

Pulikkathumbayil Muhammed Junais: Conceptualization, Software, Formal analysis, Investigation, Writing - original draft, Visualization. G. Govindaraj: Methodology, Validation, Resources, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Authors thank Central instrumentation facility (CIF), Pondicherry University, for providing characterisation facilities.

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