Elsevier

Journal of Alloys and Compounds

Volume 773, 30 January 2019, Pages 470-481
Journal of Alloys and Compounds

Preparation and dielectric properties of Ba4RFe0.5Nb9.5O30 (R = La, Nd, Eu, Gd) unfilled tungsten bronze ceramics

https://doi.org/10.1016/j.jallcom.2018.09.241Get rights and content

Highlights

  • New BRFN relaxor ceramics were prepared.

  • Both the V and Tm increase as rare earth radius decrease.

  • The Nb–O stretching changed as the A1-site ion size decreased.

Abstract

Lead-free relaxor ferroelectrics with the general structural formula Ba4RFe0.5Nb9.5O30 (R = La, Nd, Eu, Gd) (BRFN) were prepared using a standard solid-state sintering route, and the influence of the A site ion occupation on their electrical properties and dielectric characteristics was systematically discussed based on their structure. All compounds presented tetragonal tungsten bronze (TTB) structure with space group P4bm, where the A1 sites were exclusively occupied by R3+ ions and the A2 sites were exclusively occupied by Ba2+ ions, while the Fe3+ and Nb5+ ions were randomly distributed between the B1 and B2 sites. Furthermore, frequency and temperature caused BRFNs to present a broad permittivity peak accompanied by wide frequency dispersion. The maximum dielectric constant temperature gradually increased and the frequency dispersion gradually decreased as the A1-site ion size decreased. Complex impedance spectroscopy indicated that only the grain boundaries contributed to the conductive behavior of BRFN ceramics at high temperature, and the activation energy increased as the A1-site ion size decreased. The direct current (DC) conductivity of all compounds originated from the electrons generated by the second ionization of the oxygen vacancies and might have been caused by the hybrid conduction mechanism of the oxygen-vacancy ions and electrons. Raman spectra of the BRFN ceramics mostly contained multi-component and broad bands related to the internal vibrations of the octahedral NbO6, and as the size of the A1-site ion decreased, the Nb-O stretching changed.

Introduction

Ferroelectrics play an important role in the field of electronic materials because of their wide applications in photovoltaic, thermoelectric, piezoelectric, and nonlinear optical devices [[1], [2], [3]]. Modern applications of dielectrics and ferroelectrics are still based on perovskite materials. This is mainly due to the possibility of rationalizing the structure/properties through tilting systems and geometric tolerance factors [4]. The development of the electronics industry has facilitated the study of other structures, and allowed researchers to discover new materials to replace the existing lead-containing compounds. Due to its variable composition, the structure and physical properties of tetragonal tungsten bronze (TTB) have been extensively studied. The perovskite structure can be transformed into a tungsten bronze (TB) structure by tilting an oxygen octahedron to form a lattice with three different types of voids (quadrangular A1, pentagonal A2, and triangular C), thus the general formula of the TB structure would be (A1)2(A2)4C4(B1)2(B2)8O30 [5]. The A1 and A2 sites would be occupied by the larger cations, the B1 and B2 sites in the center of the oxygen octahedra would generally be occupied by small-radius and high-valence cations, and the C position would usually be empty.

Previous investigations of this structure have focused mainly on filled TTBs. Research on KBa3RNb10O30 (R = La, Pr, Nd, Sm, Eu, Gd) filled TB ceramics determined that KBa3RNb10O30 (R = La, Pr, Nd, Sm, Eu) ceramics are relaxors, while KBa3GdNb10O30 is a normal ferroelectric [6]. Research on the Ba5RZr3Nb7O30 (R = La, Nd, Sm) filled TB ceramics determined that as the difference in radius between the A1-and A2-site ions decreased, the peak temperature of dielectric relaxation became lower and the permittivity peak exhibited a more diffuse nature [7]. Researchers have studied Ba2MTi2Nb3O15 (M = Bi, La, Nd, Sm, Gd) with filled TTB structure and concluded that BNTNO15, BSTNO15, and BGTNO15 were first order ferroelectrics, while BLTNO15 and BBTNO15 exhibited relaxor behavior [8]. Furthermore, Ba4RETiNb9O30 (RE = La, Pr, Nd, Sm, Eu, Gd) unfilled TB compounds and ceramics have been investigated, and it was found that as the radius of the rare earth metal ion decreased, the tetragonality and phase transition temperature of the TB structure increased [9].

So far, there have only been few studies focused on the structure and properties of materials exhibiting unfilled TB structure. In this work, the effects of structural modulation by different rare earth elements occupying the A1-sites on the crystal-chemical environment, dielectric, and ferroelectric properties of unfilled TB BRFN-based ceramics were researched in detail. Our study aimed to enrich the understanding of the dielectric properties and crystal structure of unfilled TTB ferroelectrics and to offer guidance for designing new ferroelectric materials.

Section snippets

Materials and methods

Ba4RFe0.5Nb9.5O30 (R = La, Nd, Eu, Gd) ceramics were manufactured based on a conventional mixed-oxide technique using BaCO3 (99.95%), La2O3 (99.99%), Nd2O3 (99.99%), Eu2O3 (99.99%), Gd2O3 (99.99%), Fe2O3 (99.99%), and Nb2O5 (99.99%) reagent-grade powders as raw materials. The mixing ratio of the high-purity powders was obtained using the stoichiometric-ratio method. The powders were mixed with industrial alcohol and thereafter ground for 12 h using zirconia balls. The well-mixed raw material

Results and discussion

The geometric tolerance factor (t) can be used to evaluate the stability of the TB structure system:t=(tA1+2tA2)3and the tolerance factors for A1 and A2 sites are:tA1=(RA1+RO)2(RB+RO)tA2=(RA2+R0)23123(RB+RO)where RA1, RA2, and RB are the effective radii of the A1-, A2-, and B-site ions, while RO is the radius of the oxygen ion [10,11]. The effective tolerance factors were 0.94, 0.92, 0.91, and 0.90 for the Ba4LaFe0.5Nb9.5O30 (BLFN), Ba4NdFe0.5Nb9.5O30 (BNFN), Ba4EuFe0.5Nb9.5O30 (BEFN), and Ba4

Conclusions

A new type of lead-free ferroelectric Ba4RFe0.5Nb9.5O30 (R = La, Nd, Eu, Gd) ceramics presenting unfilled TTB structure with P4bm space group were prepared using a solid phase synthesis method. The calculated geometric tolerance factors indicated that the structural stability gradually decreased as the radii of the A1-site rare earth cations decreased. Moreover, XRD analysis revealed that the cell volume increased gradually as the radii of the A1-site rare earth cations increased. Typical

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (11564009), Natural Science Foundation of Guangxi (2013GXNSFBA019230, 2014GXNSFAA118350), and open founding of the Guangxi Ministry-Province Jointly-Constructed Cultivation Base for the State Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials (13KF-17, 15KF-12).

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