Elsevier

Materials Research Bulletin

Volume 60, December 2014, Pages 111-117
Materials Research Bulletin

Photoluminescence and electrical characterization of unfilled tetragonal tungsten bronze Ba4La1  xEuxTiNb9O30

https://doi.org/10.1016/j.materresbull.2014.08.005Get rights and content

Highlights

  • Unfilled TTB structure BLTN: Eu3+x ceramics have been synthesized.

  • Photoluminescenct properties of the BLTN: Eu3+x ceramics have been first reported.

  • Bright red emission excited by NUV light has been observed at room temperature.

  • Obvious variations of dielectric characteristics have been confirmed.

  • Relaxor-like ferroelectric phase transitions have been detected.

Abstract

Unfilled tetragonal tungsten bronze (TTB) structure Ba4LaTiNb9O30 doped by Eu3+ (BLTN: Eu3+x) with different x have been prepared, and their structural, photoluminescence, dielectric, and ferroelectric properties are carefully investigated in this work. Bright red emission, originating from 5D0  7F1 and 5D0  7F2 transitions of Eu3+ ions, has been observed by naked eyes at room temperature under near ultraviolet (NUV) light excitation. Optimized emission intensity is obtained when x = 1.00 for present unfilled TTB-type BLTN: Eu3+x samples. Furthermore, with increasing x, the dielectric and ferroelectric characteristics of the unfilled TTB-type BLTN: Eu3+x samples also display remarkable variation. When x  0.50 relaxor-like ferroelectric phase transitions are detected above room temperature, it is believed that unfilled TTB-type BLTN: Eu3+x = 1.00 involving bright photoluminescence and enhanced ferroelectric properties may act as a potentially multifunctional optical-electro material.

Graphical abstract

PL spectra of the unfilled TTB structure BLTN: Eu3+x samples (x = 0.00, 0.25, 0.50, 0.75, and 1.00) excited by 399 nm. The inset is a schematic diagram of the unfilled TTB structure.

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Introduction

Tetragonal tungsten bronze (TTB) oxides, which are generally expressed as (A2)4(A1)2(C)4(B1)2(B2)8O30 [1], [2], [3], [4], [5], have received considerable attention of the researchers throughout the world owing to their excellent dielectric, ferroelectric, pyroelectric, piezoelectric, and electro-optic properties [6], [7], [8], [9], [10]. The structure of TTB oxides consists of (B1/B2)O6 octahedra sharing corners to produce a framework structure with three types of interstices (square A1, pentagonal A2, and trigonal C). According to the occupation of cations in TTB structure, TTB oxides can be divided into three categories: fully filled TTB oxides (A1-, A2-, and C-sites are all filled) [11], filled TTB oxides (A1- and A2-sites are both filled, while C-sites are empty) [1], [2], [3], [4], [5], [6], [7], [8], [9], and unfilled TTB oxides (A2-sites are fully filled, A1-sites are partly occupied, and C-sites are empty, as shown in Fig. 1) [12]. For the TTB structure, studies have shown that different ionic substitutions at the above mentioned sites can have significant influence on their physical features.

Particularly, rare earth (Re) ions-doped TTB oxides have recently attracted much attention due to their interesting electrical-related properties [1], [2], [3], [4], [5], [6], [7], [8], [9]. Kirk et al. reported that Ba2LaTi2Nb3O15 is a relaxor ferroelectric with Tc  200 K in comparison with Ba2NdTi2Nb3O15 and Ba2SmTi2Nb3O15 which both show normal first order ferroelectric phase transitions with Tc  430 K and 540 K, respectively [2], [13], [14]. The re-entrant relaxor behavior of Re (Re = La, Nd, Sm) doped Ba5ReTi3Nb7O30 TTB ceramics was demonstrated by Li et al. [15]. Moreover, Fang et al. reported the high dielectric permittivity (ϵr) of 110 and 87.6, low dielectric loss (tan δ) of 0.0010 and 0.0012 (at 1 MHz), and low temperature coefficient of dielectric permittivity (τϵ) of −30 and −63 ppm/°C for Re (Re = Pr and Eu, respectively) doped Ba4Re2Fe2Ta8O30 ceramics [16]. In addition, many Re-doped TTB compounds (e.g. Sr4Re2Ti4Nb6O30, Re = Nd, Sm, Eu) displayed one or more dielectric anomalies with frequency dispersion below the ferroelectric transition temperature which may be related with the subtle structure of TTB materials [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

To date, many research works concerning the Re-doped filled TTB oxides have been reported in the literature[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]; however, our knowledge on the unfilled TTB compounds doped by Re ions has been very limited up to now [12], [18], [19]. Furthermore, it is worth noting that previous works relating with TTB compounds mainly focused on their structural and electrical properties. Referring to the important role of Re ions in photoluminescence materials and electrical adjustment of the filled TTB oxides [20], [21], [22], it is of interest and significance to simultaneously investigate photoluminescence and electrical properties of the Re-doped unfilled TTB compounds from the aspect of designing multi-functional materials [23].

In the present work, Eu3+-doped Ba4LaTiNb9O30 compounds (BLTN: Eu3+x), in which Eu3+ ions are taken as the photoluminescence activator, have been synthesized. The structural, photoluminescence, dielectric, and ferroelectric properties have been systematically investigated as a function of Eu3+ ions concentration. Involving bright red emission and enhanced ferroelectric properties, BLTN: Eu3+x may act as a potentially multifunctional optical-electro material.

Section snippets

Experimental details

The polycrystalline Ba4La1  xEuxTiNb9O30 (BLTN: Eu3+x, x = 0.0, 0.25, 0.50, 0.75, and 1.00) ceramics were synthesized by a conventional solid-state reaction method [24], [25]. BaCO3(99%), La2O3(99.99%), and Nb2O5(99.5%) were supplied by Sinopharm Chemical Reagent Beijing Co. Ltd., Eu2O3(99.99%) and TiO2(99%) were supplied by Aladdin Industrial Corporation. All chemicals were mixed in stoichiometric ratio. After mixing by milling in alcohol for 24 h using agate pots and agate balls in a planetary

Results and discussions

Fig. 2 presents typically XRD patterns of the unfilled TTB-type polycrystalline BLTN: Eu3+x samples (x = 0.00, 0.50, and 1.00). By the GSAS, all BLTN: Eu3+x samples are determined as single TTB phase with space group P4bm in which larger Ba2+ ions fill the A2-sites, relative smaller La3+ and Eu3+ ions occupy the A1-sites, while Ti4+ and Nb4+ ions fill the B-sites. In Fig. 2, no other phases are detected with the introduction of Eu3+ ions in BLTN: Eu3+x samples. The final difference between

Conclusion

A series of BLTN: Eu3+x ceramics have been synthesized by the solid-state reaction method. We have investigated their structural, photoluminescence, dielectric, and ferroelectric properties. BLTN: Eu3+x samples belong to the single unfilled TTB phase with space group P4bm in which larger Ba2+ ions fill the A2-sites, relative smaller La3+ and Eu3+ ions occupy the A1-sites, while Ti4+ and Nb4+ ions fill the B-sites. The photoluminescence properties of the BLTN: Eu3+x ceramics are first reported

Acknowledgments

This work was supported by the Natural Science Foundation of China (No. 51102277), the Tianjin Reasearch Program of Application Foundation and Advanced Technology (No. 14JCQNJC03700), and the National Undergraduate Training Programs for Innovation and Entrepreneurship (No. 201410059032).

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