Photoluminescence and electrical characterization of unfilled tetragonal tungsten bronze Ba4La1 − xEuxTiNb9O30
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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Structure and upconversion analyses of Er<sup>3+</sup>-doped Ba<inf>6</inf>Ti<inf>2</inf>Nb<inf>8</inf>O<inf>30</inf>-Sr<inf>6</inf>Ti<inf>2</inf>Nb<inf>8</inf>O<inf>30</inf> solid solutions
2015, Ceramics InternationalCitation Excerpt :One can see that very small difference between the measured and refined XRD patterns is shown for each case in Fig. 1. The reliability of the Rietveld structural refinement is demonstrated by the high-quality refinement parameters (Rp and Rwp) as shown in Table 1 [25]. Moreover, the structural parameters of SBTNx are also given in Table 1.