Materials Research Bulletin 60 (2014) 111–117

Photoluminescence and electrical characterization of unfilledtetragonal tungsten bronze Ba4La1 � xEuxTiNb9O30

T. Wei a,*, Y.Q. Wang a, C.Z. Zhao b, L.Q. Zhan a

aCollege of Science, Civil Aviation University of China, Tianjin 300300, Chinab School of Electronics and Information Engineering, Tianjin Polytechnics University, Tianjin 300160, China

A R T I C L E I N F O

Article history:Received 3 June 2014Received in revised form 27 July 2014Accepted 7 August 2014Available online 8 August 2014

PACS:78.55.-m77.84.-s

Keywords:CeramicsLuminescenceDielectric propertiesFerroelectricity

A B S T R A C T

Unfilled tetragonal tungsten bronze (TTB) structure Ba4LaTiNb9O30 doped by Eu3+ (BLTN: Eu3+x) withdifferent x have been prepared, and their structural, photoluminescence, dielectric, and ferroelectricproperties are carefully investigated in this work. Bright red emission, originating from 5D0! 7F1 and5D0! 7F2 transitions of Eu3+ ions, has been observed by naked eyes at room temperature under nearultraviolet (NUV) light excitation. Optimized emission intensity is obtained when x = 1.00 for presentunfilled TTB-type BLTN: Eu3+x samples. Furthermore, with increasing x, the dielectric and ferroelectriccharacteristics of the unfilled TTB-type BLTN: Eu3+x samples also display remarkable variation. Whenx � 0.50 relaxor-like ferroelectric phase transitions are detected above room temperature, it is believedthat unfilled TTB-type BLTN: Eu3+x = 1.00 involving bright photoluminescence and enhanced ferroelectricproperties may act as a potentially multifunctional optical-electro material.

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1. Introduction

Tetragonal tungsten bronze (TTB) oxides, which are generallyexpressed as (A2)4(A1)2(C)4(B1)2(B2)8O30[1–5], have received con-siderable attention of the researchers throughout the world owingto their excellent dielectric, ferroelectric, pyroelectric, piezoelec-tric, and electro-optic properties [6–10]. The structure of TTBoxides consists of (B1/B2)O6 octahedra sharing corners to produce aframework structure with three types of interstices (square A1,pentagonal A2, and trigonal C). According to the occupation ofcations in TTB structure, TTB oxides can be divided into threecategories: 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-sitesare empty) [1–9], and unfilled TTB oxides (A2-sites are fully filled,A1-sites are partly occupied, and C-sites are empty, as shown inFig.1) [12]. For the TTB structure, studies have shown that differentionic substitutions at the above mentioned sites can havesignificant influence on their physical features.

Particularly, rare earth (Re) ions-doped TTB oxides haverecently attracted much attention due to their interestingelectrical-related properties [1–9]. Kirk et al. reported that

* Corresponding author.E-mail address: weitong.nju@gmail.com (T. Wei).

http://dx.doi.org/10.1016/j.materresbull.2014.08.0050025-5408/ã 2014 Elsevier Ltd. All rights reserved.

Ba2LaTi2Nb3O15 is a relaxor ferroelectric with Tc� 200 K incomparison with Ba2NdTi2Nb3O15 and Ba2SmTi2Nb3O15 whichboth show normal first order ferroelectric phase transitions withTc� 430 K and 540 K, respectively [2,13,14]. The re-entrant relaxorbehavior of Re (Re = La, Nd, Sm) doped Ba5ReTi3Nb7O30 TTBceramics was demonstrated by Li et al. [15]. Moreover, Fang et al.reported the high dielectric permittivity (er) of 110 and 87.6, lowdielectric loss (tan d) of 0.0010 and 0.0012 (at 1 MHz), and lowtemperature coefficient of dielectric permittivity (te) of �30 and�63 ppm/�C for Re (Re = Pr and Eu, respectively) doped Ba4Re2-Fe2Ta8O30 ceramics [16]. In addition, many Re-doped TTBcompounds (e.g. Sr4Re2Ti4Nb6O30, Re = Nd, Sm, Eu) displayedone or more dielectric anomalies with frequency dispersion belowthe ferroelectric transition temperature which may be related withthe subtle structure of TTB materials [7–17].

To date, many research works concerning the Re-doped filledTTB oxides have been reported in the literature[2–17]; however,our knowledge on the unfilled TTB compounds doped by Re ionshas been very limited up to now [12,18,19]. Furthermore, it is worthnoting that previous works relating with TTB compounds mainlyfocused on their structural and electrical properties. Referring tothe important role of Re ions in photoluminescence materials andelectrical adjustment of the filled TTB oxides [20–22], it is ofinterest and significance to simultaneously investigate photo-luminescence and electrical properties of the Re-doped unfilled

Fig. 1. A schematic diagram of the unfilled TTB structure. The A1-, A2-, B1-, B2-, andC-sites are labeled.

Fig. 2. Rietveld structural refinement results for BLTN: xEu3+ (x = 0.00 (a), 0.50 (b),and 1.00 (c)). The open circle dots represent the measured XRD reflections and thepink solid lines are the Rietveld refined results. The black lines show the differencebetween the measured data and Rietveld refined data. The short vertical solid linescorrespond with the Bragg positions.

112 T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117

TTB compounds from the aspect of designing multi-functionalmaterials [23].

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

2. 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 aconventional solid-state reaction method [24,25]. BaCO3(99%),La2O3(99.99%), and Nb2O5(99.5%) were supplied by SinopharmChemical Reagent Beijing Co. Ltd., Eu2O3(99.99%) and TiO2(99%)were supplied by Aladdin Industrial Corporation. All chemicalswere mixed in stoichiometric ratio. After mixing by milling inalcohol for 24 h using agate pots and agate balls in a planetary mill,the as-prepared powders were dried and then calcined at 1300 �Cfor 8 h. The resultant powders were reground and pelletized under10 MPa pressure into disks of 13 mm in diameter and sintered at1350 �C for 8 h.

The phase identification was determined by an X-ray diffrac-tometer (XRD) (X'pert-MPD, Philips) using Cu Ka radiation, withworking current and voltage of 40 mA and 40 kV, respectively. XRDmeasurements were carried out over an angular range from 8� to120� with scanning step of 0.02�. The general structure analysissystem (GSAS) program was used for Rietveld structural refine-ment [26,27]. The photoluminescence spectrum at room temper-ature was recorded by using F7000 fluorescencespectrophotometer (Hitachi, Japan). Silver electrodes were depos-ited on the surfaces to perform electrical measurements. Thetemperature dependence of relative dielectric permittivity wasmeasured by using a dielectric spectrometer (TH2828S, Tonghuielectronic Co., Ltd.) connected to a tubular furnace (GSL-1100X)with heating rate of 2 K/min. The ferroelectric hysteresis loops atroom temperature were measured with a Radiant Precision

Table 1Structural parameters and reliability factors of BLTN: Eu3+x.

x 0.00 0.25 0.50 0.75 1.00

a (Å) 12.47680(8) 12.46938(10) 12.46096(10) 12.45200(8) 12.44285(10)c (Å) 3.94600(4) 3.94459(5) 3.94200(5) 3.94024(4) 3.94140(5)ð

ffiffiffiffiffiffiffiffi

10cp

=aÞ 1.00012 1.00036 1.00038 1.00065 1.00168Vunit (Å3) 614.276(10) 613.326(12) 612.096(12) 610.944(9) 610.225(12)Rp 4.61% 4.50% 4.20% 4.33% 4.12%Rwp 6.09% 5.93% 5.48% 5.67% 5.33%x2 3.199 3.243 2.719 2.881 2.558

Table 2Final atomic positions and thermal parameters of BLTN: Eu3+x (x = 0.00).

Atom x/a y/b z/c 100 � Uiso Position Occupies

La1 0.0(0) 0.0(0) 0.033(4) 0.39 (13) 2 a 0.5Ba2 0.17261(13) 0.67261(13) 0.0013(34) 3.34(6) 4 c 1.0Nb1/Ti1 0.0(0) 0.5(0) 0.511(5) 2.68(15) 2 b 0.9/0.1Nb2/Ti2 0.07313(16) 0.21200(15) 0.5026(30) 1.44(5) 8 d 0.9/0.1O1 0.1355(9) 0.0701(10) 0.564(6) 1.32(48) 8 d 1.0O2 0.3383(12) 0.0055(7) 0.579(5) 0.92(45) 8 d 1.0O3 0.0715(11) 0.2140(9) 0.049(6) 2.38(39) 8 d 1.0O4 0.2893(9) 0.7893(9) 0.447(8) 3.25(76) 4 c 1.0O5 0.0(0) 0.5(0) 0.025(22) 1.41(88) 2 b 1.0

Table 3Final atomic positions and thermal parameters of BLTN: Eu3+x (x = 0.50).

Atom x/a y/b z/c 100 � Uiso Position Occupies

La1/Eu1 0.0(0) 0.0(0) 0.012(4) 0.64(48) 2 a 0.25/0.25Ba2 0.17262(12) 0.67262(12) �0.0103(32) 2.98(6) 4 c 1.0Nb1/Ti1 0.0(0) 0.5(0) 0.510(4) 2.34(23) 2 b 0.9/0.1Nb2/Ti2 0.07271(15) 0.21125(14) 0.4810(26) 1.16(7) 8 d 0.9/0.1O1 0.1345(9) 0.0715(10) 0.519(9) 4.28(53) 8 d 1.0O2 0.3357(12) 0.0052(7) 0.572(4) 1.43(49) 8 d 1.0O3 0.0724(11) 0.2128(9) �0.001(15) 4.02(37) 8 d 1.0O4 0.2889(8) 0.7889(8) 0.436(8) 1.67(73) 4 c 1.0O5 0.0(0) 0.5(0) 0.033(15) 2.41(1.08) 2 b 1.0

Table 4Final atomic positions and thermal parameters of BLTN: Eu3+x (x = 1.00).

Atom x/a y/b z/c 100 � Uiso Position Occupies

Eu1 0.0(0) 0.0(0) 0.024(4) 0.13(5) 2 a 0.5Ba2 0.17285(13) 0.67285(13) �0.0138(29) 2.38(5) 4 c 1.0Nb1/Ti1 0.0(0) 0.5(0) 0.504(4) 1.21(14) 2 b 0.9/0.1Nb2/Ti2 0.07241(16) 0.21071(15) 0.4796(24) 0.58(5) 8 d 0.9/0.1O1 0.1348(9) 0.0723(11) 0.556(5) 1.56(55) 8 d 1.0O2 0.3388(12) 0.0061(7) 0.562(4) 0.42(26) 8 d 1.0O3 0.0698(12) 0.2131(10) 0.043(6) 3.19(48) 8 d 1.0O4 0.2872(9) 0.7872(9) 0.413(6) 0.69(47) 4 c 1.0O5 0.0(0) 0.5(0) �0.004(24) 3.19(1.07) 2 b 1.0

Table 5Selected bond lengths of BLTN: Eu3+x (x = 0.0, 0.25, 0.50, 0.75, and 1.00).

Bond distances (Å) x = 0.00 x = 0.25 x = 0.50 x = 0.75 x = 1.00

Nb (1)/Ti(1)��O(2) 2.037(15) 2.048(16) 2.062(15) 2.023(14) 2.020(15)Nb (1)/Ti(1)��O(5) 1.92(9) 1.84(6) 1.88(6) 1.94(8) 2.00(9)

2.03(9) 2.10(6) 2.06(6) 2.00(8) 1.94(9)Nb (2)/Ti(2)��O(2) 1.881(13) 1.876(14) 1.865(13) 1.909(12) 1.897(13)Nb (2)/Ti(2)��O(1) 1.949(13) 1.928(14) 1.911(13) 1.950(13) 1.913(14)

2.040(12) 2.025(14) 2.040(13) 2.014(12) 2.055(14)Nb (2)/Ti(2)��O(4) 1.981(6) 1.977(5) 1.985(5) 2.004(5) 2.007(6)Nb (2)/Ti(2)��O(3) 1.790(26) 1.97(6) 1.90(6) 1.89(6) 1.722(23)

2.156(26) 1.98(6) 2.04(6) 2.05(6) 2.220(23)

T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117 113

Fig. 3. PLE spectra of BLTN: Eu3+x (x = 0.00, 0.25, 0.50, 0.75, and 1.00) monitored by592 nm emission (a). PL spectra of BLTN: Eu3+x (x = 0.00, 0.25, 0.50, 0.75, and 1.00)excited by 399 nm (b).

Fig. 4. Simplified energy level diagram of Eu3+. The arrows show representativeemissions of Eu3+.

Fig. 5. Variation of PL emission and PLE excitation integrated intensity with x forBLTN: Eu3+x system.

114 T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117

Multiferroic Tester (Radiant Technologies Ltd., Albuquerque, NM)in a standard mode.

3. Results and discussions

Fig. 2 presents typically XRD patterns of the unfilled TTB-typepolycrystalline BLTN: Eu3+x samples (x = 0.00, 0.50, and 1.00). Bythe GSAS, all BLTN: Eu3+x samples are determined as single TTBphase with space group P4bm in which larger Ba2+ ions fill the A2-sites, relative smaller La3+ and Eu3+ ions occupy the A1-sites, whileTi4+ and Nb4+ ions fill the B-sites. In Fig. 2, no other phases aredetected with the introduction of Eu3+ ions in BLTN: Eu3+x samples.The final difference between experiment and the Rietveldrefinement results are shown in Fig. 2. Moreover, the refinedstructural parameters are given in Tables 1–5.

As shown in Table 1, the lattice parameters (a, c, and Vunit)gradually decrease with increasing x from 0.00 to 0.75 for BLTN:Eu3+x system. It is qualitatively understandable because theeffective radius Eu3+ ion (1.066 Å, CN8) is smaller than La3+ ion(1.160 Å, CN8), resulting in the lattice shrinkage [28]. However,abnormal jump in c-axis appears as x ranging from 0.75 to 1.00 inTable 1. Generally, the lattice parameters will continuously shrink

Fig. 6. Variation of er with T of BLTN: Eu3+x under different external AC electric field frequencies (1 kHz–1 MHz).

T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117 115

with increasing concentration of small ions, such as Eu3+ ions inthis work. The largest tetragonality ð

ffiffiffiffiffiffiffiffi

10cp

=aÞof BLTN: Eu3

+x = 1.00 may suggest that severe structural distortion occurs inthis composition which can lead to the above abnormal jump inc-axis.

Table 5 gives the selected bond lengths of BLTN: Eu3+x samples.It can be seen the existence of local lattice distortion in Nb(1)/Ti(1)O6 and Nb(2)/Ti(2)O6 octahedra which is related with thepolarization features of this material. For the unfilled TTB-typeBLTN: Eu3+x samples, it is believed that Nb(1)/Ti(1) and Nb(2)/Ti(2)ions act as the ferroelectric-active cations [29]. As shown inTable 5, the equatorial Nb(1)/Ti(1)��O(2) bonds in Nb(1)/Ti(1)O6

octahedra have equal bond lengths, however, the lengths of theapical Nb(1)/Ti(1)��O(5) bonds are unequal, which indicates anoff-center displacement of Nb(1)5+ and Ti(1)4+ ions along the c-axis. On the other hand, for Nb(2)/Ti(2)O6 octahedra, the differenceof the apical Nb(2)/Ti(2)��O(3) bond lengths suggests the similaroff-center displacement of Nb(2)/Ti(2) ions with that of Nb(1)/Ti(1). It is believed that the above Nb(1)5+, Ti(1)4+, Nb(2)5+, and Ti(2)4+ ions displacement is tightly correlated to the ferroelectricordering of BLTN: Eu3+x samples as confirmed in the later section ofthis paper. Moreover, as shown in Table 5, it is worth noting thatthe off center displacement in ab-plane of Nb(2)/Ti(2) ions alsoexists referring to the different Nb(2)/Ti(2)��O(1) bond lengths

which suggests the distortion of Nb(2)/Ti(2)O6 octahedra is largerthan that of Nb(1)/Ti(1)O6 octahedra.

Fig. 3(a) shows the photoluminescence excitation (PLE) spectraof BLTN: Eu3+x (x = 0.00, 0.25, 0.50, 0.75, and 1.00). By monitoringthe 592 nm emission, strong excitation peaks of BLTN: Eu3+x(x 6¼ 0.0) are detected as given in Fig. 3(a). The remarkably strongand sharp excitation peaks in the wavelength range between 370and 550 nm are owing to the typical f–f absorption of Eu3+ [30–33].The intense high-energy side excitation peak around 399 nm isassigned to the 7F0! 5L6 transition. The weak excitation peakaround 416 nm corresponds with the 7F0! 5D3 transition. Fur-thermore, the low-energy side excitation peaks around 464 nmand 530 nm should be corresponded to the transitions from the 7F0ground state to the 5D2, 5D1, and 5D0 excited states of Eu3+ asillustrated in Fig. 3(a). No shifts of the excited peaks for all of theBLTN: Eu3+x samples are observed. In addition, it should be notedthat the remarkable excitation (370–550 nm) for present BLTN: Eu3

+x samples locates around the emission wavelength region ofcommercial near ultraviolet (NUV) LEDs (350–420 nm) and blueLEDs (450–470 nm) which indicates BLTN: Eu3+x can act as apotential NUV and blue exciting phosphor [34].

Fig. 3(b) displays the photoluminescence (PL) spectra of BLTN:Eu3+x (x = 0.00, 0.25, 0.50, 0.75, and 1.00) excited by NUV light(399 nm) at room temperature. Under the resonant excitation

Fig. 7. P–E hysteresis loops of BLTN: Eu3+x (x = 0.00, 0.50, and 1.00) at roomtemperature.

116 T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117

at 399 nm, the luminescence spectra of BLTN: Eu3+x dominated byseveral emission peaks locating around 592 nm, 616 nm, 650 nm,and 680 nm owing to intra f–f transitions of Eu3+ ions are obtained[35,36]. These peaks in the PL spectra presents characteristic5D0! 7FJ (J = 1, 2, 3, and 4) transitions as shown in Fig. 3(b). Toclearly illustrate the PL process, Fig. 4gives the simplified energylevel diagram of Eu3+. It is worth noting that the emission from theexcited levels above 5D0 level is not observed in Fig. 3(b) due to thecross relaxation and non-radiative relaxation by the emission ofphonons. The prominent 5D0! 7F1 transition (at 593 nm) inFig. 3(b) corresponds with the magnetic dipole allowed transition,and its intensity hardly varies with the environment of the Eu3+

ions. However, the 5D0! 7F2 transition (at 614 nm) is a forbiddenelectric dipole transition according to the theory of Judd and Ofelt,which is sensitive to chemical bonds in the vicinity of Eu3+ [37,38].Referring to the relative weak emission intensity of 5D0! 7F2compared with that of 5D0! 7F1 and almost unchanged intensityratio of 5D0! 7F2 to 5D0! 7F1 as shown in Fig. 3(b), it is believedthat the crystal field circumstance around Eu3+ in BLTN: Eu3+x maybe scarcely influenced with increasing x. Moreover, to intuitivelydisplay the emission, the inset of Fig. 3(b) gives the luminescencephotograph of BLTN: Eu3+x = 1.00 obtained in darkness by acommon digital camera under the excitation of a NUV commercialLED (1 W, 400–405 nm). One can see bright red light emission bynaked eyes at room temperature. In addition, to optimize the Eu3+

concentration, Fig. 5 provides the variation of the emission andexcitation integrated intensity with x. It is observed that theemission and excitation light intensity continuously increases withincreasing x in the whole doping concentration region. Noconcentration quenching effect is detected with Eu3+ totallysubstituting for La3+. The maximum emission intensity is obtainedfor BLTN: Eu3+x = 1.00 sample.

Besides the bright PL feature, we have also carried out electricalproperties measurement. Fig. 6 presents the variation of relativedielectric permittivity (er) with temperature (T) under differentexternal alternating current (AC) electric field frequencies (1 kHz–1 MHz). It is shown that er decreases monotonously withtemperature increase up to about 450 K [21], then, an extremeraise of er with increasing temperature is detected for BLTN: Eu3

+x = 0.0. One can see that the extreme raise of er appears for allBLTN: Eu3+x samples, and it should be resulted from the activedefect in the high temperature region [39].

As displayed in Fig. 6(b), similar er–T curves with that of BLTN:Eu3+x = 0.0 are obtained for BLTN: Eu3+x = 0.25. No ferroelectricphase transition is achieved above room temperature for BLTN:xEu3+ with x = 0.0 and 0.25 [24,25]. However, with x furtherincreasing, BLTN: Eu3+x samples with x = 0.50, 0.75, and 1.00 showremarkable dielectric permittivity peaks which should correspondwith the ferroelectric–paraelectric phase transitions[5,7,9,24,25,40]. The phase transition temperature is rapidlyelevated with increasing x. For example, the transition tempera-ture is about 333 K and 437 K at 1 MHz for BLTN: Eu3+x = 0.50 and1.00, respectively. It is believed that higher tetagonality ð

ffiffiffiffiffiffiffiffi

10cp

=aÞofthe present BLTN: Eu3+x system with increasing x shouldcontribute to the enhancement of ferroelectric phase transitiontemperature as shown in Fig. 6 [5,7].

Furthermore, it is worthy noting that the ferroelectric phasetransitions of BLTN: Eu3+x samples also display relaxor-likebehavior because the temperature (Tm) corresponding the peakof er shifts towards higher-T side and the peak value (er,max) of erdecreases with the increasing of frequency [24]. It can be seen thatstrong frequency dispersion appears for BLTN: Eu3+x withx = 0.50 and 0.75 as given in Fig. 6(c and d), respectively.Specifically, when frequency is 1 kHz, Tm and er,max is about353 K and 1151 for BLTN: Eu3+x = 0.75 sample; however, Tm and er,max separately changes into 387 K and 960 when frequency is

1 MHz. Moreover, it is observed that the frequency dispersionbecomes weaker with x further increasing as revealed in Fig. 6(e)which indicates the more stable normal ferroelectric phase in thiscomposition [24,25]. In addition, although no phase transitionsexists for BLTN: Eu3+x with x = 0.00 and 0.25 above roomtemperature, the relaxor-like behavior around room temperaturecan still be detected in Figs. 6(a and b) which alludes the low-Trelaxor-like ferroelectric behavior [5,7,24,25].

T. Wei et al. / Materials Research Bulletin 60 (2014) 111–117 117

Fig. 7 gives the polarization-electric field (P–E) hysteresis loopsat room temperature. All BLTN: Eu3+x samples indicate P–Ehysteresis loops which confirm the ferroelectricity in the presentceramics [21,24,25,40]. The remnant polarization (Pr) increasesfrom 0.78 mC/cm2 for BLTN: Eu3+x = 0.00 to 2.8 mC/cm2 for BLTN:Eu3+x = 1.00. Accordingly, the coercive field (Ec) also increases from11 kV/cm for BLTN: Eu3+x = 0.00 to 21 kV/cm for BLTN: Eu3+x = 1.00.The values of Pr and Ec increase as the concentration of Eu3+

increase for BLTN: Eu3+x system, which is consistent with thevaried trend of dielectric features as illustrated in Fig. 6 [24,25].

4. Conclusion

A series of BLTN: Eu3+x ceramics have been synthesized by thesolid-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 spacegroup P4bm in which larger Ba2+ ions fill the A2-sites, relativesmaller 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 in this work. Under NUV lightradiation, bright red emission mainly originating from 5D0! 7F1(592 nm) and 5D0! 7F2 (616 nm) transitions of Eu3+ ions has beenobserved at room temperature by naked eyes. Furthermore, thedielectric and ferroelectric characteristics of BLTN: Eu3+x alsodisplay obvious variation with the incorporation of Eu3+. Thetetragonality of BLTN: Eu3+x changes from 1.00012 to 1.00168 with xincreasing from 0.00 to 1.00, accordingly the phase transitiontemperature is promoted from 333 K (at 1 MHz) to 437 K (at 1 MHz).The Pr and Ec increases from 0.78 mC/cm2 and 11 kV/cm for BLTN:Eu3+x = 0.00 to 2.8 mC/cm2 and 21 kV/cm for BLTN: Eu3+x = 1.00. It isbelieved that unfilled TTB-type BLTN: Eu3+x = 1.00 involving brightphotoluminescence and enhanced ferroelectric properties may actas a potentially multifunctional optical-electro material.

Acknowledgments

This work was supported by the Natural Science Foundation ofChina (No. 51102277), the Tianjin Reasearch Program of Applica-tion Foundation and Advanced Technology (No. 14JCQNJC03700),and the National Undergraduate Training Programs for Innovationand Entrepreneurship (No. 201410059032).

References

[1] D.C. Arnold, F.D. Morrison, B-cation effects in relaxor and ferroelectrictetragonal tungsten bronzes, J. Mater. Chem. 19 (2009) 6485–6488.

[2] M. Prades, H. Beltrán, N. Masó, E. Cordoncillo, A.R. West, Phase transitionhysteresis and anomalous Curie–Weiss behavior of ferroelectric tetragonaltungsten bronzes Ba2RETi2Nb3O15: RE = Nd, Sm, J. Appl. Phys.104 (2008) 104118.

[3] S. Lanfredi, D.H.M. Gênova, I.A.O. Brito, A.R.F. Lima, M.A.L. Nobre, Structuralcharacterization and Curie temperature determination of a sodium strontiumniobate ferroelectric nanostructured powder, J. Solid State Chem.184 (2011) 990.

[4] A. Simon, J. Ravez, Solid-state chemistry and non-linear properties oftetragonal tungsten bronzes materials, C. R. Chimie 9 (2006) 1268.

[5] I. Levin, M.C. Stennett, G.C. Miles, D.I. Woodward, A.R. West, I.M. Reaney,Coupling between octahedral tilting and ferroelectric order in tetragonaltungsten bronze-structured dielectrics, Appl. Phys. Lett. 89 (2006) 122908.

[6] E.Castel,P.Veber, M.Albino,M.Velázquez, S.Pechev, D. Denux, J.P. Chaminade, M.Maglione, M. Josse, Crystal growth and characterization of tetragonal tungstenbronze FerroNiobates Ba2LnFeNb4O15, J. Cryst. Growth 340 (2012) 156.

[7] X.L. Zhu, S.Y. Wu, X.M. Chen, Dielectric anomalies in (BaxSr1 �x)4Nd2Ti4Nb6O30

ceramics with various radius differences between A1- and A2-site ions, Appl.Phys. Lett. 91 (2007) 162906.

[8] Y.B. Yao, C.L. Mak, Effects of Ca-dopant on the pyroelectric, piezoelectric anddielectric properties of (Sr0.6Ba0.4)4Na2Nb10O30 ceramics, J. Alloys. Comp. 544(2012) 87.

[9] Y. Bai, X.L. Zhu, X.M. Chen, X.Q. Liu, Dielectric and ferroelectric characteristicsof Ba5NdFe1.5Nb8.5O30 tungsten bronze ceramics, J. Am. Ceram. Soc. 93 (2010)3573–3576.

[10] S. Zhang, F. Yu, Piezoelectric materials for high temperature sensors, J. Am.Ceram. Soc. 94 (2011) 3153.

[11] F.W. Ainger, J.A. Beswick, W.P. Bickley, R. Clarke, G.V. Smith, Ferroelectrics inthe lithium potassium niobate system, Ferroelectrics 2 (1971) 183.

[12] C.S. Pandey, J. Schreuer, M. Burianek, M. Mühlberg, Relaxor behaviorof CaxBa1 � xNb2O6 (0.18 = x = 0.35) tuned by Ca/Ba ratio andinvestigated by resonant ultrasound spectroscopy, Phys. Rev. B. 87 (2013)094101.

[13] C.A. Kirk, M.C. Stennett, I.M. Reaney, A.R. West, A new relaxor ferroelectricBa2LaTi2Nb3O15, J. Mater. Chem. 12 (2002) 2609–2611.

[14] M.C. Stennett, I.M. Reaney, G.C. Miles, D.I. Woodward, A.R. West, C.A. Kirk, I.Levin, Dielectric and structural studies of Ba2MTi2Nb3O15 (BMTNO15, M = Bi3+,La3+, Nd3+, Sm3+, Gd3+) tetragonal tungsten bronze-structured ceramics, J.Appl. Phys. 101 (2007) 104114.

[15] K. Li, X.L. Zhu, X.Q. Liu, X.M. Chen, Re-entrant relaxor behavior ofBa5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics, Appl. Phys. Lett.102 (2013) 112912.

[16] L. Fang, X.Y. Peng, C.C. Li, C.Z. Hu, B.L. Wu, H.F. Zhou, Ba4Ln2Fe2Ta8O30 (Ln = Pr,Eu): temperature–stable low loss dielectrics with a tungsten bronze structure,J. Am. Ceram. Soc. 93 (2010) 945–947.

[17] X.L. Zhu, X.M. Chen, X.Q. Liu, Dielectric abnormity of Sr4Nd2Ti4Nb6O30

tungsten bronze ceramics over a broad temperature range, J. Mater. Res. 22(2007) 2217.

[18] L. Wang, Y. Sakka, D.A. Rusakov, Y. Mozharivskyj, T. Kolodiazhnyi, Novelincipient ferroelectrics based on Ba4MNbxTa10 �xO30 where M = Zn, Mg, Co, Ni,Chem. Mater. 23 (2011) 2586.

[19] C.Z. Hu, L.J. Hou, L. Fang, L.J. Liu, Preparation and dielectric properties ofunfilled tungsten bronze ferroelectrics Ba4RETiNb9O30, J. Alloys Compd. 581(2013) 547.

[20] M.H. Lia, T.C. Chong, X.W. Xu, H. Kumagai, Growth and spectracharacterization of Ce and Eu doped SBN crystals, J. Cryst. Growth. 225(2001) 479–483.

[21] T. Wei, Y.Q. Wang, C.Z. Zhao, X.X. Dong, J.H. Wang, Upconversion photo-luminescence and dielectric properties in Er3+ and Yb3+ co-doped Sr4La2-Ti4Nb6O30, Mater. Lett. 128 (2014) 152–155.

[22] T. Wei, X.D. Wang, C.Z. Zhao, M.F. Liu, J.M. Liu, Correlation betweenupconversion photoluminescence and dielectric response in Ba substituted(Sr1 � xBax)4(La0.85Ho0.025Yb0.125)2Ti4Nb6O30, Appl. Phys. Lett. 104 (2014)261908.

[23] F. Gao, G.J. Ding, H. Zhou, G.H. Wu, N. Qin, D.H. Bao, Combination of strong blueup-conversion photoluminescence and greatly enhanced ferroelectric polari-zation in Tm3+–Yb3+–W6+-doped Bi4Ti3O12 thin films, J. Electrochem. Soc. 158(2011) G128–G131.

[24] T. Wei, C. Zhu, K.F. Wang, H.L. Cai, J.S. Zhu, J.M. Liu, Influence of A-site codopingon ferroelectricity of quantum paraelectric SrTiO3, J. Appl. Phys. 103 (2008)124104.

[25] T. Wei, Q.J. Zhou, Q.G. Song, Z.P. Li, S.Q. Guo, Y.R. Guo, Y.F. Chen, X.L. Qi, J.M. Liu,The interaction of multifold polar orderings in Ba-doped Sr0.7Ca0.3TiO3, Mater.Res. Bull. 47 (2012) 1316–1322.

[26] A.C. Larson, R.B. Von Dreele Los Alamos National Laboratory Report LAUR 200086–748

[27] B.H.Toby,EXPGUI,agraphicaluser interfaceforGSAS,J.Appl.Cryst.34(2001)210.[28] R.D. Shannon, Revised effective ionic radii and systematic studies of

interatomie distances in halides and chaleogenides, Acta. Crystallogr. A. 32(1976) 751.

[29] C.J. Huang, K. Li, X.Q. Liu, X.L. Zhu, X.M. Chen, Effects of A1/A2-sites occupancyupon ferroelectric transition in (SrxBa1 �x)Nb2O6 tungsten bronze ceramics, J.Am. Ceram. Soc. 97 (2014) 507.

[30] W.J. Park, S.G. Yoon, D.H. Yoon, Photoluminescence properties of Y2O3 co-doped with Eu and Bi compounds as red-emitting phosphor for white LED, J.Electroceram. 17 (2006) 41.

[31] S. Neeraj, N. Kijima, A.K. Cheetham, Novel red phosphors for solid statelighting; the system BixLn1 �xVO4; Eu3+/Sm3+ (Ln = Y, Gd), Solid State Commun.131 (2004) 65.

[32] T. Wei, C.Z. Zhao, C.P. Li, Y.B. Lin, X. Yang, H.G. Tan, Photoluminescence andferroelectric properties in Eu doped Bi4Ti3O12–SrBi4Ti4O15 intergrowthferroelectric ceramics, J. Alloys Compd. 577 (2013) 728.

[33] T.S. Chan, C.C. Kang, R.S. Liu, L. Chen, X.N. Liu, J.J. Ding, J. Bao, C. Gao,Combinatorial study of the optimization of Y2O3: Bi, Eu red phosphors, J. Comb.Chem. 9 (2007) 343.

[34] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters andLasers, Springer, Berlin, 1997.

[35] A.K. Pradhan, K. Zhang, S. Mohanty, J. Dadson, D. Hunter, G.B. Loutts, U.N. Roy,Y. Cui, A. Burger, A.L. Wilkerson, Luminescence and spectroscopic behavior ofEu3+-doped Y2O3 and Lu2O3 epitaxial films grown by pulsed-laser deposition, J.Appl. Phys. 97 (2005) 023513.

[36] B. Mercier, C. Dujardin, G. Ledoux, C. Louis, O. Tillement, P. Perriat, Observationof the gap blueshift on Gd2O3:Eu3+ nanoparticles, J. Appl. Phys. 96 (2004) 650.

[37] R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962)750.

[38] G.S. Ofelt, Intensities of crystal spectra of rare-earth ions, J. Chem. Phys. 37(1962) 511.

[39] Z. Wang, X.M. Chen, L. Ni, X.Q. Liu, Dielectric abnormities of complexperovskite Ba(Fe1/2Nb1/2)O3 ceramics over broad temperature and frequencyrange, Appl. Phys. Lett. 90 (2007) 022904.

[40] T. Wei, J.M. Liu, Q.J. Zhou, Q.G. Song, Coupling and competition betweenferroelectric and antiferroelectric states in Ca-doped Sr0.9 � xBa0.1CaxTiO3:multipolar states, Phys. Rev. B. 83 (2011) 052101.