JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014, P. 879

Foundation item: Project supported by Fund for Fostering Talents in Basic Science of the National Natural Science Foundation of China (J1103207), the National Natural Science Foundation of China (11274288, 21002097), the National Basic Research Program of China (2011CB932801, 2012CB933702), and Ministry of Education of China (20123402110034)

* Corresponding authors: ZHANG Zengming, WANG Zhongping (E-mail: zzm@ustc.edu.cn, zpwang@ustc.edu.cn; Tel.: +86-551-63607671, +86-551-63601850)

DOI: 10.1016/S1002-0721(14)60157-5

Pressure and temperature dependent up-conversion properties of Yb3+-Er3+ co-doped BaBi4Ti4O15 ferroelectric ceramics

QI Xingguo (戚兴国)1, SUI Zhilei (随志磊)1, DENG Yuhang (邓宇航)1, DAI Rucheng (代如成)2, WANG Zhongping (王中平)2,*, ZHANG Zengming (张增明)2,*, DING Zejun (丁泽军)1,3 (1. Department of Physics, University of Science and Technology of China, Hefei 230026, China; 2. The Centre for Physical Experiments, University of Science and Technology of China, Hefei 230026, China; 3. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China)

Received 25 September 2013; revised 24 April 2014

Abstract: Yb3+ and Er3+ co-doped BaBi4Ti4O15 (BBT) ceramic samples showed brighter up-conversion photoluminescence (UC-PL) under excitation of 980 nm. The monotonous increase of fluorescence intensity ratio (FIR) from 525 to 550 nm with temperature showed that this material could be used for temperature sensing with the maximum sensitivity to be 0.0046 K–1 and the energy dif-ference was 700 cm–1. Moreover, the sudden change of red and green emissions around 400 ºC might imply a phase transition. With increasing pressure up to 4 GPa, the PL intensity decreased but was still strong enough. These results illustrated the wide applications of BBT in high temperature and high pressure conditions.

Keywords: BaBi4Ti4O15; piezoelectric ceramic; rare earths; up-conversion; pressure; temperature

Up-conversion material doped with lanthanide ions have attracted much attention due to its wide application in biosensors[1], light devices[2], solar cells[3] and oth-ers[4,5]. As to the high temperature sensing technologies, the piezoelectric ceramic is considered as one of the best candidates by comprehensive analyzing cost, sensitivity and design. Rare earth doped piezoelectric ceramic is an important multifunctional material because of its ferro-electric properties and strong up-conversion photolumi-nescence (UC-PL). Peng et al.[6,7] have synthesized and investigated many rare earth doped ferroelectric oxides, such as, CaBi4Ti4O15:Pr, BaBi4Ti4O15:Er and CaBi2Ta2O9: Er, which show a wonderful application potential. Due to the f-f forbidden transition, however, single doped up- conversion materials often exhibit lower luminescent ef-ficiency which can be resolved by co-doping method. BaBi4Ti4O15, as a member of the Aurivillius family, has attracted considerable attention due to its high Curie temperature and excellent ferroelectric properties.

In this work BBT ceramics, with different doping ra-tios and various concentrations of Yb3+ and Er3+, were prepared by conventional solid-state reaction. Its UC-PL was investigated at high temperature and under high pressure. The result indicated that the up-conversion emission lines were sensitive to temperature and stable at high pressure, which demonstrates the wide application

related to temperature and high pressure.

1 Experimental

1.1 Preparation of Yb3+ and Er3+ co-doped BBT ce-ramics

(Ba1/3Bi2/3)(3–z)YbyErxBi2Ti4O15 (abbreviated as BBT_ YbyErx, z=x+y) ceramics were synthesized by simple solid state reaction method with modification[6,8]. The starting materials, reagent-grade oxides and carbonate of Bi2O3, TiO2, BaCO3, Yb2O3, and Er2O3, were weighed and ground finely in an agate mortar according to the ap-propriate stoichiometric ratio (5% more Bi2O3 in consid-eration of volatilization loss[9]). After that, the mixture was calcinated at 800 ºC for 2 h for three times with in-termediate grindings. The obtained powder was milled again and then pressed into pellets with 10 mm in di-ameter and 1–2 mm in thickness (10 MPa). The final product was obtained after the pellet was sintered at 1150 ºC for 30 min.

1.2 Characterization of Yb3+ and Er3+ co-doped BBT ceramics

The structure and phase information of the obtained ceramics were then confirmed by conventional X-ray dif-fraction studies (XRD) with a Cu Kα radiation (λ=

880 JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014

0.154056 nm). The Raman measurements were carried out with an excitation source of 785 nm at room tem-perature by a Raman spectrometer (LabRAM HR800, Horiba JobinYvon), with the data collected in the incre-ment of 0.526 cm–1. Emission and excitation spectra were carried out on a steady-state/lifetime spectro-fluorimeter (JOBIN YVON, FLUOROLOG-3-TAU) equipped with a 450 W xenon lamp as excitation source.

2 Results and discussion

2.1 XRD analysis

Fig. 1 shows the room temperature XRD patterns of BBT_YbyErx ceramics with the standard lines at the top (JCPDS Card No. 35-0757). With increasing calcination times, the XRD pattern gets optimized, especially for the impurity peak at ~31°. After calcinating for three times, diffraction pattern matches well with the standard one and no other secondary phase is observed, which means the doping of rare earth ions does not affect the crystal structure. From the reported literature[10], it is known that the Yb3+ and Er3+ take the place of Bi3+ because of their similar radius and same valence.

2.2 Raman analysis

The room temperature Raman spectrum of BBT specimen is depicted in Fig. 2. The excitation source is 785 nm instead of 514 nm because the BBT_YbyErx ce-ramics have strong fluorescence background due to lu-minescence of rare earth ions around 514 nm as shown in the inset of Fig. 2. The Raman spectrum is consistent with literatures reported, verifying the XRD data and the BBT structure[8,11].

2.3 Mechanism of up-conversion

Fig. 3 is the schematic energy levels of Yb3+ and Er3+ ions[18]. Because of the large absorption cross section for 980 nm pump laser, the sensitizer Yb3+ is easily excited from ground state 2F7/2 to the excitation state 2F5/2. The

Fig. 1 XRD patterns of BBTs with different calcination times

Fig. 2 Raman spectra of BBT after calcinations under 785 nm

with the inset excited by 514 nm

Fig. 3 Energy level diagrams of the Yb3+ and Er3+ ions and the

proposed UC mechanism excited Yb3+ ion excites Er3+ ion from ground state to 4I11/2 level by energy transfer (ET). Some populations at 4I11/2 transit to the lower energy level 4I13/2 by nonradia-tive process. Some populations at 4I11/2 and 4I13/2 can be excited to 4F7/2 and 4F9/2 by excitation state absorption (ESA) processes, respectively. The excited Yb3+ ions provide the energy in ESA process by ET. Subsequent nonradiative processes within the Er3+ ions populate their radiation states of 4H11/2 and 4S3/2. The green band is at-tributed to the transitions 4H11/2, 4S3/2→4I15/2. The red emis-sion band is from the transition 4F9/2→4I15/2 (665 nm).

As for up-conversion process, the dependence of emis-sion intensity on pump power is described by the fol-lowing equation[12]: I=Pn (1) where I is the intensity of the emission band, P represents the pump power and n is the photon number during the process. Fig. 4 shows double logarithmic plot for the de-pendence of the emission intensity on pump power. The slopes of red and green emission bands are 1.98 and 1.87, similar to the results of single doped BBTs[6]. This result agrees with the mechanism depicted in Fig. 3, i.e. they

QI Xingguo et al., Pressure and temperature dependent up-conversion properties of Yb3+-Er3+ co-doped BaBi4Ti4O15… 881

Fig. 4 Excitation power dependence of UC emissions of BBT

ceramic are all two photon processes.

2.4 Influence of rare earth dopant

Fig. 5 describes the up-conversion emission spectra of BBT_Yby_Er0.02 for different doping ratios of Yb3+ to Er3+ at room temperature. There are three emission bands at 525, 550 and 665 nm, corresponding to the transition of 4S3/2, 2H11/2 and 4F9/2 to 4I15/2 of Er3+ ions, respectively. It can be seen from Fig. 5 that, the UC-PL intensity slightly depends on the doping ratio and reaches the maximum at ~6. The UC emission spectra are shown in Fig. 6 for different dopant concentrations. The best dopant concentration is 20 mol.%, corresponding to Yb3+ dopant of 17 mol.% and Er3+ dopant of 3 mol.%. Mean-while, Fig. 6 also shows that the UC intensity weakly changes with the dopant concentration.

2.5 Influence of high temperature

Fig. 7 is the temperature dependent UC emission spec-tra for BBT ceramic. From Fig. 7, the intensity of red emission P3 peak increases with temperature. The inset figure shows the intensity ratio of red to green emissions (i.e. the area of P3 to that of P1+P2) obeying an expo-nential law with temperature. The sudden change of the intensity of P3 takes place at about 400 ºC. It may imply

Fig. 5 PL spectra of BBT ceramic with different doping ratios

of Yb3+ to Er3+ by 980 nm diode laser

Fig. 6 PL spectra of BBT ceramic with different doping con-

centrations with a fixed Yb3+/Er3+ molar ratio of 6

Fig. 7 Temperature dependence of UC-PL of BBT ceramics

(normalized to P2) (Inset: relationship between ratio (Red/Green) and temperature)

a phase transition from orthorhombic (space group A21am) to tetragonal (I4/mmm), some works also pointed out the transition temperature at 417 ºC[13,14]. The jump of intensity indicates the dominance of red emission at high temperature.

As for the high temperature sensing, the relative inten-sity change of P1 to P2 can be used as a sensitivity in-dex[15–17]. The ratio of P1 to P2 (FIR) is defined by,

{ } { }H H H H

S S S S

exp expI g E E

FIR R CI g kT kT

ω σ

ω σ

Δ Δ≡ = = − = −

(2) where the parameters σ and ω are respectively emission cross sections and transition angular frequency of the en-ergy level. S and H respond to the energy level 4S3/2 and 2H11/2, respectively. Parameter g is degeneracy factor. ΔE, k and T represent the energy gap, Boltzmann constant and absolute temperature, respectively[17].

Fig. 8 plots ln(R) versus 1/T curve,

ln lnE

R CkT

Δ= − (3)

with the result depicted in Fig. 6. From the slope of the

882 JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014

Fig. 8 Relation between ln(R) and 1/T and the inset: the sensi-

tivity as a function of temperature fitted line the calculated energy gap between 4S3/2 and 2H2/11 (ΔE) is about 700 cm–1. The change of R with tem-perature is defined as the temperature sensitivity:

2

d

d

R ES R

T kT

Δ= = (4)

The experimental data and Eq. (4) are shown in the inset of Fig. 8. It is seen that the experimental data agree with theoretical curve.

2.6 Influence of high pressure

BBT ceramic, as a piezoelectric material, is supposed to be sensitive to pressure and has some wonderful prop-erties under pressure. The high pressure PL and Raman studies have been carried out by using the hydrostatic pressure method, in which a diamond anvil cell (DAC) is used to produce high pressure. The pressure calibration is made with the well-known ruby luminescence R1 line.

Fig. 9 shows high pressure Raman spectra. With in-creasing pressure, some peaks (242 and 694 cm–1) are enhanced while others are weakened. The peak at 280 cm–1 is independent of pressure but other peaks have red shift with increasing pressure. However, the Raman spectra change has not shown phase transition; the mate-rial is very stable under high pressure. Fig. 10 illustrates

Fig. 9 Raman spectra of BBT with increasing pressure, normal-

ized to 888 cm–1

Fig. 10 UC-PL spectra of BBT with increasing pressure

high pressure PL spectra; the general trend of the UC-PL is diminishing with increasing pressure. However, the intensity change when pressure is not so high is more in-teresting. It is found that the intensity loss at ~4 GPa is only about half, therefore, BBT ceramics should have important application to high pressure condition.

3 Conclusions

The properties of UC-PL and Raman spectra for Yb3+ and Er3+ co-doped BBT ceramics at room temperature, high temperature and high pressure were investigated. The results illustrated that the doping did not change the structure of BBT and bright UC-PL was obtained in all situations. High temperature research revealed that the red emission was more competitive at high temperature and the temperature sensing region was up to 350 ºC. High pressure study indicated that it was very stable and still had strong luminescence up to 4 GPa.

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