Effect of phase structure changes on the lead-free Er3+-doped (K0.52Na0.48)1−xLixNbO3 piezoelectric ceramics

Journal of Alloys and Compounds 680 (2016) 467e472

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Effect of phase structure changes on the lead-free Er3þ-doped (K0.52Na0.48)1xLixNbO3 piezoelectric ceramics Yongjie Zhao a, *, Yiyao Ge b, Xuanyi Yuan c, **, Yuzhen Zhao b, Heping Zhou b, Jingbo Li a, HaiBo Jin a a Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, 100081, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR China c Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing, 100872, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 November 2015 Received in revised form 7 April 2016 Accepted 10 April 2016 Available online 11 April 2016

Er3þ doped (K0.52Na0.48)1xLixNbO3 ceramics had been prepared through a conventional solid state reaction routine and their phase structure, ferroelectric, piezoelectric, and photoluminescence properties were systematically investigated. X-ray diffraction results clearly illustrated a phase transition from the orthorhombic to coexistence and further to tetragonal phase with the increase of Li content. Via measuring the ferroelectric and piezoelectric properties of the ceramics, it found that the polymorphic phase transition (PPT) effect contributed the obvious enhancement in electrical properties of the Er3þ doped (K0.52Na0.48)0.94Li0.06NbO3 ceramic which lay in the composition region of coexistence phase structure. The up-conversion photoluminescence intensity of green emission and red emission first increased as x increase from 0 to 0.06 and then decreased for x ¼ 0.08. The most obvious enhancement in the up-conversion photoluminescence intensity was also achieved in the composition lying in the vicinity of PPT region. And the above circumstance caused the consideration that here the essence of the coexistence phase may be a phase structure with lower crystal symmetry instead of just physical combination of orthorhombic and tetragonal phase. © 2016 Elsevier B.V. All rights reserved.

Keywords: Phase structure change Ferro/piezoelectric Photoluminescence (K0.52Na0.48)1xLixNbO3

1. Introduction In recent years, rare earth-doped piezoelectric hosts have realized both photoluminescence (PL) properties and piezoelectric properties of materials, which may expand the application fields of piezoelectric materials as a multifunctional device by integrating luminescent and piezoelectric property. Meanwhile, the research also indicated that the doping of rare earth could also improve the ferroelectric and piezoelectric properties of the host materials to some extent [1e3]. And multi-property coupling, such as electromechano, electro-optic, and mechano-optic couplings, has already been realized in piezoelectric materials [4e6]. In the case of practical applications, rare earth doped piezoelectric materials are already being used in whit-emitting,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhao), [email protected] (X. Yuan). http://dx.doi.org/10.1016/j.jallcom.2016.04.098 0925-8388/© 2016 Elsevier B.V. All rights reserved.

temperature sensor, near infra-red sensors and so forth [7e9]. Moreover, rare earth-doped lead-free ferroelectrics were found to exhibit strong up-conversion emissions, with the intensity that varied with the phase transition induced by the temperature change [10e12]. Compared with traditional glasses and fluorides host, lead-free perovskite ferroelectrics possess excellent thermal and chemical stability and are environmentally friendly. Therefore, the rare earth-doped lead-free ferroelectrics are expected to be prominent in the next generation multifunctional devices. Among these ferroelectrics, potassium sodium niobate (K,Na)NbO3 (abbreviated as KNN) has inherent advantages over the other ferroelectric host materials. It has a high Curie temperature, good piezoelectric properties, and low anisotropy in electromechanical coupling coefficients [13]. Recent years, it finds that the introduction of LiNbO3 into (K,Na)NbO3-based ceramics by optimizing the sintering temperature could obviously improve the piezoelectric property, which was considered to be resulted from the effect of polymorphic phase transition [14,15].

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It knows that the variation for the crystal structure of the host materials would bring about significant influence on the crystal field around the rare earth dopant ions, and consequently affect its optical properties [16]. As mentioned above, the introduction of LiNbO3 to (K,Na)NbO3 host could effectively regulate the phase structure of the host, and accordingly the variation of the electrical properties would be induced [14]. Therefore, it is meaningful to investigate the effect of phase structure variation in rare earth doped (K,Na)NbO3 based materials which would render important potential in the application of multifunctional devices. In this paper, Er3þ-doped (Li,K,Na)NbO3 ceramics have been prepared and the effects caused by the variation of Li content in Er3þ-doped (Li,K,Na)NbO3 ceramics have been systematically investigated. The rare earth Er3þ was selected as the doping ion because of its unique characteristic in up-conversion emission [12,17,18]. In addition, for potential applications in solid state lasers, these ceramics, as a multifunctional material, should be of great importance for the fundamental study of optical-electro-mechano couplings in many application fields, including future opticalelectro-integrated materials and devices, photo-ferroelectric, and mechanical-ferroelectric devices. 2. Experimental procedures 0.75 mol% Er3þ-doped (K0.52Na0.48)1xLixNbO3 ceramics with x varying from 0, 0.06 to 0.08 (abbreviated as KNN-xLN:Er) were prepared by solid-state synthesis using analytical-grade metal oxides or carbonate powders: K2CO3 (99.9%), Na2CO3 (99.5%), Nb2O5 (99.99%), Li2CO3 (99%), and Er2O3 (99.99%). The powders in the stoichiometric ratio of the compositions were first mixed thoroughly in ethanol using zirconia balls for 24 h, and then dried and calcined at 760  C for 5 h. The calcined powders were ball-milled again for 24 h and mixed thoroughly with polyvinyl butyral (PVB) binder solution, and then pressed into the disk samples with a diameter of 10 mm. The disk samples were finally sintered at 1080  C for 2 h in the air. Silver electrodes were fired on the top and bottom surfaces of these as-sintered ceramics for the measurements of ferroelectric and piezoelectric properties. The phase structure was examined using an X-ray diffraction meter with a CuKa radiation (l ¼ 1.5418 Å) (XRD, D8 Advance, Bruker Inc., Germany). Ferroelectric hysteresis loops and field-

induced strain were measured at room temperature using a TF2000FE-HV ferroelectric test unit (aix-ACCT Inc., Germany). Upconversion photoluminescence spectra were recorded using spectrophotometer (LabRAM HR Evolution) under the excitation of a 980 nm laser diode. 3. Results and discussion Fig. 1 shows the XRD patterns for the KNN-xLN:Er ceramics (x ¼ 0, 0.06 and 0.08). All the prepared ceramics were pure perovskite phase and no secondary phase could be found within the detection resolution of the diffraction meter. It is generally accepted that the perovskite phase structure of KNN-based ceramics could be determined by assessing the relative intensities of (200) diffraction peak [19,20]. For the ceramics with x ¼ 0, the peak profiles could be fitted well with orthorhombic symmetry, because they could be characterized by two splitting peaks for {200} with weaker reflection on the higher angle and stronger reflection on the lower angle, respectively. Meanwhile, the ratio of the relative diffraction intensity of left and right peak is about 2. All these diffraction peaks become broad as the content of Li element was increased to 0.06. It is easy to deduce that the as-sintered ceramic with x ¼ 0.06 consists of a mixture of orthorhombic and tetragonal phases due to the similar magnitude of two diffraction peaks around 2q z 45 . By comparison, a distinct tetragonal symmetry can be determined for the composition with x ¼ 0.08, as featured by the appearance that the ratio of the relative intensity of left and right diffraction peak around 2q z 45 is about 0.5. Therefore, it could evidently reach the conclusion that a phase transition from orthorhombic to tetragonal phase has occurred resulting from the increment of Li content. And this was well consistent with the previous literature concerning the Li modified (K,Na)NbO3 materials [14,20]. Fig. 2 showed the room temperature P-E hysteresis loops and S-E loops of KNN-xLN:Er ceramics with the fixed frequency of 1 Hz. Fig. 2a depicted the ferroelectric hysteresis loops of the KNN-xLN:Er ceramics. According to our previously research, when the Er3þ ion concentration lies in the region of 0.5e1 mol%, the improved ferroelectric and photoluminescence property could be obtained in (K,Na)NbO3 system [2]. Therefore, the Er3þ ion concentration was fixed at 0.75 mol%. All of the ceramics exhibited the strong

Fig. 1. (a) The XRD patterns of the KNN-xLN:Er ceramics. (b)The enlarged XRD patterns near 2q ¼ 44e47.

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Fig. 2. Ferroelectric loops P-E and field-induced strain S-E measured at room temperature for the KNN-xLN:Er ceramics as a function of Li content: (a) ferroelectric loops P-E and (b) field-induced strains S-E.

ferroelectric properties, and their polarizations were saturated under the external electric field of 5 kV/mm. The coercive field Ec monotonically increased from 1.49 to 2.44 kV/mm as x was increased from 0 to 0.08. Moreover, the remnant polarization Pr increased from 19.3 to the maximum value of 21.9 mC/cm2 as x increased from 0 to 0.06. And then Pr decreased to 15.3 mC/cm2 with x further increased to 0.08. Fig. 2b illustrated the S-E loops of these ceramics. Herein, the d33* was calculated by the maximum strain (Smax) and the corresponding electric field (Emax) according to the equation of d33* ¼ Smax/Emax. d33* increased with x up to 0.06, and

achieved a maximum value of 307.8 pm/V, which reconfirmed that KNN-0.06LN:Er lying in the region of coexistence phase benefited from polymorphic phase transition effect. The results in Fig. 2 suggested that the electrical properties of the KNN-xLN:Er ceramics closely related with the content of Li and the important role of PPT effect. Fig. 3aec showed the SEM images of the as-sintered surface of these ceramics sintered at 1080  C for 2 h. The ceramics with x ¼ 0 have a homogeneous microstructure with an average grain size of 2.0 mm. The average grain size increased with increasing the Li

Fig. 3. Scanning electron microscopy images of the fractured surface of the KNN-xLN:Er ceramics sintered at 1080  C for 2 h with (a) x ¼ 0, (b) x ¼ 0.06, (c) x ¼ 0.08, and (d) The dielectric constant and loss versus temperature curves of the KNN-xLN:Er ceramics.

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content up to approximately 10 mm for the ceramics with x ¼ 0.08, as shown in Fig. 3c. As square-shaped grains with a flat surface was found in the ceramics with x ¼ 0.08, abnormal grain growth occurred in these ceramics [21]. The temperature dependence of the dielectric constant and loss at 1 kHz was shown in Fig. 3d. In the measuring temperature region, there is no phase transition could be observed for the ceramics with x ¼ 0. However, the phase transition of orthorhombic-tetragonal (TOT) could be clearly found for the ceramics with x ¼ 0.06 and 0.08. Additionally, the TOT shifted toward lower temperature with the increase of Li content. Even for the ceramics with x ¼ 0.08, the TOT was located far below the room temperature which was similar with the reported result in previous literature [14].

Upon excitation by a 980-nm laser with different pumping power, strong up-conversion (abbreviated UC) emission could be observed in the KNN-xLN:Er ceramics, as depicted in Fig. 4. All of UC emission spectra for these three kinds of ceramics included green and red emission bands. The green emission bands were made up of two main emission peaks centered at 528 and 548 nm which were produced by the transitions from excited states 2H11/2 and 4S3/ 4 2 to the ground state I15/2. Meanwhile, the red emission band centered at 660 nm was caused by the relaxation process of 4F9/ 4 2 / I15/2. Herein, the UC emission peaks of these ceramics in the visible region were consistent well with the energy-level diagram of Er3þ ion as reported in previous literature [22]. As for the other minor emission peaks around the centered emission peaks, they

Fig. 4. The Vis up-conversion emission spectra of KNN-xLN-Er ceramics with different phase structure under the 980 nm laser at power ranging from 0.36 to 1.22 W. (a) Orthorhombic phase x ¼ 0, (b) Coexistence of orthorhombic and tetragonal phase x ¼ 0.06, (c) Tetragonal phase x ¼ 0.08. (d), (e) and (f) are the corresponding emission intensities logelog plotted against the laser powers.

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Fig. 5. (a) UC emission spectra of the KNN-xLN:Er ceramics, and (b) variation of green and red emission integrated intensities of the KNN-xLN:Er ceramics with different x. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

were ascribed to the energy splitting in the crystal field around Er3þ. It was known that the higher pumping power could bring about the higher emission intensity. The intensity for each emission band had been integrated and plotted as a function of the pumping power in Fig. 4def. Based on the theoretical consideration of the multi-photon absorption process, the relationship between the emission intensity I and pumping power P could be represented by a power law [23]:

IfP n where in n is the number of photons required for the up-conversion process. For any kind of the sample, a straight line with a positive slope (i.e., n) could be obtained for each emission band in the logelog plot shown in Fig. 4. As shown in Fig. 4def, the observed n for the green and red emission bands of the ceramics with all compositions were close to 2.0, indicating a two-photon process of the up-conversion photoluminescence [24]. Fig. 5a showed the UC emission spectra with a power of 0.36 W of the KNN-xLN:Er ceramics. For a designed excitation power, it could evidently find that the relative intensity of photoluminescence spectra first increased as x increased from 0 to 0.06, and then decreased at higher x (x ¼ 0.08). These may be due to the change in the crystal field around Er3þ arisen from the doping of Liþ [25]. Thus it readily reached the conclusion that the Li doping was effective in regulating the emission intensity of up-conversion photoluminescence for the KNN-xLN:Er ceramics. From Fig. 5b showing the variations of the relative integrated intensities of emission bands for KNN-xLN:Er under 980 nm excitation, it can be seen that the enhancement in red emission was more obvious than that in the green emission. It knows that the visible 4f / 4f emissions of Er3þ are dominated by the electric dipole (ED) transition. According to the Judd- Ofelt (J-O) theory developed for explaining the ligand field of rare earth ions, the transition probabilities and then the photoluminescence intensities of the two green emission for Er3þ are dependent on intensity parameter U2 and U6, respectively, whereas those of red emission are determined by two intensity parameters U4 and U6 [26]. Jorgensen and Reisfeld have shown that U2 can be increased by decreasing the symmetry between Er3þ and the ligand field, whereas U6 is increased by decreasing the covalence between Er3þ and the hosts [27]. On the other hand, it has been reported that the bond length of EreO can be increased by Liþ [28]. The Li-doping may also change the

dimensions of the other bonds, such as KeO, NaeO, and NbeO. The local environment of Er3þ may hence be distorted, leading to a decrease in the symmetry between Er3þ and the ligand field as well as a decrease in the covalence between Er3þ and the host material. According to the XRD analysis, the KNN-xLN:Er ceramics evolved from orthorhombic phase with space group Amm2 to tetragonal phase with higher crystal symmetry P4mm with increasing x. However, obvious enhancement in the intensity of green emission and red emission was found in the coexistence phase, implying that the crystal symmetry and covalence of the host material KNN-0.06LN:Er had been decreased. This consideration suggested that the polymorphic phase transition composition region is not the simple coexistence of orthorhombic and tetragonal phase, which is generally accepted. We may considered that here the coexistence of orthorhombic and tetragonal may induce a coexistence phase with lower crystal symmetry instead of the simple combination of orthorhombic and tetragonal phase. However, now there is still no direct proof to certify this standpoint and more intensive research would be indeed needed to certify this speculation. However, what is most important is that the variation of the relative photoluminescence intensity would render us a handy approach to qualitatively judge the phase structure in Er3þ doped (Li,K,Na)NbO3 ceramics.

4. Conclusions In conclusion, the Er3þ doped(K0.52Na0.48)1xLixNbO3 lead-free ceramics have been fabricated via the conventional solid state reaction method. After the introduction of Li element, the ferro-/ piezoelectric and up-conversion photoluminescence properties had been correspondingly changed. Meantime, a phase transition from orthorhombic phase to tetragonal phase was also clearly observed via X-ray diffraction analysis. The polymorphic phase transition effect brought about the enhancement in the ferro/ piezoelectric properties as well as the photoluminescence property.

Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 51272119 and 51302145), Beijing Institute of Technology Research Fund Program for Young Scholars and the State Key Laboratory of New Ceramics and Fine Processing of Tsinghua University.

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