3)O3-0.32PbTiO3 single crystal

Accepted Manuscript 3+ Growth and property enhancement of Er -doped 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystal Wei Long, Xing Chu, Zengzhe Xi, Pinyang Fang, Xiaojuan Li, Wenwu Cao PII:

S1002-0721(18)30294-1

DOI:

10.1016/j.jre.2018.01.020

Reference:

JRE 161

To appear in:

Journal of Rare Earths

Received Date: 7 November 2017 Revised Date:

29 December 2017

Accepted Date: 2 January 2018

Please cite this article as: Long W, Chu X, Xi Z, Fang P, Li X, Cao W, Growth and property 3+ enhancement of Er -doped 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystal, Journal of Rare Earths (2018), doi: 10.1016/j.jre.2018.01.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Growth and property enhancement of Er3+-doped 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystal Wei Long1, Xing Chu1, Zengzhe Xi1, Pinyang Fang1, Xiaojuan Li1, Wenwu Cao2,3,∗ , 1 Shaanxi key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China 2 Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150080, China

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3 Department of Mathematics and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

Abstract

Er3+-modified 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-32PT) single crystals were grown by using the flux method. The growth mechanism of the crystal and influences of Er3+ ions on phase structure, electrical and optical properties were investigated. Results reveal that the crystals are still pure perovskite structure with Er3+ ions doping, but lattice enlarges slightly. The coercive electric field is increased from 4.83 to 6.37 kV/cm for [100]-oriented crystals compared

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to undoped PMN-32PT single crystals. Moreover, the crystal exhibits upconversion emission properties. Green (531 and 552 nm) and red (670 nm) emission bands are recorded under the excitation of 980 nm diode laser, which correspond to photoelectric multifunctional devices.

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the 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ ions. Our results show the feasibility of using this crystal in Keywords: PMN-PT single crystal, Er3+ ion doping, Flux method, Upconversion; Rare earths 1. Introduction

Lead magnesium niobium (PMN) with relaxor behavior and lead titanate (PT) with normal ferroelectric property were used to form the solid solution of (1–x)Pb(Mg1/3Nb2/3)O3-xPbTiO3. These relaxor-based ferroelectric single crystals are widely used in ultrasonic medical imaging transducers and sonar receivers because of their outstanding piezoelectric performance. They are also the target of fundamental studies due to the rich physical phenomena associated with compositions near the morphotropic phase boundary[1-5]. However, the low coercive field limits their applications in

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high-power transducers, such as ultrasonic motors and sonar projectors [6]. Some studies have shown that doping certain ions could effectively improve the properties of these relaxor-based ferroelectrics. Li et al. found that the coercive field of Mn-doped PMN-PT single crystals was enhanced from 250 to 420 V/mm, and the dielectric loss was decreased from 1% down to 0.5% [7]. Rare earth ions were considered to be a significant ions to optimize material performance and develop new multifunctional materials. As a type of typical rare

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earth ions, Er3+ ions have great applications in optical thermometry, solid state lighting, and bioimaging[8-10]. What’s more, they can improve the luminescence of materials because of their dense energy levels[11]. Numerous researchers have investigated the processing technique and evaluated the performance of Er3+-doped ferroelectrics[12-15]. Wang et

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al. prepared Er substituents to CaBi4Ti4O15 ceramics with the perovskite structure and found that the piezoelectric, ferroelectric, and luminescent properties were all effectively improved by the Er3+ ions modification [12]. Hao et al. demonstrated that the upconversion photoluminescence of Er/Yb co-doped BaTiO3 thin film can be enhanced via electric field [16]. Bai et al. investigated 2D layered materials of rare-earth Er-doped MoS2. Obvious NIR-to-NIR downand up-conversion photoluminescence were observed under the excitation of 488 and 980 nm, respectively[17]. They also proposed that the composites through incorporating luminescent ions into several advanced functional materials can combine the beneficial aspects of each component and be applied in many fields as a new nultifunctional materials[18].

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Project supported by the National Basic Research Program of China (973 Program) (2013CB632900), the National Natural Science Foundation of China (51472197), and the Shaanxi Provincial Education Department (14JK1333), the Key Laboratory of

Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2016DP173016) and Shaanxi Key Laboratory of Optoelectronic Functional Materials and Devices (2015SZSJ-59-5) Corresponding author: Zengzhe Xi，Tel: 86-29-86173324, E-mail address: [email protected] 1

ACCEPTED MANUSCRIPT Glass and ceramics can be used as the matrix of photoelectric materials [19-21], but their application is limited due to their poor thermal stability and less satisfactory electrical properties. Up to date, few studies have investigated the performance enhancement of relaxor-based ferroelectric single crystals by rare-earth ion doping. In the paper, Er3+-doped PMN-32PT relaxor single crystals were grown using the flux method. The influence of Er2O3 on the phase structure and electrical properties was investigated, and the upconversion luminescent properties were also characterized.

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2. Experimental

Er3+-doped PMN-32PT single crystals were grown spontaneously by the flux method. The starting materials included MgO, Nb2O5, TiO2, PbO, and Er2O3, with purity >99.9%. First, the columbite precursors MgNb2O6 (MN) was synthesized by sintering the mixtures of MgO and Nb2O5 at 1000 °C for 10 h. As much as 70 wt% excess PbO was added to the raw materials as the flux. The doping content of Er2O3 was selected as 2 wt%, which was referring to several previous literatures[22-24]. According to the stoichiometric composition of 68PMN-32PT, the raw materials

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were mixed and ball milled for 12 h. The obtained powders were placed in a Pt crucible with the size of φ40 mm×60 mm, covered with a Pt lid, and growth was done in an electric furnace. Finally, platelet sample along [100] orientation was cut from the as-grown single crystals for property measurements.

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The growth surface morphology of single crystals was characterized using the scanning electron microscopy (SEM). Phase structure was analyzed by X-ray diffraction (XRD6000, Japan) with Cu Kα radiation for crystal powders and platelet sample. Dielectric constants were measured at various temperatures (from room temperature to 300 °C) in the frequency range of 100 Hz to 100 kHz through an automatic measuring system with a temperature-controlled chamber (E4980/GJW-I). The piezoelectric constant (d33) of the crystal was measured using a quasi-static d33 meter (Institute of Acoustics, Chinese Academy of Sciences, Model ZJ-4AN). Ferroelectric hysteresis loops were measured at room temperature using a ferroelectric test system (Radiant Precision Premier II). Absorption spectra were recorded on an Agilent Technologies Cary series UV-vis-NIR spectrophotometer, in the spectral range from 300 to 1700 nm. The

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upconversion emission spectrum was measured by using the Lifetime and steady-state spectrometer (FLS980, Edinburgh, England) equipped with a power controllable 980 nm laser diode (LD). 3. Results and discussion

3.1 Phase structure of as-grown single crystals

Fig. 1 shows the XRD patterns of PMN-32PT and Er-doped PMN-32PT single crystal powders at room temperature.

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Both showed pure perovskite phase for the as-grown crystals. The diffraction peaks of Er-doped crystals slightly shifted to low angle compared with that of the PMN-32PT single crystals, indicating that Er3+ ions were well-diffused into the crystal lattice of PMN-32PT host. The shifting to lower angle was probably due to the Er3+ (0.0868 nm) substitution on

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the B-sites Mg2+ (0.072 nm) or Nb5+ (0.064 nm), which resulted in increased volumes and crystalline interplanar spacing d. It can also be obtained that the coordinated number of Mg2+, Nb5+ and Er3+ ions are 6 because of their location at B site of perovskite structure. In order to get a precise lattice change, Rietveld refinement by using a Gsas program was performed, with lattice parameters and volumes shown in Table 1. It can be seen that the lattice enlarged slightly with Er3+ doping. Fig. 2 shows the XRD pattern of the platelet sample. The diffraction peaks of {100} were observed, showing the crystal is along [100] orientation. 3.2 Morphology of as-grown single crystals The as-grown Er3+-doped PMN-32PT crystals with light yellow color are shown in Fig. 3 (a). The largest as-grown single crystal had a dimension of approximately 6 mm×5 mm×3 mm. The Er3+-doped PMN-32PT crystals exhibited pseudo-cubic shape during the initial nucleation. Further growth led to the deformation of this cubic shape, resulting in various morphologies, including arrowhead and dendritic morphologies, which can be clearly observed in Fig. 3(a). The growth surface morphology of Er3+-doped PMN-32PT was characterized by SEM. As shown in Fig. 3(b), the growth 2

ACCEPTED MANUSCRIPT surface was very rough and layer growth morphology was evident. Pitting and cavities were also observed on the surface of single crystals. The fluctuations in temperature and composition resulted in the formation of PbO packages and other inclusions during crystal growth, which were removed by 50% hot HNO3 after the growth. The inclusions served as substrates to facilitate heterogeneous nucleation and crystal growth. Therefore, suitable amounts of flux and temperature gradient are necessary to inhibit the inclusions. Fig. 3 (c) shows the scanning electron micrograph of the surface of single crystals. The EDS spectrum of the single crystals is presented in Fig. 3 (d). The spectrum shows that the chemical

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composition of the single crystal was mostly Pb, Mg, Nb, Ti and O. In addition, Er was also found, indicating that Er3+ ions diffused into the structure of single crystals. The element content of the crystals obtained by EDS is shown in Table 2. The composition of the as-grown crystals is calculated to be about 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3, indicating composition segregation happens easily during the growth of PMN-PT system. 3.3 Electrical properties of single crystals

The dielectric constant of the Er-doped PMN-32PT single crystal along [100] orientation is 4300, and dielectric loss

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is less than 1% at room temperature at 1 kHz, which are consistent with previous works for PMN-PT single crystals [25]. Fig. 4 presents the temperature dependence of the dielectric permittivity at different measuring frequencies (100 Hz to 100 kHz). Three dielectric permittivity peaks, namely peak Ι at 58.5 °C, peak II at 73.4 °C and peak III at 131.5 °C, were

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observed. Peak Ι corresponds to the transition from rhombohedral to orthorhombic phase. Some say this should be monoclinic phase, which is the evidence for the phase to be orthorhombic. Peak II and peak III are associated with orthorhombic–tetragonal and tetragonal–cubic phase transitions, respectively. The dielectric peak at approximately 131.5 °C is very broad. With increasing frequency, the dielectric permittivity peaks decreased and shifted to high temperatures. This characteristic is typical for relaxor ferroelectrics. The modified Curie-Weiss law is usually applied to describe the temperature dependence of dielectric permittivity at T > Tm [26].

1 ε ' − 1 ε m = (T − Tm ) C γ

(1)

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where C is the Curie constant, ɛm is the maximum dielectric permittivity, Tm is the temperature at ɛm, and γ is the dielectric relaxation degree varying from γ = 1 for the normal ferroelectrics to γ = 2 for relaxor ferroelectrics. Fig. 5 presents the fitting results of the temperature dependence of dielectric permittivity above Tm at 1 kHz. One can see that the experimental data are well fitted by Eq. (1). A linear relationship was found, and the slope of the fitted line is defined typical relaxors.

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as the γ value. Here the obtained γ value is 1.93, which indicates that the Er3+-doped PMN-32PT single crystals are The piezoelectric constant of the Er3+-doped PMN-32PT single crystals along [100] orientation is 1089 pC/N at the field level of 1 kV/mm. Fig. 6 shows the polarization versus electric field (P–E) of the PMN-32PT and Er3+-doped

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PMN-32PT single crystals along [100] orientation. Saturation loops were observed at the field level of 15 kV/cm. The values of the saturation polarization (Ps), remnant polarization (Pr) and coercive field (Ec) of the Er3+-doped PMN-32PT single crystals are 35.37 µC/cm2, 30.32 µC/cm2 and 6.37 kV/cm, respectively, which are higher than those of PMN-32PT single crystals. Enhancement of the coercive filed is attributed to the domain wall pinning by defects. Because of similar ionic radius between doping Er3+ ions (0.0868 nm) and Cations located at B site (Mg2+:0.072 nm; Nb5+:0.064 nm; Ti4+:0.0605 nm)，Er3+ ions preferentially occupy B site and distort crystal lattice, which can be demonstrated in XRD results with diffraction peaks shifting toward low angle. The substitution of (Mg,Nb)4+ and Ti4+ with Er3+ results in the creation of oxygen vacancies for compensating charge balance. With oxygen vacancies migrating in lattice, defect dipoles form, which inhibit the motion of domain wall and polarization rotation under an alternating field, resulting in the increase of Ec and Pr. 3.4 Luminescence of the single crystals 3

ACCEPTED MANUSCRIPT The absorption spectra of Er3+-doped PMN-32PT crystals in 300–1700 nm region are shown in Fig. 7. There are seven main peaks in the absorption spectrum, and their central wavelengths were at 490, 522, 552, 652, 796, 974 and 1548 nm, which are attributed to the transitions from the ground state 4I15/2 of Er3+ to the excited states 4F7/2, 2H11/2, 4S3/2, 4

F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. The spectrum from Er3+-doped PMN-32PT crystal has similar feature as other

Er3+-doped ferroelectric materials[27,28]. The upconversion emission spectra of Er3+-doped PMN-32PT single crystal thin plate was excited by a 980 nm LD

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with various powers as presented in Fig. 8. Green emission bands centered at 531, 552 and red emission centered at 670 nm correspond to 4H11/2→ 4I15/2, 4S3/2→ 4I15/2 and 4F9/2→ 4I15/2 transitions of Er3+. In addition, the fluorescence intensities of the green and red emissions increased with the excitation power of the 980 nm LD.

The luminescent mechanism of Er3+-doped materials was examined. The fluorescence intensity I was approximately proportional to the power of pumping laser, I∝Pn, where n is the number of photons revealing the pumping mechanism [29]. The dependence of the upconversion emission intensities on the laser power intensity of

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Er3+-doped PMN-32PT at 531, 552, and 670 nm is shown in Fig. 9. The slopes of n were 1.8, 1.7, and 1.6 for the two green and one red emissions, respectively, which indicate that they are from two-photon excitation process. The possible mechanism for the upconversion process is given in the schematic energy level diagram of Er3+ ions

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shown in Fig. 10. Under the 980 nm LD excitation, the Er3+ ions are excited from the ground state 4I15/2 to 4I11/2 state after absorbing 980 nm photons, and some Er3+ ions relax to the 4I13/2 state through non-radiative transitions. Er3+ ions at 4

I11/2 and 4I13/2 states absorb photons again and are further excited to 4F7/2 and 4F9/2 states via excited state absorption.

Then, the Er3+ ions at 4F7/2 decay to 4H11/2, 4S3/2, and 4F9/2 states. Subsequently, all Er3+ ions at 4H11/2, 4S3/2 and 4F9/2 relaxed to the ground state with radiative transition of green (4H11/2 and 4S3/2→ 4I15/2) and red (4F9/2→ 4I15/2) emission bands, respectively, through multiphoton relaxation. The mechanism described above is called a two-photon excitation process, which is in agreement with the Er3+-doped other hosts [27-30].

Our results also show that the relaxor-ferroelectric single crystal PMN-32PT can provide an ideal crystal field for 4. Conclusions

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the Er3+ ions, which is beneficial for the upconverted luminescence. Er3+-doped PMN-32PT single crystals are grown using the flux method. The crystal grows via the layer growth mechanism. Pure perovskite structure is indexed by XRD. Dielectric and piezoelectric constants of the crystal along [100] orientation are found to be 4300 and 1089 pC/N, respectively. The coercive field and dielectric relaxation degrees

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are about 6.37 kV/cm and 1.93. The rhombohedral–orthorhombic–tetragonal–cubic phase transition sequence is observed with increasing temperature. Furthermore, upconversion luminescence occurrs under the excitation of 980 nm. Luminescence intensities of the green and red emissions increases with the excitation power of the LD. Our results show

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that the Er3+-doped PMN-32PT single crystals have high potential to become the next-generation photoelectric multifunctional materials. References

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Na1/2Bi1/2TiO3 ceramics. J Am Ceram Soc. 2007; 90: 664.

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[30] Chen S, Wu MJ, An LQ, Li YX, Wang SW. Strong green and red upconversion emission in Er3+ -doped

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Table caption Table1 Lattice parameters and volumes determined by the Rietveld refinement. Lattice parameters

Crystals 3

PMN-32PT

3+

a = 0.40217 nm, c = 0.40242 nm a = 0.40229 nm, c = 0.40333 nm

3

Monoclinic

0.065087

0.065227

V (nm )

a = 0.40247 nm, b = 0.3974 nm, c = 0.4024 nm, β = 89.97°

a = 0.40353 nm, b= 0.40318 nm, c = 0.4031 nm, β =89.92°

0.06437

0.06557

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Er -doped PMN-32PT

V (nm )

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Tetragonal

Mg

Atomic percent

67.33

3.79

Weight percentage

20.17

1.72

Ti

Nb

Er

Pb

4.32

9.61

0.68

14.28

3.87

16.71

2.14

55.39

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Table2 The element content of the Er3+-doped PMN-32PT crystals obtained by EDS

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ACCEPTED MANUSCRIPT Figure caption Powder x-ray diffraction (XRD) pattern of PMN-32PT and Er3+-doped PMN-32PT single crystals.

Fig. 2

XRD pattern of the platelet sample.

Fig. 3

Photographs, surface morphology, and EDS spectrum of Er3+-doped PMN-32PT single crystal.

Fig. 4

Temperature dependence of dielectric constant of Er3+-doped PMN-32PT crystal wafer at different frequencies.

Fig. 5

Ln(1/ε'-1/εm) as a function of ln(T-Tm) at 1 kHz above Tm for Er3+-doped PMN-32PT single crystals.

Fig. 6

(P-E) hysteresis loop of PMN-32PT and Er3+-doped PMN-32PT single crystals.

Fig. 7

Absorption spectra of Er3+-doped PMN-32PT in the wavelength range of 300–1700 nm.

Fig. 8

Luminescence spectra of Er3+-doped PMN-32PT wafer under 980 nm LD excitation of different power levels.

Fig. 9

Power dependence of the upconversion emission intensities of Er3+-doped PMN-32PT wafer at 531, 552 and

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Fig. 1

670 nm.

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Fig. 10 Energy-level diagram and the illustration of luminescence mechanism of Er3+ ions.

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Fig.2

(c )

(d )

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Er -doped PMN-32PT single crystal exhibited upconversion emission properties. The green (531 and 552 nm) and red (670 nm) emission bands were recorded under 980 nm diode laser excitation, corresponding to the transitions of 4

H11/2→ 4I15/2, 4S3/2→ 4I15/2 and 4F9/2→ 4I15/2 of Er3+. In addition, the fluorescence intensities of the green and red

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emissions increased with the excitation power increasing.

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