Yb3+-modified PSN-PMN-PT crystals

Accepted Manuscript 3+ 3+ 3+ Electric and optical properties of Er - and Er /Yb -modified PSN-PMN-PT crystals Zengzhe Xi, Aiguo He, Pinyang Fang, Xiaojuan Li, Wei Long PII:

S0925-8388(17)32135-7

DOI:

10.1016/j.jallcom.2017.06.136

Reference:

JALCOM 42205

To appear in:

Journal of Alloys and Compounds

Received Date: 30 March 2017 Revised Date:

10 June 2017

Accepted Date: 12 June 2017

3+ Please cite this article as: Z. Xi, A. He, P. Fang, X. Li, W. Long, Electric and optical properties of Er 3+ 3+ and Er /Yb -modified PSN-PMN-PT crystals, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.06.136. 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

Electric and optical properties of Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals Zengzhe Xi∗ Aiguo He Pinyang Fang Xiaojuan Li

Wei Long

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Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of materials and Chemical

Engineering, Xi’an Technological University, Xi’an,710021, china

Abstract

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PSN-PMN-PT, Er3+-, and Er3+/Yb3+-modified PSN-PMN-PT crystals were grown by using the flux method. The effects of Er3+ and Er3+/Yb3+ modification on the crystal structure, electric,

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and optical properties were investigated. Phase structure of all the crystals was determined as a pure perovskite structure by the XRD. The dielectric properties of the PSN-PMN-PT crystals along [100]-orientation were enhanced significantly, and the Pr increased from 24.2 to 30.99 and 28.5 µC/cm2, respectively by the Er3+ and Er3+/Yb3+ modification. The specific absorption at the

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UV-VIS-NIR band and the strong green and red UC PL at 980 nm laser excitation were observed in the Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals. The UC emission mechanisms of

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these modified PSN-PMN-PT crystals were also discussed.

Keywords: Er3+/Yb3+-modified PSN-PMN-PT; Optical properties; Perovskite structure; UC

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emission mechanisms

∗

Corresponding author: Zengzhe Xi, Tel: +86-29-86173324, E-mail address: [email protected]

ACCEPTED MANUSCRIPT 1. Introduction Relaxor ferroelectric single crystals based on PbTiO3(PT) are a crucial functional materials. They possess ultro-high piezoelectric permittivity (d33 > 1500 pC/N) and electromechanical

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coupling factor (k33 ≈ 0.90)[1-5]. They are regarded as the most promising candidates in the applications of medical ultrasound probes, underwater acoustic transducers, and piezoelectric actuators, et al. Much money and attention have been paid on investigation of ferroelectric crystals.

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Various systems of crystals have been gradually developed. Recently, New literatures[6-11]

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reported the electric and optical properties of rare-earth (RE) -doped ferroelectric materials, especially upconversion (UC) photoluminescence (PL). It is indicated that ferroelectric materials, constituted by different oxides, can achieve strong UC PL with RE ions modification, which give us a expectation to develop ferroelectric materials into new multifunctional materials if they

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possess excellent optical and electric properties simultaneously. Han. etal[8] revealed that Er3+ can enhance the Ec of Pb(Zn1/3Nb2/3)O3-9PbTiO3 single crystalsls, but also make crystals achieve strong geeen and red UC PL. Luo, et al[9] found the colour of UC emissions can be turned by

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changing the concentration of sensitizer Yb3+ in Er3+/Yb3+-modified KNLN ceramics. Some inner

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relations were also demonstrated between UC PL and piezoelectric properties[9,10]. It showed us the UC PL intensity of ferroelectrics can be modulated through reverse piezoelectric effects. Furthermore, the UC PL properties, including emission wavelength, bandwidth and lifetime, can turned effectively via strain engineering[12]. (1-x-y)Pb(Sc1/2Nb1/2)O3-xPb(Mg1/3Nb2/3)O3-yPbTiO3 [(1-x-y)PSN-xPMN-yPT][13-16] is a typical ternary ferroelectric single crystals. It was developed through adding PSN-xPT (with a high Ec of 5 kV/cm)[17] to PMN-xPT system, which overcame the disadvantage of low coercive

ACCEPTED MANUSCRIPT field (Ec ≈ 2 kV/cm) of PMN-xPT[3-5] and cut raw-materials cost[14]. It displays excellent electric properties and high electrostrictive strain. They can experience phase transitions by applying a electric field. They also possess low phonon energy. These make PSN-PMN-PT

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crystals become the most optimal host candidate to exploit photoelectric performance of RE-doped materials. It will promote the development of PSN-PMN-PT crystals as a multifunctional materials and broaden its application into short-wavelength lasers, displays,

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RE-modified PSN-PMN-PT crystals were reported so far.

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environmental monitoring equipments, and biomedical devices. However, no any work about

In the present work, Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals were grown by using the flux method. The effects of Er3+ and Er3+/Yb3+ modification on the phase structure, electric, and optical properties of the crystals were then investigated. The UC emission

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mechanisms of Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals were also discussed.

2. Experimental procedure

PSN-PMN-PT, Er3+- , and Er3+/Yb3+-modified PSN-PMN-PT crystals were grown by the flux

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method. The raw materials of Nb2O5, PbO, Sc2O3, MgO, TiO2, Er2O3, and Yb2O3 with purity

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higher than 99.99% were prepared. The precursors of MgNb2O6 (MN) and ScNbO4 (SN) were first synthesized to prevent the formation of pyrochlore. Three groups of mixtures composed of SN, MN, TiO2, and PbO weighed as stoichiometrc ratio were prepared, with an excess PbO of 70wt% added to each group. Dopants of 2wt% Er2O3 and 2wt% Er2O3/Yb2O3 (1:1) were put into two groups respectively, whereas dopants were not addded in the ramaining group. The mixtures were mixed by ball-milling with alcohol and subsequently dried. Pre-synthesized crystal materials were charged into three Pt crucibles with a size of Φ40×40 mm, covered with Pt lids. The charged Pt

ACCEPTED MANUSCRIPT crucibles were placed in a corundum crucible, supported with alumina powders. The corundum crucibles were placed in an ultra-temperature furnace and crystal growth was carried out according to a preset program. The Pt crucibles were removed from the furnace after being grown

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over and were placed in a boiled solution of HNO3:H2O=1:1. Bright crystals were obtained after the cleaning process in ultarsonic cleaner as the crystals were removed from the platinum crucible. Crystal powders and regular crystal samples with a flat sufface were prepared. The phase

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structure was examined by using the X-ray diffractometer (XRD-6000, SHIMADZU, JAPAN) at

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room temperature ranging from 10º to 80º (2θ) with a step size of 0.02. Crystal samples were polished to a thickness of approximately 1 mm, parallel to its flat surface. Then silver paste was daubed on two sides and fired as electrodes at 550 °C for 30 min. The dielectric properties were examined over a temperature range of 25–350 °C through a computer-controlled measurement

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system (Agilent E4980A, Egilent, New Mexico, USA). The ferroelectric properties were measured by using a ferroelectric test system (Radiant Precision PremierⅡ, Radiant Technologies, Inc., Albuquerque, New Mexico). Absorption at UV-VIS-NIR bands was measured by utilizing the

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UV-VIS-NIR spectrophotometer (UV3600PLUS, SHIMADZU, JAPAN). The UC PL and

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fluorescence lifetime under 980 nm laser excitation were measured by using the Lifetime and steady-state spectrometer (FLS980, Edinburgh, England). Moreover, the UC emission mechanisms of Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals were analyzed through power laws Ilum∝Pexn and the simplified energy level diagrams of Er3+ and Yb3+ ions.

3. Results and discussion The XRD patterns of the PSN-PMN-PT, Er3+-, and Er3+/Yb3+-modified PSN-PMN-PT crystals are shown in Fig. 1. Fig. 1(a) indicated that the positions and intensity ratios of the

ACCEPTED MANUSCRIPT diffraction peaks from all the crystals were match closely with that of the Pb(Mg0.333Nb0.667)O3 (PDF#81-0861). The phase structure of all the crystals can be confirmed as the perovskite structure with Pm-3m space group. Impurity peaks corresponding to the reaction oxides or any

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other secondary phases were not detected, indicating that Er3+ and Yb3+ ions diffused into the lattices of the PSN-PMN-PT crystals to form solid solutions. The main diffraction peaks of the Er3+- and Er3+/Yb3+-modified systems exhibited an obvious shift to a lower angle compared with

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that of the pure PSN-PMN-PT (shown in Fig.1 [b]). It attributes to the lattice distortion induced by

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the introduction of Er3+ and Yb3+ ions. The XRD patterns from exposed sufface of single crystal samples were showen in Fig. 1(c). Great diffraction peaks corresponding to (100) and (200) planes were obtained. Simultaneously several impurity peaks were also detected, which may caused by lacking strict orientation. However, we can confirm the crystals are basically along or colse to

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[100]-orientation.

The temperature dependence of dielectric permittivity ε and loss tanδ for the [100]-oriented PSN-PMN-PT, Er3+-, and Er3+/Yb3+-modified PSN-PMN-PT crystals at 1 kHz was shown in Fig. 2.

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The ε evidently improved with Er3+ and Er3+/Yb3+ modification, which should be associated with

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the change of electric dipole ordering. The Tc for the PSN-PMN-PT was 144 °C. After modification with Er3+ ions, the Tc increased to 151 °C, but the modification of Er3+/Yb3+ ions had little effect for Tc. The loss peak emergence was earlier than dielectric peak that was also a demonstration of phase transition from ferroelectric phase to paraelectric phase. The tanδ was less than 0.1 for all crystals in the temperature range of 25 °C – 300 °C but increased sharply when temperature exceeded 300 °C. This result is attributed to the high electrical conductivity at high temperatures. The tanδ of Er3+ and Er3+/Yb3+ modified crystals is evidently less than pure

ACCEPTED MANUSCRIPT PSN-PMN-PT over 300 °C. Funthermore, great relaxation behavior can be observed form the Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystals. Figure 3 shows field-induced polarization (P-E) hysteresis loops of the pure, Er3+-, and

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Er3+/Yb3+-modified PSN-PMN-PT crystals at 10 Hz. Saturated P-E loops were obtained under an external field of ±40, ±35 and ±32 kV/cm at room temperature, respectively. Nearly equivalent Ec of 10 kV/cm was detected, which was much higher than that of the [001]-oriented PMN-PT (Ec~2

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kV/cm)[18] and PIN-PMN-PT (Ec~4.6 kV/cm) single crystals reported[19]. It is even higher than

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that of the Mn-doped PIN-PMN-PT single crystal (Ec~8.9 kV/cm) along [001]-orientation[20]. The Pr of the PSN-PMN-PT crystals increased from 24.2 to 30.99 and 28.5 µC/cm2 with Er3+ and Er3+/Yb3+ modification, respectively. The enhancement of Pr is attributed to the structural distortion induced by the Er3+ and Yb3+ adding as evidenced through XRD [Fig. 1(b)]. Same

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results were reported in Eu-modified SrBi2Nb2O9 ceramics[21] and Er-doped Ba0.4Ca0.6TiO3 multifunctional ceramics[22]. In addition, It is likely related to the domian switching behavior. Figure 4 shows the absorption at the UV-VIS-NIR bands 300–1800 nm for the pure, Er3+-,

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and Er3+/Yb3+-modified PSN-PMN-PT crystals. Absorption peaks were not detected for the

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PSN-PMN-PT. By contrast, a series of peaks can be observed for Er3+- and Er3+/Yb3+-modified system, indicating that the special absorption ability is induced by the Er3+ and Er3+/Yb3+ modification for the PSN-PMN-PT crystals. The peaks for the Er3+-modified PSN-PMN-PT crystals centered at 489, 522, 654, 794, 974, 1467, and 1540 nm correspond to the transition absorption of Er3+ from ground state 4I15/2 to 4F7/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2 (1467 and 1540 nm), respectively. The peak located at 974 nm is the absorption from 4I15/2→4I11/2 of Er3+, as well as the absorption of 2F7/2→2F5/2 of Yb3+ for Er3+/Yb3+-modified PSN-PMN-PT crystals. An

ACCEPTED MANUSCRIPT abnormal peak centered at 899 nm is also caused by Yb3+ because of its broad absorption at NIR bands. Absorption cross-section (σabs) is an important parameter in spectroscopy, which is defined as

the following formula

lg[I 0 (υ ) / I (υ )] 2.3D = NL NL

(3.1)

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σ abs = 2.3

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the probability that incident photons can be absorbed by materials. σabs is usually calculated using

where I0 and I are the intensities of incident light and transmitted light, respectively; N is the

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doping concentration of rare earth ions; L is the thickness of samples; D is the optical density, D=log(I0/I). The doping concentration of rare earth ions can be calculated using formula (3.2).

N=

Ax O3 wt% ρN A ×2 M Ax O3

(3.2)

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where AxO3wt% is the weight percentage of rare earth oxides, ρ is the density of samples (g/cm3), NA is the Avogadro constant, and NA=6.022×1023 mol-1, MAxO3 is the molecular weight of AxO3, ρ can be obtained by Archimedes principle. The σabs values corresponding to the largest absorption

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peak (654 nm) are 9.82×10-21 cm2 and 1.17×10-20 cm2 for Er3+- and Er3+/Yb3+-modified

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PSN-PMN-PT crystals, respectively. Therefore, Yb3+ can increase the absorption ability of Er3+-modified materials. It may attributed to the change of UC emission mechanism induced by Yb3+ addition.

The UC emission spectrums of the Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals

excited at 980 nm are shown in Fig. 5. Evident emission peaks were not observed in the detection range of 450–750 nm for the PSN-PMN-PT crystals, with curves waving near a horizontal line under different measuring powers. Three emission bands centered at 530, 564, and 665 nm were

ACCEPTED MANUSCRIPT observed for the Er3+ modified system, which correspond to the transition of Er3+ from excited state 2H11/2, 4S3/2, and 4F9/2 to ground state 4I15/2. The green emission intensity was significantly stronger than that of the red emission. The UC PL intensity increased with exciting power. The

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identical locations of emission bands were observed in the Er3+/Yb3+-modified system, but intensity is higher than the former. These findings indicated that Yb3+ ions can increase emission intensity as sensitizers.

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To investigate the dependence of the UC green (530 and 564 nm) and red (665 nm) emission

in previous works is introduced.

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intensities on pump powers under the 980 nm laser excitation, the expression [23,24] widely used

I Lum ∝ Pex

n

(3.3)

where ILum is the UC emission intensity, Pex is the pump laser power, and n is the number of

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absorbed photons from pumping source for transition. The number of absorbed photons from the pumping source can be estimated from the slope of ILum versus Pex in a log-log plot. Fig. 6 showes that the n values are 2.08, 2.05 (green), 1.96 (red), and 1.8, 1.79 (green), 1.8 (red) for the Er3+- and

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Er3+/Yb3+-modified PSN-PMN-PT crystals, respectively. These results indicate that the UC green

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and red emission is mainly caused by the two-photon process for the Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals.

To describe the UC emission mechanisms in detail, we developed simplified energy level

diagrams of Er3+ and Yb3+ ions and possible transition pathways in Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals at 980 nm laser excitation (Fig. 7). The UC emissions for the Er3+-modified PSN-PMN-PT crystals (Fig. 7[a]) are conducted through two processes, including ground state absorption (GSA) and excited state absorption (ESA). Electrons located at ground

ACCEPTED MANUSCRIPT state 4I15/2, initially are excited to 4I11/2 through the GSA process (Er3+:4I15/2→4I11/2), and subsequently electrons located at 4I11/2 reach to 4F7/2 through the ESA process (Er3+:4I11/2→4F7/2). Three pathways can be selected for electrons populating at the 4F7/2 level. Most of the electrons

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reach to the 2H11/2 and 4S3/2 levels without a radiation process and subsequently return to ground state 4I15/2, concomitantly photons with wavelength of 530 and 565 nm are emitted. A small number of electrons located at 4F7/2 return to the 4F9/2 level without a radiation, and a weak red

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emission (665 nm) is obtained when the electrons return to ground state 4I15/2.

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Yb3+ exhibits a significant absorption cross-section near the wavelength of 980 nm. Thus, energy transfer upconversion (ETU) is the dominant approach to achieve luminescence under the excitation of 980 nm laser for Er3+/Yb3+-modified PSN-PMN-PT crystals (Fig.7[b]). A electron located at ground state 4I15/2 absorbs the energy of Yb3+ located at excited state 2F5/2 and jumps to I11/2. Two pathways can be selected for electrons populating at the 4I11/2 level. Some electrons

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return to a lower level (4I13/2) without a radiation, then jump to the 4F9/2 level through the ETU2 process. Most electrons populating at 4I11/2 reach the upper level (4F7/2) through the ETU2 process,

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subsequently return to the 2H11/2 and 4S3/2 level without radiation. Finally, the electrons populating

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at the 2H11/2, 4S3/2, and 4F9/2 levels return to ground state 4I15/2. In a similar manner, photons with wavelengths of 530, 565 (green), and 665 nm (red) are emitted. In addition to the ETU process that occured in Er3+-Yb3+ ions contributing to the UC transmission of PSN-PMN-PT:Er/Yb crystals, ESA (Er3+) is also a way that is responsible for the UC luminescence. Fluorescent lifetime (τ) is essential for any luminescent materials. The decay curves of the 4

S3/2 level of Er3+ ions for Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals under 980 nm

laser excitation are shown in Fig. 8. The curves increased and decayed. The increase corresponded

ACCEPTED MANUSCRIPT to the absorption of Er3+ and Yb3+ from the pumping source, and the decay corresponded to the electron transition from an upper lever (4S3/2) to a lower level (4I15/2). The shorter increase time (0.058 ms) of Er3+/Yb3+-modified PSN-PMN-PT crystals compared with that of the Er3+-modified

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PSN-PMN-PT crystals (0.206 ms) indicated that Yb3+ ions was more efficient to absorb energy from laser to accomplish electron transitions. The decay of luminescence exhibited a double exponential decay behavior. By fitting the decay curves, the lifetimes 10.39/66.59 µs for

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Er3+-modified PSN-PMN-PT crystals, 12.19/110.87 µs for Er3+/Yb3+-modified crystals were

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obtained. It was comparable to that of the Er/Yb co-doped TiO2 nanopowders[25], which may be important for the luminescence properties of Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystals as optical devices. The results also showed that Yb3+ can prolong the luminescence lifetime of crystals.

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4. Conclusions

PSN-PMN-PT, Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals are grown by using the flux method. Our results confirme that the grown crystals are composed of a pure perovskite phase

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structure. The dielectric permitivity is enhanced significantly, and the Pr increased from 24.2 to

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30.99 and 28.5 µC/cm2, respectively for [100]-oriented PSN-PMN-PT crystals with the Er3+ and Er3+/Yb3+ modification. The Er3+- and Er3+/Yb3+-modified systems yield a special absorption at the UV-VIS-NIR band and strong green and red UC emissions irradiated by the 980 nm laser. Yb3+ as a sensitizer can significantly improve UC emission intensity. The absorption cross-section can also be expanded, and luminescence lifetime can be prolonged. The absorption cross-section and lifetime for Er3+ modification are 9.82×10-21 cm2 and 10.39/66.59 µs, respectively. By comparison, the absorption cross-section and lifetime for Er3+/Yb3+ modification are 1.17×10-20 cm2 and

ACCEPTED MANUSCRIPT 12.19/110.87 µs. These results make Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystals become a promising candidate with excellent optical and electric properties, and play a important role in optical and optoelectronic coupling devices.

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Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (Grant No. 2013CB632900), and the National Natural Science Foundation of China (Grant No.

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51472197).

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ACCEPTED MANUSCRIPT Figure captions Fig.1

X-ray diffraction patterns of pure, Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystals: (a)XRD patterns of crystal powders; (b)zoomed XRD patterns from 30.5° to 32.5°; (c)the

Fig.2

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XRD patterns of the [100]-oriented crystals Temperature dependence of dielectric permittivity ε and dielectric loss tanδ for pure, Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystal at measuring frequency 1kHz

Polarization versus electric field (P-E) of the pure, Er3+ and Er3+/Yb3+ modified

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Fig.3

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PSN-PMN-PT crystals Fig.4

The UV-VIS-NIR absorption of pure, Er3+ and Er3+/Yb3+ modified PSN-PMN-PT crystals

Fig.5

The UC emission spectra at 980 nm excitation of pure (a), Er3+ (b) and Er3+/Yb3+ (c) modified PSN-PMN-PT crystals

Dependence of UC PL intensity at different transitions (2H11/2→4I15/2, 4S3/2→4I15/2 and 4

F9/2→4I15/2) on the excitation power for Er3+(a) and Er3+/Yb3+(b) modified PSN-PMN-PT

crystals

Simplified Energy level diagrams of Er3+ and Yb3+ ions and possible transition pathways

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Fig.7

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Fig.6

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in Er3+ (a) and Er3+/Yb3+ (b) modified PSN-PMN-PT crystals at 980 nm excitation Fig.8

Decay curve of 4S3/2 level of Er3+ ions for Er3+ (a) and Er3+/Yb3+ (b) modified

PSN-PMN-PT crystals at 980 nm laser excitation

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Highlights (for review) ACCEPTED MANUSCRIPT 1. The ε of the [100]-oriented crystals was enhanced significantly with RE modification. 2. The Pr increased evidently by the Er3+ and Er3+/Yb3+ modification. 3. The UC PL was achieved in the Er3+- and Er3+/Yb3+-modified PSN-PMN-PT crystals.

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4. The Yb3+ can expand the σabs and prolong the τ of Er3+-modified PSN-PMN-PT crystals.