Eu3+-site occupation in CaTiO3 perovskite material at low temperature

Current Applied Physics 17 (2017) 24e30

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Eu3þ-site occupation in CaTiO3 perovskite material at low temperature Fengfeng Chi a, Yanguang Qin a, Shaoshuai Zhou b, Xiantao Wei a, Yonghu Chen a, *, Changkui Duan a, Min Yin a a

Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province 230026, PR China b Department of Physics, Qufu Normal University, Qufu, Shandong Province 273165, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2016 Received in revised form 4 September 2016 Accepted 26 October 2016 Available online 27 October 2016

In order to clarify the site occupancy of rare-earth ions in rare-earth doped perovskite materials, the undoped pure CaTiO3 and Eu3þ-doped CaTiO3 samples with a series of Ca/Ti ratio were synthesized via high-temperature solid-state reaction method. X-ray diffraction (XRD) powder patterns confirm that the crystal structure keeps invariant at various Ca/Ti ratios. Measurement results of unit-cell parameters and X-ray photoelectron spectroscopy (XPS) indicate that Eu3þ ions enter into the Ca2þ site. The highresolution photoluminescence spectra of Eu3þ ions at 20 K in all samples did not witness a significant change under the excitation at different wavelength, implying that Eu3þ ions occupy only one type of site. Considering the small spectral splitting range of 5D0 / 7F2 transition and the large intensity ratio of 5 D0 / 7F2/5D0 / 7F1, it can be concluded that Eu3þ occupies Ca2þ site with larger coordinate numbers rather than Ti4þ site. © 2016 Elsevier B.V. All rights reserved.

Keywords: CaTiO3 Photoluminescence Site occupation

1. Introduction Perovskite structure materials with ABO3 formula are a special class of compounds which are applied in many fields of science and technology, including multiferroic material [1e3], dielectric material [4,5], energy harvesting device [6e8], photocatalysis [9e11]. The ideal cubic structure of ABO3 formula perovskites is simple with corner-linked BO6 octahedra and the A cations sitting in the space between the octahedra. Because of external stresses or chemical substitution, most perovskites present structural distortions away from the parent cubic structure. The idiosyncratic deformation of the perovskite structure lowers the point symmetry at the alkaline-earth site which, when doped with trivalent rare earth ions (RE3þ), would enhance the 4f-4f transitions of RE3þ [12]. CaTiO3 (CTO) is a perovskite oxide material which exhibits ferroelectricity and quantum paraelectricity at low temperature [13]. It has attracted increasing interest due to the superior electronic, piezoelectric and optical properties. The bulk (Ba, Ca)TiO3 materials shows good energy storage efficiency suggesting they might be candidates for high energy density capacitor applications [14]. Ceramic solid solutions involving CTO are among the most

interesting high-quality dielectric materials at microwave frequencies [15]. Pure CTO is an incipient ferroelectric and its dielectric constant increases with decreasing temperature and saturates at temperatures below 30 K [16]. Recently, much efforts focus on optical performance of CTO doped with RE3þ for white light emitting diodes (WLEDs) and field emission displays [17]. The local environment of lanthanide ion in doped host with multiple sites available for occupancy is very important to understand structure-property correlation to optimize its performance. Trivalent rare-earth ions with ionic radius intermediate between A2þ and B4þ could be forced to occupy a specific site or both sites simultaneously. When RE3þ is incorporated at Ca site, its positive effective charge can be compensated by conduction electrons, calcium vacancies, and titanium vacancies. When substitution occurs at the Ti site, compensation can involve oxygen vacancies or electron holes. When both cation sites are occupied, self-compensation is possible. For the incorporation of a generic M2O3 oxide (M ¼ Al, Ga, Fe, Lu, Yb, Er, Y, Gd, Eu, Sm, Nd, La), the following defect reactions have been considered in detail [18]. (1) Substitution of M3þ at the Ca2þ site with calcium vacancy compensation,

* Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.cap.2016.10.018 1567-1739/© 2016 Elsevier B.V. All rights reserved.

F. Chi et al. / Current Applied Physics 17 (2017) 24e30

CaTiO3

00

M2 O3 þ3TiO2 ƒƒƒƒƒƒ ƒ!3CaTiO3 þ2MCa þVCa : (2) Substitution of M3þ at the Ti4þ site with oxygen vacancy compensation, CaTiO3

0

M2 O3 þ 2CaOƒƒƒƒƒƒ ƒ!2CaTiO3 þ2MTi þV O:

25

lock-in amplifier and stored into computer memories. The decay curves were recorded with a Tektronix TDS2024 digital storage oscilloscope. For the measurements at low temperature, the powder sample was first uniaxially pressed into pellet in a steel die, and then mounted on a Cu sample pedestal, which is loaded in a closedcycle He cryostat. 3. Results and discussion 3.1. XRD characters

(3) Substitution at the Ca2þ compensation, CaTiO3

and Ti4þ sites

with self-

0

M2 O3 ƒƒƒƒƒƒ ƒ!CaTiO3 þMTi þMCa : V. Buscaglia et al. reported that changing the Ca/Ti ratio to force Yb3þ on Ca site (Ca/Ti < 1), on Ti4þ site (Ca/Ti > 1), or on both sites (Ca/Ti ¼ 1) [18]. Besides, the simulations for CTO by J. A. Dawson using a new potential model show that large to mid-size RE3þ ions (La to Eu) energetically favor Ca site doping with Ca vacancy charge compensation and smaller ions dope via self-compensation [19]. Moreover, Dong et al. claimed three different charge compensation mechanisms by substituting Eu3þ ion at different sites [20]. These researches, to be honestly, remain somehow doubts and deserve further exploration. Eu3þ is often used as a structure probe to study the local site characters, because of the relative simplicity of its energy-level structure and its emission spectra showing evident dependence on its site symmetry in the matrix. The 5D0 / 7F1 transition is magnetic dipole allowed, and its intensity is generally accepted to be fairly constant for Eu3þ in different crystal field [21]. While the forced electric dipole transition of 5D0 / 7F2 is hypersensitive to the degree of deviation from centro-symmetry of Eu3þ site in a matrix. The intensity of electric dipole transition is higher than magnetic dipole transition in a site without inversion symmetry. In this paper, the low temperature photoluminescence investigations of Eu3þ doped CTO were firstly carried out to probe the local site environment of Eu3þ ion in CTO.

The crystal structures of the as-prepared samples were identified by XRD. Fig. 1(a) shows the XRD patterns of the pure CTO, CTOA, CTO-B and CTO-AB samples together with the standard pattern of CaTiO3 as the reference. The diffraction peaks of all the samples are in good agreement with the standard JCPDS Card No. 22-0153 and no other impurity peak is detected. A strongest peak around 33.1 is observed in these samples, which corresponds to the crystalline plane with Miller indices of (121). The doped Eu3þ ions modify the lattice parameters due to the differences in the ionic radius between the dopant and substituted elements, which can be seen from the amplification of the strongest diffraction peak (Fig. 1(b)). It is worth noting that, the diffraction peaks of CTO-A, CTO-B and CTO-AB samples shift to the larger diffraction angle in comparison to the pure CTO sample, implying that the crystal lattice of CTO-A, CTO-B and CTO-AB have been contracted. We suggest that Eu3þ ions enter into Ca site, which has a larger ionic radius in comparison to Eu3þ (r ¼ 1.066 Å) [22]. The unit-cell parameters of the pure CTO, CTO-A, CTO-B and CTO-AB samples were calculated by the Program Unit Cell software (T. J. B. Holland, S. oA. T. Redfern, Department of Earth Sciences, Cambridge, U.K., 1995) and the results are given in Table 1. The unit cell volumes of CTO-A, CTO-B and CTO-AB were slightly smaller than that of CTO. Because of the ionic radius of Eu3þ ions is between Ca2þ (r ¼ 1.34 Å) [22] and Ti4þ (r ¼ 0.605 Å) [22] ions, the substitution of Eu3þ for Ca2þ may explain the decrease in unit cell volume. 3.2. Crystal structure

2. Experimental details The nominal compositions of the samples were taken on the basis of the following formulas: Ca1-3x/2EuxTiO3, CaEuxTi1-xO3-x/2 and Ca1-x/2EuxTi1-x/2O3. Three different 1% Eu3þ doped Ca0.985TiO3 (CTO-A), CaTi0.99O2.995 (CTO-B) and Ca0.995Ti0.995O3 (CTO-AB) samples of different Ca/Ti ratios and the pure CTO were prepared by the high-temperature solid state reaction method. The starting materials were CaCO3, TiO2 and Eu2O3. The stoichiometric weights of raw materials were completely mixed and crushed in an agate mortar. The well-ground chemicals were put into an alumina crucible and sintered at 1300  C for 4 h in air [17]. The materials were cooled to room temperature and then crushed into fine powders. The crystalline phases of the synthesized powders were characterized by an X-ray diffractometer (MAC Science Co. Ltd. MXP18AHF) using nickel-filtered Cu Ka radiation (l ¼ 0.15418 nm) in the 2q range from 10 to 70 . X-ray photoelectron spectroscopy (Thermo ESCALAB 250) was carried out with Al Ka line of 1486.6 eV and 150 W. For high-resolution spectra measurements, a tunable laser system (Opolette 355 LD OPO system) was used as the excitation source with the wavelength range of 410e2200 nm, the spectral linewidth of 4e7 cm1, the pulse duration of 7 ns and the repetition rate of 20 Hz. The visible emission of the sample was dispersed by a Jobin-Yvon HRD1 double monochromator with a spectral resolution of 0.014 nm and detected by a Hamamatsu R928 photomultiplier. The signal was analyzed by an EG&G 7265 DSP

The structure of the CTO unit cell is shown in Fig. 2(a). The unit cell was modelled through a program called Diamond. This shows that CTO has an orthorhombically distorted perovskite structure. The Ca2þ ion is at the center of 12 nearest neighbour O2 anions forming a distorted icosahedron (Fig. 2(b)). Each Ti4þ ion is surrounded by six O2 anions resulting in only one kind of TiO6 octahedral structure (Fig. 2(b)). 3.3. XPS analyzing In order to study the ionic replacement of Eu3þ in the CTO-A, CTO-B and CTO-AB samples, the XPS spectra for CTO, CTO-A, CTO-B and CTO-AB are shown in Fig. 3. The content of Eu3þ is too small to be detected by XPS. It was possible to identify and quantify in each spectra the atomic concentrations of peaks corresponding to the binding energies of Ca2p, Ti2p, O1s and C1s (Table 2). Table 2 presents the experimental values determined by XPS. As shown in Table 2, the atomic concentration ratio of Ca/Ti becomes obviously less than 1 in the CTO-A, CTO-B and CTO-AB samples in comparison to the ratio in CTO. We suggest that Eu3þ ions substitute Ca2þ site in the CTO-A, CTO-B and CTO-AB samples. 3.4. 20 K high-resolution spectra The sample CTO-A, which has the ratio of Ca/Ti < 1, was

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F. Chi et al. / Current Applied Physics 17 (2017) 24e30

Fig. 1. (a) XRD patterns of the pure CTO and Eu3þ doped CTO samples under different Ca/Ti ratio. (b) Variation of the position of the main (121) peak.

Table 1 The calculated lattice parameters of the corresponding sample. Sample

a (Å)

b (Å)

c (Å)

V (Å3)

CTO CTO-A CTO-B CTO-AB

5.404 5.400 5.404 5.405

5.416 5.417 5.418 5.416

7.647 7.638 7.641 7.643

223.828 223.431 223.703 223.720

subjected to detailed emission spectra investigation since the local site symmetry of Eu3þ in the sample with lower dopant concentration suffered minimal distortion. Fig. 4 shows the emission spectra of CTO-A under 466.4 nm, 528.1 nm, 581.6 nm wavelength excitation which are normalized on the emission intensity at 614.7 nm. The emission spectra mainly cover the transitions 5 D0 / 7F0, 1, 2, of which we are mainly concerned. There are not any remarkable differences either in the 5D0 / 7F1 or the 5D0 / 7F2 transitions from comparison of spectra under different excitation. The emissions corresponding to the 5D0 / 7F1 and the 5D0 / 7F2 transitions are split into three peaks centered at 590.8 nm, 594.1 nm, 596.9 nm and five peaks centered at 614.7 nm, 616.2 nm, 617.5 nm, 619.1 nm and 626.5 nm, respectively. The identical emission spectra indicate that all the emission spectra of Eu3þ under different excitation are from one origin. The nondegenerate

Fig. 2. The CTO host structure (a) and the coordination environment of O2 anions around Ca2þ (b) and Ti4þ (b) cations in CTO.

5 D0 / 7F0 transition peak found at 581.3 nm reveals itself in two emission spectra with shorter wavelength excitation. Under the excitation of 466.4 nm, besides the intense emission from 5D0, several weak emission peaks originating from 5D1 / 7F3 are also observed at 584.8 nm, 586.9 nm and 588.3 nm. The excitation spectra were recorded by monitoring the emission peak at 591.1 nm at 20 K, as plotted in Fig. 5(a). The excitation

F. Chi et al. / Current Applied Physics 17 (2017) 24e30

27

Fig. 4. Emission spectra of CTO-A at 20 K under 466.4 nm, 528.1 nm and 581.6 nm wavelength excitation.

Fig. 3. XPS spectra of the CTO, CTO-A, CTO-B and CTO-AB samples.

Table 2 The experimental values determined by XPS.

CTO

CTO-A

CTO-B

CTO-AB

590.5 nm and 591.1 nm. Besides, as shown in Fig. 5(b), the excitation peaks of 7F0 / 5D0 transition monitoring at 589.9 nm, 590.5 nm and 591.1 nm are located at 581.1 nm, 581.4 nm and 581.6 nm respectively. We attribute the small differences of excitation peaks to the distortion of lattice site from one origin. In order to identify more carefully the crystal-field environment of Eu3þ ions in this material, the emission spectra of CTO-A sample were measured under the excitation from 581.1 nm to 581.7 nm with every 0.1 nm wavelength increment (Fig. 6). There is a variation tendency of emission spectra as the excitation wavelength changing from 581.1 nm to 581.7 nm. The slight differences among these emission spectra can be attributed to the emission from differently distorted lattice sites. On the basis of these emission spectra, we suggest that they are all from one origin. Notably, emission spectra were nearly identical under the 581.6 nm and 581.7 nm excitation wavelength, indicating a single kind of sites. 3.5. Lifetime measurement

Name

Start BE

Peak BE

End BE

PP Hgt (N)

PP At. %

C1s Ca2p Ti2p O1s C1s Ca2p Ti2p O1s C1s Ca2p Ti2p O1s C1s Ca2p Ti2p O1s

292.72 353.17 467.72 534.97 292.44 353.19 467.59 534.69 292.89 353.14 467.79 535.04 292.33 353.28 467.68 535.08

284.8 346.35 458.32 529.59 284.8 346.38 458.38 529.65 284.8 346.38 458.37 529.62 284.8 346.39 458.36 529.59

282.27 343.22 454.52 526.97 281.79 343.09 454.79 526.84 282.09 342.94 453.64 527.14 282.23 343.03 454.38 526.98

0.0368 0.0184 0.0181 0.0316 0.0404 0.0208 0.0225 0.0397 0.0438 0.0225 0.0249 0.0448 0.036 0.0182 0.0196 0.0348

35.08 17.54 17.28 30.1 32.78 16.83 18.21 32.18 32.22 16.52 18.3 32.96 33.13 16.77 18.09 32.02

corresponding to the 7F0 / 5D2, 7F0 / 5D1 and 7F0 / 5D0 transitions are composed of three, three and one lines, respectively. The excitation spectra monitoring at 589.9 nm and 590.5 nm emissions were also recorded. However, we observe no clear differences among the excitation spectra in the wavelength range of the 7 F0 / 5D2 and 7F0 / 5D1 transitions when monitoring at 589.9 nm,

As shown in Fig. 7(a), typical decay curves of the emission of Eu3þ were obtained by monitoring the emission from 5D0 (614.9 nm), 5D1 (540.9 nm) and 5D2 (511.4 nm) under the excitation of 466.4 nm for CTO-A sample. The decay curves can be well fitted by a single exponential function. The rising edges at the beginning of 5D0 and 5D1 decay curves can be ascribed to the feeding from the upper energy levels. The fitted decay lifetimes obtained by monitoring at 614.9 nm, 540.9 nm and 511.4 nm are 0.673 ms, 0.079 ms and 0.012 ms, respectively. As seen in Fig. 7(b), (c), the lifetimes of 5 D0 and 5D1 obtained under the excitation of 528.1 nm are 0.712 ms and 0.075 ms, respectively. And the lifetime of 5D0 under 581.6 nm light excitation is 0.711 ms. On the basis of these results, it can be concluded that emission spectra of Eu3þ under the excitation of 466.4 nm, 528.1 nm and 581.6 nm light are all from one origin. Thus, the origin of all peaks and energy levels can be identified by measuring their decay lifetimes. For a further analysis, we considered the relaxation of the lower excited level with feeding from the upper level. The decay curves are well fitted by a second order exponential decay equation:

    t t  A2 exp  : IðtÞ ¼ A1 exp 

t1

t2

28

F. Chi et al. / Current Applied Physics 17 (2017) 24e30

Fig. 5. (a) Excitation spectra were recorded by monitoring 5D0 / 7F1 emission bands with peak at 591.1 nm at 20 K. (b) Excitation spectra of 7F0 / 5D0 transition monitored at 589.9 nm, 590.5 nm and 591.1 nm.

Here we relaxed the restriction of A2 ¼ A1 (>0). By fitting the decay curve of 5D0 with the second order exponential decay equation, the obtained t1 and t2 are the radiative lifetimes of 5D1 and 5D0, respectively. As the radiative lifetimes of 5D1 and 5D0 have been measured to be 0.075 ms and 0.712 ms, for an elementary study, we fit the decay curve by fixing t2 ¼ 0.712 ms, then the obtained t1 is 0.075 ms and the fitting correlation R2 is 0.999. The similar case is found between 5D2 and 5D1 under the excitation of 466.4 nm. By fixing t2 ¼ 0.079 ms, we obtained the lifetime t1 of 5 D2 is 0.011 ms. From the coordination diagram of anions around Ca2þ and Ti4þ cations (Fig. 2(b)), we observed twelve and six O2 anions round Ca2þ and Ti4þ cations respectively. Considering the ionic radius size of Eu3þ ions is between Ca2þ and Ti4þ ions, which shows that Eu3þ ions have the ability to occupy Ca2þ and Ti4þ ions. According to the research results of Eu3þ emission spectra at different sites which have different coordination number [23,24], it has been suggested that the crystal field strength of EuO12 should usually be notably lower than that of EuO6. Thus when Eu3þ ions substitute the Ti4þ ions, the spectrum splitting range of 5D0 / 7F2 transitions should be larger than the situation of Eu3þ ions substituting the Ca2þ ions. While the spectral splitting range of 5D0 / 7F2 transitions (DE ¼ 325 cm1) of CTO-A in our research are far less than the situation under strong crystal field [23]. We suggest that Eu3þ ions substitute the Ca2þ ions in the sample of CTO-A. On the basis of transition selection rules, the magnetic dipole (MD) transition 5D0 / 7F1 is allowed, its transition probability is insensitive to the local environment. Its emission intensity (marked as I1) is constant in different host lattices and serves as a reference for the determination of the emission intensities of other transitions. Meanwhile, the 5D0 / 7F2 is a forced electric dipole (ED) transition for which the intensity (marked as I2) is hypersensitive to the local environment. When Eu3þ ions are located on the sites deviating from centrosymmetry, the ED transitions become possible due to the relaxation of selection rules. The ratio of I2/I1 (R) can be taken as a probe of the ligand symmetry around Eu3þ ions. High values of R indicate low ligand symmetry and high bond covalency [25]. As shown in the emission spectra of CTO-A (Fig. 4), the emission intensity of 5D0 / 7F2 is higher than 5D0 / 7F1 which indicates that there is no inversion center at the local environment of Eu3þ. According to the coordination diagram of anions around Ca2þ and Ti4þ cations (Fig. 2(b)), there is no inversion center around Ca2þ cations. Through above analysis, we conclude that Eu3þ ions occupy the Ca2þ ions in CTO-A sample. We next measured the spectral properties and lifetime characteristic of CTO-B and CTO-AB samples, which has the ratio of Ca/ Ti > 1 and Ca/Ti ¼ 1. Significantly, we observe no clear differences for excitation spectra, emission spectra and lifetimes among the three samples with different stoichiometric ratios. In conclusion, we find that Eu3þ ions only occupy Ca2þ site in our samples and the insignificant differences of emission spectra are due to the distortion of lattices. Changing the Ca/Ti ratio doesn't affect the site occupancy of Eu3þ ions in our research. 4. Conclusions

Fig. 6. Emission spectra measured under the excitation of the tunable OPO laser from 581.1 nm to 581.7 nm.

Un-doped and 1% Eu3þ ions doped CaTiO3 samples with different Ca/Ti ratios were synthesized using solid-state reaction method. Measuring results of unit-cell parameters and X-ray photoelectron spectroscopy (XPS) show that Eu3þ ions enter into the Ca2þ site. The high-resolution photoluminescence spectra of Eu3þ ions at 20 K in all samples did not witness a significant change under the excitation at different wavelength, implying that Eu3þ ions occupy only one type of site. Considering the small spectral splitting range of 5D0 / 7F2 transition and the large intensity ratio

F. Chi et al. / Current Applied Physics 17 (2017) 24e30

Fig. 7. Decay curves of the emission of Eu3þ obtained under the excitation wavelength of 466.4 nm (a), 528.1 nm (b) and 581.6 nm (c), respectively.

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F. Chi et al. / Current Applied Physics 17 (2017) 24e30

of 5D0 / 7F2/5D0 / 7F1, it can be concluded that Eu3þ occupies the 12-coordinated Ca2þ site without inversion symmetry rather than Ti4þ site. Changing the Ca/Ti ratio doesn't affect the site occupancy of Eu3þ ions in our research. These results may help clarifying the site occupancy of rare-earth ions, and optimizing the performance of rare earth doped perovskite materials.

[10]

[11]

Acknowledgements

[12]

This work was financially supported by the National Key Basic Research Program of China (Grant No. 2013CB921800), the National Natural Science Foundation of China (Nos. 11274299, 11374291, 11574298 and 11404321), and Anhui Provincial Natural Science Foundation (Grant No. 1308085QE75).

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