Photoluminescence spectroscopy of Eu3+: an economical technique for the detection of crystal phase transformation in PbZr0.53Ti0.47O3 ceramics

Prasenjit Prasad Sukul; Kaushal Kumar; H. C. Swart

doi:10.1364/OSAC.1.000971

1. Introduction

X-ray diffraction is commonly used technique to determine the crystal structure of materials. Several situations limit the use of this technique for prepared samples in specific form (pellet). Also XRD system is costlier and could not be made available to each laboratory. Hence there is a search for alternative techniques that could easily probe the crystal structure or detect any secondary phase change in crystal structure. The understanding of materials physical processes decodes its crystal structural information. Over the past few decades’ different sophisticated techniques like X-ray diffraction [1,2], transmission electron microscopy [3] and neutron diffraction have been used to probe crystal phase structure in different ferroelectric hosts. These conventional techniques to monitor phase transitions in ceramics are reliable but are costlier and cumbersome.

The photoluminescence (PL) of rare earth (RE) ions is one of the sensitive techniques to be used to detect minor changes in the sample property [4,5]. Photoluminescence spectroscopy is also advantageous because it is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. Generally, the spectral emission intensity and peak of emission wavelength vary with concentration, and site symmetry around RE ion in a host. So if one can have access to photoluminescence modification trajectory in a material, the crystal phase structure can be easily probed. Meanwhile using PL to detect crystal phase finds recent enlightens due to its high precision, economically low among all other phase probe technologies and much simpler. Suarez et al. [6] have used photoluminescence as a tool to detect phase transition in Eu and Tb containing metallomesogens. Zhang et al. [7] have found that the photoluminescence of Pr³⁺ was responsive to polarization & temperature induced phase transition in ferroelectric (Ba_0.77Ca_0.23)TiO₃ ceramics. Moreover, Polman [8] has predicted if crystal phase correlated study is made with the optical emission properties then optical emission could be an easy tool using Er³⁺ as a probe. It has also been found that photoluminescence of RE ions in pervoskites host are much responsive to their elctro-optics, mechano-optics, piezoelectric properties. Recently, Wei et al. [9] have found an intimate correlation between PL of Ho³⁺ ions with its dielectric response of the host. Therefore, the photoluminescence of RE ions activated PZT host materials allows to detect crystal phase transition. Yao et al. [10] have identified the crystal phase transformation in PMN-xPT:Er (x = 0.15-0.40) with the help of photoluminescence. Noheda et al. have discussed the existence of monoclinic phase as intermediate in between the tetragonal to rhombohedral phases in PbZr_1−xTi_xO₃ ceramics [11].

The Pb(Zr_1-xTi_x)O₃ ceramic has been extensively investigated because of its ultrahigh piezoelectric properties near the morphotropic phase boundary (MPB) [1]. A transition region in their composition phase diagrams where the crystal structure changes abruptly and the electromechanical properties are maximal known as a morphotropic phase boundary. At room temperature the solid solution of PbTiO₃ (PT) and PbZrO₃ (PZ) represents the combination of two ferroelectric (tetragonal-rhombohedral) phases at MPB. The crystal structure of PZT with composition around MPB is complex because of the coupling of ferroelectric domain and crystal structure. To understand such complex phenomena numerous sophisticated phase probing methods have been used earlier by researchers [2,3]. These methods also determine the origin of the high maxima of electrical properties at MPB in PZT ceramics. Though current research on PZT materials is focused on the phase transition near MPB, still it is under debate with a lack of proper method of crystal phase detection.

In this article, authors are reporting the use of photoluminescence of Eu³⁺ as a probe to detect the crystal phase transition near MPB of PZT ceramic where other characteristics properties do not reveal the crystal phase change directly. High purity (99.99%) powders of TiO₂, PbO, ZrO₂ and Eu₂O₃ were purchased from Sigma Aldrich, Germany and used as the raw materials to synthesize samples according to the stoichiometric condition of Eu³⁺ doped Pb(Zr_1-xTi_x)O₃. The samples are denoted as PbZr_0.53Ti_0.47O₃:Eu for simplicity. Eu concentration is taken 0.03 mol%, whereas x = 0.47 mol% (concentration of B site in ABO₃ structure) [12]. At first, all the raw materials are mixed in an agate mortar via isopropanol media and grinded together according to the stoichiometric ratio. After 6 h of grinding, the homogeneously mixed materials were calcined at 900 °C and sintered at four different temperatures (1000 °C, 1100 °C, 1200 °C, 1300 °C) for 6 hours by following the solid state reaction route [13]. The sample were collected in powder form and later processed in pellet form using PVDF binder. The crystal structure & morphology of prepared ceramic was examined by XRD (Bruker D8 Advanced, Germany) with Cu K_α radiation and FESEM (SUPRA 55, Carl Zeiss) respectively. Polarization vs. electric field (P-E) hysteresis loops were obtained at 1.0Hz using Bruker’s multimode 8 AFM workstation in Piezo Response (PR) phase mode. The piezoelectric coefficient d₃₃ of each sample was measured using quasistatic piezoelectric meter (ZJ-4B, China). Dielectric constants e₃₃ were measured at 1.0 kHz using impedance analyzer (TH2827, Tonghui, Changzhou, China). The photoluminescence (PL) studies were carried out on a Hitachi Fluorescence Spectrometer F-2500 with a 150 W Xe lamp as an excitation sources in the range 220–750 nm with band pass filter for different excitation. Fluorescence decays were measured on FLS980 spectrophotometer (Edinberg Instruments, UK).

2. Results and discussion

Figure 1

Fig. 1 XRD patterns of PZT:Eu³⁺ at different sintering temperatures.

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shows XRD patterns measured for the PZT:Eu³⁺ samples sintered at four different temperatures. Pure perovskite phase has found within the detection range of diffractometer. The miller indices highlighted in the region of 30°≤2θ≤32° show noticeable change in peak intensities and hence there is a crystal phase transition. The sample sintered at 1200 °C shows tetragonal phase (P4mm) confirmed by JCPDF No. 33-0784. Whereas XRD pattern of the sample sintered at 1300 °C matches with rhombohedral phase (R3m) confirmed by JCPDF No. 73-2022. The highlighted portion in the Fig. 1 shows the crystal phase transformation, as the intensity of the peak (101)_c increases with sintering temperature. Change in the intensity of (101)_c peak can be ascribed to the transition from tetragonal to rhombohedral phase in PZT:Eu ceramic [10,12]. Similar report of crystal phase transition in PMN-xPT:Er from rhombohedral to tetragonal phase near MPB region has been confirmed by Yao et al. [10].

The field emission scanning electron micrographs (FESEM) of samples annealed at four different temperatures are shown in Fig. 2

Fig. 2 FESEM images of the PZT:Eu³⁺ ceramic at different sintering temperatures at 50 KX magnification.

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at same magnification (50 KX). As shown in figure, the shape of the particles is not much affected by the change in sintering temperature of the ceramic but size of the particle definitely increases.

Due to magnetic-dipole & electric-dipole transitions four characteristic absorption peaks of the Eu³⁺ ions centered at 320, 380, 392 and 412 nm referred to ⁷F₀→⁵D₄, ⁷F₀→⁵G_{J(J = 2−5)}, ⁷F₀→ ⁵L₆ and ⁷F₀→⁵D₃ transitions, respectively has been observed [14]. Any of these referred wavelengths can excite Eu³⁺ ion directly to the higher states and later they relax to the ground state through the radiative emission at 594 nm and 617 nm. With excitation at 320 nm wavelength strong PL emission was observed from the samples as shown in the Fig. 3

Fig. 3 Photoluminescence spectra of PZT:Eu³⁺ ceramics at different sintering temperatures.

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.

The PL emission spectra include reddish orange (594 nm) and red (617 nm) emission bands. The emission peak centered at 594 nm is due to the radiative ⁵D₀→⁷F₁ transition whereas, the red emission centered at 617 nm caused by the ⁵D₀→⁷F₂ transition. Four samples sintered at different temperature show variation in their emission peak positions. Samples sintered at 1000 °C and 1200 °C show only intensity variation with no obvious shift in emission wavelength. PL emission intensity is found to increase with the change in sintering temperature of the ceramic. Surprisingly, the sample sintered at 1300 °C shows lower frequency shift with increase in emission intensity. This shift in emission wavelength could be related to the crystal phase transition. In support of this assumption, Raman spectra of PbZrTiO₃:Eu³⁺ samples were taken and recorded spectra are shown in Fig. 4

Fig. 4 Raman spectroscopy of PZT:Eu³⁺ at different sintering temperatures.

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.

The peaks obtained from Raman spectra at room temperature agrees well with reported results [15,16]. Labeling of the Raman modes has taken from work on tetragonal lead lanthanum zirconium titanate (PLZT) [15] and PbTiO₃ [17,18]. In the first region below 200 cm⁻¹, modes related to Pb atom are seen. The identification of E(TO1) and A₁(TO1) modes at frequencies similar to modes in PbTiO₃ (∼80 and 150 cm⁻¹, respectively) is easy. The middle region between 200 and 450 cm⁻¹ are the modes (Slater type) from Zr and Ti atoms are observed. In tetragonal sample, three main bands correspond to the E(TO2), F2u and A₁(TO2) modes. In the last frequency region, the bands between 500 and 650 cm⁻¹ can be assigned to the E(TO3) and A₁(TO3) modes, a mixture of vibrations of oxygen and B-site atoms. Longitudinal optic modes are also detected in the spectra, the weak E(LO2) and A₁(LO2) modes are located between 420 and 450 cm⁻¹. Above 700 cm⁻¹, E(LO3) and A₁(LO3) modes due to nonpolar oxygen breathing mode are also found [19]. It is clear that intensity of the modes related to tetragonal phase increases with the change in sintering temperature up to 1200 °C. At 1300 °C the mode frequency [E(TO2) and B₁ + E(TOs)] not only increases in intensity but shifts towards higher energy side. This behavior of mode shifting towards the higher energy side can be related to the tetragonal to rhombohedral phase transition [20].

Figure 5(a)

Fig. 5 (a): Hysteresis Loops of PZT:Eu³⁺ ceramics at different sintering temperatures. (b) Dependence of the coercive field E_c and remnant polarization P_r of PZT:Eu³⁺ ceramics extracted from (a) with sintering temperature variation.

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shows the ferroelectric P-E hysteresis loops of the PZT:Eu³⁺ sample. All four ceramic samples show strong ferroelectricity within polarization saturation limit under maximum electric field of 46kV/cm. The variation of coercive field E_c and remnant polarization P_r as a function of sintering temperature is shown in the Fig. 5(b). E_c increases monotonically from 4 to 18kV/cm as the sintering temperature changes 1000 °C to 1300 °C. Simultaneously, P_r increases from 14 to the maximum of 30 mC/cm² with the variation of sintering temperature upto 1200 °C. There is sharp decrease in P_r for sample annealed at 1300 °C. The variation of piezoelectric coefficient d₃₃ and di-electric constant e₃₃ of the PZT:Eu³⁺ sample as a function of sample sintering temperature is shown in the Fig. 6

Fig. 6 Piezoelectric coefficient d₃₃ and dielectric constant e₃₃ of the PZT:Eu³⁺ ceramic at different sintering temperatures.

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. d₃₃ increases with sintering temperature to 359 pC/N and then there is a decrease for sample annealed at 1300 °C. It can be seen that the piezoelectric properties show the optimum when sintering temperature is between 1000 and 1200 °C. It is due to the fact that the well-arranged microstructures in the proposed phase transition region. Specifically, the ceramics sintered at 1200 °C have relatively low porosity inside the grains and the grain boundaries are well stick together [21]. The piezoelectric constant is supposed to be low for orthorhombic phase compared to the tetragonal phase. The ceramic shows phase transition with sintering temperature upon Eu³⁺ probing due to the reason of change porosity which also decreases the Piezoelectric properties when sintering temperature changes 1200 °C to 1300 °C. In contrary, e₃₃ of the sample decreases as sintering temperature increases up to 1300 °C and there is no abrupt change which can be related to the crystal phase change.

Interestingly, the evolution of the phase transformation near MPB in the PZT:Eu³⁺ sample are not directly correlated with its ferroelectric, dielectric and piezoelectric properties as seen from Fig. 5 & 6. The truth behind this depends on the fact that the electrical properties of the ferroelectric ceramics are affected not only by the crystal structure but also by the domain and grain size [22]. Emission intensity is strongly influenced by domain and grain sizes both because of light scattering by the domain and grain boundaries, so intensity of the emission also could not be used to detect phase transformation as it does not share any direct relation with crystals structure. Therefore, it seems that it is difficult to identify direct correlation between crystal structures and emission intensity. As the conventional phase probing techniques are economically costlier and hence new phase detection probes must satisfy two conditions. One is probe should be economically cheaper than conventional techniques like XRD and second is that it should share the same precision & accuracy like conventional one. Here, the results of XRD and Raman spectroscopy are supporting the fact of crystal phase transformation (tetragonal to rhombohedral) in the PZT:Eu³⁺ sample. Authors have tried to show photoluminescence as phase probing technique which is economically viable and at the same time shares a great precision using the magnetic dipole coupling in Eu³⁺. Though emission intensity variation is not directly correlated to crystals structure, the emission wavelength shift observed here can be a parameter to predict phase change. Change in sintering temperature up to 1200 °C does not show emission wavelength shift except intensity enhancement. The peak position shifting in emission spectra at 1300 °C leads to clear phase transformation to rhombohedral (R3m) phase near MPB. It is reasonable to conclude that shift in PL emission for PZT:Eu³⁺ at 1300 °C could be an economically cheaper phase probing technique with greater accuracy factor and its ease of simplicity.

To get an idea on the nature of rare earth ion occupancy & stability, lifetime measurement was done. The average lifetime values (t₁₁₀₀^o_C = 24.9µs, t₁₂₀₀^o_C = 65.2µs & t₁₃₀₀^o_C = 92.9µs) are shown in the Fig. 7

Fig. 7 Decay time of ⁵D₀ level of Eu³⁺ at different sintering temperature of the ceramic.

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, where nonlinear fitting is used to extract the lifetime values from the well-known double exponential equation,

I 〈 t 〉 = I_{o} + A_{_{1}} e^{- \frac{t}{τ_{1}}} + A_{_{2}} e^{- \frac{t}{τ_{2}}}

where, I(t) and I_o denote the luminescence intensities at time t and t = 0, respectively. A₁ and A₂ are fitting parameters and τ₁, τ₂ are the two components of the luminescence decay times. The increase in average lifetime values correlates and supports the idea of detectable change in PL spectrum.

In Eu³⁺, electric dipole transitions within 4f→4f shell are permitted if the surroundings of the ion are such that its nucleus is not situated at a center of inversion. It is well known fact that ⁵D₀→⁷F₁ is magnetic dipole transition and according to Judd-Ofelt (J-O) theory [23] the magnetic dipole transition rate of the ⁵D₀→⁷F₁ can be expressed as follows:

A_{m d} = \frac{64 π^{4} k_{m d}^{3} n^{3} S_{m d}}{3 h (2 J^{'} + 1)}

where, k_md is the transition energy for ⁵D₀→⁷F₁ in wavenumber unit, h is Planck’s constant and

(2 J^{'} + 1)

is the degeneracy of the initial state (1 for ⁵D₀). The factor n is the refractive index of host, S_md is a constant independent of the host [24]. The ⁵D₀→⁷F_J (J = 2, 4, 6) transition are electric dipole transition of Eu³⁺ and the radiative rate can be written as:

A j = \frac{64 π^{4} e^{2} k_{m d}^{3} n^{3}}{3 h (2 J^{'} + 1)} \frac{n {(n^{2} + 2)}^{2}}{9} \sum_{λ = 2, 4, 6} Ω_{λ} 〈 Ψ J ‖ U^{λ} ‖ Ψ^{'} J^{'} 〉

where e is the electronic charge, k is the transition energy of electric transition in cm⁻¹,

Ω_{λ}

is the intensity parameter, which depends on crystal field and radial integration of electron.

〈 Ψ J ‖ U^{λ} ‖ Ψ^{'} J^{'} 〉

values are the squared reduced matrix elements. The red emission (617 nm) band in Fig. 3 is a hypersensitive transition of Eu³⁺. Such hypersensitive transition is determined by J-O parameter Ω₂ (Ω₂>Ω₄) which confirms the asymmetry of Eu³⁺ ion [25]. In addition to the electric-dipole transition, authors have observed another intense emission at 594 nm, which is due to magnetic-dipole transition ⁵D₀→⁷F₁. Consequently the observed variation of emission intensity of the PZT:Eu³⁺ ceramics can be attributed to the Eu³⁺ site symmetry induced by the change of crystal structure caused by sintering temperature variation.

The emission intensities for the electric dipole and magnetic dipole transitions between multiplet states of 4f^N configuration of lanthanide ions can be related to the electric and magnetic dipole moments, respectively [26]. For the ⁵D₀→⁷F_J (J = 0–6) transitions of Eu³⁺ ions, the intensity calculation is particularly simple due to the simplification of angular-moment selection rule which limits electric dipole transitions to J = 2, 4, 6 and magnetic dipole transition to J = 1. We denote the following ratio as R_J, i.e.

R_{J} = \frac{S^{E D} ({}^{7}F_{J}, {}^{5}D_{0})}{S^{M D} ({}^{7}F_{1}, {}^{5}D_{0})}

where S^ED(⁷F_J, ⁵D₀) is the electric dipole line strength (the square of the electric dipole matrix element between ⁷F_J and ⁵D₀ multiplets), and S^MD(⁷F₁, ⁵D₀) is the magnetic dipole line strength between ⁷F₁ and ⁵D₀. The intensity ratios of ⁵D₀–⁷F_J (J = 2, 4, 6) to ⁵D₀–⁷F₁ transitions can then be expressed as i.e.

\frac{I^{E D} ({}^{5}D_{0} \to {}^{7}F_{J})}{I^{M D} ({}^{5}D_{0} \to {}^{7}F_{1})} = {[\frac{σ ({}^{5}D_{0} \to {}^{7}F_{J})}{σ ({}^{5}D_{0} \to {}^{7}F_{1})}]}^{4} . \frac{n f^{2}}{n^{3}} . R_{J}

where σ(⁵D₀–⁷F_J) is the photon energy for the ⁵D₀–⁷F_J emission, n is the refractive index at the transition wavelength, and f is the local-field enhance factor. A similar type of calculation of the relative intensity ratio for Eu³⁺ can be found in this report [26]. With increase of sintering temperature in PZT:Eu³⁺ ceramic, the crystal phase evolves from tetragonal phase(P4mm) to rhombohedral phase(R3m). As the lower energy shift is observed in the PL emission spectra we can conclude change in crystal phase because of decrease of crystal symmetry around Eu³⁺ environment. Also this result reconfirms that the MPB is not co-existence of tetragonal phase and rhombohedral which is conventionally right [27]. MPB is more likely consider being an unstable monoclinic phase with lower crystal symmetry, which lasts for a very short interval than tetragonal and rhombohedral phase. So the observation of MPB through detectable change in PL emission as low frequency shift may provide us, an intuitive method to detect crystal phase transition with change of crystal symmetry in PZT:Eu³⁺ ceramics.

3. Conclusion

In summary, PbZr_0.53Ti_0.47O₃:Eu³⁺ ceramic samples with different sintering temperatures were prepared and their dielectric, ferroelectric, piezoelectric and photoluminescence properties were studied. Through XRD and Raman spectroscopy it is concluded that PZT:Eu³⁺ undergoes crystal phase transformation near MPB. The dielectric, ferroelectric and piezoelectric properties had no direct relation with the phase structure of PZT:Eu³⁺. However, through lower energy shift in PL emission is seen strongly correlated with the phase structure of the host. The well-known magnetic dipole transition & electric dipole transition of Eu³⁺ was strongly correlated with phase structure on the basis of J-O theory. This investigation suggests that the photoluminescence of Eu³⁺ can be used as an economically reliable & accurately précised phase probing technique as compared to conventional XRD & Raman phase probing methods.

Funding

DST-SERB, New Delhi (EMR/2017/000228/18); DST-FIST under project [No. DST-FIST/PS1-184/13].

Acknowledgment

Authors are grateful to Dr. A.K. Kar, Associate Professor, IIT(ISM), Dhabad, India by guiding in the ferroelectric analysis. The authors would like to acknowledge the financial support from DST-SERB, New Delhi (EMR/2017/000228/18) and the financial support from DST-FIST under project [No. DST-FIST/PS1-184/13]. All authors listed on the title page have contributed significantly to the work, have read the manuscript, attest to the validity and legitimacy of the data and its interpretation, and agree to its submission to the journal.

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Photoluminescence spectroscopy of Eu³⁺: an economical technique for the detection of crystal phase transformation in PbZr_0.53Ti_0.47O₃ ceramics

Abstract

1. Introduction

2. Results and discussion

3. Conclusion

Funding

Acknowledgment

References

Cited By

Figures (7)

Equations (5)

OSA Continuum