Optical properties of undoped NdTaO4, ErTaO4 and YbTaO4 ceramics

Journal of Luminescence 179 (2016) 146–153

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Optical properties of undoped NdTaO4, ErTaO4 and YbTaO4 ceramics Kisla P.F. Siqueira a,n, Alexandre P. Carmo b, Maria José V. Bell c, Anderson Dias a a b c

Departamento de Química, Universidade Federal de Ouro Preto, Campus Morro do Cruzeiro, ICEB II, 35400-000, Brazil Instituto Federal Fluminense, Campus Cabo Frio, 28909-971, Brazil Departamento de Física, Universidade Federal de Juiz de Fora, 36036-330, Brazil

art ic l e i nf o

a b s t r a c t

Article history: Received 13 May 2016 Received in revised form 19 June 2016 Accepted 21 June 2016 Available online 30 June 2016

The optical properties of undoped polycrystalline ceramics NdTaO4 (NTO), ErTaO4 (ETO) and YbTaO4 (YTO) were investigated for the first time to our knowledge. The ceramics were prepared by solid-state reaction and exhibited two different crystalline structures, according to x-ray diffraction: M-fergusonite type (I2/a, #15), for NTO, and M0 -fergusonite type (P2/a, #13), for ETO and YTO. The results from photoluminescence (PL) absorption and emission showed that the optical properties of these compounds have some variation in accordance with the rare earth ion present on the TaO4 host matrix. In these ceramics, the intraconfigurational f–f transitions were observed. The specific emissions in the infrared (IR) regions were assigned to all ceramics, as well as emission in the visible range for ETO. Under near-IR excitation (at 980 nm), the ETO ceramic is characterized by intense purplish pink up-conversion luminescence with coordinates x ¼0.460 and y¼0.271 in the chromaticity diagram (CIE), and about 47% of color purity. Under visible excitation at 532 nm, clear evidences of the down-conversion process in ETO through the cross-relaxation have been found. Fluorescence lifetimes for dominated IR-emission were determined as 81.5, 122.1 and 131.2 μs for NTO, YTO and ETO, respectively. The broad full width at halfmaximum (FWHM) and optical results indicate that the polycrystalline ceramics NTO, ETO and YTO investigated here are promising luminescent inorganic materials for solid state lasers. & 2016 Elsevier B.V. All rights reserved.

Keywords: Orthotantalates Luminescence Crystal structures Solid state lasers Rare earth

1. Introduction The rare-earth tantalates (RETaO4) are being extensively studied because of their interesting structural properties and large technological applications. RETaO4 are used in x-ray imaging systems, mercury free fluorescent lamps and field emission display devices due to their strong irradiation hardness, good x-ray absorption, high luminescence efficiency, and insignificant reduction in light output under low voltage cathode rays respectively [1–3]. Recent studies reveal that the RETaO4 having a relatively high rare earth concentration would be new promising candidates for microchip lasers [4]. The tantalates have been considered appropriate as host matrices and they have attracted wide attention as potential rare earth doped laser hosts, due to their high chemical durability and thermal stability [4,5]. Doped and undoped RETaO4 have chemical stability and show many other promising characteristics, such as photoelectronic activity, ion conductivity and luminescence [6]. Many works have been reported the luminescence properties of polycrystalline as well as single crystalline rare earth tantalates [4,7–9]. Nevertheless, n

Corresponding author. Fax: þ55 31 35591707. E-mail address: [email protected] (K.P.F. Siqueira).

http://dx.doi.org/10.1016/j.jlumin.2016.06.054 0022-2313/& 2016 Elsevier B.V. All rights reserved.

relatively less information has been published based on tantalate phosphors if we compare with the information available about other phosphor materials. In this work, the optical properties of NTO, ETO and YTO ceramics were investigated aiming to look for new promising solid state lasers. This study was based on the good performance already reported for rare earth tantalates [10]. The study of the optical properties of rare earth in ceramic matrices has been of great interest from both scientific and technological points of view [11]. Recent developments in the field of solid-state lighting have motivated the development and demand for novel efficient inorganic luminescent materials. In this respect, single phase rare earth tantalates are difficult to be produced because they exhibit several polymorph forms and crystallize only at high temperatures [1]. Nevertheless, to the best of our knowledge, no work concerning optical properties of neodymium (Nd), erbium (Er) and ytterbium (Yb) orthotantalates in undoped materials was previously reported in the literature so far. In the present work, all undoped ceramics were prepared by conventional solid-state reactions. Specific conditions of temperature and time were employed to allow the ceramics to be crystallized in monoclinic structures with two different arrangements, in agreement with the previous work by Siqueira et al. [2]. The photoluminescence (PL) properties of each sample were

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studied in detail by PL absorption, emission and decay time measurements, in order to understand the luminescence mechanisms in tantalate matrices containing a high concentration of the rare earths Nd, Er and Yb (undoped materials). Moreover, we will compare the optical properties exhibited by undoped polycrystalline ceramics with other doped materials, as well as investigated their probable use as solid state lasers.

2. Experimental Tantalates (TaO4) were used as host matrices in undoped polycrystalline ceramic powders: NdTaO4 (NTO), ErTaO4 (ETO) and YbTaO4 (YTO). The ceramics were prepared by conventional solidstate reactions. The starting materials were Nd2O3, Er2O3, Yb2O3 and Ta2O5 ( 499.9% Sigma–Aldrich), which were thoroughly mixed according to the desired stoichiometric ratios for each sample. The oxides were mixed in a mortar and pestle with ideal parameters of temperature and time to obtain crystalline samples. The experimental conditions were 1300 °C, for 6 h, in order to produce single phase ceramics. The crystalline structures of the assynthesized samples were studied by x-ray diffraction (XRD) using a Shimadzu D-6000 diffractometer with graphite monochromator and a nickel filter in the range of 10–60°2θ (15 s/step of 0.02°2θ), operating with FeKα radiation (λ ¼0.1936 nm), 40 kV and 20 mA. All results were automatically converted to CuKα radiation for data treatment and manipulation. The room temperature absorption spectra were acquired on a Bruker FT-NIR Multi Purpose Analyzer (MPA), operating in the range from 850 to 1700 nm with resolution of 0.5 nm, by reflection in integrating sphere. The emission spectra were acquired at room temperature on a DIGIKROM480 Tzerny Turner monochromator with resolution of 1 nm, excited by a He–Cd laser (325 nm, 40 mW). The luminescence signal was acquired by a photomultiplier model (R928), operating in the range from 400 to 900 nm or an InGaAs detector, in the range from 800 to 1700 nm. Signal was amplified by a SRS 530 lock-in with reference signal provided by an optical chopper (SR540), operating in the range from 5 to 4000 Hz and collected by a computer. Luminescence decay curves were obtained in the same experimental setup of the luminescence experiments, where the SRS lock-in was substituted by a SR445A 350 MHz Preamplifier and the amplified signal was collected by a computer. The excitation laser was pulsed by the use of a chopper, with frequencies in the range from 10 to 100 Hz.

3. Results and discussion Polycrystalline ceramics using TaO4 as host matrices were produced and the XRD results are presented in Fig. 1. The samples crystallized in fergusonite-type structures, but with different arrangements as a function of rare earth ionic radius. NTO exhibited a M-type structure, belonging to the space group I2/a (C 62h , #15), with four units per unit cell (Z¼4). On the other hand, ETO and YTO exhibited a M0 -type structure, space group P2/a (C 42h , #13), with two units per unit cell (Z¼ 2). XRD patterns were indexed according to ICDD (International Committee for Diffraction Data) card numbers #01-072-0906, #01-072-2015 and #01070-9018, for NTO, ETO and YTO, respectively. According to Fig. 1, we can observe that single phase undoped rare earth tantalates were obtained and indexed according to the ICDD cards. The main crystallographic planes were assigned and the crystallographic data are listed in Table 1, which shows the characteristics of each polymorphic form assumed by the RETaO4 ceramics investigated in this work.

Fig. 1. X-ray diffraction patterns for NdTaO4 (NTO), ErTaO4 (ETO) and YbTaO4 (YTO), with the respective ICDD cards (red) and crystallographic planes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Crystallographic data for the polycrystalline ceramics NTO, ETO and YTO. Orthotantalate

Space group

Cell parameters (Ǻ)

NdTaO4

Monoclinic, I2/a (#15) Z¼4 M-type

a¼ 5.51 b¼ 11.23 c¼ 5.11 β¼ 95.7° V ¼314.84 Å3

ErTaO4

Monoclinic, P2/a (#13) Z¼2M'-type

a¼ 5.29 b¼ 5.44 c¼ 5.10 β¼ 96.4° V ¼146.01 Å3

YbTaO4

Monoclinic, P2/a (#13) Z¼2M'-type

a¼ 5.07 b¼ 5.25 c¼ 5.42 β¼ 96.2° V ¼143.50 Å3

It is known that the crystal structure affects directly the luminescent properties of solid materials [12]. Thus, it is important to define the structural characteristics of the material before investigating the optical properties, e.g., the polymorphic form M0 -type exhibited by the TbTaO4 ceramics seems to be the most suitable for luminescence devices, as evidenced by Siqueira et al. [12]. This behavior could be explained by the strongest emission and highest fluorescence lifetime showed by the M0 –TbTaO4 ceramics, if compared with the M–TbTaO4 analogous [12]. In the present investigation, the main difference between the arrangements from NTO (M-type) and ETO/YTO (M0 -type) is related to the coordination of tantalum ions, which is four in NTO and six in ETO/YTO ceramics [12]. Furthermore, in the NTO the tantalum ions have a tetrahedral coordination, while a distorted octahedron is observed in ETO/YTO ceramics [2,13]. The average Ta–O distance is longer in the NTO structure, which results in a higher unit cell volume for M-type arrangements [2,13]. Now, we will discuss the photoluminescence absorption behavior for the polycrystalline orthotantalates. Relatively scarce information about optical properties of undoped orthotantalates has been reported. Blasse et al. [10] observed broad absorption bands for YTaO4, LaTaO4 and LuTaO4, which were attributed to the TaO4 absorption. Fig. 2 shows the absorption spectra at room temperature for NTO ceramics in the visible (VIS) region (Fig. 2a), as well as in the infrared (IR) region (Fig. 2b). At right of Fig. 2, we can observe the partial

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Fig. 2. Room temperature absorption spectra for polycrystalline NTO with the partial energy diagram from Nd3 þ ions. The states involved on the emission for NTO are in red. (a) VIS region; (b) IR region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

energy level diagram from Nd3þ ions. The absorption spectrum of Nd3þ approximately consists of 12 absorption bands centered at 469, 525, 570, 588, 630, 684, 748, 808, 837, 880, 1646 and 2495 nm, corresponding to the absorption from the ground state, i.e. 4I9/2, to the excited states 2P3/2, 2D5/2 þ 2P1/2, 4G11/2, 2G9/2 þ 2K13/2, 4G7/2, 4G5/2 þ 2 G7/2, 2H11/2, 4F7/2 þ 4S3/2, 4F5/2 þ 2H9/2, 4F3/2, 4I15/2 and 4I13/2, respectively. The absorption spectra show possible transitions in a broad region of the electromagnetic spectrum. The 4F3/2, 4I13/2 and 4I11/2 energy levels were identified in red because these transitions jointly with the fundamental state are involved in the PL emission of the NTO. All the absorption bands are relatively narrow and are related to the presence of Nd3 þ ions in the host TaO4 matrix. Both the wavelength scales (nm and cm  1) were displayed in order to facilitate the comparison data. Fig. 3 exhibits the absorption spectrum for the ETO ceramics. The absorption spectrum revealed approximately 11 distinct bands centered at 379, 406, 450, 488, 525, 654, 671, 795, 830, 970 and 1500 nm, which are related to the transitions from the 4I15/2 ground state to the 2G7/2, 4G9/2, 4G11/2, 2H9/2, 4F3/2 þ 4F5/2, 2H11/2,

S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 excited states for the Er3 þ ions, respectively. In Fig. 3a, we can observe the levels from the Er3 þ involved in the visible region, while Fig. 3b shows the absorption levels in the IR region. The partial energy level diagram (right panels) shows the respective transitions from Er3 þ . In the absorption spectrum in Fig. 3a, a range of bands can be observed. They are assigned to the transitions from the ground state, i.e., 4 I15/2 to the different excitation states. The excited states 4I9/2, 4F9/2, 4 S3/2 and 2H11/2 were highlighted in red once they correspond to the PL emission recorded for ETO ceramics, as well as the 4I13/2 and 4 I11/2 states (Fig. 3b). The absorption spectrum for the YTO ceramics is shown in Fig. 4. In this case, we can observe only the transitions in IR region related to the rare earth ytterbium. The partial energy diagram shows the absorption from the ground state 2F7/2 to the excited state 2F5/2, with its respective Stark components. We can observe the influence of the symmetry or strength of the crystal field from the rare earth in all orthotantalates transitions investigated. In the YTO ceramics, this effect is most evident since each component 4

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Fig. 3. Room temperature absorption spectra for the polycrystalline ETO ceramics with the partial energy diagram from Er3 þ . The states involved on the emission of ETO are in red. (a) VIS region; (b) IR region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

from the 2F5/2 multiplet could be observed. The presence of these components is due to the complete “Jþ1/2” degeneracy. Thus, it is expected three Stark levels to the multiplet 2F5/2. In the orthoceramics, the interaction of the crystal field is responsible for the appearance of Stark levels as well as the shift of the multiplets [14]. Now, we will present the emission spectra for the NTO, ETO and YTO ceramics. For RETaO4 compounds, luminescence is an inherent feature, and this phenomenon occurs due to their chemical nature, i.e., extrinsic emission centers could not be created, since no activators (dopants) are added in the compounds. In the RETaO4 systems, three types of energy transitions prevail [15]. The first one arises from the presence of the rare earth and its unique intraconfigurational f―f transition (Nd3 þ , Er3 þ or Yb3 þ ), which occurs as a sharp and intense emission lines. Second, the 4fn4fn  15d interconfigurational transitions and, finally the third type are constituted by the charge transfer transitions or charge

transfer band (CTB). In the RETaO4 systems, we can consider that the conduction band is composed by Ta5 þ ―4d orbital, while the valence band is formed by the O2  ―2p orbital, as verified for niobates [16]. CTB originates from the charge transfer transitions Ta5 þ ―O2  , similarly to what is observed in niobates and antimonates [10,17,18]. In this work we investigated only the first kind of energy transitions, i.e., f―f intraconfigurational from rare earths; once it is the main responsible by luminescent properties in polycrystalline RETaO4 and the CTB emission were not accessible to our detector. The room temperature IR emission spectrum for the NTO excited at 808 nm is shown in Fig. 5. There are three distinct emission bands with range of 864–940 nm, 1030–1124 nm and 1302–1426 nm, which originate from the state 4F3/2 to the state 4 I9/2, 4I11/2 and 4I13/2, respectively. The asterisks at 1175 nm and 1256 nm are related to laser contributions. It can be noted the strongest emission corresponds to the 4F3/2-4I11/2 at 1064.5 nm,

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Fig. 4. Room temperature absorption spectra for the polycrystalline YTO ceramics with the partial energy diagram and Stark components from Yb3 þ .

Fig. 5. Emission spectrum (λexc ¼ 808 nm).

for

the

NTO ceramics at

room temperature

which exhibited a broad bandwidth with the full width at halfmaximum (FWHM) of 9.1 nm. This value is about 9 times larger than the well known laser Nd:YAG (FWHM¼ 1.1 nm at 1064.2 nm) [19]. According to Ning et al., [4] larger values of FWHM could be beneficial to the development of tunable or ultrafast lasers. The presence of the Stark level is due to the complete “Jþ1/2” degeneracy. Thus, it is expected a maximum of 10, 12 and 14 Stark levels for the transitions F3/2-4I9/2, 4F3/2-4I9/2 and 4F3/2-4I9/2, respectively. In order to analyze in detail the fine spectral structure of Nd3 þ induced by the crystal field splitting, the spectra were deconvoluted using Lorentzian curves. Fig. 6 shows the PL profiles for NdTaO4 ceramic, which was divided in three regions. The transitions 4 F3/2-4I9/2 (860–940 nm) and 4F3/2-4I11/2 (1015–1127 nm) were better adjusted by 7 and 8 peaks, as it can be seen in Fig. 6a and Fig. 6b, respectively. On the other hand, the transition 4F3/2-4I13/2 (1300–1425 nm) was better adjusted by 9 peaks, as it is shown in Fig. 6c. The difference among the number of peaks expected and exhibited at each transition suggests that the Stark levels are overlapping. It is important to note that in all ceramics studied in this work, i.e. NTO, ETO and YTO, the rare earth transitions are affected by crystal field strength. However, we will exhibit the deconvoluted spectra only for NTO ceramic, due to the high number of Stark components involved in ETO and YTO transitions.

Fig. 6. Deconvoluted emission spectra for the NTO ceramics at room temperature in the range 860–1425 nm (λexc ¼808 nm). Experimental data are in open circles, whereas the fitting curves are represented by red lines. Green lines represent the Lorentzian curves necessary to the mathematical adjustment. (a) F3/2 -4I9/2, (b) 4 F3/2-4I9/2, and (c) 4F3/2-4I9/2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7 shows the room temperature emission spectra for the ETO ceramics. In Fig. 7a, we can observe the up-conversion luminescence under 980 nm, which exhibits the typical sharp emission lines corresponding to the excited states 2H11/2, 2S3/2, 4F9/2 and 4I9/2 to the ground state 4I15/2 for Er3 þ . The main emission is due to the 4 F9/2-4I15/2 transitions, which are responsible by the red emission

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Fig. 7. Emission spectra for the ETO ceramics at room temperature. (a) Up-conversion emission (λexc ¼ 980 nm); inset: CIE chromaticity diagram coordinates of ETO. (b) Down-conversion emission (λexc ¼ 532 nm). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

in the range 630–705 nm. This strong band exhibits a FWHM value of 9.7 nm, which was similar to the value found for NTO ceramics (see Fig. 5). The color of up-conversion emission from polycrystalline ETO ceramics is in accordance with CIE chromaticity diagram, as exhibited in the inset of Fig. 7a. CIE parameters, such as color coordinates (x, y) and color correlated temperature were calculated to characterize the emitted color. The coordinates x ¼0.460 and y ¼0.271 were calculated by a software [20]. Thus, the up-conversion luminescence from ETO ceramics is in the purplish pink region. The color correlated temperature was calculated by the McCamy empirical formula [21]: T ¼  437n3 þ 3601n2  6861n þ 5514:31 ;

ð1Þ

where n¼ (x  0.332)/(y  0.186). Therefore, the calculated color correlated temperature is 1857 K. It is believed that the colorimetric properties could be useful for the design of luminescent devices and so they were presented by the ETO ceramics, since it was only polycrystalline ceramic that emitted on visible range. To characterize the light sources property, color purity of the emitted color was calculated as per the following formula [22,23]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx  xi Þ2 þ ðy  yi Þ2 P ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi:100; ð2Þ ðxd  xi Þ2 þ ðyd  yi Þ2 where (x,y) are the color coordinates of the sample, (xd,yd) are the coordinates of the dominant wavelength and (xi,yi) are the

151

coordinates of the illuminant point. Thus, the calculated color purity for the ETO up-conversion emission in the purplish pink region is about 47%. Among the rare earth ions using the useful near infrared (NIR) to visible up-conversion luminescence, Er3 þ has been receive more attention [24,25]. In this context, much research effort has been devoted to the up-conversion materials, based on erbium ions in various hosts due to their potential applications in biosensors, IR detector, optical storage, light emitting displays and upconversion lasers [25]. Up-conversion is a process where the adsorption of two or more lower-energy photons leads to the emission of higher energy photon [26]. Materials containing Er3 þ provide efficient up-conversion, as well as were observed in the Fig. 7a. Erbium ions can absorb light at around 980 nm, which corresponds to the emission of well-developed InGaAs laser diodes and emit in both the green ( 540 nm) and the red ( 650 nm) spectral regions [26]. Due to the special structure of Er3 þ energy levels (see Fig. 3), the excited states for up-conversion can be populated by several well-known mechanisms such as: excited-state absorption (ESA), cross-relaxation (CR) and energytransfer (ET) [27,28]. In general, several processes are involved in an up-conversion luminescence process: ground state absorption (ESA), radioactive transition, multiphoton relaxation and ET [29]. For the ETO ceramics, Er3þ ions are excited from the ground state 4I15/2 to active state 4I11/2 by absorbing a 980 nm laser photon. The ions in the 4I11/2 level sequentially absorb another 980 nm photon and are raised to the 4F7/2 level by an ESA process. The ions in the 4F7/2 level return to the splitting levels of 2H11/2 and 4S3/2 via multiphoton relaxation. Finally, the Er3þ ion in the metastable state decays to the ground state by the emission of green (2H11/2-4I15/2, 2S3/2-4I15/2) emission bands [28,29]. This is called a two-photon process, which is in agreement with the up-conversion emissions in other Er3þ doped samples [29]. Fig. 7b shows the NIR down-conversion luminescence under VIS excitation (532 nm). Down-conversion (frequently called quantum cutting) is a process where one high-energy photon (UV or VIS) is cut into two lower energy photons (NIR) [26]. Down-conversion is useful when considering the problem of spectral mismatch between the solar cells and solar spectrum [26]. For the ETO ceramics, the downconversion emission can be attributed to the electrons of Er3 þ which were excited to high energy levels and then the activated electrons got back to the steady ground state via some nonradioactive and radioactive transitions [29]. We can observe three distinct emission bands in range 930–1070 nm, 1130–1225 nm and 1413–1689 nm, which originated from 4I11/2-4I15/2, 2H11/2-4I11/2 and 4I13/2-4I15/2 transitions, respectively. It can be noted that the down-conversion emission line at 1004 nm shows a strong, broad bandwidth with FWHM of 11.1 nm. Fig. 8 shows the emission spectrum for the YTO ceramics under 980 nm excitation. The broad band in the range 995–1096 nm is related to the 2F5/2-2F7/2 transition from Yb3 þ . The YTO exhibited a FWHM value of 58.8 nm, which is the largest value found among the RETaO4 studied here. The asterisk at 992 nm is related to laser contribution. In this work, we investigated the optical properties of the undoped polycrystalline ceramics; however, it is important to note that the spectral region of the 2F7/2-2F5/2 emission from Yb3 þ ions overlaps that of 4I15/2-4I11/2 absorption from Er3 þ ions [30]. This fact favors an effective Yb3 þ to Er3 þ energy transfer and as a consequence, Yb3 þ ions are often used as dopant in materials contend Er3 þ in your host matrix. PL decay curves were obtained from kinetic measurements and they are also an important parameter for luminescence materials. The decay of the PL intensity was monitored after the excitation interruption and, on the basis of the exponential formula, the fluorescence lifetime was determined, i.e., the time after which the intensity is dropped to 1e from the initial value. Fig. 9 shows the PL

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K.P.F. Siqueira et al. / Journal of Luminescence 179 (2016) 146–153 Table 2 Comparison between the fluorescence lifetimes at room temperature for some doped materials and the polycrystalline ceramic investigated in this work. Sample

Nature

Excited-state

NdTaO4 NdTaO4 1 at% Nd:YAG ErTaO4 ErTaO4 K2ErF5 K2ErF5 YbTaO4 K2YbF5

PP SC SC PP PP SC SC PP SC

4

F3/2 F3/2 F3/2 4 F9/2 4 I11/2 4 F9/2 4 I11/2 2 F5/2 2 F5/2 4 4

Lifetime (μs) 81.5 o 8.4 225 67.1 131.2 79 61 122.1 580

Reference This 4 19 This This 26 26 This 26

work

work work

work

*PP¼ polycrystalline powder. *SC¼ Single crystal.

efficiency, and this peculiarity indicates the suitability for laser operation [26]. Some works have investigated the concentration quenching for tantalate host matrices using the rare earths with activators [31– 34]. For example, Nd3 þ was used as dopant in M0 –LuTaO4 resulting in a potential heavy scintillation material with relatively fast decay time (E 260 ns) [31,32]. By comparing the undoped GdTaO4 ceramic with the doped Nd:GdTaO4 single crystal, Peng et al. [33] also observed that the doped crystal exhibited faster scintillation decay. Besides, the light yield of Nd:GdTaO4 could be estimated to be equal to that of GdTaO4 [33]. These results showed that we could use tantalate host matrices (including YTaO4, LaTaO4, GdTaO4, LuTaO4) and additionally we could dope them using Nd3 þ , Er3 þ and Yb3 þ ions as activators. However, the resulting materials will exhibit different optical properties. In this work, we investigate only the optical behavior of the undoped polycrystalline ceramics NTO, ETO and YTO, and our results show that they can be used as luminescent materials for applications in luminescent devices, despite they have a higher rare earth concentration, i.e., in these materials, the rare earth ions are not a dopant.

Fig. 8. Emission spectrum for the YTO ceramics (λexc ¼ 980 nm).

Fig. 9. PL decay curves for NTO (excited at 808 nm), ETO and YTO (excited at 980 nm) monitored at 1064 nm (NTO), 671 nm and 1007 nm (ETO) and 1004 nm (YTO). The black lines represent the mathematical adjustment.

4. Conclusions decay curves for NTO, ETO and YTO ceramics. For NTO, the monitored emission wavelength was at 1064 nm under excitation of 808 nm. The transition 4F3/2-4I11/2 was chosen since it presents the best signal/noise relation. For ETO, the monitored emissions were at 671 nm, in the visible range, and at 1007 nm, in the infrared one, which correspond to the 4F9/2-4I15/2 and 4I11/24 I15/2 transitions, respectively. In this case, the most intense upconversion (see Fig. 7a) and down-conversion emissions (see Fig. 7b) exhibited the best signal/noise relations. For ETO, lifetimes measurements used the 980 nm excitation. Finally, YTO ceramic had the emission at 1004 nm monitored and, in this case, also was used the 980 nm excitation. In all cases, decay curves can be fitted by single-exponential function: I ¼ I 0 expð  t=τÞ ;

ð3Þ

where I 0 is the initial intensity at t ¼ 0 and τ is the lifetime. Fluorescence lifetimes for NTO, ETO (visible emission), ETO (infrared emission) and YTO were determined as (81.5 70.4) μs, (67.1 70.2) μs, (131.270.5) μs and (122.1 70.6) μs, respectively. In general, there are several factors, e.g., crystalline phase, crystal structure, crystallinity, particle size, surface defects, and impurities that influence in the lifetimes of the materials [25]. As the comparison purposes, we present in Table 2 the fluorescence lifetimes at room temperature for some luminescent materials. It is known that shorter fluorescence lifetimes suggest greater energy transfer

For the first time, the optical properties of polycrystalline undoped ceramics NdTaO4, ErTaO4 and YbTaO4 were investigated. XRD results proved the structural quality of the samples and the different arrangements assumed by them, i.e., M-type fergusonite (I2/a, #15), for NTO, and M0 -type (P2/a, #13), for ETO and YTO, in perfect agreement with the ICDD cards. In all samples, a great influence of crystal field could be observed, which is responsible by appearance of Stark levels to each multiplet in absorption and emission spectra. The absorption spectra were investigated for all samples, and the results are related with the rare earth present on the host matrix (TaO4). The emission spectra showed the intraconfigurational f–f transitions from rare earth ions in tantalates. For NTO, the main emission is related to the 4F3/2-4I11/2 at 1064.5 nm. ETO exhibited an up-conversion emission under 980 nm excitation, and a down-conversion emission under 532 nm. Colorimetric parameters were determined to ETO, such as color coordinates (x¼ 0.460, y¼0.271), color correlated temperature (1857 K) and color purity (about 47%). The PL emission for YTO exhibited the largest FWHM, i.e., 58.8 nm, due to the 7F5/2-2F5/2 transition from Yb3 þ . Fluorescence lifetimes for dominant emissions were determined as 81.5, 67.1, 131.2 and 122.1 μs to NTO, ETO (VIS), ETO (NIR) and YTO, respectively. Nevertheless, the results indicate the undoped polycrystalline ceramics with a new promising to solid state lighting devices.

K.P.F. Siqueira et al. / Journal of Luminescence 179 (2016) 146–153

Acknowledgments The authors acnowledge the financial support from the Brazilian agencies CNPq, FINEP and FAPEMIG.

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