Luminescence and EPR studies of YBO3:U

ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 1261–1266

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Luminescence and EPR studies of YBO3:U T.K. Seshagiri a, M. Mohapatra a, T.K. Gundu Rao b, R.M. Kadam a, V. Natarajan a,, S.V. Godbole a a b

Radiochemistry Division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India Indian Institute of Technology, Powai, Mumbai 400076, India

a r t i c l e in fo

abstract

Article history: Received 21 July 2008 Received in revised form 30 June 2009 Accepted 5 July 2009

Yttrium borate doped with uranium was prepared by mixing and heating yttrium oxide obtained through oxalate precipitation route, boric acid and requisite amount of nuclear-grade uranium oxide at high temperature. Photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on gamma-irradiated doped/undoped yttrium borate samples in the temperature range 300–600 K. TSL studies showed the presence of two glow peaks at 414 and 471 K. PL studies along with lifetime decay investigation suggested uranium goes in  the matrix as UO2+ 2 . EPR studies showed the presence of O2 radical ion along with electron trapped in defect centres, which might have been produced for charge compensation. Apart from this, CO 2 radical was also observed in the system having its origin from residual oxalate ion. Temperature dependence EPR studies of the observed radical confirmed the involvement of the CO 2 and dioxide radical ion in the observed glow peaks. By correlating the TSL, PL and ESR data, probable mechanism is proposed for the observed TSL glow in the system. & 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds D. Defects D. EPR D. Luminescence

1. Introduction Lanthanide-doped phosphor systems have found considerable interest due to their wide range of applications in various fields ranging from luminescence to dosimetry [1,2]. Ortho-borates, RBO3 (R ¼ Y, La, Gd and Lu) doped with different lanthanide ions (Eu3+, Ce3+, Dy3+ and Tb3+) are among the various phosphor materials attracting much attention, due to their high luminescence yield under vacuum ultraviolet (VUV) excitation [3]. These are potential candidates for luminescent materials to be used in flat displays, mercury-free fluorescent tubes, plasma display panels and detector system [4]. Yttrium borates and yttrium aluminum borate (YAl3B4O12) have excellent non-linear optical properties and their doped crystals are important for building microchip lasers [5,6]. Apart from this, Ce3+-doped systems have particularly interesting characteristics for applications as radiation detectors [7]. Further, the near tissue equivalent absorption coefficient of ortho-borates makes them potential candidates for dosimetric studies. These classes of materials present a better sensitivity as compared to LiF:Mg,Ti (TLD-100). In this context, the study of their thermally stimulated luminescence (TSL) characteristics and mechanism becomes very much important. Various synthesis methods such as solid state reaction, co-precipitation, microwave heating, spray pyrolysis, sol–gel and hydrothermal method have been used to prepare RBO3 doped with rare-earth

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E-mail address: [email protected] (V. Natarajan). 0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.07.011

phosphor materials so as to achieve desirable properties like enhancement of photoluminescence (PL) and thermoluminescence (TL) intensity and better chromaticity [8,9]. From these studies, it is clear that RBO3 is a good host material with excellent luminescent properties when doped with a suitable activator. For example, recently Eu3+-doped YBO3 was shown to be a promising orange red-emitting luminescent material [10,11]. Apart from rare-earth ions, actinide ions such as U, Am and Cm are also known to give efficient photoluminescence. Among the actinide ions, uranium in its 6+ and 4+ oxidation states gives a good luminescence yield, under UV excitation. In this regard, luminescence and optical properties of U (VI, V, IV and III) ions and the radiation-induced changes have been studied in various systems [12–16]. These investigations have revealed radiation-induced changes in the oxidation states of uranium. Though many uranium-containing compounds show efficient luminescence from the uranyl group (UO2+ 2 ), octahedral and tetrahedral uranium and U6+) are also reported to have intense groups (UO66, UO2 4 luminescence peaks [17]. There is a wealth of information available in the literature on the luminescence properties of various uranium-doped systems; but as far as we know, there are no reports on the luminescence studies of uranium doped in YBO3. The present paper focuses on the synthesis, characterisation, TSL, PL and Electron Paramagnetic Resonance (EPR) measurements on undoped and uranium-doped YBO3 samples. TSL studies provide information regarding the trap depth or activation energy and thermal escape probabilities or frequency factor, while information on the microscopic structure and chemical nature of the traps involved can be obtained by studying their thermal stabilities

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using EPR spectroscopy. The complimentary nature of information obtained from these two techniques can be utilized for understanding the processes leading to TSL. PL studies help to identify the nature of the species responsible for the glow and its site symmetry. The PL data along with life time decay measurements, when used in conjunction with TSL and EPR data, can throw light on the dopant ion species responsible for the luminescence process.

2. Experimental All the chemicals used were of analytical reagent (AR) or nuclear grade. Yttrium oxide (Aldrich) was dissolved in highpurity nitric acid and then precipitated as oxalate by adding a saturated solution of oxalic acid. The oxalate precipitate after drying was sintered at 820 K to get pure yttrium oxide. Boric acid (Fluka) was then added to yttrium oxide prepared and the mixture was ground thoroughly in a mortar, heated in a muffle furnace at 1070 K for about 15 h. Uranium-doped samples (in the range of 0.05 to 2 mol%) were prepared in a similar manner by dissolving requisite amounts of nuclear-grade uranium oxide powder (U3O8) in highly pure HNO3, followed by precipitation as outlined earlier. Phase purity of the samples was confirmed by X-ray diffraction measurements carried out on a Phillips instrument (PW1071) operating with Cu-Ka radiation (l ¼ 1.5418 A1) fitted with a graphite crystal monochromator. The scan rate was kept at 0.051/s in the scattering angle range (2y) of 10–651. Trace metallic constituents in the sample were determined by spectroscopic analysis of the samples using an axial inductively coupled plasma–atomic emission spectrometer (ICP-AES, Jyovin-Von, France). TSL studies were carried out on an indigenously built thermoluminescence reader with an EMI6255S photomultiplier tube having quartz window with S11 response. Glow curves were obtained at variable heating rates. PL and lifetime decay investigations were carried out on an Edinburgh F-900 unit having a Xe microsecond flash lamp as the excitation source and equipped with a M-300 monochromator (Resolution ¼ 1 nm). Irradiations were carried out in a 60Co gamma chamber at a dose rate of 1.88 kGy/h. EPR measurements were carried out on a Varian E-112 E-line Century series X-band EPR spectrometer. The resonance magnetic field positions were determined at appropriate places in the spectrum (corresponding to gJ and g? values) using a reference field marker sample. In our case, TCNE (tetracyanoethylene) with a g-value of 2.00277 was used as the standard for g-factor measurements. From these measured magnetic field values, gJ and g? values of respective radicals were determined by employing the formula, hn ¼ gbH. For the temperature varying experiments, step heat treatments were performed in situ in the EPR cavity using the Varian variable temperature accessory up to a temperature of 3001C to follow the decay and evolution of the defect centres. Although we used the Varian variable temperature accessory in the initial experiments, the actual thermal annealing experiments were carried out using an external muffle furnace, as we had to go to higher temperatures (above 3001C) and the EPR spectra were recorded using a cavity, which was at room temperature for all the thermal annealing experiments.

3. Results and discussion 3.1. Physicochemical characterisation The phase purity of the samples was established by X-ray powder diffraction measurements. The diffraction patterns for a

500 YBO3

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40 2 theta

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Fig. 1. XRD pattern of the 1 mol% YBO3:U.

typical sample that is YBO3 with 1% U obtained are shown in Fig. 1, which indicated crystallization of the product in a hexagonal structure (space group P63/m). The observed lattice parameters were found to be in good agreement with those reported in the literature (JCPDS card no. 16-0277). There was no impurity phase formation even in the 2% U-doped YBO3 sample. ICP-AES results confirmed the absence of any trace metallic impurity beyond the range of 10 ppm. The structure of YBO3 is well reported in the literature, where boron is fourfold coordinated to the oxygen forming a [BO4] tetrahedra [18]. The [BO4] tetrahedra are of two types, the first one (I) is a very regular tetrahedron with average B–O distances in the range 1.37–1.57 A˚ and the second one (II) is a slightly distorted ˚ and two large tetrahedron with two similar short bonds (1.37 A) ˚ On the other hand, yttrium is bonds ranging from 1.89 to 1.92 A. eightfold coordinated by oxygen atoms in an arrangement that can be best described as a trigonal bicapped antiprism. The oxygen atoms surrounding the Y3+ ions occupy two crystallographic sites: six oxygen (O1) atoms in (4f) and two oxygen (O2) atoms in (6 h) with partial occupancy of 1/3. The Y–O1 and Y–O2 ˚ The structure of YBO3 distances are, respectively, 2.39 and 2.32 A. consists of a 3D network obtained by connecting [YO8] polyhedra, wherein each [BO4] group is linked with two adjacent [BO4] groups by O2 atoms and with two adjacent [YO8] groups by O1 atoms. Here in case of YBO3, deviation from the ideal S6 symmetry is reported due to the existence of two types of O2 atoms. 3.2. Photoluminescence studies PL studies were carried out on 0.05, 0.1, 0.5, 1 and 2 mol% uranium-doped YBO3 before and after gamma irradiation. PL spectra obtained with 0.05, 0.1, 0.5 mol% samples were weak and hence further measurements were done on the 1 mol% doped sample. PL spectrum of the 1% doped sample under 230 nm excitation wavelength is shown in Fig. 2. It shows emission maxima at 499, 520, 545 and 570 nm, characteristic of uranium in +6 oxidation state, wherein the peaks at higher wavelengths correspond to the vibrational fine structures [12]. The emission peak at 499 nm can be attributed to the well-known zero phonon (zp) line of U(VI). For the ion U(VI), the electron transfer transition Pg-Sg+ from oxygen to one of the non-bonding uranium orbital is generally referred to as the zero phonon (zp) transition. The observation of zp line at 499 nm and the occurrence of vibrational

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Fig. 4. PL emission spectra of YBO3:U with different delay times.

Fig. 2. Photoluminescence spectrum of 1 mol % YBO3:U.

component of the decay curve may be due to a uranyl species present in the vicinity of defect centres. On the other hand the relatively longer decay time of 275 ms may be coming from uranyl species that are far from the defect centres and thus having a longer lifetime.

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3.3. TSL studies

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Fig. 3. Decay time curve for the 1 mol % YBO3:U.

fine structures (u1790 cm1) at room temperature indicated that U(VI) is getting stabilized as uranyl (UO2+ 2 ) in the matrix. The fluorescence lifetime decay curve of the sample is shown in Fig. 3. The decay profile could be fitted into a bi-exponential component, out of which one was a dominant fraction with shorter lifetime (t ¼ 40 ms) and the other was a weaker part having longer lifetime (t ¼ 275 ms). The predominant lifetime of 40 ms may be attributed to the uranyl species present in an asymmetric environment. To probe further into the different types of U(VI) species present in the system, time-resolved emission spectrometry was performed on the system where emission spectra were recorded with two different delay times (0.3 and 1.8 ms). Fig. 4 illustrates the photoluminescence spectrum of the 1 mol% YBO3:U sample recorded with these two delay times. It can be seen from the figure that the two spectra are identical, indicating that they are coming from the same species. However, the occurrence of two different lifetimes can be explained on the basis of the following argument. It is well known that defect centres around a luminescent species provide an easy path for non-radiative decay and generally the lifetime of such species is greatly reduced. Though the two uranium luminescent centres are identical, the fast decaying

TSL studies were carried out on gamma-irradiated undoped as well as uranium-doped samples in the temperature range 300–700 K. Prior to gamma irradiation, no TSL was observed in undoped as well as in doped samples. TSL studies were carried out on samples with 0.05, 0.1, 0.5, 1 and 2 mol% uranium. In all the uranium-doped samples two glow peaks around 414 and 471 K were observed (total dose ¼ 4 kGy), latter being the more intense one. A typical glow curve for the 1% uranium-doped sample is shown in Fig. 5. The TSL yield was maximum in the sample with 1 mol% uranium and thereafter the TSL intensity decreased due to concentration quenching as illustrated in Fig. 6. Gamma dose dependence studies have shown that the glow peaks saturate after a dose of 4 kGy. The glow peaks were obtained at various heating rates of 2.5. 1.5, 1 0.5 and 0.3 K/s and thereafter an Arrhenius plot of (ln T2m/b) versus 1/Tm (where b corresponds to the heating rate and Tm represents the peak temperature in K) was obtained. A least-square fit program was employed to get the best fitting for the trap parameters, viz trap depth (activation energy) and frequency factor values. The trap depth value and frequency factors obtained for the 414 K peak and 471 K are 0.88 eV, 6.2  109 s1, and 0.94 eV, 1.1 109 s1, respectively. Alternatively the trap depth values were also evaluated by conducting TSL experiments connected with the initial rise method. The values obtained for the 414 and 471 K peaks are 0.83 and 0.89 eV, respectively. Thermal bleaching of the low-temperature glow peak was employed to obtain reliable data for the high-temperature glow peak. To study the spectral composition of the glow peaks, the glow curves of the samples were recorded using transmittance and narrow band interference filters. These studies showed emission groups around 488 and 547 nm, characteristic of UO2+ 2 ion as reported in the literature [19,20]. 3.4. Electron paramagnetic resonance studies Unirradiated uranium-doped samples showed no EPR signal at room temperature (RT) as well as at liquid nitrogen temperature,

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Fig. 6. Concentration dependence of TSL intensity.

Fig. 5. TSL glow peak of 1 mol% YBO3: U (Heating rate ¼ 2.5 K/s, Dose ¼ 4 kGy.)

indicating the absence of any paramagnetic species. This also rules out the formation of U(III) or U(IV) in the matrix, which is in accordance with the PL data. Fig. 7(a) shows the ESR spectrum of the 1% uranium-doped samples recorded at RT after irradiation (8 kGy dose). A cursory examination indicates that the spectrum may arise from two major centres with one of the centre exhibiting an axially symmetric g-tensor (indicated as centre I in figure). This centre, characterized by the principal g-values gJ ¼ 2.0290 and g? ¼ 2.0066 and lack of any hyperfine structure, was assigned to O 2 radical ion based on the known characteristics of this ion as reported in other ESR studies of zeolites and metal oxides [21,22]. In an earlier report on Eu3+-doped YBO3, this radical was observed to have gJ ¼ 2.0295 and g? ¼ 2.0040. This deviation in the g-values, because of the change in the nature of the dopant ion is quite predictable for this type of radicals. Centre II (discussed in the latter part) is attributed to CO 2 radical. Both  O 2 and CO2 radicals have displayed reasonably well-separated spectra (Fig. 7). The powder spectra have the characteristic shapes of ions, which have an axially symmetric g-tensor and do not exhibit any hyperfine structure. Centre III shows a single symmetric ESR line and this could be seen clearly without any overlap from CO 2 lines after thermal anneal at 600 1C. A step annealing technique was used to measure the thermal stability of the O 2 radical. The sample was returned to room temperature and the signal intensity was recorded after each step. The thermal annealing behaviour (shown in Fig. 7(b), (c), (d) and (e)) indicated a drastic reduction in the radical intensity in the temperature range 400–460 K beyond which the radical was barely visible. When correlated with the TSL data, it can be understood that the radical got completely destroyed in the above

g = 2.0028

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(d)

(e)

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H (GAUSS) Fig. 7. EPR Spectra for the samples at different temperatures (a) immediately after irradiation. (b), (c), (d) and (e) refer to spectra from samples annealed at 453, 563, 623 and 713 K, respectively.

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Fig. 8. C hyperfine lines (shown in parts b and c) along with the field-marker (shown in part a). Part a was recorded at a receiver gain of 1.2  103 while the hyperfine lines of 13C (parts b and c) were recorded at a receiver gain of 4.1 104. Thus the observed break in the EPR spectrum is evident due to the difference in the receiver gain settings.

temperature range, releasing the trapped holes. It is speculated that the thermally released holes recombine with electrons trapped at other sites in the lattice. The recombination energy is transferred to the uranyl ion, which acts as the luminescent centre emitting light and yields the glow peak at 414 K. It became evident during thermal annealing experiments that the remaining part of the spectrum (Fig. 7(a)) arises from two distinct species labeled as centres II and III. Centre II is characterized by an axially symmetric g-tensor, with principal values gJ ¼ 1.9987 and g? ¼ 2.0022. The observed g-values in the present study are similar to the reported values for the paramagnetic ion CO 2 in NaHCO3 [23] and in KHCO3 [24]. The g-value 1.9987 is characteristic of CO 2 radical and is reasonably independent of the matrix surrounding the radical. Further support for this assignment comes from the observation of 13C hyperfine lines shown in Fig. 8. As 13C lines are supposedly weak, they were recorded at higher gain settings of the receiver amplifier (4.1 104) than the central main radical line (1.2  103) shown in part a. These two hyperfine lines (parts b and c) are due to the interaction of the unpaired electron with 13C nucleus (I ¼ 1/2, isotopic abundance 1.1%). CO 2 radical is likely to be formed in the present matrix as the synthesis of the compound involves the oxalate precipitation method. During the annealing of the sample, even a small quantity of residual oxalate is sufficient to produce a significant concentration of the radical. The thermal annealing behaviour of the CO 2 radical (Fig. 9) showed a reduction in intensity of the corresponding ESR lines in the temperature range 440–620 K, indicating complete destruction of the ion. This is in accordance with the second TSL glow peak observed at 471 K. The trapped electrons released from this centre interact with holes trapped at other sites and the recombination energy is transferred to the luminescent uranyl ion, which in turn gives rise to the glow peak. We could not obtain the signature for BO2 3 radical as reported in most of the borate matrices [25]. In a borate centre, the interaction of the unpaired electron with the nucleus of 11B atom (I ¼ 3/2; isotopic abundance ¼ 81.2%) may give rise to hyperfine

420

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Fig. 9. EPR intensity of centre I and centre II as a function of annealing temperature.

structure in the observed spectrum depending on the electronic structure of the radical. However, centre II did not show any hyperfine lines, indicating the absence of BO2 3 . It is worth noting here that in our earlier work concerning YBO3 doped with Eu [11], the destruction of the borate radical produced a separate set of TSL peaks than observed in the present case, further ruling out the formation of the borate-centered radical in the system. The ESR line labeled as III in Fig. 7(b) is due to a centre characterized by a single ESR line [seen clearly in Fig. 7(e)] with an isotropic g-value 2.0034 and 5 gauss line width. One of the probable centres, which can be trapped in the present system, is the F+ centre where an electron is trapped at an anion vacancy. Hutchison [26] first observed such a centre in neutron-irradiated lithium fluoride matrix. In such matrices, a single broad line (width ¼ 100 Gauss) with a g-factor 2.008 is well reported. X-ray or gamma irradiation also produces such centres in other systems like alkali halides [27–29]. Such centres are characterized by (i) a small g-shift, which may be positive or negative, (ii) large line widths mostly coming from unresolved hyperfine structure and (iii) saturation properties characteristic of an inhomogenously broadened ESR line. In the present case, the defect centre III was also characterized by a small g-shift and a reasonably large line width. The centre also did not exhibit any resolved hyperfine structure. On the basis of these observations and considerations of the characteristic features of the defect centres likely to be formed in a system such as YBO3, centre III is tentatively assigned as an F+ centre. Thermal annealing studies showed no specific correlations of this centre with the observed TL peaks, as the centre was still observable after annealing at 713 K. Based on the above discussion, the mechanism for the observed TSL glow curves can be formulated as follows:

 On gamma irradiation; YBO3 -O 2 ; CO2 ðcoming from residual oxalateÞ

2 On heating; O 2 -O2 þ h ðat 414 KÞ

CO 2 -CO2 þ e ðat 470 KÞ e þ h-hn 2þ  UO2þ 2 þ hn1 -ðUO2 Þ 2þ  ðUO2þ 2 Þ -UO2 þ hn2 ðTSL GlowÞ

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4. Conclusion TSL studies on uranium-doped YBO3 samples prepared using Y2O3 derived from the oxalate precipitation route showed two TSL glow peaks at 414 and 470 K. Photoluminescence data revealed in the matrix. EPR data the stabilisation of uranium as UO2+ 2  showed the formation of O 2 and CO2 radical ions in the matrix. The destruction of these radical ions and the appearance of the TSL glow peak around the same temperature range suggested their role in the TSL process. The radicals got completely destroyed releasing trapped holes and electrons. The energy released in the electron hole recombination process is transferred to the UO2+ 2 ion, the luminescence centre, which gets de-excited by giving emissions at 499, 520, 545 and 573 nm. References [1] W.D. Horrocks, D.R. Sudnick, Acc. Chem. Res. 14 (1981) 384–392. [2] G. Blasse, Chem. Mater. 1 (1989) 294–301. [3] M. Ren, J.H. Lin, Y. Dong, L.Q. Yang, M.Z. Su, L.P. You, Chem. Mater. 11 (1999) 1576–1580. ¨ [4] T. Justel, J.C. Krupa, D.U. Wiechert, J. Lumin. 93 (2001) 179–189. [5] F. Kellendonk, G. Blasse, J. Chem. Phys. 75 (1981) 561–571. [6] C. Gorller-Warland, P. Vandevelde, I. Hendrickx, P. Porcher, J.C. Krupa, G.S.D. King, Inorg. Chim. Acta 143 (1988) 259–270. [7] M. Raukas, S.A. Basun, W. van Schaik, W.M. Yen, U. Happek, Appl. Phys. Lett. 69 (1996) 3300–3302. [8] Xiao-Cheng. Jiang, Chun-Hua. Yan, Ling-Dong. Sun, Zheng-Gui. Wei, ChunSheng. Liao, J. Solid State Chem. 175 (2003) 245–251.

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