Investigation of uranium luminescence in SrB4O7 matrix by time resolved photoluminescence, thermally stimulated luminescence and electron spin resonance spectroscopy

Journal of Alloys and Compounds 611 (2014) 74–81

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Investigation of uranium luminescence in SrB4O7 matrix by time resolved photoluminescence, thermally stimulated luminescence and electron spin resonance spectroscopy M. Mohapatra, B. Rajeswari, R.M. Kadam, M. Kumar, T.K. Seshagiri, N.K. Porwal, S.V. Godbole, V. Natarajan ⇑ Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 26 November 2013 Received in revised form 13 May 2014 Accepted 14 May 2014 Available online 22 May 2014 Keywords: SrB4O7 Uranium Photoluminescence TSL ESR

a b s t r a c t The luminescence of uranium in strontium borate (SrB4O7, SBO) matrix was investigated by time resolved photoluminescence, thermoluminescence (TSL) and electron spin resonance techniques (ESR). The samples were synthesized using solid state fusion reaction route and characterized by X-ray diffraction. Photoluminescence excitation and emission data suggested the stabilization of uranium as uranate (UO6 6 ) in the matrix. Luminescence decay time data suggested the stabilization of uranium at two different sites in the matrix. By giving suitable delay times and choosing proper gate widths, the two emission spectra due to the two uranate species could be obtained. Thermoluminescence investigation on the gamma-rays irradiated sample showed a strong glow peak at 415 K and a weak glow peak at 505 K. The dose response behavior, the trap parameters along with the order of kinetics for the strong glow peak were determined. To pinpoint the exact chemical nature of the defect centers responsible for the observed glow peaks, electron spin resonance technique was employed. Based on the ESR-TSL correlation data and the observed photoluminescence results, a plausible mechanism for the origin of the luminescence in the system was proposed. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Noncentrosymmetric alkaline earth borates are the workhorses of modern day high power laser systems [1,2]. Because of their appropriate non linear optical coefficients, wide transparency throughout the spectral range and high optical damage thresholds, these materials find extensive use as non linear optical crystals [3,4]. One such compound is strontium tetra borate (SrB4O7 = SBO), which has excellent non linear optical properties with high mechanical strength, non-hygroscopicity and high optical damage threshold [5]. In the recent past, another alkaline earth and alkali metal borate, namely, NaSrBO3, doped with various rare earths such as Tb3+ [6], Sm3+ [7] and Eu3+ [8] have been reported for their excellent luminescence properties. Especially when doped with trivalent europium, the system can be excited at 394 nm by UV-LED which opens up many applications in the light industry [8]. In addition, several lithium containing borate compounds of ⇑ Corresponding author. Address: Spectroscopy Section, Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Tel.: +91 22 25594580; fax: +91 22 25505151. E-mail address: [email protected] (V. Natarajan). http://dx.doi.org/10.1016/j.jallcom.2014.05.096 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

the type LiMBO3 (M = Sr, Ca) have also been reported to have excellent optical properties when doped with suitable rare earths [9,10]. Compared to these borates, the SBO system possess a rigid three dimensional (B4O7) network of corner linked BO4 tetrahedrons, where the strontium atoms are positioned in the large cages of the structure coordinated by nine oxygen atoms [11,12]. These cages are large enough to accommodate any activator/dopant ions such as rare earths and other large cations without disruption of the borate network. Thus, these materials have been doped with various activator ions targeting different technological applications. On one hand, SBO has been used as a host matrix for luminescence applications by doping with Eu [4,13–16], Sm [16–18], Tm [19,20], Yb [21], Pb [22], Cu [23] and Ag [24] ions etc. On the other hand, this material has been used as a host material for thermoluminescence dosimetric applications [25]. One particularly interesting aspect of this material is that due to its unique crystal structure, many trivalent lanthanide ions such as Eu3+, Sm3+, Yb3+ and Tm3+ can be reduced to their corresponding divalent species without introduction of reducing agent in air at high temperatures [26]. In some cases, the change in the valence state can also be brought about by external irradiation such as gamma rays and X-rays.

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H ¼ bHj Sj g j

The undoped sample did not show any PL. On doping, if the uranium atom goes to the lattice positions of the Sr or B, then UO9 or UO4 type of polyhedra will be expected. However, considering the ionic radii of B (III) and U (IV and VI), the substitution at B site is improbable. The earlier report by Tanner et al. [33] had suggested

(1 2 1)

(2 1 1)

(0 2 0)

(0 0 2)

(1 4 1) (1 4 0) (2 0 0)

A more detailed discussion regarding the sample preparation is mentioned elsewhere [20]. The phase purity of the prepared samples was characterized by powder X-ray diffraction (XRD) investigations carried out on a Phillips PW1071 spectrometer with Cu Ka (k = 1.5405 Å) as the source and graphite crystal monochromator. The scan rate was kept at 0.05 Å/s in the scattering angle range (2h) of 10–55°. The chemical compositions of the samples were obtained by emission spectrometric method using Inductively Coupled Plasma (ICP-AES, Jobin–Yvon, France) as an excitation source. Chemical analysis have shown that Sr and B are in the molar ratio of 1:4 indicating that strontium borate have the composition SrO.2(B2O3). The other non metallic impurities and quenchers were below the detection limit. TRPLS investigations were done on an Edinburgh FLS-900 unit with CD-920 controller. The unit is equipped with a micro second Xe flash lamp as the excitation source and M 300 monochromators. Gamma irradiation of the samples were done in a 60Co gamma chamber with a dose rate of 1.05 kGy/h. TSL studies were carried out on an indigenously built unit with an EMI 6255S photomultiplier tube having quartz window with S11 response. Glow curves were obtained at variable heating rates. ESR studies were carried out on Bruker EMX series (EMM1843) spectrometer operated at X-band frequency with 100 kHz field modulation. The ‘g’ values were evaluated relative to a 2,2 diphenyl-1 picryl hydrazil (DPPH) sample with g = 2.0036. The various ESR parameters for different radicals were determined precisely from the calculated spectra by simulation using Bruker SIMFONIA program based on perturbation theory. The theoretical ESR signals were calculated using the spin Hamiltonian parameter (H) as per the following equation:

3.2. Photoluminescence (PL) investigations

(0 4 0)

ð1Þ

The XRD pattern of the blank SBO samples heated at 900 °C is shown in Fig. 1. The patterns matched with the ICDD standard pattern no-15-0801 corresponding to the orthorhombic system of strontium tetra borate confirming the formation of a single phase compound. The crystallographic parameters determined from the SBO sample are presented in Table 1. For comparison, the standard pattern is also shown in Fig. 1. The structure of this orthorhombic SBO system has been well reported in literature. It was discovered in the binary system of SrO-B2O3 by Block et al. [34,35]. As discussed already, the crystal structure of SBO is formed by the corner sharing borate (BO4 tetrahedron) groups, which form a three dimensional network of (B4O7)1 containing channels parallel to the b and c-axis with the Sr ions in the cage formed by them. The Sr atoms are in the SrO9 polyhedrons with Cs geometry. The nine nearest neighboring ‘O’ atoms has a bond distance of 0.253–0.284 nm with 0.269 nm representing only one interaction, while the other bonds are repeated by mirror plane building a mono-capped square prism polyhedron type of system [16]. The shortest Sr–B distance is 0.311 nm and the closest Sr atom is situated at 0.424 nm distance. The SBO space group is Pnm21. A schematic of the crystal structure is presented here in Fig. 2 which was drawn using the VESTA visualization program [36].

(1 0 1)

SrCO3 þ 4H3 BO3 ! SrB4 O7 þ 6H2 O þ CO2

3.1. Crystal structure

(1 3 0)

Samples of un-doped SBO were prepared by solid state fusion reaction using SrCO3 and H3BO3 in the molar ratio 1:4. To avoid loss of boron, 5% excess boric acid was added to the reaction mixture. In case of uranium doped sample, varying amounts of uranium oxide dissolved in 4 M HNO3 was added to the (SrCO3 + H3BO3) mixture, as slurry in acetone. The amount of the actinide element was varied so as to get the final concentration from 0.1 mol% to 2 mol%. In all the cases, the resultant mixtures were dried, finely ground and then heated in a muffle furnace at 600 °C for 2 h. The mixtures were cooled to room temperature, ground again and then heated at 900 °C for 3 h. The overall reaction taking place can be represented by the following equation.

3. Results and discussion

(0 1 1)

2. Experimental

the signal. The line width of each component was optimized in order to obtain a best fit between the simulated and the experimental data. Temperature variation studies were done on a Bruker variable temperature accessory Eurotherm BVT-2000.

(1 1 0)

Like lanthanides, the actinide element, uranium is known to have variable oxidation states, which can be stabilized in a particular host. In this regard, luminescence and optical properties of different oxidation states of uranium, namely VI, V, IV and III, and the radiation-induced changes in the oxidation state have been reported [27–31]. Though in most uranium-containing compounds, the luminescence results due to the uranyl group (UO2þ 2 ), 2 6+ octahedral and tetrahedral uranium groups (UO6 6 , UO4 and U ) are also reported to have intense luminescence [32]. The luminescence properties of U doped in SBO matrix has been reported by Tanner et al. [33]. The authors have recorded the emission and excitation spectra of the system at room temperature and at 77 K, wherein they had observed a structure-less ‘green’ luminescence. This was similar to the emission of an uranate system arising due to the transitions from oxygen-derived orbitals to uranium 5f and 6d orbitals. In the present paper, we have carried out a detailed time resolved photoluminescence study (TRPLS) of the uranium doped SBO system. In addition, the system was investigated for its thermally stimulated luminescence (TSL) behavior on gamma irradiation. The order of the kinetics for the observed TSL glow peak was also determined. To determine the chemical nature of the defect centers, electron spin resonance (ESR) technique was used. By the help of ESR–TSL correlation, a plausible mechanism for the glow peak is proposed.

(2 3 0)

ICDD File No-15-0801

ð2Þ

Here j is the component along one of the axis x, y and z; H is the applied field, S is the total spin of the electron, g is the g factor and b is Bohr magneton. In the SBO samples, ESR signals were simulated by generating 9000 random orientations of the magnetic field and by summing the corresponding 9000 absorption signals. The final signal was obtained by performing a convolution (Gaussian or Lorentzian line shape) of each transition line adding all contributions and calculating the first derivative of

10

15

20

25

30

35

40

45

50

55

2θ Fig. 1. XRD pattern of the blank SBO sample along with the ICDD standard pattern no-15-0801.

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Table 1 XRD data of SBO (k = 0.15406 nm).

1x105

I/I0

d value (Å)

hkl

Lattice parameters

33.703 32.789 16.556 21.662 22.547 29.148 32.194 33.444 39.274 40.718 42.651 44.23 44.947 46.057 46.231 47.069 48.346 30.355

100 50 25 30 30 30 45 16 18 25 30 12 65 14 16 20 25 15

2.657 2.729 5.35 4.09 3.94 3.61 2.78 2.67 2.29 2.21 2.11 2.046 2.015 1.97 1.96 1.93 1.88 2.942

121 03 1 020 110 011 101 130 040 140 200 002 220 141 022 201 211 230 111

a = 4.426, b = 10.70, c = 4.23

8x104

Intensity (a.u.)

2h values

λex = 246 nm SBO:U (1 mol%)

6x104

4x104

2x104 480

520

560

600

Wavelength (nm) Fig. 3A. PL emission spectrum for the 1 mol% uranium doped SBO with 246 nm excitation.

that on doping, uranium primarily goes as U(VI) and sits in the matrix in a disordered manner. Fig. 3A shows the PL emission spectrum of the SBO doped with 1 mol% of U with kex = 246 nm. The spectrum revealed the superposition of strong emission peak at 520 nm and lower intensity bands at 500 and 540 nm. A small hump was also seen at 560 nm. The spectrum has similar characteristics to that reported by Tanner et al. [33] for the matrix at 77 K. Fig. 3B shows the intensity of the 520 nm peak as a function of uranium concentration. It can be observed from the figure that, the emission intensity is saturated beyond 1 mol% of the dopant ion indicating quenching. Such spectrum corresponds to the transitions from orbitals derived primarily from oxygen (2p) to uranium (6d) orbitals. The transitions are thus parity allowed, but not necessarily electric dipole allowed. Commonly, the first emission peak of a U (VI) species known as the zero phonon (zp) line corresponding to the P electron transfer transition Pg ? þ g (Dh) from oxygen to the non-bonding orbital of uranium. With 4, 5 or 6 equatorial coordination in crystalline materials, the position of the zp line normally varies from 520 to 470 nm. In the present case, the observed band starts at 500 nm. This strongly indicates that the equatorial coordi12 nation is in between 4 (as in UO6 6 ) and 7 (as in UO9 ) [27]. It is usually observed that, for uranyl species (UO2þ ), the zp line (t0) 2

is strongest peak, but for uranates, the t1 or in some case, the t2 is the most intense peak. In the present case, as it can be seen, the t1 peak is more intense. Further, the vibronic couplings, which are usually more prominent in case of uranyl species, are not clearly observed. The emission spectrum observed here also closely resembles that of uranates (UO6 6 ) as reported by Blasse et al. [27]. This type of emission was also reported for uranium doped in SrLaMgNbO6 matrix by Bleijenberg [37]. All these previous reports have attributed the green luminescence to the moiety UO6 6 placed in the matrix in a disordered manner. Considering the above relations, the possible arrangement formed to replace the SrO9 polyhedra could be the UO6O3 group, where the first number denotes the nearest neighbors. The saturation behavior observed in case of the emission intensity might be due to cross relaxation between the uranium ions via a non-radiative energy transfer process. This non-radiative energy transfer can take place via two different process (a) exchange interaction also known as coalitional energy transfer or Dexter mechanism (b) multipole–multipole interaction also known as Forster resonance energy transfer or fluorescence resonance energy transfer (FRET) [38]. FRET is a dynamic process based on dipole–dipole interactions that are dependent on the donor acceptor distance (L)

Fig. 2. A schematic of the crystal structure of SBO.

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M. Mohapatra et al. / Journal of Alloys and Compounds 611 (2014) 74–81

6x103

Intensity (a.u.)

5x103

4x103

3x103

4

1x10

λex= 246 nm, λem = 520 nm

10000

Intensity (a.u.)

Intensity in linear scale(a.u.)

2x104

1000

100

6x103 10

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2x103 0.0

0.5

1.0

1.5

Time (m sec)

2.0

U conc (mol %)

0 0.5

Fig. 3B. The intensity of the 520 nm peak as a function of U concentration.

and the probability of this decreases at a rate of 1/L6. Moreover, it is also dependent on the donor–acceptor spectral overlap and the relative orientation of the dipole moments. The typical distance range over which FRET can occur is up to 100 Å. On the other hand, Dexter mechanism is also a dynamic quenching process but takes place over a short range of distance (typically 610 Å). The probability of this process decreases as (exp)L and depends on the spatial overlap of donor–acceptor orbitals. A rough estimation of the critical transfer distance (Lc) for energy transfer can be calculated using the relation given by Blasse [39].



3V 4pNX C

13 ð3Þ

Here V is the unit cell volume, XC the critical concentration and N the number of sites. Taking the critical concentration value to be 1 mol% and values of V and N obtained from the XRD data analysis, Lc was evaluated to be 25 Å. This value suggests that multipolar interaction is the dominant mechanism of luminescence quenching in this particular matrix. For further studies, this 1 mol% U doped sample of the system was taken into consideration.

The PL decay time or the fluorescence life time (sf) of any system usually reveals the total lifetime of a particular energy level (the time taken for the particular state population to become 1/e of the initial value). PL decay time measurements for the 1 mol% U: SBO system was carried out with kex = 246 nm and kem = 520 nm that is shown in Fig. 4. The luminescence decay curve was recorded on 4 ms time scale and fitted using the following exponential decay equation by an iterative process.

ð4Þ

Here Ai is a scalar quantity, tis are the times of measurement and sis are the decay time values. The decay curve could be best fitted into bi-exponential components of 200 ls (major component) and 490 ls (minor component) with v2 = 1.414. This suggested that, there are two types of uranate species present in the system. The percentage of ‘U’ ions exhibiting a specific life-time obtained using Eq. (5) revealed the major component of 85% and the minor component of 15%.

  ðAi  si Þ  100 % of species ¼ P Ai  s i

2.0

2.5

3.0

3.5

4.0

Fig. 4. PL decay time curve of the 1 mol% U doped SBO in linear scale; the inset figure shows the decay profile in log scale.

In order to identify the individual species exhibiting a particular life-time, TRPLS was carried out. For this, a TRPLS scan was taken with 246 nm excitation and 450–650 nm emission range. Two data slices were taken from this scan having different time-delays, but constant integration time so that the two slices represent the two species. Fig. 5 shows the individual emission spectrum for the two species obtained by the data slicing. For the short lived component, a delay time of 100 ls was given where as for the longer lived component delay time of 1 ms was given keeping similar gate widths for both the species. It can be observed from the figure that, the two species obtained from TRPLS, corresponding to the two life time values are identical. So, it confirms that the two species present in the SBO matrix are similar in nature. The difference observed in the decay time values can be due to the difference in the nature of the charge compensating defect centers surrounding a particular uranium ion. 3.4. Thermally stimulated luminescence (TSL) investigations TSL investigations of the 1 mol% U doped samples were carried out before and after gamma irradiation. For the initial set of

3.3. PL decay time and TRPLS investigations

IðtÞ ¼ A0 þ A1 expðt1 =s1 Þ þ A2 expðt 1 =s2 Þ þ   

1.5

Time (m sec)

ð5Þ

1x105

Intensity (a.u.)

LC ¼ 2

1.0

Minor Species 490 μs Major Species 200 μs

1.9x104

8x104

1.5x104

6x104

1.1x104

4x104

7.6x103

2x104

3.8x103 460 480 500 520 540 560 580 600

Wavelength (nm) Fig. 5. Individual emission spectrum for the two uranate species present in the SBO matrix obtained by TRPLS. Delay time for the short lived species – 100 ls and delay time for long lived species – 1 ms with similar gate widths for both the species.

M. Mohapatra et al. / Journal of Alloys and Compounds 611 (2014) 74–81

experiments a gamma radiation dose of 0.5 kGy was given to the system. Prior to gamma irradiation, no TSL signal was observed in the sample. There was no change in the PL properties of the system after gamma irradiation. This indicated that the oxidation state of uranium was not influenced by gamma irradiation. TSL investigations on the undoped as well as U doped SBO have been reported earlier [4,25]. Manam and shrama [40] have done detailed TSL studies on another alkaline earth borate (barium borate) matrix. In case of the strontium borate matrix, no TSL was observed in the gamma irradiated undoped sample up to a dose of 10 KGy. In ‘U’ doped and c-irradiated SBO sample at room temperature, a single TSL glow peak at 415 K (heating rate – 2.5 K/s) was observed as shown in Fig. 6. Along with this intense glow peak, another weak glow peak was observed at 505 K. This suggested that two types of trap centers were created in the system by gamma irradiation. The 1 mol% U containing sample was irradiated by different gamma doses to evaluate the TL dose response of the 415 K peak. The TSL intensity maximum was found to be at 2 kGy. This is presented in the Fig. 6 inset. Spectral studies of the TSL glows were carried out using narrow band interference filters and transmittance filters. The interference filters used in the range 380–600 nm had a band pass of ±20 nm. The emission groups around 510 and 540 nm were observed indicating that uranate ion acts as luminescent center. The trap parameters viz. activation energy (Ea) and frequency factor (s) for the 415 K glow peak were determined using variable heating rates method [41,42]. For this, 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 T 2m /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. The trap depth values for 415 K and 505 K peaks were evaluated to be 0.85 eV and 0.94 eV respectively. The frequency factors were determined to be 2  1010 s1 and 5  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 415–505 K peaks were 0.83 eV and 0.90 eV respectively. Thermal bleaching of the low temperature glow peak was employed to obtain a reliable data for the high temperature glow peak.

TSL Intensity (a.u)

10

14

TSL Intensity (a.u)

12

8

12

10

δ

τ

8

Im/2

ω

6

4

2

T1

T2

0 350

400

Tm

450

500

Temperature (K) Fig. 7. TSL glow curve of the U:SBO system showing the order of the TSL process, the red curve shows the Gaussian fit of the original TSL curve (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The order of the TSL kinetics (b) for the major glow peak was evaluated by calculating the symmetry factor (lg) of the glow peak based on the peak shape method [42]. According to this method, the symmetry factor can be represented as lg = d/x. Here Tm is the Peak temperature and d = T2–Tm, s = Tm–T1 and x = T2–T1. T1 and T2 are the temperatures at the half maximum (Im/2) of the TSL intensity at the lower and higher temperature side of the TSL peak maxima. This is represented in Fig. 7. In case of first order peaks, symmetry factor is close to 0.4. On the other hand, in case of second order kinetics, the peaks are almost symmetric with identical d and s values with symmetric factors close to 0.5. As it can be seen from Fig. 7 that the glow peak is almost symmetric with lg = 0.409. This indicates that the major TSL glow peak of the system follows second order kinetics. 3.5. Electron spin resonance (ESR) investigations

6 5 4 1

2

3

4

Gamma Dose (kGy)

8

6

4

2

0 375

μg = δ / ω

9

10

300

Im 14

Intensity (a.u.)

78

450

525

600

675

Temperature (K) Fig. 6. TSL glow curve of the 1 mol% U: SBO system; the inset figure shows the TSL dose response of the 415 K peak (TSL signal intensity Vs gamma dose).

No ESR signal was observed prior to irradiation suggesting the absence of any paramagnetic species in the prepared samples. The room temperature ESR spectrum of c-irradiated (2 kGy dose) 1 mol% uranium doped sample showed mainly a broad signal superimposed on few relatively sharp signals at g  2.00 as given in Fig. 8. The broad signal showed unresolved structures on it, which could be resolved by recording the spectrum in second derivative mode. The second derivative spectrum of c-irradiated sample (shown as an inset in Fig. 8) clearly showed at least three distinct radicals. The different radicals observed in the ESR spectra were identified on the basis of their ‘g’ values, power saturation behavior and thermal stabilities. Fig. 9 shows the ESR power variation experiments done on the gamma irradiated U:SBO system in the range 0.2–20 mW. The intense signal seen with the highest power (20 mW) with quartet hyperfine structure having orthorhombic symmetry was attributed to the well known boron oxygen hole trapped center (BOHC, g1 = 2.014, g2 = 2.0037 and g3 = 1.9960; A1 = 6.25 G,

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4x104 20000

d 2I/dH 2

3x104 4

dI/dH

2x10

0

-20000

1x104

-40000 3300

3350

3400

3450

3500

3550

H (Gauss)

0 -1x104 -2x104 -3x104 3200

3300

3400

3500

3600

3700

Magnetic Field (Gauss) Fig. 8. Room temperature ESR spectra of the 1 mol% U: SBO, gamma irradiated (dose = 2 kGy); the inset figure shows the spectrum in second derivative mode.

structure in present case was attributed to the interaction of unpaired electron of the radical with the most abundant isotope of boron, 11B (I = 3/2, natural abundance 80.2%). The hyperfine structure due to 10B nucleus (I = 3; isotopic abundance 19.80%) is unresolved and adds to the line width of ESR signal (H = 7 G). This signal is more prominent in the temperature variation experiments done on the 1 mol% U doped system (with 20 mW power) in the range 300–475 K as shown in Fig. 10. This type of center has been previously reported in several borate and borophosphate based crystalline as well as amorphous matrices [44–47]. The ESR signal intensity of this BOHC signal as a function of temperature is given in Fig. 11. In addition to this BOHC (BO2 3 ) signal, two more centers, one with a hyperfine structure and other without any hyperfine structure were also observed. The weak ESR signal with very large quartet hyperfine structure (Aiso = 115 G) was attributed to boron oxygen electron trapped center (BOEC). In addition to these boron based radicals, a axial signal without any hyperfine structure was also observed in the composite signal, which could be clearly seen at higher annealing temperatures. The

N

A A2 = 6 G and A3 = 6.5 G). Usually, in case of borates and borophosphates, the major radiation induced defect formed on irradiation is identified as the BOHC, in which the unpaired spin density resides on the 2p orbital of the oxygen atom next to a boron atom, which is strongly coupled to the electron spin. Griscom et al. [43] were the first to determine the spin parameters for this center in a binary borosilicate glass (B2O33SiO2). The observed hyperfine

g gA

300 K

g 0

(BOHC) 3300

3400

3500

350 K

20 mW

0

0

375 K

3300

03300

3400

3500

3300 0

3400

3500

03300

3400

3500

3300 0

3400

3500

6.33500 mW

3400

0

400 K

2 mW 0 3300

3400

3500

425 K

0.63 mW 0 3300

3400

450 K

3500

0.20 mW

475 K

0 3300

3300

3400

3350

3400

3500

3450

3500

H (GAUSS) Fig. 9. ESR power variation experiments done on the gamma irradiated uranium (1 mol%) doped SBO system in the range 0.2–20 mW.

0

3300

3300

3400

3350

3400

3500

3450

3500

H (GAUSS) Fig. 10. ESR temperature variation experiments on the gamma irradiated U (1 mol%) doped SBO system.

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M. Mohapatra et al. / Journal of Alloys and Compounds 611 (2014) 74–81

EPR Intensity (a.u)

4x10

observed that uranium was stabilized in the system as uranate (UO6 6 ) at two different sites surrounded by different numbers of charge compensating defect centers. The gamma irradiated doped samples showed a strong TSL glow peaks at 415 K and a weak peak at 505 K. The dose response for the strong glow peak was determined. The trap parameters such as the energy of activation and frequency factors for the two peaks were evaluated. Order of the kinetics for the major glow peak was evaluated using peak shape method. A detailed ESR power variation and temperature variation studies were carried out to investigate the chemical nature of the defect centers responsible for the observed thermoluminescence in the system. It was found that, boron based hole centers and oxygen radical were getting formed in the gamma irradiated system. Based on these data, a plausible mechanism for the observed glow peak was proposed.

3

3x103

1x103

300

350

400

450

References

Temperature (K) Fig. 11. ESR signal intensity of the BOHC signal as a function of temperature.

radical ions without any hyperfine structure are usually known to  þ be associated with oxygen containing centers (O, O 2 , O3 , O2 , etc.). It may be mentioned that the O (gx, gy (g\) > gz (g||)) and O 2 (g|| > g\) radical ions can be distinguished on the basis of the trend in their g values, while for O 3 radical the g anisotropy is very small. Thus the species with g|| > 2.000 and g\ = 2.000 is tentatively assigned to O 2 from the known characteristics of this center from ESR studies [44,46,47]. 3.6. Plausible mechanism of the TSL glow peak

[1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11]

TSL arises when the trapped electron and holes (charge carriers) created by external irradiation are released from the trap centers, combine and transfer the energy to the activator sites. As it was observed from the ESR data, upon external gamma irradiation, SBO produces BOHC and BOEC in the system along with oxygen ion vacancies which act as trapping sites for holes and electrons. From PL experiments, the luminescent centers associated with the TSL process were identified as the uranate species. Further from the ESR temperature variation studies it was found the BOHC species is getting destroyed at the TSL peak temperature of 415 K confirming its role in the peak. Based on these data, a plausible mechanism could be proposed for the 415 K peak as follows.

SrB4 O7 ðon gamma irradiationÞ ! BOHC; BOEC;O2

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

On heating the sample at 415K : BOHCðhole releasedÞ þ 6 UO6 6 þ hm ! ðUO6 Þ  ðUO6 6 Þ

!

UO6 6

O2 ðelectron

[25]

releasedÞ ! hm



þ hm1 ðglow peak at 415 KÞ

[26] [27] [28] [29] [30] [31]

The mechanism of 505 K peak could not be identified due to its weak intensity.

[32]

4. Conclusion

[34] [35] [36] [37] [38]

To conclude, the alkaline earth tetra borate, SrB4O7 doped with uranium was synthesized via solid state reaction route. The optimum concentration of the dopant ion giving maximum luminescence was determined along with the mechanism of the luminescence quenching. From TRPLS investigations, it was

[33]

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