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Influence of defects on thermoluminescence in pristine and doped LiMgPO4
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Influence of defects on thermoluminescence in pristine and doped LiMgPO4 a,⁎
b,c
b,c
a
a
M. Kalinkin , R. Abashev , A. Surdo , N. Medvedeva , D. Kellerman a b c
T
Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, Ekaterinburg, Russia Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Ekaterinburg, Russia Department of Experimental Physics, Ural Federal University, Ekaterinburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Defects Phosphates Dosimetry Thermoluminescence LiMgPO4
Pure and terbium-doped lithium-magnesium phosphate LiMgPO4 was investigated with respect to its thermoluminescent properties. The samples were synthesized by solid state, ultrasonic spray pyrolysis, as well as by melting and quenching methods. To predict the terbium site, ab initio calculations were carried out. Additional treatment in atmospheres with different oxygen partial pressures was used. The larger is the concentration of oxygen in the atmosphere and, accordingly, the smaller is the quantity of surface oxygen vacancies, the greater is the intensity of thermoluminescence. For a better understanding of thermoluminescence in LiMgPO4 and LiMgPO4:Tb, the TL spectra were recorded. It was concluded that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals of the matrix and activator ions.
1. Introduction
2. Experimental
The lithium-magnesium phosphate LiMgPO4 with olivine structure is one of the most promising matrices for ionizing radiation detectors [1–8]. Recently, it was considered as an alternative to the commercially available dosimetric material Al2O3:C [3]. Most often, terbium ion (Tb3+) is used as an activator of thermally or optically stimulated luminescence (TL, OSL) in LiMgPO4; sometimes, together with terbium, boron ions are introduced into the lithium magnesium phosphate lattice [9–10]. Different suggestions regarding the terbium site have been considered in the experimental works, in which Tb3+ was assumed to replace only the Li+ site [11] or only the Mg2+ site [12]. There are some reports on the properties of dosimetric material based on LiMgPO4 doped with Eu, Sm, and Tm [5,13,14]. However, even without activators, pure phosphate exhibits the thermoluminescent properties formed due to defects [14–18], although the dose sensitivity of such material is significantly lower than that of activated one. A similar situation is described for Al2O3 [19], LiAlO2 [20], Li2B4O7 [21], and CaB4O7 [22]. In this paper, we discuss the effect of surface and bulk defects on the thermoluminescence of pure and terbium-doped lithium-magnesium phosphate and show that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals from the matrix and activator ions.
2.1. Preparation of samples
⁎
Series of samples of pristine phosphate LiMgPO4 and those doped with terbium were synthesized by the conventional solid state reaction method. The stoichiometric amounts of raw materials Li2CO3 (lithium carbonate, 99.99%), 3MgCO3*Mg(OH)2*3H2O (magnesium carbonate, 99.9%), NH4(H2)PO4 (ammonium dihydrogen phosphate, 99.99%), and TbCl3 (terbium chloride, 99.99%) were mixed and milled until the average particle size was less than 1 mm. The mixture was pre-heated in platinum crucibles in air at 300 °C for 5 h. After the second homogenization in an agate mortar, the sample was heated to 500 °C and kept at this temperature for 20 h. After the third homogenization, the sample was pressed into a tablet and thermally treated at 800–900 °C for 20 h in gas atmospheres of air, Ar, and O2. To obtain a high-density sample, melting followed by quenching was carried out. The tablet of LiMgPO4 was melted in ambient air at 1250 °C for 1 h in alumina crucible. The melt was quenched by a pair of stainless-steel plates and formed into the polycrystalline film with a thickness of 0.2 mm. For ultrasonic spray pyrolysis synthesis, all the components taken in stoichiometric ratio for LiMgPO4 were dissolved in diluted nitric acid. Synthesis was carried out from a solution with a concentration of 100 g/l at 850 °C; the air consumption was of 14.6 ml/s.
Corresponding author. E-mail address: [email protected] (M. Kalinkin).
https://doi.org/10.1016/j.nimb.2019.12.021 Received 30 October 2019; Received in revised form 26 November 2019; Accepted 20 December 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al.
2.2. Characterisation of samples The X-ray diffraction data were collected at room temperature using a STADI-P transmission diffractometer (STOE) equipped with a scintillation detector. The phase purity of the sample was proved by comparing its XRD pattern with that in the PDF2 database (ICDD, USA, release 2016). The specific surface area of the powder samples was obtained using a Gemini VII Analyzer (Micromeritics) from the N2 adsorption–desorption isotherms and was calculated by the Brunauer–Emmett–Teller (BET) method. The diffuse absorbance spectrum was obtained with a Shimadzu UV-2401 PC UV–VIS spectrophotometer using BaSO4 as a reference. 2.3. Ab initio calculations To predict the terbium site, we carried out ab initio calculations using the density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) within the projector augmented waves (PAW) formalism [23,24]. Exchange and correlation potential was described by the generalized-gradient approximation (GGA) with the Perdew–Becke–Ernzerhof functional [25]. The kinetic energy cutoff of 500 eV was set for the plane-wave expansion and the Brillouin zones were sampled with the Г-centered 4x4x4 k-point grids. We employed a 112-atom supercell, where the substituting Tb atom corresponds to a doping concentration of 0.9 at.%. Due to computational problems, we did not study a smaller amount of Tb doping, which requires significantly larger supercells. Structural optimization, when only atomic positions were relaxed with the convergence criterion for the total energy and interatomic forces of 10−5 eV and 0.02 eV/Å, respectively, was used. 2.4. Radioluminescence and thermoluminescence measurements TL and RL measurements were carried out by exciting the samples with different radiation doses from an Eclipse IV Lab Rh X-ray tube (Oxford Instruments, Ua = 0–45 kV, I = 0–45 μA, Rh anode). All experiments were carried out at Ua = 30 kV and I = 30 μA. The TL and RL emission was recorded using a Varian Cary Eclipse spectrophotometer. The TL glow curves and spectra were collected using an automated laboratory setup, which provided the regimes of linear heating from 50 to 900 °C or isothermal treatment of the sample at any temperature within the given range. 10 mg samples placed in a silver cup were used for the measurements.
Fig. 1. The XRD patterns of LiMgPO4 synthesized in various ways and LiMgPO4:Tb.
necessary for the substitution of Tb3+ for Mg2+. Two configurations are possible for replacing Li+ by Tb3+, when one vacancy is in the Mg site (□Mg) and when there are two lithium vacancies (2*□Li). The conclusions on the preferred configuration were made from a comparison of their total energies, taking into account the chemical potentials of magnesium and lithium as simple metals in stable states. We find that the Tb3+ → Mg2+ substitution remains the most energetically preferable one even taking into account the formation of a compensating lithium vacancy. The configuration with the lithium vacancy closest to the terbium ion is by 0.14 eV lower in energy than its distant position, which indicates attractive bonding in the Tb3+- □Li pair. Replacement of Tb3+ → Li+ by two lithium vacancies (2*□Li) is less favorable by 2.98 eV. The Tb3+ → Li+ substitution, accompanied by the appearance of a magnesium vacancy, has an even higher energy of 1.3 eV and is the most unlikely one among the considered configurations. Thus, we believe that terbium with a concentration of ~1 at.% should replace magnesium with the formation of a bound lithium vacancy. In addition to lithium vacancies, the LiMgPO4 lattice can contain oxygen ones, both charged and neutral. Their energy levels are located inside the band gap of 5.5 eV [16]. This is well illustrated by the diffuse absorption spectra, in which, along with the fundamental absorption edge (≈220 nm), an additional band is clearly visible. It starts at ≈320 nm and is most pronounced for the sample annealed in argon when the oxygen vacancies are most probable. In oxygen atmosphere, this band almost disappears (Fig. 2). Since pre-prepared samples were annealed in various atmospheres, it can be assumed that oxygen defects are mainly on the surface of phosphate grains. It was found with the use of Raman spectroscopy [17]
3. Results and discussion Fig. 1 shows the XRD patterns for pure polycrystalline LiMgPO4 synthesized under different conditions, as well as for LiMgPO4:Tb (0.1%, 0.25% 0.5%, 0.7%). All the reflections are well indexed to an olivine-type orthorhombic structure with the space group Pnma (ICDD file 0–084-0342). For all samples except LiMgPO4 with 0.7% Tb, no impurities are detected. For the latter compound, a peak belonging to TbPO4 is observed at 2Θ = 19.365°. The solubility limit of terbium in lithium magnesium phosphate close to 0.5% agrees well with the results of other works [3,6]. Note that the authors of Ref. [26] reported that about 10% of Tb can be introduced into LiMgPO4, which seems implausible. To clarify the terbium site in the LiMgPO4 structure, ab initio calculations were carried out. Our calculations for a 112-atom supercell of LiMgPO4 without charge compensation predict that Tb would prefer to replace Mg with a high energy gain of 3.0 eV that corresponds to a difference in the ionic radii of doping Tb3+ (0.93) and substituted Li+ (0.58) or Mg2+ (0.72). The formation of compensating vacancies can change the preferred site. The terbium substitution is accompanied by the formation of defects, which provide the charge compensation. To keep the electroneutrality of the cell, one lithium vacancy (□Li) is 2
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al.
Fig. 4. The TL glow curves of LiMgPO4 synthesized in various ways.
Fig. 2. The absorbance spectrum of LiMgPO4 synthesized in various gas atmospheres.
Fig. 5. The TL spectra of LiMgPO4:Tb exposed to 3 Gy X-ray. Inset: The TL glow curves of LiMgPO4:Tb.
Fig. 3. The intensity of the glow curve components (peaks) depending on the annealing atmosphere. Inset: The TL glow curves of LiMgPO4 annealed at different atmospheres after irradiation with the dose of 2 Gy.
that, in addition to oxygen vacancies, carbon-containing groups are also present on the LiMgPO4 surface. The impact of these surface defects on the thermoluminescence is demonstrated in Fig. 3, which presents the glow curves for the samples annealed in argon, oxygen, and air. It is seen that the TL glow curve consists of overlapping peaks in temperature range of 300–600 K. There are three rather intense peaks at 365, 410 and 570 K and two peaks with a lower intensity at about 460 and 480 K, which are difficult to separate. Note that the TL glow curves shown in Fig. 3 (inset) are similar to those obtained in [2] for single
Fig. 6. The TL spectra of LiMgPO4 exposed to 3 Gy X-ray.
crystals grown by the micro pulling down (MPD) technique. The complex form of the TL glow curves evidently shows that there is a large number of various traps in LiMgPO4, which are associated with intrinsic defects and defects arising during irradiation. It is owing to these 3
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al.
Fig. 7. The RL and TL spectra of LiMgPO4 and LiMgPO4:Tb.
defects capable of storing energy that the dosimetric application of LiMgPO4 becomes possible. Fig. 4 shows the thermoluminescent glow curves of LiMgPO4 synthesized in different conditions (solid state reaction, spray pyrolysis, melting) and irradiated with a dose of 3 Gy. The samples with the same composition radically differ in the specific surface area (BET). For the solid state and spray pyrolysis LiMgPO4, the specific surface area is 0.525 m2/g and 8.001 m2/g, respectively. For a sample obtained by melting of a powder, this value is negligibly small (~0.001 m2/g). Obviously, the larger is the surface, the higher is the concentration of surface defects in the sample. From Fig. 4 it can be clearly seen that the maximum intensity of thermoluminescence is observed for the molten and quenched sample, while the minimum intensity is found for the nanosized sample obtained by the spray pyrolysis method. The thermoluminescence of pure lithium-magnesium phosphate is insufficient for its application in dosimeters. To increase the intensity, terbium ions are most often introduced. The effect of terbium on the thermoluminescence of lithium-magnesium phosphate is clearly shown in Fig. 5 (inset). In addition to a sharp increase in TL intensity for LiMgPO4: Tb, a simplification of the glow curves shape can be noted. There is a single wide peak with a maximum at ≈500 K. This agrees with the results of other works [9,27] and apparently indicates the existence of only one dominant trap type. For a better understanding of the thermoluminescence in LiMgPO4 and LiMgPO4:Tb, we recorded the TL spectra (Figs. 5–7). In Fig. 6, a 3D spectrum for a pure matrix LiMgPO4 is given. At all temperatures, it consists of two bands with centers at 375 nm (UV) and 625 nm (red), the intensity of which varies in accordance with the TL glow curve (Fig. 3). Transitions causing thermoluminescence are due to intrinsic defects in LiMgPO4. The main features of the LiMgPO4: Tb spectrum are determined by intra-4f-shell transitions in the terbium ion (Tb3+ 4f8): 5D3–7F5 (417 nm), 5D3–7F4 (438 nm), 5D4–7F6 (490 nm), 5D4–7F5 (543 nm), 5 D4–7F4 (591 nm), 5D4–7F3 (625 nm). Transitions are indicated on the spectrum recorded at 140 °C (Fig. 7). The dotted line shows the TL spectrum of the matrix recorded at the same temperature. It is clearly seen that two broad spectral lines characteristic of pure LiMgPO4 are also present in the spectrum of LiMgPO4: Tb. Thus, we can conclude that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals of the matrix and activator ions. The thermoluminescence spectra (Figs. 5, 6) reflect the release of
energy stored during irradiation, but for LiMgPO4 and LiMgPO4:Tb there is also a direct response to x-ray irradiation. We have measured the optical spectra using X-ray excitation at room temperature (Fig. 7). The energy of X-rays is high enough to excite all optical transitions not only of activator ions, but also of the host crystal. There is an exceptional similarity between the thermoluminescence and x-ray luminescence spectra of LiMgPO4 and LiMgPO4:Tb, including the additive character of the latter. 4. Conclusions An experimental study was carried out to demonstrate that defects of different origin can be observed in LiMgPO4. We found that terbium replaces magnesium with the formation of a bound lithium vacancy. In this study, the samples of LiMgPO4 synthesized by the solid state reaction method were additionally treated in argon and oxygen atmospheres. Besides, for the synthesis of samples with different specific surfaces, ultrasonic spray pyrolysis, as well as melting and quenching methods were used. TL of the samples exposed to 2 Gy X-ray irradiation was found to correlate with the oxygen content in the applied atmosphere and with the surface area. We found out that the TL spectra of LiMgPO4:Tb are additively composed of the signals of the matrix and activator ions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Russian Foundation for Basic Research (Grant No. 18-08-00093 -a) and AAAA-A19-119031890025-9, AAAA-A19-119062590007-2 programs. References [1] N.S. Rawat, B. Dhabekar, K.P. Muthe, D.K. Koul, D. Datta, Nucl. Instrum. Meth. Phys. Res., B 397 (2017) 27. [2] W. Gieszczyk, D. Kulig (Wrobel), P. Bilski, B. Marczewska, M. Kłosowski, Rad. Meas. 85 (2016) 88.
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Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al. [3] B. Dhabekar, S.N. Menon, E. Alagu Raja, A.K. Bakshi, A.K. Singh, M.P. Chougaonkar, Y.S. Mayya, Nucl. Instr, Methods Phys. B 889 (2011) 1844. [4] M. Kumar, B. Dhabekar, S.N. Menon, M.P. Chougaonkar, Y.S. Mayya, Nucl. Instrum, Meth. Phys. B 269 (2011) 1849. [5] M. Gai, Z. Chen, Y. Fan, S.-Y. Yan, Y.-X. Xie, J. Wang, Y. Zhang, Radiat. Meas. 78 (2015) 48. [6] B. Marczewska, P. Bilski, D. Wrobel, M. Kłosowski, Radiat. Meas. 90 (2016) 265. [7] S.N. Menon, A.K. Singh, B. Dhabekar, M.P. Chougaonkar, D.A.R. Babu, J. Lumines. Appl. 4 (2014) 92. [8] W. Gieszczyk, D. Kulig Wróbel, P. Bilski, B. Marczewska, M. Kłosowski, Radiat. Meas. 106 (2017) 100. [9] N.S. Bajaj, C.B. Palan, K.A. Koparkar, M.S. Kulkarni, S.K. Omanwar, J. Lumines. 175 (2016) 9. [10] B. Marczewska, A. Sas-Bieniarz, P. Bilski, W. Gieszczyk, M. Kłosowski, M. Sądel, Radiat. Meas. 129 (2019) 106205. [11] W. Gieszczyk, B. Marczewska, M. Kłosowski, A. Mrozik, P. Bilski, A. Sas-Bieniarz, P. Goj, P. Stoch, Materials 12 (2019) 2861. [12] C.B. Palan, N.S. Bajaj, A. Soni, S.K. Omanwar, Bull. Mater. Sci. 39 (2016) 1157. [13] M. Gai, Z. Chen, Y. Fan, J. Wan, J. Rare Earths 31 (2013) 551. [14] A. Sas-Bieniarz, B. Marczewska, M. Kłosowski, P. Bilski, W. Gieszczyk, J. Lumines 218 (2020) 116839.
[15] S.N. Menon, T.K. Gundu Rao, D.K. Koul, S. Watanabe, Radiat Eff. Defects Solids 173 (2018) 210. [16] D.G. Kellerman, N.I. Medvedeva, M.O. Kalinkin, A.I. Syurdo, V.G. Zubkov, J. Alloy. Compd. 766 (2018) 626. [17] M.O. Kalinkin, R.M. Abashev, E.V. Zabolotskaya, I.V. Baklanova, A.I. Surdo, D.G. Kellerman, Mater. Res. Express 6 (2019) 046206. [18] N.I. Medvedeva, D.G. Kellerman, M.O. Kalinkin, Mater. Res. Express 6 (2019) 106304. [19] V.S. Kortov, S.V. Zvonarev, V.A. Pustovarov, A.I. Slesarev, Rad. Meas. 61 (2014) 74. [20] B. Dhabekar, E.A. Raja, S. Menon, G. Rao, R.K. Kher, B.C. Bhatt, J. Phys. D Appl. Phys. 41 (2008) 115414. [21] Sangeeta, K. Chennakesavulu, D.G. Desai, S.C. Sabharwal, Mary Alex, M.D. Ghodgaonkar, Nucl. Instrum. Meth. Phys. Res., Sect. A 571 (2007) 699. [22] M. Erfani, Haghiri, E. Saion, N. Soltani, W. Saffiey wan Abdullah, Adv. Mater. Res. 832 (2014) 189. [23] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) (1999) 1758. [24] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [25] J.P. Perdew, S. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [26] P.C. de Sousa Filho, O.A. Serra, J. Phys. Chem. C 115 (2011) 636. [27] S.N. Menon, B. Dhabekar, E. Alagu Raja, M.P. Chougaonkar, Rad. Meas. 47 (2012) 236.
5
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Influence of defects on thermoluminescence in pristine and doped LiMgPO4 a,⁎
b,c
b,c
a
a
M. Kalinkin , R. Abashev , A. Surdo , N. Medvedeva , D. Kellerman a b c
T
Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, Ekaterinburg, Russia Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Ekaterinburg, Russia Department of Experimental Physics, Ural Federal University, Ekaterinburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Defects Phosphates Dosimetry Thermoluminescence LiMgPO4
Pure and terbium-doped lithium-magnesium phosphate LiMgPO4 was investigated with respect to its thermoluminescent properties. The samples were synthesized by solid state, ultrasonic spray pyrolysis, as well as by melting and quenching methods. To predict the terbium site, ab initio calculations were carried out. Additional treatment in atmospheres with different oxygen partial pressures was used. The larger is the concentration of oxygen in the atmosphere and, accordingly, the smaller is the quantity of surface oxygen vacancies, the greater is the intensity of thermoluminescence. For a better understanding of thermoluminescence in LiMgPO4 and LiMgPO4:Tb, the TL spectra were recorded. It was concluded that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals of the matrix and activator ions.
1. Introduction
2. Experimental
The lithium-magnesium phosphate LiMgPO4 with olivine structure is one of the most promising matrices for ionizing radiation detectors [1–8]. Recently, it was considered as an alternative to the commercially available dosimetric material Al2O3:C [3]. Most often, terbium ion (Tb3+) is used as an activator of thermally or optically stimulated luminescence (TL, OSL) in LiMgPO4; sometimes, together with terbium, boron ions are introduced into the lithium magnesium phosphate lattice [9–10]. Different suggestions regarding the terbium site have been considered in the experimental works, in which Tb3+ was assumed to replace only the Li+ site [11] or only the Mg2+ site [12]. There are some reports on the properties of dosimetric material based on LiMgPO4 doped with Eu, Sm, and Tm [5,13,14]. However, even without activators, pure phosphate exhibits the thermoluminescent properties formed due to defects [14–18], although the dose sensitivity of such material is significantly lower than that of activated one. A similar situation is described for Al2O3 [19], LiAlO2 [20], Li2B4O7 [21], and CaB4O7 [22]. In this paper, we discuss the effect of surface and bulk defects on the thermoluminescence of pure and terbium-doped lithium-magnesium phosphate and show that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals from the matrix and activator ions.
2.1. Preparation of samples
⁎
Series of samples of pristine phosphate LiMgPO4 and those doped with terbium were synthesized by the conventional solid state reaction method. The stoichiometric amounts of raw materials Li2CO3 (lithium carbonate, 99.99%), 3MgCO3*Mg(OH)2*3H2O (magnesium carbonate, 99.9%), NH4(H2)PO4 (ammonium dihydrogen phosphate, 99.99%), and TbCl3 (terbium chloride, 99.99%) were mixed and milled until the average particle size was less than 1 mm. The mixture was pre-heated in platinum crucibles in air at 300 °C for 5 h. After the second homogenization in an agate mortar, the sample was heated to 500 °C and kept at this temperature for 20 h. After the third homogenization, the sample was pressed into a tablet and thermally treated at 800–900 °C for 20 h in gas atmospheres of air, Ar, and O2. To obtain a high-density sample, melting followed by quenching was carried out. The tablet of LiMgPO4 was melted in ambient air at 1250 °C for 1 h in alumina crucible. The melt was quenched by a pair of stainless-steel plates and formed into the polycrystalline film with a thickness of 0.2 mm. For ultrasonic spray pyrolysis synthesis, all the components taken in stoichiometric ratio for LiMgPO4 were dissolved in diluted nitric acid. Synthesis was carried out from a solution with a concentration of 100 g/l at 850 °C; the air consumption was of 14.6 ml/s.
Corresponding author. E-mail address: [email protected] (M. Kalinkin).
https://doi.org/10.1016/j.nimb.2019.12.021 Received 30 October 2019; Received in revised form 26 November 2019; Accepted 20 December 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al.
2.2. Characterisation of samples The X-ray diffraction data were collected at room temperature using a STADI-P transmission diffractometer (STOE) equipped with a scintillation detector. The phase purity of the sample was proved by comparing its XRD pattern with that in the PDF2 database (ICDD, USA, release 2016). The specific surface area of the powder samples was obtained using a Gemini VII Analyzer (Micromeritics) from the N2 adsorption–desorption isotherms and was calculated by the Brunauer–Emmett–Teller (BET) method. The diffuse absorbance spectrum was obtained with a Shimadzu UV-2401 PC UV–VIS spectrophotometer using BaSO4 as a reference. 2.3. Ab initio calculations To predict the terbium site, we carried out ab initio calculations using the density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) within the projector augmented waves (PAW) formalism [23,24]. Exchange and correlation potential was described by the generalized-gradient approximation (GGA) with the Perdew–Becke–Ernzerhof functional [25]. The kinetic energy cutoff of 500 eV was set for the plane-wave expansion and the Brillouin zones were sampled with the Г-centered 4x4x4 k-point grids. We employed a 112-atom supercell, where the substituting Tb atom corresponds to a doping concentration of 0.9 at.%. Due to computational problems, we did not study a smaller amount of Tb doping, which requires significantly larger supercells. Structural optimization, when only atomic positions were relaxed with the convergence criterion for the total energy and interatomic forces of 10−5 eV and 0.02 eV/Å, respectively, was used. 2.4. Radioluminescence and thermoluminescence measurements TL and RL measurements were carried out by exciting the samples with different radiation doses from an Eclipse IV Lab Rh X-ray tube (Oxford Instruments, Ua = 0–45 kV, I = 0–45 μA, Rh anode). All experiments were carried out at Ua = 30 kV and I = 30 μA. The TL and RL emission was recorded using a Varian Cary Eclipse spectrophotometer. The TL glow curves and spectra were collected using an automated laboratory setup, which provided the regimes of linear heating from 50 to 900 °C or isothermal treatment of the sample at any temperature within the given range. 10 mg samples placed in a silver cup were used for the measurements.
Fig. 1. The XRD patterns of LiMgPO4 synthesized in various ways and LiMgPO4:Tb.
necessary for the substitution of Tb3+ for Mg2+. Two configurations are possible for replacing Li+ by Tb3+, when one vacancy is in the Mg site (□Mg) and when there are two lithium vacancies (2*□Li). The conclusions on the preferred configuration were made from a comparison of their total energies, taking into account the chemical potentials of magnesium and lithium as simple metals in stable states. We find that the Tb3+ → Mg2+ substitution remains the most energetically preferable one even taking into account the formation of a compensating lithium vacancy. The configuration with the lithium vacancy closest to the terbium ion is by 0.14 eV lower in energy than its distant position, which indicates attractive bonding in the Tb3+- □Li pair. Replacement of Tb3+ → Li+ by two lithium vacancies (2*□Li) is less favorable by 2.98 eV. The Tb3+ → Li+ substitution, accompanied by the appearance of a magnesium vacancy, has an even higher energy of 1.3 eV and is the most unlikely one among the considered configurations. Thus, we believe that terbium with a concentration of ~1 at.% should replace magnesium with the formation of a bound lithium vacancy. In addition to lithium vacancies, the LiMgPO4 lattice can contain oxygen ones, both charged and neutral. Their energy levels are located inside the band gap of 5.5 eV [16]. This is well illustrated by the diffuse absorption spectra, in which, along with the fundamental absorption edge (≈220 nm), an additional band is clearly visible. It starts at ≈320 nm and is most pronounced for the sample annealed in argon when the oxygen vacancies are most probable. In oxygen atmosphere, this band almost disappears (Fig. 2). Since pre-prepared samples were annealed in various atmospheres, it can be assumed that oxygen defects are mainly on the surface of phosphate grains. It was found with the use of Raman spectroscopy [17]
3. Results and discussion Fig. 1 shows the XRD patterns for pure polycrystalline LiMgPO4 synthesized under different conditions, as well as for LiMgPO4:Tb (0.1%, 0.25% 0.5%, 0.7%). All the reflections are well indexed to an olivine-type orthorhombic structure with the space group Pnma (ICDD file 0–084-0342). For all samples except LiMgPO4 with 0.7% Tb, no impurities are detected. For the latter compound, a peak belonging to TbPO4 is observed at 2Θ = 19.365°. The solubility limit of terbium in lithium magnesium phosphate close to 0.5% agrees well with the results of other works [3,6]. Note that the authors of Ref. [26] reported that about 10% of Tb can be introduced into LiMgPO4, which seems implausible. To clarify the terbium site in the LiMgPO4 structure, ab initio calculations were carried out. Our calculations for a 112-atom supercell of LiMgPO4 without charge compensation predict that Tb would prefer to replace Mg with a high energy gain of 3.0 eV that corresponds to a difference in the ionic radii of doping Tb3+ (0.93) and substituted Li+ (0.58) or Mg2+ (0.72). The formation of compensating vacancies can change the preferred site. The terbium substitution is accompanied by the formation of defects, which provide the charge compensation. To keep the electroneutrality of the cell, one lithium vacancy (□Li) is 2
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
M. Kalinkin, et al.
Fig. 4. The TL glow curves of LiMgPO4 synthesized in various ways.
Fig. 2. The absorbance spectrum of LiMgPO4 synthesized in various gas atmospheres.
Fig. 5. The TL spectra of LiMgPO4:Tb exposed to 3 Gy X-ray. Inset: The TL glow curves of LiMgPO4:Tb.
Fig. 3. The intensity of the glow curve components (peaks) depending on the annealing atmosphere. Inset: The TL glow curves of LiMgPO4 annealed at different atmospheres after irradiation with the dose of 2 Gy.
that, in addition to oxygen vacancies, carbon-containing groups are also present on the LiMgPO4 surface. The impact of these surface defects on the thermoluminescence is demonstrated in Fig. 3, which presents the glow curves for the samples annealed in argon, oxygen, and air. It is seen that the TL glow curve consists of overlapping peaks in temperature range of 300–600 K. There are three rather intense peaks at 365, 410 and 570 K and two peaks with a lower intensity at about 460 and 480 K, which are difficult to separate. Note that the TL glow curves shown in Fig. 3 (inset) are similar to those obtained in [2] for single
Fig. 6. The TL spectra of LiMgPO4 exposed to 3 Gy X-ray.
crystals grown by the micro pulling down (MPD) technique. The complex form of the TL glow curves evidently shows that there is a large number of various traps in LiMgPO4, which are associated with intrinsic defects and defects arising during irradiation. It is owing to these 3
Nuclear Inst. and Methods in Physics Research B 465 (2020) 1–5
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Fig. 7. The RL and TL spectra of LiMgPO4 and LiMgPO4:Tb.
defects capable of storing energy that the dosimetric application of LiMgPO4 becomes possible. Fig. 4 shows the thermoluminescent glow curves of LiMgPO4 synthesized in different conditions (solid state reaction, spray pyrolysis, melting) and irradiated with a dose of 3 Gy. The samples with the same composition radically differ in the specific surface area (BET). For the solid state and spray pyrolysis LiMgPO4, the specific surface area is 0.525 m2/g and 8.001 m2/g, respectively. For a sample obtained by melting of a powder, this value is negligibly small (~0.001 m2/g). Obviously, the larger is the surface, the higher is the concentration of surface defects in the sample. From Fig. 4 it can be clearly seen that the maximum intensity of thermoluminescence is observed for the molten and quenched sample, while the minimum intensity is found for the nanosized sample obtained by the spray pyrolysis method. The thermoluminescence of pure lithium-magnesium phosphate is insufficient for its application in dosimeters. To increase the intensity, terbium ions are most often introduced. The effect of terbium on the thermoluminescence of lithium-magnesium phosphate is clearly shown in Fig. 5 (inset). In addition to a sharp increase in TL intensity for LiMgPO4: Tb, a simplification of the glow curves shape can be noted. There is a single wide peak with a maximum at ≈500 K. This agrees with the results of other works [9,27] and apparently indicates the existence of only one dominant trap type. For a better understanding of the thermoluminescence in LiMgPO4 and LiMgPO4:Tb, we recorded the TL spectra (Figs. 5–7). In Fig. 6, a 3D spectrum for a pure matrix LiMgPO4 is given. At all temperatures, it consists of two bands with centers at 375 nm (UV) and 625 nm (red), the intensity of which varies in accordance with the TL glow curve (Fig. 3). Transitions causing thermoluminescence are due to intrinsic defects in LiMgPO4. The main features of the LiMgPO4: Tb spectrum are determined by intra-4f-shell transitions in the terbium ion (Tb3+ 4f8): 5D3–7F5 (417 nm), 5D3–7F4 (438 nm), 5D4–7F6 (490 nm), 5D4–7F5 (543 nm), 5 D4–7F4 (591 nm), 5D4–7F3 (625 nm). Transitions are indicated on the spectrum recorded at 140 °C (Fig. 7). The dotted line shows the TL spectrum of the matrix recorded at the same temperature. It is clearly seen that two broad spectral lines characteristic of pure LiMgPO4 are also present in the spectrum of LiMgPO4: Tb. Thus, we can conclude that the thermoluminescence of LiMgPO4:Tb is additively composed of the signals of the matrix and activator ions. The thermoluminescence spectra (Figs. 5, 6) reflect the release of
energy stored during irradiation, but for LiMgPO4 and LiMgPO4:Tb there is also a direct response to x-ray irradiation. We have measured the optical spectra using X-ray excitation at room temperature (Fig. 7). The energy of X-rays is high enough to excite all optical transitions not only of activator ions, but also of the host crystal. There is an exceptional similarity between the thermoluminescence and x-ray luminescence spectra of LiMgPO4 and LiMgPO4:Tb, including the additive character of the latter. 4. Conclusions An experimental study was carried out to demonstrate that defects of different origin can be observed in LiMgPO4. We found that terbium replaces magnesium with the formation of a bound lithium vacancy. In this study, the samples of LiMgPO4 synthesized by the solid state reaction method were additionally treated in argon and oxygen atmospheres. Besides, for the synthesis of samples with different specific surfaces, ultrasonic spray pyrolysis, as well as melting and quenching methods were used. TL of the samples exposed to 2 Gy X-ray irradiation was found to correlate with the oxygen content in the applied atmosphere and with the surface area. We found out that the TL spectra of LiMgPO4:Tb are additively composed of the signals of the matrix and activator ions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Russian Foundation for Basic Research (Grant No. 18-08-00093 -a) and AAAA-A19-119031890025-9, AAAA-A19-119062590007-2 programs. References [1] N.S. Rawat, B. Dhabekar, K.P. Muthe, D.K. Koul, D. Datta, Nucl. Instrum. Meth. Phys. Res., B 397 (2017) 27. [2] W. Gieszczyk, D. Kulig (Wrobel), P. Bilski, B. Marczewska, M. Kłosowski, Rad. Meas. 85 (2016) 88.
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