Luminescence properties of Na₃LuSi₃O₉:Ce³⁺ as a potential scintillator material

Abstract

Lutetium based oxyorthosilicate (Lu₂SiO₅:Ce³⁺/Lu_2−xY_xSiO₅:Ce³⁺) and pyrosilicate (Lu₂Si₂O₇:Ce³⁺) scintillators exhibit good scintillation performances in terms of light-yield and decay properties. Nevertheless, both of them have a high melting point (≥1900 °C) which is unfavorable to crystal growth. In this work, an alternative lutetium based silicate scintillator was proposed with a lower melting point (about 1500 °C): Na₃LuSi₃O₉:Ce³⁺. The structure and thermal quenching properties of Na₃LuSi₃O₉:Ce³⁺ were investigated. The emission spectra of Na₃Lu_0.99Ce_0.01Si₃O₉ under UV and X-ray excitation as well as pulsed X-ray measurements were also determined. The phosphors show a broad emission band from 320 nm to 475 nm. Due to the suitable emission wavelength range, the fast decay properties and relative low melting point, Na₃LuSi₃O₉:Ce³⁺ may be a potential novel inorganic scintillator material.

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Introduction

Scintillation materials have been developed rapidly in recent years because they have been widely used in nuclear medical equipment,¹ high energy physics, the detection of high energy rays and non-destructive material testing,² among which Ce³⁺ doped scintillators play an important role due to the high light output and fast scintillation decay. Ce³⁺ has parity allowed f–d transitions which makes it a special and important lanthanide ion. This parity-allowed feature ensures that the Ce³⁺ can absorb energy efficiently and has an intense emission with a very short decay time (usually dozens of ns),³ although the f–d transition of Ce³⁺ has a great dependence on the crystal field of the host.

Rare earth silicate-based materials have attracted much attention due to their high chemical and physical stability,⁴ excellent thermal stability and high luminescence efficiency.⁵ Many Ce³⁺ doped silicates have already been studied thoroughly due to their good properties, such as Lu₂SiO₅:Ce,^6,7 Lu₂Si₂O₇:Ce,⁸ Y₂SiO₅:Ce,⁹ Y₂Si₂O₇:Ce¹⁰ and Gd₂Si₂O₇:Ce.¹¹ Lutetium based oxyorthosilicate (Lu₂SiO₅:Ce³⁺/Lu_2−xY_xSiO₅:Ce³⁺) and pyrosilicate (Lu₂Si₂O₇:Ce³⁺) scintillators present high light-yield, good energy resolution and fast decay time properties, but both of them have high melting point which is unfavorable to crystal growth. Among silicates, Na₃YSi₃O₉ is an interesting host for rare earth activated phosphors, such as a green-emitting phosphor (Na₃YSi₃O₉:Tb³⁺) for plasma display panels (PDP),¹² a single-phased white-emitting phosphor (Na₃YSi₃O₉:Tm³⁺, Dy³⁺) for light emission diodes (LEDs)¹³ and a red-emitting phosphor (Na₃YSi₃O₉:Eu³⁺).¹⁴ The concentration of Eu³⁺ in Na₃YSi₃O₉ at which quenching occurs at extraordinarily high concentration due to the isolated YO₆ octahedra in the Na₃YSi₃O₉ host.¹⁴ This is attractive information for the rare earth doping with high concentration to improve luminescence intensity. As we knew, the synthesis and luminescence properties of Na₃LuSi₃O₉:Ce³⁺ have not been reported.

In this work, a series of Na₃LuSi₃O₉:Ce³⁺ samples were prepared with traditional high temperature solid-state reaction method. And the structures of as-synthesized samples were refined and their luminescence and decay properties under X-ray excitation were investigated.

Experimental

Synthetic procedures

Na₃Lu_(1−x)Ce_xSi₃O₉ samples were prepared with a high temperature solid-state reaction method in different atmosphere (air and CO reducing atmosphere). The raw materials included Na₂CO₃ (analytical reagent, A.R.), SiO₂ (4 N) and Lu₂O₃ (4 N). Thus, two series of products synthesized in different conditions were obtained: synthesized in air and in CO reducing atmosphere.

(a) The starting materials according to the composition Na₃Lu_(1−x)Ce_xSi₃O₉ (x = 0, 0.005, 0.01, 0.03, 0.05, 0.07, 0.09) were thoroughly ground and calcined at 1150 °C for 8 h in air;

(b) The starting materials according to the composition Na₃Lu_(1−x)Ce_xSi₃O₉ (x = 0, 0.005, 0.01, 0.03, 0.05, 0.07, 0.09) were thoroughly ground and calcined at 1150 °C for 8 h under CO reducing atmosphere.

Measurements

The phase purity of the samples was examined by X-ray diffraction (XRD) using a BRUKER D8 ADVANCE powder diffractometer with Cu K-alpha radiation (λ = 0.15418 nm) operating at 40 kV and 40 mA. Data for Rietveld refinement were collected over a 10°–100° 2θ range at an interval of 0.02°. The structure analysis of Na₃Lu_0.91Ce_0.09Si₃O₉ was carried out using the Topas Academic software.¹⁵

The UV excitation and emission spectra were recorded on an Edinburgh FLS920 spectrometer equipped with a CTI-cryogenics temperature control system and a 450 W xenon lamp was used as the excitation source.

Radioluminescence (RL) spectra measurements were obtained by irradiating the samples with a Philips X-ray tube with tungsten anode set at 30 kV and 30 mA at room temperature. The emitted light was collected via an optical fibers and detected by an Andor Newton 970 CCD camera coupled to a monochromator (Andor Shamrock 500i) working in the 200–1000 nm interval. Radioluminescence (RL) time decay of the samples was obtained with the Pico-X (voltage 30 kV, laser frequency 1 MHz), without using any filter in front of the photomultiplier. The accumulation time for the decays was set to 10 minutes.

Results and discussion

XRD characterization and structure analysis

To confirm the structure and phase purity, Rietveld refinement for Na₃Lu_0.91Ce_0.09Si₃O₉ sample was performed using the diffraction data (Fig. 1). It is obvious that the sample has good phase purity. The XRD patterns of Na₃Lu_(1−x)Ce_xSi₃O₉ with different Ce³⁺ content were very similar. Thus only the XRD pattern of Na₃Lu_0.91Ce_0.09Si₃O₉ was presented. The melting point of Na₃LuSi₃O₉ was confirmed to be about 1500 °C which is much lower than that of Lu₂SiO₅ (2100 °C) and Lu₂Si₂O₇ (1900 °C).¹⁶ And the density of the investigated composition was calculated to be 3.65 g cm⁻³.

Fig. 1 Experimental (red crosses) and calculated (green solid line) XRD patterns and their difference (blue solid line) for Na₃Lu_0.91Ce_0.09Si₃O₉. The inset shows the unit cell of the Na₃LuSi₃O₉ structure. The LuO₆ octahedra (blue) and SiO₄ tetrahedra (cyan) are highlighted. R_wp = 2.829%, R_B = 0.902%.

Na₃LuSi₃O₉ crystallizes in the orthorhombic space group P2₁2₁2₁ and it is characterized by four non-equivalent Lu sites. One interesting result can be noted from the refined structure parameters is that 90% Ce³⁺ ions enter in the Lu1 and Lu4 sites (29% in Lu1 site and 61% in Lu4 site) and the remaining mostly embedded in Lu3 site. Ce³⁺ ions prefer to embed in big sites because the radius of Ce³⁺ is bigger than that of Lu³⁺. It can be seen from Table 1 that the average Lu–O bond distance is the biggest in Lu4 site (2.263(8) Å) and then Lu1 site (2.250(8) Å). The inset in Fig. 1 displays unit cell of the Na₃LuSi₃O₉ structure. It is obvious that the distorted LuO₆ octahedra are discrete and they are separated from one another by the SiO₄ tetrahedra.

Table 1 Lu³⁺ sites and Lu–O bond lengths

Bond	Distance (Å)	Bond	Distance (Å)
Lu1–O34	2.009(8)	Lu2–O25	2.105(7)
Lu1–O17	2.276(7)	Lu2–O32	2.204(7)
Lu1–O3	2.288(7)	Lu2–O31	2.216(7)
Lu1–O33	2.306(8)	Lu2–O27	2.275(7)
Lu1–O1	2.309(7)	Lu2–O11	2.280(7)
Lu1–O22	2.312(8)	Lu2–O24	2.287(7)
Average	2.250(8)	Average	2.227(7)
Lu3–O4	2.087(7)	Lu4–O23	2.168(8)
Lu3–O30	2.206(7)	Lu4–O18	2.179(8)
Lu3–O35	2.215(7)	Lu4–O2	2.184(7)
Lu3–O15	2.232(7)	Lu4–O29	2.274(7)
Lu3–O8	2.270(7)	Lu4–O9	2.345(8)
Lu3–O6	2.351(7)	Lu4–O36	2.427(7)
Average	2.226(7)	Average	2.263(8)

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Photoluminescence properties of Na₃LuSi₃O₉: 1.0 at% Ce³⁺

To investigate the photoluminescence properties of Na₃Lu_0.99Ce_0.01Si₃O₉ at room temperature, the normalized excitation (λ_em = 375 nm, 380 nm and 390 nm) and emission (λ_ex = 310 nm, 350 nm and 360 nm) spectra of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO are displayed in Fig. 2.

Fig. 2 Normalized excitation (λ_em = 375 nm, 380 nm and 390 nm) and emission (λ_ex = 310 nm, 350 nm and 360 nm) spectra of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO reducing atmosphere. The dot line is normalized excitation spectrum (λ_em = 395 nm) of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air.

For comparison, the excitation spectrum (λ_em = 395 nm, dot line) of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air was also given in Fig. 2. The broad excitation band from 250 nm to 330 nm was attributed to the absorption of oxygen defects in the host. Because the intensity of this band decreased significantly after recalcination in air and the broad band nearly disappeared when directly synthesized in air (dot line in Fig. 2). The centers of excitation bands of Ce³⁺ from 4f to lowest 5d state locate at 350 nm and 360 nm for Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO and air, respectively. The reason for the different excitation positions of Ce³⁺ in Na₃Lu_0.99Ce_0.01Si₃O₉ under different atmosphere may be that the oxygen defects changed the crystal field, which led to different Ce³⁺ 5d level splitting. The energy level of the lowest 5d orbit became higher and the excitation band of Ce³⁺ shifted to shorter wavelength when synthesized in CO.

The centers of emission bands of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO locate at 375 nm, 380 nm and 390 when excited at 310 nm, 350 nm, and 360 nm, respectively. But when synthesized in air, the emission spectra of Na₃Lu_0.99Ce_0.01Si₃O₉ overlapped very well when the excitation wavelength changed. This obvious redshift was mainly caused by oxygen defects in the host rather that Ce³⁺ at different sites.

The influence of temperature on the luminescence of Na₃LuSi₃O₉: 1.0 at% Ce³⁺

The UV emission spectra of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air under 360 nm excitation were determined from 300 K to 500 K, as shown in Fig. 3A. It can be observed that the emission intensity decreased and the FWHM broadened with the temperature increasing from 300 K to 500 K. The emission intensity of Na₃Lu_0.99Ce_0.01Si₃O₉ at 500 K was 26.5% of its original value at 300 K. To fully understand the temperature dependence of the emission intensity and to determine the activation energy for thermal quenching, the Arrhenius equation¹⁷ (eqn. (1)) was fitted to the thermal quenching data of Na₃Lu_0.99Ce_0.01Si₃O₉ and the graph of ln[(I₀/I_T) − 1] vs. 1/T was plotted in Fig. 3B.

I_T = I₀/(1 + c exp(−ΔE/kT)) (1)

Fig. 3 (A) Temperature dependence on the photoluminescence emission of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air (λ_ex = 360 nm); (B) Arrhenius plot of the temperature dependence of the PL emission intensity of Na₃Lu_0.99Ce_0.01Si₃O₉.

In eqn (1), I₀ is the initial emission intensity, I_T is the intensity at a given temperature T, c is a constant for a certain host, k is Boltzmann constant (8.629 × 10⁻⁵ eV), ΔE is activation energy of thermal quenching which represents the energy difference between the lowest 5d excited state and the bottom of the host lattice conduction band.¹⁸ The activation energy was calculated to be 0.3386 eV.

X-ray excited luminescence (XEL) properties of Na₃LuSi₃O₉: 1.0 at% Ce³⁺

Fig. 4 displays the comparison between the X-ray excited luminescence spectra of the Bi₄Ge₃O₁₂ (BGO) and Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO reducing atmosphere. As clearly visible, the amplitude is rather similar for the two samples. However, the intensity, i.e. the area underneath the curves, is surely going to be larger (at least by a factor 2) in the case of BGO because of its wider emission spectrum. The slight kink at about 480 nm visible in the BGO spectrum is likely related to a non-perfect correction of the raw data.

Fig. 4 X-ray excited luminescence spectra (RT) of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in CO and BGO powders. The spectra have been corrected for the system response.

Scintillation decay properties of Na₃LuSi₃O₉: 1.0 at% Ce³⁺

Fig. 5 shows the scintillation decay curve of Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air under X-ray excitation. The curve can be well fitted with a single exponential equation:

I_t = A + I₀ exp(−t/τ) (2)

where I_t and I₀ are the luminescence intensity, A is a constant, t is the time and τ is the decay time. The value of τ was calculated to be 31.27 ns which is much faster than that of BGO (about 300 ns).¹⁹

Fig. 5 Pulsed X-ray measurements (RT) performed on Na₃Lu_0.99Ce_0.01Si₃O₉ synthesized in air.

The decay curve was characterized by a very fast beginning which is not related to the sample, but likely to the instrumentation. This peak intensity, in fact, appears to be practically independent on the sample emitted light, and the same pulse was also obtained by completely removing the sample holder. This peak has a full width at half maximum of about 1 ns, which seems rather close to the PMT time response. This signal is, however, not related to the optical excitation given by the laser, since no light was detected by the PMT with the X-ray tube shut down. It seems reasonable that this signal could be due to a direct hit of the pulsed X-rays on the PMT window.

Conclusions

A series of Na₃Lu_(1−x)Ce_xSi₃O₉ phosphors were prepared with a high temperature solid-state reaction method under different atmosphere (air and CO reducing atmosphere). The influence of temperature on the luminescence of Na₃Lu_0.99Ce_0.01Si₃O₉ was investigated. The emission intensity decreased with the temperature increased from 300 K to 500 K and the activation energy was calculated to be 0.3386 eV. The amplitude of emission intensity under X-ray excitation is rather similar for Na₃Lu_0.99Ce_0.01Si₃O₉ and BGO samples. Na₃LuSi₃O₉: 1.0 at% Ce³⁺ showed very fast scintillation decay time of 31.27 ns under X-ray excitation. Taking into account the fast scintillation decay time and good luminescence properties under X-ray excitation, Na₃LuSi₃O₉:Ce³⁺ may be a potential novel inorganic scintillator material.

Acknowledgements

This work was financially supported by the “973” Programs (2014CB643801), Science & Technology Project of Guangdong Province (No. 2015A050502019), Science & Technology Project of Guangzhou (No. 2015100110296), State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University) and China Scholarship Council (No. 201406385019).

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