1. Introduction

The development of efficient inorganic scintillators has been driven by applications in medical imaging and industrial measuring systems. Myriad host lattices containing rare-earth ions as luminescent activators have been investigated because the 4f-5d transitions of these ions are parity-allowed. The recent work has been motivated by a phenomenological desire to further understanding 5d→4f luminescence processes[1,2] and by a technological desire to develop improved tunable solid-state laser, scintillator, and phosphor materials in the vacuum ultraviolet (VUV), ultraviolet (UV)[3], and blue[4]. The most researches are focused on Ce³⁺ ion, whose 5d→4f transition is electric dipole permitted. Consequently, doping Ce³⁺ ion is one major way to get fast response inorganic scintillating materials. It has long been known that the emission efficiency of 5d→4f transition of Ce³⁺ varies widely and randomly with the host lattice. Many studies have shown that in some host (e.g., Lu₂SiO₅), Ce³⁺ emits with high quantum efficiency, while in other host lattices (e.g., Y₂O₃, Lu₂O₃, La₂O₃ etc.), Ce³⁺ emission is completely quenched, even at low temperature[5,6]. But the application of pressure to Ce:Lu₂O₃ crystal leads to the generation of luminescence[7]. The effort to investigate the luminescence of Ce³⁺ in nano-size Y₂O₃ was also failed[8]. Due to larger radius of Ce³⁺ ion (101.6 pm) than that of Re³⁺ ion, Ce doping concentration is usually low in most Ce³⁺-doped inorganic scintillating materials, including aluminate compounds Re₃Al₅O₁₂ (ReAG) , ReAlO₃ (ReAP) (Re=Y, Lu, Gd etc) and silicate compounds Re₂SiO₅ and Re₂Si₂O₇. Recently as we studied Nd:Y_2-2xLa_2xO₃ transparent ceramics, we doped Ce³⁺ into Y_2-2xLa_2xO₃ transparent ceramics by chance and found it was easy for Ce³⁺ ion to dope heavily in Y^2-2xLa^2xO₃ ceramics due to the similar radii of Ce³⁺ ion and La³⁺ ion[9]. At the same time, we surprised to observe the luminescence of Ce³⁺ in the solid solution of Y₂O₃ and La₂O₃, though Ce³⁺ emission completely quenched in Y₂O₃, Lu₂O₃ and La₂O₃, respectively.

Compared to single crystal, transparent ceramics has several advantages, for example, ease of fabrication, lower cost, and fabrication of large size, etc. Another advantage of transparent ceramics is that it is very easy to fabricate optical materials with high melting points. It is extremely difficult to grow Y₂O₃ single crystal using conventional crystal growth methods because of its high melting point (2450 °C). But it is much easier to fabricate Y₂O₃ into a ceramics because the sintering temperature is about 700 °C lower than its melting point[10]. Y_2-2xLa_2xO₃, a solid solution of Y₂O₃ and La₂O₃, has lower liquidus temperature than Y₂O₃, so the sintering temperature of Y_2-2xLa_2xO₃ transparent ceramics is further lowered[11].

Lu₂O₃ has high density and this is important for scintillator materials. But the solid solubility limit of La₂O₃ in Lu₂O₃ is much lower than that in Y₂O₃, so we use Lu₂O₃, Y₂O₃ and La₂O₃ to form three components solid solution to solve the low solubility limit of La₂O₃ in Lu₂O₃ and get high density ceramic material at the same time.

2. Experimental

Transparent Ce-doped and undoped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics were fabricated with nanopowders and sintered at 1650∼1700°C for 25 h in H₂ atmosphere. Ce³⁺ ion is 0.5 at.%. The apparent densities of the sintered samples were measured by Archimedes method. The phase composition of the sintered specimen was identified by X-ray diffraction (Model D/MAX-3C, Rigaku, Japan). Microstructures were observed with optical microscope (BX60, OLYPMUS, Japan). The optical transmission spectra were measured with a spectrophotometer using Xe light as pump source (Model V-570, JASCO) at room temperature. The fluorescence spectra were measured with a fluorescence spectrum analyzer (Fluorolog-3, Jobin Yvon Spex, France) at room temperature. The luminescent decay curve was measured with a pulse x-ray spectrophotometer (designed by Tongji University and Shanghai Institute of Ceramics, Chinese Academy of Sciences).

3. Results and discussion

Figure 1 is the photograph of Ce:(Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics. The specimen has high transparency and the letters under the ceramic can be seen distinctly. The colour of Ce:(Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics is deep red, which is different from those of Ce:Y₂O₃ (colorless) or Ce: La₂O₃ (yellow). Both Ce³⁺ doped and undoped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics have the similar apparent densities of 8.10g/cm³ (higher than that of Ce:YAG, 4.68g/cm³). An ideal scintillator should have high density and high effective atomic number. As high temperature oxide ceramics (Lu_0.7Y_0.25La_0.05)₂O₃ has long term stability.

 

Fig. 1. Photograph of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics (3 mm thick)

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Fig. 2. X-ray diffraction profiles of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics

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Figure 2 shows XRD pattern of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics. All peaks are corresponding to cubic Lu₂O₃ phase and no other phases were detected. Since both Lu₂O₃ and Y₂O₃ have the same cubic crystal structure, and the radii of Lu³⁺ and Y³⁺ is similar, Y³⁺ can substitute Lu³⁺ sites, Lu₂O₃ and Y₂O₃ can form extensive solid solution, so only Lu₂O₃ phase can be detected.

Figure 3 shows the optical microstructure of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics. It reveals that there are almost no pores in or between the grain boundaries and the average grain size is about 20 μm.

 

Fig. 3. Optical microscopic photograph of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics

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Figure 4 is the absorption spectra of Ce³⁺ doped and undoped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics. There is one absorption peak centered at 223 nm in the undoped sample and two absorption peaks centered at 230 nm and 400 nm, an absorption shoulder at 486 nm in Ce³⁺ doped specimen. The strong absorption peak of 223 nm in both samples is associated with the fundamental host absorption and the peaks centered at 230 nm and 400 nm is a characteristic absorption peak of Ce³⁺ ion and assigned to the lowest energy 4f→5d transitions of Ce³⁺ ion, which is similar to the results of Ref 5.

 

Fig. 4. Absorption spectra (a).Ce³⁺ doped Y₂O₃ crystal,⁵ (b). Ce-doped Lu₂O₃ crystal,⁵ (c). Ce³⁺ doped and undoped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics

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Fig. 5. Emission spectra of Ce: (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics at room temperature (λ_ex=230 nm)

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Figure 5 shows the luminescence of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics stimulated at 230 nm at room temperature and no luminescence of undoped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics at the same condition was observed. A luminescence band centered at 480 nm, which covers wavelength range from 380 nm to 650 nm, has been observed from this spectrum. The full width at half maximum (FWHM) of this broad luminescence band is 137 nm. Considering the asymmetric feature of the spectrum, it could be decomposed into three Gaussian peaks, viz., at 471 nm (blue), 532 nm (green) and 590 nm (yellow-red), with a FWHM of 57 nm, 74 nm and 127 nm, respectively, which are corresponding to 5d→4f luminescence transition of Ce³⁺.

The decay curve of the 480 nm emission at room temperature is demonstrated in Fig. 6. The curve shows a fair consistency with the double exponential decay. The decay time expression equation is as follow:

So the lifetimes were calculated to be 7.15 ns and 26.92 ns. The ratio between fast decay time and slow decay time was calculated to be 0.43 and the fast decay time is 30% of the total time. The decay times of Ce: YAG single crystal is 119 ns and 1004 ns and Ce: YAG ceramics is 85 ns and 1050 ns[12].

 

Fig. 6. Luminescence decay curve of Ce: (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics at room temperature

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Fig. 7. Schematic of energy levels governing the 5d→4f luminescence transition of Ce³⁺. CB and VB represent the conduction and valence bands of the host lattice, respectively. 5d and 4f represent the emitting state and the ground state (² F _5/2) of Ce³⁺, respectively. (a) totally quenched luminescence; (b) efficient luminescence.

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Lu₂O₃ and Y₂O₃ both are sesquioxide crystal with a cubic bixbyite structure (space group Ia3). The crystal contains 32 octahedrally coordinated Lu³⁺ or Y³⁺ dopant sites per unit cell, 75% and 25% occupy sites with C ₂ and C _3i symmetries, respectively. Triply ionized rare-earth ions enter these sites randomly during the ceramic sintering process[13]. In the (Lu_0.7Y_0.25La_0.05)₂O₃ structure, there are two sites (C ₂, C _3i) for the Ce³⁺ cation which are the sites statistically occupied by Lu³⁺, Y³⁺ and La³⁺ ion. Due to the difference between the values of the atomic radius of Lu³⁺/Y³⁺ and La³⁺ (86.1/90.0 pm and 103.2 pm, respectively), it is indeed clear that a disordered occupation of the sites is expected. Since the radii of La³⁺ ion (103.2 pm) and Ce³⁺ ion (101 pm) have near size, Ce³⁺ can substitute both the lattice sites of Lu³⁺/Y³⁺ ion and La³⁺ ion in (Lu_0.7Y_0.25La_0.05)₂O₃ structure. Thus, two different groups of optical centers (Ce1 and Ce2) are present, which results in two luminescence decay times.

The electronic structure of Ce³⁺:Y₂O₃ must be considered in an attempt to explain the origin of the emission bands. Ce³⁺ has a 4f ¹5d ⁰ ground state and a 4f ⁰5d ¹ first excited state, and often exhibits broad 5d→4f emission bands similar in appearance to the emission bands observed in our experiment. It is believed that the solid solution of Lu₂O₃, Y₂O₃ and La₂O₃ influence the energy of the conduction-band edge of Lu₂O₃/Y₂O₃ or La₂O₃ as well as the energy of the 4f and 5d states of Ce³⁺ in (Lu_0.7Y_0.25La_0.05)₂O₃ host lattice. Since the 4f orbitals of lanthanides are well shielded and interact only weakly with the surrounding lattice, we expect only weak variations in the energy of the 4f state of Ce³⁺. The 5d state, however, is well extended spatially and interacts strongly with the lattice[69]. As a result, we expect a shift of the 5d state of Ce³⁺ in (Lu_0.7Y_0.25La_0.05)₂O₃ host lattice. From the emission spectrum of Ce³⁺ doped (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics, it can be deduced that the bottom of the conduction band is shifted upward Ce³⁺ 5d levels as shown in Fig. 7, which makes it possible to have emission peaks in (Lu_0.7Y_0.25La_0.05)₂O₃ ceramics. This will be further investigated theoretically and experimentally.

4. Conclusions

The large-sized bulky Ce-doped (Lu_0.7Y_0.25La_0.05)₂O₃ transparent ceramics were successfully fabricated and the luminescence of Ce³⁺ in the solid solution of Lu₂O₃, Y₂O₃ and La₂O₃ have been found, though Ce³⁺ emission completely quenched in Y₂O₃, Lu₂O₃ and La₂O₃, respectively. Ce³⁺:(Lu_0.7Y_0.25La_0.05)₂O₃ with high density 8.10g/cm³ and short fluorescence lifetimes (7.15 ns) can be easy to dope Ce³⁺ heavily. It is expected to be a good fast response high temperature inorganic scintillating materials. And we also conclude that this compound is a future candidate for phosphor materials in the UV-visible bands.