Ce3+:(Lu0.7Y0.25La0.05)2O3 transparent ceramics were fabricated with nanopowders and sintered in H2 atmosphere. The spectral properties of Ce3+:(Lu0.7Y0.25La0.05)2O3 transparent ceramics were investigated and the luminescence of Ce3+ in the solid solution of Lu2O3, Y2O3 and La2O3 has been found. The ceramics has high density of 8.10g/cm3 and short fluorescence lifetimes of 7.15 ns and 26.92 ns. It is expected to be a good fast response high temperature inorganic scintillating materials.
©2008 Optical Society of America
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), and blue. 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. The effort to investigate the luminescence of Ce3+ in nano-size Y2O3 was also failed. 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:Y2-2xLa2xO3 transparent ceramics, we doped Ce3+ into Y2-2xLa2xO3 transparent ceramics by chance and found it was easy for Ce3+ ion to dope heavily in Y2-2xLa2xO3 ceramics due to the similar radii of Ce3+ ion and La3+ ion. 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. Y2-2xLa2xO3, a solid solution of Y2O3 and La2O3, has lower liquidus temperature than Y2O3, so the sintering temperature of Y2-2xLa2xO3 transparent ceramics is further lowered.
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.
Transparent Ce-doped and undoped (Lu0.7Y0.25La0.05)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:(Lu0.7Y0.25La0.05)2O3 transparent ceramics. The specimen has high transparency and the letters under the ceramic can be seen distinctly. The colour of Ce:(Lu0.7Y0.25La0.05)2O3 transparent ceramics is deep red, which is different from those of Ce:Y2O3 (colorless) or Ce: La2O3 (yellow). Both Ce3+ doped and undoped (Lu0.7Y0.25La0.05)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 (Lu0.7Y0.25La0.05)2O3 has long term stability.
Figure 2 shows XRD pattern of Ce3+ doped (Lu0.7Y0.25La0.05)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 (Lu0.7Y0.25La0.05)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.
Figure 4 is the absorption spectra of Ce3+ doped and undoped (Lu0.7Y0.25La0.05)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.
Figure 5 shows the luminescence of Ce3+ doped (Lu0.7Y0.25La0.05)2O3 transparent ceramics stimulated at 230 nm at room temperature and no luminescence of undoped (Lu0.7Y0.25La0.05)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.
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. In the (Lu0.7Y0.25La0.05)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 (Lu0.7Y0.25La0.05)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 (Lu0.7Y0.25La0.05)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. As a result, we expect a shift of the 5d state of Ce3+ in (Lu0.7Y0.25La0.05)2O3 host lattice. From the emission spectrum of Ce3+ doped (Lu0.7Y0.25La0.05)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 (Lu0.7Y0.25La0.05)2O3 ceramics. This will be further investigated theoretically and experimentally.
The large-sized bulky Ce-doped (Lu0.7Y0.25La0.05)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+:(Lu0.7Y0.25La0.05)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.
We acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 60578041 and 60425516).
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