Synthesis and Luminescent Properties of Strontium Cerium Oxide Phosphors Doped with Rare Earths

In this paper we report the synthesis and luminesce nt properties of Sr 2CeO4 doped with rare earths (0.5% concentration of La, Eu and Dy ions) are reported. The powder samples of rare earths doped strontium cerium oxide (Sr 2CeO4: La, Eu and Dy ions) was synthesized by a standard solid state reaction method in air at 1200C. These samples were characterized by X-ray diffraction, scanning electr on microscope (SEM) and photoluminescence (PL) techniques, particle size an alysis. The XRD data revealed that the structure of the Sr 2CeO4 as orthorhombic and the calculated average crystal lite size is ~10nm. Scanning electron microscopy revealed that t e Sr2CeO4 phosphor particles had non-spherical shape and the irregular shaped partic les are highly agglomerated. PL excitation and emission spectra of Sr 2CeO4 doped with rare earths are recorded at room temperature. The CIE coordinates depicted on 1931 c hart of Eu (0.5%) doped Sr 2CeO4 shown that the emission colour varies from blue to white. This phosphor has a good potential for application in white light LEDs.


Introduction
Recently various phosphor materials have been actively investigated to improve their luminescent properties and to meet the development of different display and luminescence devices.Inorganic compounds doped with rare earth ions form an important class of phosphors as they possess a few interesting characteristics such as excellent chemical stability, high luminescence efficiency, and flexible emission colors with different activators [1,2].Rare earth ions doped phosphors have been used in varied fields based on their electronic and optical characters arising from their 4f electrons.Among the rare earth elements, europium is a special element as dopant, because it exhibits the property of valence fluctuation, i.e., the valence state is divalent or trivalent [3].In 1998, a blue phosphor compound, Sr 2 CeO 4 possessing one-dimensional chain of edge-sharing CeO 6 octahedra, was identified by Danielson and his co-workers with combinatorial chemistry [4].It exhibited a blue-white emission band that peaks at 485nm at 254nm excitation and the luminescence was suggested to originate from a ligand-to-metal Ce 4+ charge transfer.This material was analyzed by Rietveld structural refinement, and the luminescent phase was characterized as Sr 2 CeO 4 with orthorhombic structure containing one-dimensional chains of edge-sharing CeO 6 octahedra that are linked together by Sr 2+ ions.This material exhibits excellent luminescent property and maximum emission band around 485nm, whose intensity changes depending on the employed heat treatment.Some impurity phases such as SrCO 3 , SrCeO 3 and CeO 2 were also detected [5][6][7].Some authors [8][9][10] have assigned the luminescence of the Sr 2 CeO 4 material to the charge transfer between Ce 4+ and O 2-because the position and shape of the absorption band and the emission band width are characteristic for this type of transition.Some authors have observed that the strong luminescence of Sr 2 CeO 4 makes it potentially applicable as blue phosphor material in low pressure mercury vapor lamps [11] and FED [12,13] as well as other types of luminescence devices [14].Following this observation, several studies about this luminescent material have been conducted, and some synthetic routes have been developed for the preparation of Sr 2 CeO 4 powders.Here we report the preparation and characterization of blue phosphor Sr 2 CeO 4 doped with rare earths (0.5 mol% concentration only) La, Eu and Dy ions via standard solid state reaction method in air.

Experimental Method
Strontium nitrate [Sr(NO 3 ) 2 ], Cerium oxide (CeO 2 ), Lanthanum oxide (La 2 O 3 ), Europium oxide (Eu 2 O 3 ) and Dysprosium oxide (Dy 2 O 3 ) were used as starting materials to prepare Sr 2 CeO 4 and rare earth doped phosphors.Every starting material was of greater than 99.9% purity.All the starting materials were weighed according to the stoichiometric ratio.Sr 2 CeO 4 phosphor and other samples of Sr 2 CeO 4 doped with rare earths (0.5% concentration of La, Eu and Dy ions) were prepared via solid state reaction method (SSR).A stoichiometric mixture of these staring powders was thoroughly homogenized in an agent mortar for 1hr and then put into an alumina crucible.The homogenized mixture was heated in air at 12000C for 3h in a muffle furnace with a heating rate of 50C/min.Finally the samples were allowed to cool down to room temperature for about 20h [15,16].All the samples were again ground into fine powder using agate mortar and pestle about an hour.
Several complementary methods were used to characterize the prepared phosphors.To identify the crystal phase, X-ray diffraction (XRD) analysis was carried out with a powder X-ray diffractometer (Indus beam line-II (ADXRD BL-12),RRCAT, Indore, India), Photoluminescence (PL) emission spectra were measured by a Spectrofluorophotometer (SHIMADZU, RF-5301 PC using Xenon lamp as excitation source.The morphologies of particles were investigated by using Scanning electron microscope (PHILIPS XL 30 CP).The particle size was measured by using Laser based particle size analysis (Malvern Instrument Ltd (U.K), The CIE (Commission International de l'Eclairage) co-ordinates were calculated by the Spectrophotometric method using the spectral energy distribution.

X-ray diffractometry
The typical XRD pattern of Sr 2 CeO 4 is shown in Fig. 1    ab plane is ~5 times larger than that along c-axis.This phenomenon can be easily understood from the periodic bond chain (PBC) theory.The fundamental building units a r e in the orthorhombic structure.Fig. 4 shows the SEM photograph of Sr 2 CeO 4 : Eu (0.5%) and it exhibits the grains like irregular sizes and shapes which are highly agglomerated.

Photoluminescence study
Fig. 5 shows the PL excitation and emission spectra of Sr 2 CeO 4 particles recorded at room temperature.The emission spectra of Sr 2 CeO 4 excited at 250nm show a broad band in the region of 350-650nm with a peak at 470nm.The CT band can be assigned to the f→t 1 g transitions of Ce 4+ ions.The emission spectra of Sr 2 CeO 4 observed with 260nm excitation is similar to that observed with 250nm excitation.The only difference is the emission intensity under 260nm excitation is higher than 250nm.The emission at 470nm is considered as a potential blue emitting phosphor.The excitation spectrum recorded for Sr 2 CeO 4 displays a broad band and peaks at 250nm and 260nm which is attributed to the Ce 4+ -O distances in the lattice.

Wavelength (nm) 1 Emission for 250nm Ex 2 Emission for 260nm Ex
Fig. 6 shows the PL excitation and emission spectra of La (0.5%) doped Sr 2 CeO 4 phosphor.The sample exhibit the emission peak at 470nm upon the excitation at 250nm and 260nm.The PL emission intensity at 260nm excitation is 40% more than that of 250nm excitation.Due to the Ce sites were replaced by La ions, the charge transfer emission intensity decreased by 60% than that of Sr 2 CeO 4 phosphor.Fig. 7 shows the PL excitation and emission spectra of Dy (0.5%) doped Sr 2 CeO 4 phosphor.The sample exhibit the peaks at 469nm and 575nm upon the excitation at 250nm and 260nm.These peaks are assigned to the 4 F 9/2 -6 H 15/2 and 4 F 9/2 -6 H 13/2 transitions, respectively.Dy is well resolved in the host phosphor at 575nm in yellow region.The PL emission intensity at 260nm excitation is sharper than that of 250nm excitation.Fig. 8 shows the PL excitation and emission spectra of Eu (0.5%) doped Sr 2 CeO 4 phosphor.The sample exhibit the emission peaks at 467, 491, 511, 537, 557, 587,601 and 617nm upon the excitation at 250nm and 260nm.These peaks are assigned to the 5 D 2 → 7 F 1, 2, 3 , 5 D 1 → 7 F 1, 2, and also from 5 D 0 → 7 F 1, 2 transitions respectively.The peak around 610 -620nm is due to the electric dipole transition of 5 D 0 → 7 F 2 , which is induced by the lack of inversion symmetry at the Eu 3+ sites.It is well known that the 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 intensity ratio is a good measure of the site symmetry of rare-earth ions in a doped material.The observed PL emission of Eu doped Sr 2 CeO 4 may be due to crystal field effect and unusual luminescent property due to low vibration energy of Sr 2 CeO 4 host lattice and different energy transfer process from host to dopant.The high efficiency energy transfer allows us to expect that the Sr 2 CeO 4 crystal structure could form the base for phosphors with different spectral emissions [18][19][20][21].The Eu (0.5%) doped Sr 2 CeO 4 phosphor displays all the colours led to white light emission.Therefore this phosphor can be good has a good potential for application in white light LED's.

Particle size analysis
The particle size distribution histogram of Sr 2 CeO 4 and La, Eu, Dy doped Sr 2 CeO 4 phosphors shown in Figs. 9 to 12.The particle size of prepared phosphor specimen was measured by using laser based system, Malvern Instrument, U.K. The mean diameter of the particle size of Sr 2 CeO 4 phosphor specimen is 26µm, La doped Sr 2 CeO 4 specimen is 18.31µm, Eu doped Sr 2 CeO 4 specimen is 17µm and Dy doped Sr 2 CeO 4 specimen is 32µm.The average particle diameter of europium doped Sr 2 CeO 4 phosphor is 17µm and the crystallite size is around~ 10nm.

CIE coordinates
Based on the emission spectra, it was possible to see the color of the emission of each sample in the CIE diagrams 1931.Fig. 13 shows the CIE diagram 1931.The CIE coordinates were calculated by the spectrophotometric method using the spectral energy distribution of Sr 2 CeO 4 phosphor.The CIE chromaticity index for the Sr 2 CeO 4 phosphor (A) are x = 0.16 and y = 0.18, which is better than that reported by Danielson (x=0.20 and y=0.30) [22] and all the diffraction peaks can be well indexed based on the ICDD No.89-5546[17].All the prepared samples are characterized to be single phase Sr 2 CeO 4 of orthorhombic structure.Except this, no other phase exists in the XRD spectra, indicating that the as-prepared Sr 2 CeO 4 phosphor is single phase.XRD study confirms the single phase Sr 2 CeO 4 with the orthorhombic structure.The calculated crystallite sizes using Scherer's formula D = K.λ/ β cosθ, where k the constant (0.94), λ the wavelength of the X-ray (0.895 Å), β the full-width at half maxima (FWHM), θ the Bragg angle of the XRD is around ~9nm (un-doped), La doped Sr 2 CeO 4 is around ~ 11nm, Eu and Dy doped Sr 2 CeO 4 is around ~ 10nm.No change in the crystal structure on doping rare earths in host material.

Fig. 2 Fig. 1 .
Fig.2shows the SEM photograph of Sr 2 CeO 4 phosphor particles.The Sr 2 CeO 4 phosphor particles had non-spherical shape and the irregular shaped particles are highly agglomerated.Fig.3is the typical SEM photograph of the synthesized Sr 2 CeO 4 : Dy phosphor.The multi layered structure was observed.The prominent characteristic of Sr 2 CeO 4 : Dy crystal growth is a serious anisotropic growth rate.The growth rate along the

Sr 2
CeO 4 phosphor sample is prepared by a standard solid state reaction method in air at 1200 0 C. The XRD study confirms the structure of the system as orthorhombic and single phase was obtained due to the calcination temperature.The calculated average crystallite size using Scherer's formula is ~10nm.The excitation spectrum recorded for Sr 2 CeO 4 displays a broad band and peaks at 250nm and 260nm which is attributed to the Ce4+ -O distances in the lattice The CIE coordinates depicted on 1931 chart reveals that the Sr 2 CeO 4 and La doped phosphors exhibit blue colour, Dy doped phosphor exhibit Blue Yellow (cyan) colour and Eu doped Sr 2 CeO 4 phosphor displays all colours.Therefore