LUMINESCENT BEHAVIOR OF Tb3+ AND Pr3+ IONS IN RARE EARTH OXIDE

TAIKAR D.R.¹*, JOSHI C.P.², MOHARIL S.V.^3
1Department of applied physics, J.L. Chaturvedi College of Engineering, Nandanvan , Nagpur-440009, India.
²Physics Department, Shri Ramdeobaba K. N. Engineering College, Katol Road, Nagpur, India
³Department of Physics, Nagpur University, Nagpur, 440010, India
* Corresponding Author : deepak_taikar@rediffmail.com, deepak_taikar@yahoo.

Received : 28-02-2012     Accepted : 06-03-2012     Published : 15-03-2012
Volume : 3     Issue : 1       Pages : 41 - 43
Int J Knowl Eng 3.1 (2012):41-43

Conflict of Interest : None declared

Trivalent rare earth ions are used as an efficient emitting center in a variety of commercial phosphors. Photoluminescence spectra of Lanthanum oxide (La2O3) doped with (1%) Tb3+ and (1%) Pr3+ ions were investigated. The phosphor were synthesized by co-precipitation method and annealed at 10000C. X-ray powder diffraction (XRD) analysis confirmed the formation of La2O3. Photoluminescence measurements indicate that the La2O3: Tb3+ phosphor exhibit efficient green emission around 543 and 550 nm and La2O3: Pr3+ phosphor shows emission around 495 and 508 nm under UV excitation. Photoluminescence measurements indicate that the phosphor exhibit efficient characteristic emission under UV excitation.

photoluminescence; rare earth ion; Lanthanum oxide; co-precipitation.

Several oxides based materials are well known commercial phosphors e.g. Y_1.94Eu_0.06O₃ is red tri-colour lamp phosphor, Y_1.9Eu_0.1O₃ is red emitting CTV phosphor. Mg₂SnO₄:Mn is green emitting phosphor, Lu₂O₃:Eu³⁺ is X-ray scintillator. ZnO is FED phosphor host and is also used in solid state lighting. In fact ZnO:Zn is one of the earliest known green emitting phosphors with near UV excitation. Unfortunately, no energy transfer from ZnO to RE³⁺ result [1] . Strong Mn²⁺ emission with near UV excitation is reported in SrZnO₂ and K₂Cd₃S₄ [2] host. Eu³⁺ doped oxides were studied earlier in context of lasers [3] and CTV [4] . Eu³⁺ doped oxides were extensively studied by Blasse and coworker [5-8] and Ropp [9] . Gd₂O₃: Eu³⁺ is red emitting PDP (plasma display panel) phosphor, Al₂O₃:C²⁺ is optical stimulated luminescence (OPL) and thermoluminescence dosimetry (TLD), Al₂O₃:Ti is used in laser. La₂O₃ has largest band gap of the rare earth oxide, while also having the lowest lattice energy, with very high dielectric constant, ε=27. Lanthanum oxide has p-type semiconducting properties because its resistivity decreases with an increase in temperature, average room temperature resistivity is 10 KΩ-cm. The melting point of La₂O₃ is 2315°C. La₂O₃ is widely used in industry as well as in the research laboratory. It is used to develop optical materials and ferroelectric materials. La₂O₃ is an ingredient for the manufacture of piezoelectric and thermoelectric materials. It is also used in X-ray imaging intensifying screens.
In this paper we report synthesis and photoluminescence of La₂O₃: Tb3+ (1mol%) and La₂O₃: Pr³⁺ (1mol%).

La₂O₃ phosphor doped with different impurities was prepared by co-precipitation method using oxalic acid. The La₂O₃ (99.99%, IRE) and dopants (Tb4O7) (99.99%, IRE) and (Presidium nitrate) (99.99%, IRE) were weighed as per the stoichiometric calculations. The weighed quantities of La₂O₃ and dopent were dissolved in dil. HNO₃ (s.d.fine chem. AR) separately. The solutions were mixed. Stoicheometric amount of Oxalic acid (GR) solution was then slowly added to the mixture. The precipitate so obtained in the form of oxalate was separated by filtering, dried and thoroughly crushed to yield fine powder. It was decomposed by heating in air at 1000°C for 90 min. X-ray diffraction patterns were recorded on Philips PANalytical X’pert Pro diffractometer. Photoluminescence (PL) spectra in the spectral range 220-700 nm were recorded at room temperature on Hitachi F-4000 spectro-fluorimeter with spectral slit widths of 1.5 nm.

At low temperatures, La₂O₃ has an A-M₂O₃ hexagonal crystal structure space group p-3m1 shown in [Fig-1] . The La³⁺ metal atoms are surrounded by a 7 coordinate group of O^2- atoms, the oxygen ions are in an octahedral shape around the metal atom and there is one oxygen ion above one of the octahedral faces [10] . On the other hand, at high temperatures the Lanthanum oxide converts to a C-M₂O₃ cubic crystal structure. The La³⁺ ion is surrounded by a 6 coordinate group of O2- ions [11] .

[Fig-2] shows the comparison of XRD patterns of the as-prepared La₂O₃ powder with the ICDD data file 83-1345. An excellent match is seen.

La₂O₃:Tb³⁺
Terbium shows strong excitation corresponding to allowed transition between ⁷F₆ ground state of 4f⁸ configuration to the levels of 4f⁷5d¹ configuration. Quite often, this falls in deep UV or VUV region of the spectrum. Intense UV-excited luminescence of Tb³⁺ can be observed only if f–d excitation band is located in this region, or if there is energy transfer from the sensitizer or host, which absorbs in UV region, to one of the levels of 4f⁸ configuration of Tb³⁺. From the excited state the ion relaxes in several steps to ⁵D_j levels of 4f⁸ configuration. Line emission corresponding to f-f transitions is observed. Commonly observed, most intense lines are around 485 nm and 543 nm corresponding to ⁵D₄ à ⁷F₆ and ⁵D₄ à ⁷F₅ transitions, respectively. At low concentrations blue emission is dominant. Near UV emission around 385 nm corresponding to ⁵D₃ à ⁷F₆ transition is also observable. For high concentrations, these emissions are quenched by cross relaxation ⁵D₃ (Tb³⁺) + ⁷F₆ (Tb³⁺)à ⁵D₄ (Tb³⁺) + ⁷F₀ (Tb³⁺) and the green emission becomes dominant.
[Fig-3] shows results of PL measurements on La₂O₃ activated with (1mol%)Tb. Emission spectrum ( [Fig-3] , curve a) shows some broad lines due to transitions ⁵D₄ à ⁷F_J. The 489 nm emission line is attributed to ⁵D₄ à ⁷F₆ transition, 543 and 550 nm peaks attributed to ⁵D₄ à ⁷F₅, small peak at 586 nm attributed to ⁵D₄ à ⁷F₄ transition and 620 nm attributed to ⁵D₄ à ⁷F₃ transitions [12-14] . The excitation spectrum ( [Fig-3] , curve b) shows two peaks. The excitation maxima occur at 275 nm which corresponds to the intra- configurational transition from the ground state ⁷F₆ of 4f⁸ configuration to the lowest state of 4f⁷5d¹ configuration and 229 nm peak is probably a matrix excitation band.

La₂O₃:Pr³⁺
Like Tb³⁺, intense UV excited luminescence of Pr³⁺ can be observed only if f-d excitation band is located in this region or if there is energy transfer from host which absorbs in UV region to one of the levels of 4f² configuration of Pr³⁺. Pr³⁺ emission can be observed in UV, blue, green or red region depending on the position of the f-d excitation band [15] . In most cases this band is situated in VUV region.
For La₂O₃:Pr³⁺ (1 mol%), emission peak ( [Fig-4] , curve a) around 495 and 508 nm was observed which can be attributed to ³P₀ à ³H₄ transition of Pr³⁺. Some other peaks of much smaller intensities around 538, 554, 625 nm are also observed. The excitation spectrum ( [Fig-4] , curve b) contains two bands around 228 and 290nm. The excitation maxima occurring around 228 nm is the matrix excitation band while the longer wavelength 290 nm band may be attributed to f-d excitation.

La₂O₃:Tb³⁺ and La₂O₃:Pr³⁺ phosphors are successfully prepared by co-precipitation method using oxalic acid. The 230 nm peak which is always present in excitation spectra irrespective of the activator used is probably a matrix excitation band. The excitation spectra of both these phosphor shows good overlap over a region around 254 nm which make it good candidate for tricolor lamp. The La₂O₃: Tb³⁺ phosphor exhibit efficient green emission around 543 and 550 nm and La₂O₃: Pr³⁺ phosphor shows emission around 495 and 508 nm under UV excitation. Both these phosphors mentioned in this paper were found to be stable; no change in the PL properties was observed for the phosphors stored for several months. Samples were also found to be stable against UV irradiation that was used for the PL measurements. No changes in spectral positions or intensities were observed during several, successive PL runs.

[1] Weiyi Jia, Karem Monge, Felix Fernandez (2003) Optical Materials 23, 27-32.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[2] Axtell E.A.,III, Hanko J., Cowen J. A. and Kanatzidis M.G. (2001) Chem. Mater., 13, 2850-2863.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[3] Wickersheima K. and Lefever R.A. (1964) J. Electrochem. Soc. 111, 47.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[4] Bril A., Wanmaker W.L. and de Laut C.D.J.C. (1965) J. Electrochem. Soc. 112, 111.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[5] Blasse G. (1966) J.Chem. Phys. 45, 2356.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[6] Blasse G., Bril A. and Nieuport W.C. (1966) J. Phys. Chem. Solids, 27, 1587.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[7] Blasse G. and de Vries J. (1967) J. Electrochem.Soc. 114, 875.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[8] Blasse G. and Bril A. (1966)(a) Phil. Res.Rept. 21, 368, (1966) (b) J.Chem. Phys. 45, 2350.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[9] Ropp R.C. (1965) J. Electrochem. Soc. 111, 311 and 112, 181.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[10] Wells A.F. (1984) Structural Inorganic Chemistry.Oxford: Clarendon Press. 546.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[11] Wyckoff R.W.G. (1963)Crystal Structures: Inorganic Compounds RXn, RnMX2, RnMX3. New York: Interscience Publishers.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[12] Hongmei Yang, Jianxin Shi, Menglian Gong (2005) Journal Of Mat.Sci. 40, 6007-6010.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[13] Linjiu Xiao, Mingrui He, Yanwen Tian, Yongjie Chen, Tomoaki Karaki, Liqing Zhang, And Ning Wang (2007) Japanese Journal Of Applied Physics 46(9a), 5871-5873.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[14] Lin Lin, Chen Kun, Wang Zhifang, You Baogui, Chen Yonghu, Zhang Weiping, Shi HAOSHU (2008) Journal Of Rare Earths, 26(5), 648.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

[15] G.Blasse (1988) Prog.Solid State Chem 18 79.  
» CrossRef   » Google Scholar   » PubMed   » DOAJ   » CAS   » Scopus  

	Fig. 1- 7- coordinated La in La2O3 structure
	Fig. 2 XRD of La2O3
	Fig. 3- PL spectra of La2O3:Tb3+ (1 mol%) a) Emission curve maxima at 543nm b) Excitation curve maxima at 275 nm
	Fig. 4- PL spectra of La2O3:Pr3+ (1 mol%) a) Emission curve maxima at 508nm, b) Excitation curve maxima at 228 & 290 nm