PHOTOLUMINESCENCE STUDIES OF NA6(SO4)2CLF:RE (RE=Ce, Tb and Cu)

BHUSHAN KORE¹, DHOBLE N.S.², DHOBLE S.J.³*
¹Department of Physics, RTM Nagpur University, Nagpur, MS, India
²Department of Chemistry, Sevadal Mahila Mahavidhyalaya, Nagpur, MS, India
³Department of Physics, RTM Nagpur University, Nagpur, MS, India
* Corresponding Author : sjdhoble@rediffmail.com

Received : 28-02-2012     Accepted : 06-03-2012     Published : 15-03-2012
Volume : 3     Issue : 1       Pages : 132 - 134
Int J Knowl Eng 3.1 (2012):132-134

Luminescent materials based on lanthanide phosphors have made outstanding progress in revolutionizing the display and lighting industry. The optical properties of rare-earth ion (RE2+and RE3+) ions are valuable when these ions are doped into host lattices. The electronic transitions taking place within the partially filled 4f shell, or involving the 4f and 5d shells, gives rise to the luminescence. In preset study the preparation of Na6(SO4)2ClF doped with rare earth ions Tb, Ce and transition metal ion Cu by wet chemical method was described. All the compounds were prepared by wet chemical method as it requires very less instrumentation and less electrical power too. Under UV excitation (250nm) of Na6(SO4)2ClF:Ce3+ showed emission (359nm) in UV range. When Na6(SO4)2ClF:Tb3+ phosphor was excited at 379 nm, the emission spectrum showed intense bands at 490 nm (blue) and 546 nm (green). The peaks at 490, 546, can be attributed to 5D4-7F6 and 5D4-7F5 transitions, respectively. In Cu activated Na6(SO4)2ClF phosphor, the emission spectrum showed a dominant peak at 357 nm when excited at 252 nm.

Phosphor, halosulphate, photoluminescence.

In the past centuries, once a new kind of light source with higher energy efficiency and lower power consumption was invented, like incandescent filament and fluorescent lamp, it would make a great difference in the human’s life. In recent years, the studies on solid-state light sources, especially white light emitting diodes (w-LEDs), attracted increasing attention and became a hot issue in the energy saving, environment-friendly materials research [1] . The sulphates are an important mineral class and include some very interesting and attractive specimens. Although many minerals belong to this class only barite, gypsum, and anhydrite can be considered common. The basic chemical unit is the (AO₄) complex anion with a charge of negative two (2-) [2] . Thakre etal reported Na₆(SO₄)₂FCl:RE (RE=Dy, Ce or Eu) by solid state method [3] . Moharil and co-workers reported some suphate based materials such as KMgSO4Cl:Eu; KMgSO₄Cl:Ce, Na5(PO₄)SO₄:Ce, KZnSO₄Cl:Ce; NaMgSO₄F:Ce and Na₃SO₄F:Ce phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics [3,4] .

Na₆(SO₄)₂ClF (pure), Na₆(SO₄)₂ClF:Ce, Na₆(SO₄)₂ClF:Cu, and Na₆(SO₄)₂ClF:Tb phosphors were prepared by a wet chemical method. NaCl, Na₂SO₄ and NaF of analar grade were taken in a stoichiometric ratio and dissolved separately in single distilled de-ionized water, resulting in a solution of Na₆(SO₄)₂ClF (Eq. (1)). Water soluble sulphate salts of cerium, copper and terbium were then added to the solution to obtain Na₆(SO₄)₂ClF:Ce, Na₆(SO₄)₂ClF:Cu, and Na6(SO4)2ClF:Tb. confirming that no undissolved constituents were left behind and all the salts had completely dissolved in water, and thus reacted:
₂Na₂SO₄ + NaCl + NaF à Na₆(SO₄)₂ClF (1)
The compounds Na₆(SO₄)₂ClF (pure), Na₆(SO₄)₂ClF:Ce, Na₆(SO₄)₂ClF:Cu, and Na₆(SO₄)₂ClF:Tb in their powder form were obtained by evaporating at 80°C for 8 h. The dried samples were then slowly cooled to room temperature. The resultant polycrystalline mass was crushed to fine particles in a crucible. The powder was used in further study. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm.

I] Na₆(SO₄)₂ClF:Ce [Fig-1.1] shows the excitation and 1.2 shows emission spectra of Ce doped Na₆(SO₄)₂ClF .
PL excitation spectra of Na₆(SO₄)₂ClF:Ce³⁺ phosphor shows broadband is observed at 250 nm with λem =359 nm. [Fig-1.2] show the PL emission spectra of Ce ³⁺ ions in Na₆(SO₄)2ClF phosphors with different concentrations under excitation at 250 nm wavelength of light. Single peak is observed at 359nm, which are assigned to the 5d-4f transition of Ce³⁺ ions. With increasing concentration of Ce³⁺ ions, the peak intensity of 359 nm peak decreases and maximum intensity is observed for 0.1mol% of Ce³⁺ ions. This indicates that the Na6(SO4)2ClF lattice is more suitable for lower concentrations of Ce³⁺ ions.
Emission and excitation spectra of Cu doped Na6(SO4)2ClF are shown in [Fig-2.1] & [Fig-2.2] . The excitation wavelength 252nm was used for recording spectra of the samples. The presence of intense emission at 357nm, which is a characteristic of Cu⁺ ions, suggests the presence of Cu⁺ emission centers in the phosphor [5] .
On exciting the phosphor with 379nm light, emission bands at 490nm, 546nm & 587nm were observed, [Fig-3.1] on exciting the Tb³⁺ doped Na₆(SO₄)₂ClF phosphor at 379nm emission bands at 490nm and 546nm & 587nm have been observed. The emission is stronger under 379nm excitation than 254 nm excitation. Among these the intensity of 546nm emission band dominates over 490nm and 587nm. When Na₆(SO₄)₂ClF:Tb³⁺ phosphor was excited at 379 nm, the emission spectrum showed intense bands at 490 nm (blue) and 546 nm (green). The peaks at 490, 546, can be attributed to ⁵D₄^-7F₆ and ⁵D₄^-7F₅ transitions, respectively [6] .

Authors are grateful to Board Of Research in Nuclear Sciences (BRNS), Department of atomic Energy, Govt. Of India, for providing financial assistance to carry out this work under research project (sanctioned letter no 2011/37P/10/BRNS/144).

[1] Zhiguo Xia, Jiaqing Zhuang, Haikun Liu and Libing Liao, IOP, (in press).  
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[2] Gedam S.C., Dhoble S.J., Moharil S.V. (2007) Journal of Luminescence, 124, 120-126.  
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[3] Thakre P.S., Gedam S.C., Dhoble S.J., Atram R.G. (2011) Journal of Luminescence, 131, 2683-2689.  
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[4] Gedam S.C., Dhoble S.J., Moharil S.V. (2007) Journal of Luminescence, 126, 121-129.  
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[5] Kulkarni M.S., etal Journal of Luminescence (in press).  
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[6] Zhilong Liu, Lianxiang Yu, Qin Wang, Yanchun Tao, Hua Yang, (2011) Journal of Luminescence, 131, 12-16.  
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	Fig. 1.1- Excitation spectra of Na6(SO4)2ClF:Ce monitored at 357nm emission wavelength.
	Fig. 1.2- Emission spectra of Na6(SO4)2ClF:Ce at 250nm excitation wavelength.
	Fig. 2.1- Excitation spectra of Na6(SO4)2ClF:Cu monitored at 357nm emission wavelength.
	Fig. 2.2- Emission spectra of Na6(SO4)2ClF:Cu at 252nm excitation wavelength.
	Fig. 3.1- Excitation spectra of Na6(SO4)2ClF:Tb monitored at 546nm emission wavelength.
	Fig. 3.2- Emission spectra of Na6(SO4)2ClF:Tb at 379nm excitation wavelength.