Laura Muresan1, Elisabeth-Jeanne Popovici1, Amalia Hristea1

STUDIA UNIVERSITATIS BABES BOLYAI, PHYSICA, SPECIAL ISSUE, 2003

Spectral investigation on terbium activated

yttrium oxysulphide phosphors

Laura Muresan¹, Elisabeth-Jeanne Popovici¹, Amalia Hristea¹,

Emil Indrea² and Marilena Vasilescu³

¹“Raluca Ripan “ Institute for Research in Chemistry, 30 Fantanele, 3400-Cluj Napoca, Romania; [e-mail]

²“National Institute for R&D of Isotopic & Molecular Technology, 3400- Cluj Napoca, Romania;

³“I.G. Murgulescu” Institute of Physical Chemistry of Romanian Academy, 202 Spl.Independentei, Bucuresti, Romania
Abstract

Samples of terbium activated yttrium oxysulphide phosphors were prepared by solid-state reaction route. The structural characteristics and photoluminescence (PL) properties were evaluated on the basis of X-ray diffraction patterns and excitation or emission spectra. The paper investigates the influence of the synthesis conditions on optical and structural properties of Y₂O₂S: Tb phosphor.

Key words: phosphor; yttrium oxysulphide; luminescence
1. INTRODUCTION

Under different types of radiation such as UV and X-ray, terbium activated yttrium oxysulphide shows bright green luminescence related to the terbium ion presence. Terbium activated yttrium oxysulphide (Y₂O₂S: Tb) phosphor is of great interest for the manufacture of X-ray intensifying screens for medical diagnosis [1,2]. The utilisation of oxysulphide phosphors depends on the powder particle size distribution and luminescent properties that are adjusted during the phosphor synthesis stage [3]. Phosphor parameters are extremely sensitive to synthesis conditions.

In order to establish the optimum synthesis conditions for the manufacture of an efficient phosphor for X-ray intensifying screens, the correlation composition-structure-properties was studied for samples prepared by solid state reaction route. The paper presents the influence of some preparative conditions on the structural and photoluminescence properties of Y₂O₂S: Tb phosphor.

2. EXPERIMENTAL PART

Homogeneous mixtures of Y₂O_3, Tb₄O₇, Na₂CO₃, sulphur and Na₃PO₄ were fired at 1200⁰C for 4 hours, air (closed system). Samples were carefully washed with diluted hydrochloric acid, dried and sieved. Phosphor samples were characterised by emission and excitation spectra registered, under UV excitation, with 204 Perkin Elmer Spectrofluorimeter and by X-ray diffraction spectra taken with Philips PW 1050 Diffractiometer.

3. Results and discussion

Phosphor synthesis was performed by solid state reaction from mixtures containing oxide material (Y₂O_3, Tb₄O₇) and sulphuring flux mixture (Na₂CO₃, S, Na₃PO₄). The phosphor synthesis could be described by the following reaction:

Tb4O7						2Tb2O3	+	½ O2
4Na2CO3	+10S					Na2SO4	+	3Na2S3		+ 4CO2
(1-x)Y2O3	+	xTb2O3		+	S				Y2(1-x)Tb2xO2S + 1/2O2

Some of synthesis conditions and general characteristics of phosphor samples are presented in table 1.

Table 1

General characteristics of terbium activated yttrium oxysulphide samples

Sample code	Sulphuring mixture Na2CO3: Na3PO4:S	Phosphor formula	Powder colour	Photoluminescence colour
Y2O2S: Tb samples prepared with different amounts of terbium oxide
Y11	2 : 0.2 : 6.5	Y1.99Tb0.01O2S	Dark cream	White green
Y12	2 : 0.2 : 6.5	Y1.97Tb0.03O2S	Brick-red	Yellow green
Y13	2 : 0.2 : 6.5	Y1.95Tb0.05O2S	Light grey	Bright green
Y01	2 : 0.2 : 6.5	Y1.93Tb0.07O2S	Light cream	Bright green
Y15	2 : 0.2 : 6.5	Y1.9Tb0.1O2S	White	Bright green
Y2O2S: Tb samples prepared with various amounts of sulphur in the sulphuring flux mixture
Y16	2 : 0.2 : 0	Y1.97Tb0.03O2S	Dirty cream	Dirty green
Y17	2 : 0.2 : 2	Y1.97Tb0.03O2S	Light cream	Light green
Y18	2 : 0.2 : 4	Y1.97Tb0.03O2S	Orange	Yellow-green
Y19	2 : 0.2 : 6	Y1.97Tb0.03O2S	White	White
Y20	2 : 0.2 : 8	Y1.97Tb0.03O2S	White	White-green

Photoluminescence (PL) properties of as prepared samples were checked by emission spectra. The presence of terbium ions into the yttrium oxysulphide phosphors generates four principal emission bands in the visible spectrum, the strongest one showing a maximum at about 545 nm (figure 1 and figure 2).

Figure 1. PL spectra of samples prepared with various activator amounts	Figure 2. PL spectra of samples prepared with variable sulphuring flux mixture
Figure 3. Influence of the activator amount on PL emission (I545nm )	Figure 4. Influence of the host lattice sulphuration degree on PL emission (I545nm )

These bands could be assigned to the following electronic transition that occur between the excitation and the fundamental levels of the terbium ions: ⁵D₄  ⁷ F ₆ (~ 487 nm); ⁵D₄  ⁷ F₅ (~ 544 nm); ⁵D₄  ⁷ F₄ (~ 585 nm) and ⁵D₄  ⁷ F₃ (~ 620 nm). The emission bands relative ratio determines the apparent PL colour.

The overall PL emission depends of activator concentration i.e. large terbium amounts determines the intensification of the specific green bands (figure 3). The increase of sulphur content into the sulphuring flux mixture is in the favour of yttrium oxide conversion into the desired yttrium oxysulphide host lattice thus resulting in an increase of the green emission (figure 4).

All samples possess a good excitability at around 310 nm. The excitation maximum is influenced by the preparative conditions.

X-ray patterns illustrate the change of the crystalline structure from cubic yttrium oxide to hexagonal yttrium oxysulphide; the later is observable at samples prepared with more than 6 mole S/ mole phosphor. The formation of a multiphase luminescent material was put in evidence (figure 5).

Figure 5. X-Ray diffraction spectra for Y20 phosphor sample (CuK radiation)

4. CONCLUSIONS

The preparation conditions such as the quality of oxide mixture, concentration of terbium activator and composition of the sulphuring mixture, thermal synthesis conditions and conditioning regime are factors that influence the morpho–structural and photoluminescence characteristics of Y₂O₂S: Tb phosphor. The optimum concentration into the host lattice and the recommended sulphur amount into the sulphuring flux mixture could be established for the indicated thermal synthesis regime.

Acknowledgements:

The work was supported by MATNANTECH National Research Program, under contract no. 70/2001.
References:

1. H. Degenhardt, “Utilisation de substances luminophores a la base de terre rares dans les ecrans renforcateurs al la haute definition”, Electromedica, 1980, 3, 76-79;

2. G. Blasse, B.C. Grabmaier “Luminescence materials”, 1994, Berlin- Heidelberg

3. Y. Shu-Hong, H. Zhao-Hui, “Synthesis and formation mechanism of La₂O₂S via a Novel solvothermal-pressure-Relief Process”, Chem. Mater. 1999, 11, 192-194.