Mechanoluminescence and Photoluminescence of Pr3+

MECHANOLUMINESCENCE AND PHOTOLUMINESCENCE OF 3+ Pr ACTIVATED KMgF3 PHOSPHOR

S.J. Dhoble1, R.S. Kher2, and C. Furetta3* 1Kamla Nehru College, Sakkardara Square, Nagpur-440 009, India. 2Government Autonomous Post-Graduate College, Bilaspur-495 001, India. 3Department of Physics, Rome University “La Sapíenza”, Rome, Italy *Dep. Física, Universidad Autónoma Metropolitana-I, México D.F.

Abstract

3+ A Czocharaski method for the preparation of crystalline KMgF3 : Pr phosphors are reported. Photoluiminescence (PL) and mechanoluminescence (ML) characteristics are studied. Photoluiminescence 3+ 3+ of Pr activated KMgF3 shows the strong emission of Pr ions were observed at 498 and 650 nm by excitation of 213 nm. ML of KMgF3 : Pr3+ shows two peaks, which have been observed in ML intensity versus time curve. The ML peak shows the recombination of electrons with free radical (anion radical produced by -irradiation) released from two type 3+ traps during the mechanical pressure applied on KMgF3 : Pr phosphor. It has a supralinear ML response with -ray exposure and a negligible fading. These properties of phosphor should be suitable in dosimetry of ionization 3+ relation using ML technique. Therefore the KMgF3 : Pr phosphor proposed for ML dosimetry of ionization radiations.

1. Introduction The phenomenon of mechanoluminescence (ML) deals with the emission of light as a result of the mechanical deformation of solids. This technique offers a number of interesting possibilities such as detection of cracks in solids and the mechanical activation of various traps present in solids. Many organic and inorganic crystals, polymers, ceramics and glasses exhibit ML phenomena. Recently, ML has been investigated in metals [1], which had previously been considered to be fundamentally impossible. Panigrahi et al [2] show the mechanism of ML in sulphate based phosphor. Most of the substances exhibit ML during fracture. However, x- or -ray irradiated alkali halide crystals show ML during their elastic and plastic deformations, as well as, during their fracture [3].

Phosphors are important characteristics of efficient absorption due to excitation by different sources, such as x-ray, -ray, laser etc. KMgF3 crystals efficiently store ionizing energy [4-6]. In spite of the fact that the band gap (Eg) in such fluoropervoskites as KMgF3, NaMgF3, RbMgF3 is large ( 11eV). Therefore, ABX3, type materials are sensitive to irradiation with energies smaller than Eg.

The trivalent praseodymium (Pr3+) ion is an attractive optical activator, since its energy level spectrum contains several metastasble 1 1 3 multiplets GH, G2, Po,1,2 that offer the possibility of simultaneous emission in the blue, green orange, red and infrared (IR) [7]. Furthermore, the broad

1 3+ band interconfigurational 4f2 ! 4f5d transition of Pr which are used in scintillator detectors of ionizing radiation [8,9]. There has been a renewed interest in studying Pr3+doped materials for laser purposes both continuous 3+ 3+ wave and pulsed in crystals such as YAlO3 : Pr [10] and LaCl3 : Pr [11], as well as optical fibres [12]. Blue laser operation at room temperature in 3+ LiYF4 : Pr crystal has been reported by Esterowitz et al [13]. In the present 3+ paper, we report PL and ML characterization of KMgF3 : Pr phosphors.

2. Experimental 3+ KMgF3 : Pr crystals were prepared by the melt of KF and MgF2 in stoichiometric ratio, using the Czocharaski method in an argon atmosphere. The dopant in suitable amounts was added to the initial powder before growth.

PL emission and excitation spectra were recorded at room temperature on using Hitachi-2000 fluorescence spectrometer. The ML was excited impulsively by using a technique [14] in which a load was dropped onto the phosphor was a particular height using a guiding cylinder. The ML was monitored by an RCA 931 photomultiplier tube connected to SCIENTIFIC HM 203 oscilloscope having a phosphorescent screen. All the measurements were carried out in a dark room. Samples were exposed to -rays using a 60Co source.

2. Results and Discussion 3+ The photoluminescence of KMgF3 : Pr phosphor shown in Figs. 1, 2 (excitation and emission curve) was recorded and it was observed that it gives maximum absorption at 213 nm ( em = 498 nm). Strong emission 3 3 1 3 peak observed at 498 nm and 650 nm due to Po ! H4 and D2 ! H4 transition of Pr3+ ion. The emission intensity of 498 and 650 nm peaks is decreasing for higher concentration of Pr3+ ion, may be due to the concentration quenching. As Pr3+ is a very special case for concentration quenching, luminescence of rare earth ions such as Eu3+, Gd3+, Ce3+ and Tb3+ consists of energy migration to killer sites. The quenching of the luminescence occurs in ionic pairs and not by migration.

Figure 3 shows the characteristic of ML induced by the impact of 3+ moving piston (280 g) onto -irradiated KMgF3 : Pr phosphor. It is seen that after the impact of piston, two ML peaks are observed in ML intensity versus time curve. These two peaks are indicated in two types of traps are 3+ formed during -irradiation in the KMgG3 : Pr phosphor. At these two positions, recombination of electrons with free radicals (anion radicals produced by -irradiation) released from traps during the mechanical 3+ pressure applied on the Pr activated KMgF3 phosphor. This possible ML mechanism is proposed for -irradiated fluorides based phosphor. Several workers [15,16] have proposed that the mechanism of ML for -ray irradiated alkali halide crystals and the moving dislocation under the action of external stresses interact with F-center and capture electrons. If a dislocation containing an electron encourters an impurity center containing a hole, the electron is captured by this center and ML arises. Figure 4 shows 3+ the ML rersponse curve of KMgF3 : Pr phosphor with exposure of -rays.

2 The ML intensity of first peak increases supralinearly up to 0.4 C/Kg and ML intensity is saturated at higher exposure.

3+ Figure 5 shows the ML spectra of -irradiated KMgF3 : Pr phosphor. We have found that two peaks are observed around 490 and 655 nm due to Pr3+ ions.

Recently, Panigrahi et al [2] proposed ML mechanism of (K2 : Dy) Mg2 (SO4)3 phosphors. Dy activated K2Mg2(SO4)3 mixed sulphate was exposed to -rays the trivalent dysprosimum acting as electron trap get reduced to - divalent dysprosium with the production of trapped hole centres (like SO4 , - 2+ SO2 , etc), elsewhere. Recombination with electrons at the Dy site leads to termination of Dy3+ in electronically excited state. Luminescence is observed during de-excitation of the excited Dy3+ ion. The proposed mechanism of ML in Dy activated K2Mg2 (SO4)3 phosphor is as shown below :

On -irradiation and ML processes -2 - - SO4 ! SO4 + e - - - SO4 ! SO2 + O2 Dy3+ + e- ! Dy2+ Dy2+ + hole ! [Dy3+]* [Dy3+] * ! Dy3+ + h

3+ Similar ML mechanism may be possible in Pr activated KMgF3 3+ phosphor. In the KMgF3 : Pr phosphor defect centers are formed in the form of fluorine radicals during -ray irradiation and ML peaks may be correlated to these radicals. The formation and stability of defect center may depend on the Pr3+ in K+ and Mg2+ lattice and high ML sensitivity due to the efficient formation of defect centers in the lattice.

3. Conclusion In the present work, we have found that –

3+ 1. Pr ions show PL emission peak at 498 and 650 nm in KMgF3 host. 2. ML curve shows two peaks due to two types of defect center formed during - ray irradiation. 3. ML spectra show the two peaks at 495 and 655 nm. Hence ML is observed due to Pr3+ ion. Here the results of PL and ML are correlated. 3+ 4. KMgF3 : Pr phosphor shows the supralinear response with -ray 3+ exposure and negligible fading. Therefore KMgF3 : Pr phosphor, one of the possible candidates for ML dosimetry of ionization radiation.

Acknowledgements One of us, SJD is thankful to Dr (Mrs) Suhasini Wanjari, Director, Kamla Nehru College, Nagpur for encouragement and facilities.

3 References [1] M.L. Molotskii, Sov. Sci. Rev. Chem. 13, 1 (1989). [2] A.K. Panigrahi, S.J. Dhoble, R.S. Kher and S.V. Moharil Phys. Stat. Sol. (a) 198 (2), 322 (2003). [3] B.P. Chandra, Nucl. Track. 10, 825 (1985). [4] C. Furetta, C. Bacci, B. Rispoli and A. Scacco, Rad. Prot. Dosim, 33, 107 (1990). [5] N.V. Shiran, A.V. Gektin, V.K. Komar, I.M. Krasovitskaya and V.V. Shlyaskhturov, Cryst. Rad. Measure. 24, 435 (1995). [6] A.V. Gektin, I.M. Krasovitskaya and N.V. Shiran, J. Lum. 72, 644 (1997). [7] A.A. Kaminskii, Laser Cryst., 2nd edn. (Springler-Verlag, Berlin, 1990). [8] J.S. Chivan, W.E. Case and D.D. Eden, Appl. Phys. Lett. 35, 124 (1979). [9] E.G. Gumanskaya, M.V. Korzhik, S.A. Smirnova, V.B. Pavlenko and A.A. Fedorov, Opto. Spectro. (USSR) 72, 86 (1991). [10] A. Bleckmann, F. Heine, J.P. Meyn, T. Danger, E. Heumann and G., Huber, OSA Proc. on Advanced Solid State Laser, edited by Albert A. Pinto and Tso Yee Fan (Optical Society of America, Washington, DC, 1993) Vol. 15, pp. 199-201. [11] M.E. Koch, A.W. Kueny and W.E. Case, Appl. Phys. Lett. 56, 1083 (1990). [12] B.J. Anislie, S.P. Craig and S.T. Davey, J. Lightwave Technol. 6, 287 (1988). [13] L. Estesowitz, F.J. Bartoli, R.E. Allen, D.E. Wortman, C.A. Morrison and R.P. Leavit, Phys. Rev. B, 19, 6422 (1979). [14] B.P. Chandra and J.I. Zink, Phys. Rev. B 21, 816 (1980). [15] N.A. Atari, Phys. Lett. A 90, 93 (1982). [16] M.S.K. Khokhar, R.S. Kher, D.P. Bisen and B.P. Chandra, Ind. J. Pure App. Phys. 31, 952 (1993).