Effect of Bi ion on Eu2+ ↔ Eu3+ conversion in CaF2:Eu phosphors for RPL dosimetry
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- Dhoble, S.J., Nagpure, I.M., Dhoble, N.S. et al. J Mater Sci (2011) 46: 7253. doi:10.1007/s10853-011-5685-3
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UV photo-excited Eu2+ and Eu3+ luminescence from CaF2:Eu and CaF2:Eu, Bi phosphors were investigated. The effect of gamma exposure followed by temperature effect shows conversion mechanism between Eu2+ ↔ Eu3+ ion in CaF2:Eu and CaF2:Eu, Bi phosphors which were studied for RPL dosimetry. The photoluminescence and X-ray diffraction characterization are reported in this article.
Fluorides are well known as efficient host lattices of phosphors involved in numerous applications, instance as a media for laser action [1, 2], optical frequency upconverters , neutron scintillation detectors , and lamp phosphors [5, 6]. Fluorides have several specific features in the field of luminescent materials such as: good transparency in the vacuum ultraviolet (VUV) and visible range, weak crystal field, small refractive index, low phonon frequencies, and long lifetime of the excited states. These lead to a weak probability of non-radiative transitions and therefore it is very good material for luminescent and laser properties . Moreover, fluorides are suitable host matrices for studying the luminescent properties of rare earth ions . In the past few years, there has been an increased interest in the study of the VUV spectroscopy of rare earth ions which is the part of the effort to design new phosphors for displays, lamps, and scintillator materials [9, 10]. Among the fluoride compounds, CaF2 is an attractive material because of its stability and non-hygroscopic behavior. Besides, recently CaF2 gained a renewed interest as a laser material when doped with RE ions. In the case of CaF2:Yb single crystal for instance, due to its broad emission bands, very good performances as tunable laser and ultra-short pulse laser generator have been demonstrated [11, 12]. Since Boutonnet et al.  reported the synthesis of monodispersed metallic particles, micro-emulsions have been successfully used for the synthesis of several undoped and RE-doped compounds.
The luminescence of europium, has been intensively studied during the last decades and several applications have been reported . As a specific fact europium is able to occur both as divalent and trivalent ion and both of them are of practical interest: BaFBr:Eu2+ is an important X-ray storage phosphor in image plate detection systems and Y2O3:Eu3+ is used in the display technology. Blue light emitting BaMgAl10O17:Eu2+ is a very efficient lamp phosphor. Most of the luminescence studies of europium in solids were carried out on compounds where europium is coordinated by fluorine, chlorine, bromine, and/or oxygen. Recently, luminescence of materials containing europium coordinated by nitrogen has been reported . In some cases, it is quite difficult to stabilize Eu2+ because of its lower stability compared with Eu3+ . Rare earth ions are known to exist in various valence states although the trivalent state is the most prevalent. In particular, Sm and Eu ions are known to be stable in trivalent as well as divalent states. It is also known that exposure to ionizing radiation often results in RE3+ → RE2+ conversion . In fact in the two well-known phosphors used in dosimetry of ionizing radiations using thermoluminescence technique, namely CaF2:Dy and CaSO4:Dy, such conversion has been proposed to play a major role in the thermoluminescence processes . Several RE-doped phosphors such as CaSO4:Dy, LiYF4:Tm3+, Tb3+, Eu3+, MgB4O7:Dy3+ find important applications in various fields. Only few studies have been made on the radiation effect of fluoride based materials.
In this article, we have reported CaF2:Eu and co-doped Bi phosphors that possess the properties which are useful for radio photoluminescence dosimetry for ionizing radiations. Rare earth doped calcium fluoride based phosphor shows the Eu2+ ↔ Eu3+ valencies transfer mechanism due to irradiation effect followed by temperature treatment.
In the preparation of CaF2:Eu and CaF2:Eu, Bi phosphors, we used calcium chloride dehydrate (CaCl2·2H2O, Merk-99.9%), europium oxide (Eu2O3, Himedia, 99.9%), nitric acid (conc. HNO3, Merk-99.99%), ammonium fluoride (NH4F, Merk-99.50%), bismuth nitrate (Bi(NO3)3·5H2O, Merk-99.50%), and ethanol as raw materials. The CaF2:Eu was prepared in the following method
Dehydrated calcium chloride and europium nitrate were mixed into alcohol to form transparent solution, and then ammonium fluoride was added in excess in the calcium chloride solution. Whitish solution was obtained, which was then stirred for 10–12 h. After that calcium chloride was mixed completely with ammonium fluoride solution. Then ethanol was added into the solution and kept on high speed centrifuge at 3000 rpm for 3 h at 22–25 °C, finally paste was formed in white color. This paste was dried at 60 °C for 12 h to get white luminescent powder. For the preparation of CaF2:Eu, Bi, bismuth nitrate was added as impurity in the mixture of dehydrated calcium chloride, ammonium fluoride, and europium nitrate and the same procedure was repeated.
The prepared powder samples were characterized for their phase purity and crystallinity by X-ray powder diffraction (XRD) using PAN-analytical diffractometer (Cu-Kα radiation) at a scanning step of 0.01°, continue time 20 s, in the 2θ range from 10° to 120°. Formation of the compound confirmed by XRD pattern matched with the standard data available in JCPDS file. PL characteristics were studied for 2 mg of sample each time using a RF-5301PC Shimadzu spectrofluorophotometer, at room temperature, using 1.5 nm spectral slit width in the range of 200–700 nm. To study the effect of gamma irradiation on their photoluminescent properties of the prepared phosphors, the samples were exposed to gamma rays from a 60Co source at room temperature at the rate of 0.995 kGy/h.
Results and discussion
X-ray diffraction properties of CaF2 host
Photoluminescent properties of CaF2:Eu phosphor
Photoluminescent properties of CaF2:Eu, Bi phosphor:
The Eu2+ and Eu3+ activated CaF2:Eu and co-doped Bi phosphors were prepared by precipitation followed by centrifuge technique. At the same range of excitation wavelength, CaF2:Eu and CaF2:Eu, Bi gives Eu2+ emission in the blue region and Eu3+ emission in the red region of the visible spectrum. Effect of gamma exposure on PL spectrum gives transfer of valencies from Eu2+ → Eu3+ ion and reverse effect were observed from Eu3+ → Eu2+ ion under temperature effect. Due to Bi3+ ions the PL intensity of Eu2+ and Eu3+ ions increases by two times and same ratio is observed during the conversion of Eu3+ → Eu2+ ions. Here, Bi3+ ions play important role for enhancement of luminescence intensity due to energy transfer mechanism. Hence, it is concluded that it is possible to obtain both Eu2+ as well as Eu3+ ion emission in calcium fluoride based phosphors. It proves that calcium fluoride based phosphors are more susceptible for gamma irradiation and followed by temperature treatment. Therefore, both CaF2:Eu and CaF2:Eu, Bi phosphors may be useful for radio photoluminescent dosimetry phosphor.