International Journal of Physics
and Research (IJPR)
ISSN 2250-0030
Vol. 3, Issue 4, Oct 2013, 25-30
© TJPRC Pvt. Ltd.
CHARACTERIZATION AND PHOTOLUMINESCENCE STUDIES OF CaF2.5Al2O3:Eu3+
AND CaF2.5Al2O3: Ce3+ PHOSPHORS
M. A. KALE1, C. P. JOSHI2, S. V. MOHARIL3 & S. V. GAIKWAD4
1
Department of Physics, Dr. Babasaheb Ambedkar College of Engineering & Research, Nagpur, Maharashtra, India
2
Department of Physics, Ramdeobaba College of Engineering & Management, Nagpur, Maharashtra, India
3
Department of Physics, RTM Nagpur University, Nagpur, Maharashtra, India
4
Department of Chemistry, Dr. Babasaheb Ambedkar College of Engineering & Research, Nagpur, Maharashtra, India
ABSTRACT
The quality of calcium difluoride (fluorite, CaF2) as an optical material has been greatly improved over the past
years following a demand for highest quality materials used as lens and window material in ultraviolet laser lithography
systems. CaF2:Eu have been investigated as efficient luminescent materials with potential applications in solid state
lighting, fiber amplifiers, solid state lasers, and radiation detection CaF2:Eu2+ is one of the earliest known
thermoluminescent materials and was among the earliest used for TL dosimetry. Wet chemical synthesis and combustion
synthesis were used to prepare CaF2.5Al2O3. The X-ray diffraction pattern of the powder synthesized by combustion
synthesis of aluminium nitrate and calcium fluoride in the mole ratio of 1:10:26.66.
All the peaks are well indexed to CaF2.5Al2O3 phase (Acta Cryst. 6 (1953) 363). Luminescence studies have been
carried out frequently for CaF2 but there are no data on CaF2.5Al2O3. Three strong emission peaks of Eu3+ doped
CaF2.5Al2O3 is found at around 591 nm, 614 nm and 646 nm for the excitation at around 300 nm. The PL spectrum for
Ce3+ exhibits broad band peaking at 330 nm at prominent band of excitation around 263 nm wavelengths.
KEYWORDS: Aluminate, Photoluminescence, Combustion Synthesis
INTRODUCTION
The quality of calcium difluoride (fluorite, CaF2) as an optical material has been greatly improved over the past
years following a demand for highest quality materials used as lens and window material in ultraviolet laser lithography
systems[1]. Wide-band-gap fluoride CaF2 provide attractive hosts for those rare earth (RE) ions that are capable of
producing fast and efficient emissions under ionizing irradiation[2,3,4,5]. Doped phosphors such as CaF2:Eu and MgF2:Eu
have been investigated as efficient luminescent materials with potential applications in solid state lighting, fiber amplifiers,
solid state lasers, and radiation detection[6,7]. CaF2:Eu2+ is one of the earliest known thermoluminescent materials and was
among the earliest used for TL dosimetry[8]. CaF2:Eu2+ acts as a scintillator which is frequently used in investigations for
rare events such as dark matter and neutrinoless double beta decay. Recently, spectral hole burning has been reported in
CaF2:Eu phosphors[9,10].
Though luminescence studies have been carried out frequently for CaF 2 but there are no data on CaF2.5Al2O3.
Only a small note on the crystal structure of CaF2.5Al2O3 is given by Harald Perlitz and Gunnar Gunther[11]. Attempt was
made to prepare CaF2.5Al2O3 for the first time by combustion synthesis. We have attempted activation of this host using
Eu3+ and Ce3+.
26
M. A. Kale, C. P. Joshi, S. V. Moharil & S. V. Gaikwad
EXPERIMENTAL
Synthesis of CaF2.5Al2O3:Eu3+ and CaF2.5Al2O3: Ce3+ Phosphors
1 mol% Eu3+ doped CaF2.5Al2O3 and 1 mol% Ce3+ doped CaF2.5Al2O3 has been prepared by combustion
technique using Calcium fluoride (CaF2), Aluminium nitrate (Al(NO3)3.9H2O) as oxidizer, urea (NH2.CO.NH2) as a fuel
and dopant Europium nitrate and cerium nitrate. CaF2 was first prepared by wet-chemical synthesis. The starting chemicals
were mixed in an agate mortar to form a paste which was then transferred to a china dish. In a typical combustion reaction,
a china dish containing paste is introduced into a muffle furnace preheated to 500±10 oC. Initially, the paste melts, boils and
froths, followed by the appearance of flame yielding a voluminous product. The entire combustion process was completed
in less than 5 min. the dish was then taken out and the foamy product is crushed into fine powder and was used for
characterization without any further processing.
Characterization
Powder XRD pattern of CaF2.5Al2O3 was recorded on Philips PANalytical X’pert Pro diffractometer. Room
temperature Photoluminescence (PL) measurement was carried out using a Hitachi F-4000 spectrofluorimeter, at room
temperature, using 1.5nm spectral slit width in the range of 200–700 nm.
RESULTS
X-Ray Diffraction
The X-ray diffraction pattern of the powder synthesized by combustion synthesis of aluminium nitrate and
calcium fluoride in the mole ratio of 1:10:26.66 is shown in Figure 1. All the peaks are well indexed to CaF2.5Al2O3 phase
(Acta Cryst. 6 (1953) 363). The host lattice CaF2.5Al2O3 has a hexagonal structure with space group D16h-C6/mmm or
D36h-C6/mcm and its lattice parameters are a=5.529Ao and c= 21.79Ao.
Figure 1: XRD Pattern of CaF2.5Al2O3
Photoluminescence Studies and Rare Earth Activators
Ce3+
The 5d-level spectroscopy of Ce3+ is very simple. The 4f shell is empty and there is only one single 5d electron
interacting with the crystalline environment. The Ce3+ ion has the [Xe] 4f1 configuration, which results in only two 4f1
energy levels: the 2F5/2 ground state and 2F7/2 excited state. These energy levels are approximately 2000 cm-1 apart. At
higher energy, the 4f05d1 bands can be found. The energy of the bands is strongly dependent on the host lattice. The 4f 1
ground state is separated about 51,000 cm-1 from the excited 5d1 configuration. In a crystalline environment, the 5d
configuration may split by as much as 25,000 cm-1 into at most five distinct 5d states. In addition, the average energy of
the five 5d levels may shift downwards by 22,000 cm-1. The redshift of the first f–d-transition in Ce3+ when introduced in a
Characterization and Photoluminescence Studies of
CaF2.5Al2O3:Eu3+ and CaF2.5Al2O3: Ce3+ Phosphors
27
crystalline host is a result of two mutually independent contributions: (1) The centroid shift, defined as the lowering of the
average energy of the Ce3+ 5d configuration relative to the value for Ce3+ as a free ion. (2) The total crystal field splitting;
defined as the energy difference between the lowest and highest 5d level. The 4f05d1–4f1 emission is parity-allowed with a
decay time of 3–50 ns. Both absorption and emission have a usually broad band character, showing splitting characteristic
of 2FJ states. As the position of 5d band itself depends on the host, not only the Stoke’s shift but also the spectral positions
of both the excitation and emission bands are host dependent. Luminescence of Ce3+ gets quenched above the
concentration of about 5%. The quenching temperature is usually high. The PL spectra of CaF2.5Al2O3:Ce3+ phosphor is
shown in Figure 2. The PL spectrum for Ce3+ exhibits broad band peaking at 330 nm, which is due to the transition from 5d
level to the ground state of the Ce3+ ion. The characteristics doublet of Ce3+ ion is not clearly observed in the emission
spectrum, but it can be slightly resolved into two emission bands at 330 nm and around 350 nm corresponding to the
transitions of 5d states to 2F5/2 and 2F7/2 of Ce3+ ion respectively. The excitation spectrum is relatively narrow showing a
prominent band around 263 nm wavelength.
Figure 2: PL Spectra of CaF2.5Al2O3:Ce3+
a) Ce3+ Emission for 263 nm Excitation
b) Ce3+ Excitation for 330 nm Emission
Eu3+
Europium can act as an activator in two forms, viz. Eu2+ and Eu3+. Eu3+ or Eu2+ can be identified from the
characteristic PL they exhibit. Eu2+ emission arises from the lowest band of 4f65d1 configuration to 8S7/2 state of 4f7
configuration. The excitation arises from the transition from 8S7/2 state of 4f7 configuration to the states belonging to the
4f65d1 configuration. Due to the allowed nature of the transition, PL is intense. Spectral positions of these bands vary a
great deal from lattice to lattice. f–f transitions of Eu3+, on the other hand, are forbidden and Eu3+ PL is in general weak,
unless there is excitation by charge transfer or energy transfer from a sensitizer. In general, narrow emission bands may be
observed at about 570, 590, 610, 650 and 700nm corresponding to transitions 5D0-7F0, 7F1, 7F2, 7F3, 7F4, respectively. Eu3+
emission usually occurs from 5D0-7FJ transitions. There are three transitions which are of prime importance 5D0-7F0 (around
570 nm), 5D0-7F1 (around 595 nm) and 5D0-7F2 (around 610 nm). The first one is strongly forbidden transition and yet
observed with appreciable intensity in some hosts. 5D0-7F1 transition is forbidden as electric dipole, but allowed as
magnetic dipole. This is the only transition when Eu3+ occupies a site coinciding with a centre of symmetry. When Eu3+ ion
is situated at a site, which lacks the inversion symmetry, then the transitions corresponding to even values of J (except 0)
are electric dipole allowed and red emission can be observed. 5D0-7F1 transition can also be observed as magnetic dipole
allowed transition. Further, all the lines corresponding to these transitions split into number of components decided by the
local symmetry.
28
M. A. Kale, C. P. Joshi, S. V. Moharil & S. V. Gaikwad
Figure 3. shows the Photoluminescence emission and excitation spectra of CaF 2.5Al2O3 doped Eu3+ activator. In
this result we found three strong emission peaks of Eu3+ doped CaF2.5Al2O3 compound as shown in Figure 3. The emission
peaks are found at around 591 nm, 614 nm and 646 nm for the excitation at around 300 nm. The excitation is quite broad
as compare to the emission spectra. No Eu2+ emission is found.
Figure 3: PL Spectra of CaF2.5Al2O3:Eu3+
a) Eu3+ Emission for 300 nm Excitation
b) Eu3+ Excitation for 614 nm Emission
After getting confirmation of formation of single phase compound by X-ray diffraction pattern, attempt was made
to study the photoluminescence characteristics of CaF2.5Al2O3 by doping various activators in the same compound.
CaF2.5Al2O3:Eu3+, CaF2.5Al2O3:Tb3+, CaF2.5Al2O3:Ce3+, and CaF2.5Al2O3:Pb2+ were prepared by using combustion
synthesis and we found appreciable luminescence in all these samples. We also tried with CaF2.5Al2O3:Bi3+ but found no
emission.
X-ray diffraction pattern of the powder synthesized by combustion synthesis of aluminium nitrate and calcium
fluoride in the mole ratio of 1:10:26.66. All the peaks are well indexed to CaF2.5Al2O3 phase (Acta Cryst. 6 (1953) 363).
The photoluminescence characteristics of CaF2.5Al2O3 by doping activators in the same compound were studied.
CaF2.5Al2O3:Eu3+, CaF2.5Al2O3:Ce3+ were prepared by using combustion synthesis. Photoluminescence spectra at room
temperature were recorded in the range 220-700nm on Hitachi F-4000 spectro-fluorimeter with spectral slit widths of 1.5
nm. Three strong emission peaks of Eu3+ doped CaF2.5Al2O3 is found at around 591 nm, 614 nm and 646 nm for the
excitation at around 300 nm. The PL spectrum for Ce3+ exhibits broad band peaking at 330 nm at prominent band of
excitation around 263 nm wavelengths.
Figure 4
Characterization and Photoluminescence Studies of
CaF2.5Al2O3:Eu3+ and CaF2.5Al2O3: Ce3+ Phosphors
29
CONCLUSIONS
Results on luminescence in CaF2.5Al2O3 are reported for the first time. CaF2.5Al2O3: Eu3+ and CaF2.5Al2O3:Ce3+
powder phosphors has been prepared as a single-phase material by combustion route. Phosphor prepared from the facile
combustion route is simple, fast and easily available material. The process appears to be energetically economic and
attractive. The emission peaks for Eu3+ doped CaF2.5Al2O3 are found at around 591 nm, 614 nm and 646 nm for the
excitation at around 300 nm.
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