Na2SO4, Na2SO4: Li, and Na2SO4: Li, Eu, Dy phosphors were prepared by using slow evaporation technique followed by subsequent calcination at 400°C for 4 h. Doping with Li+ ion stabilized the thenardite phase of host matrix, while codoping with RE3+ stabilized the phase transformation from stable thenardite to metastable mirabilite crystal structure. The microstructure and morphology were studied by using scanning electron microscopy and transmission electron microscopy. The thermoluminescence studies revealed that isovalent doping of Li+ served as a quencher and addition of codopant introduces the additional trap sites in the host matrix. The room temperature emission spectra of Li-doped, RE3+-codoped, and undoped Na2SO4 were studied under ultraviolet radiation. For pure Na2SO4 the two peaks which appeared are at 364 and 702 nm, respectively. The emission intensities of RE3+-codoped samples increase with increase in dopant concentration.
Alkali sulphates have been known for a long time as versatile and excellent phosphor materials. These sulphates have attracted the attention of many workers in view of their potential applications in radiation dosimetry, TV screens, cathode ray tubes, and so forth. A variety of defect centres are likely to be formed in sulphate based phosphors [
In the present work, fading TL and PL behaviour of pure and RE3+-codoped Na2SO4 matrix are explained in detail. The host matrix shows the existence of bicrystalline phase after codoping with hypervalent ion. The codoped and pure Na2SO4 samples have been well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques.
Na2SO4 and Li2SO4 (0.5 mol%) were stoichiometrically dissolved in double-distilled deionized water (A). A known quantity of RE2O3 (RE = Eu and Dy) was dissolved in concentrated H2SO4 (B) and added to solution (A) to get RE/Na mole ratio in the concentration range of 0.08 to 0.5 mol%. The solution is then dried in an oven. The sample thus obtained in powder form was crushed and calcined at 400°C for 4 h in furnace. After natural cooling to RT, it is crushed to fine powder and pressed into pellets (80 kg/cm2).
The PXRD pattern of sample is obtained using Philips PW/1050/70/76 X-ray diffractometer which was operated at 30 KV and 20 mA using CuK
TL glow curves were measured with system in the temperature range from 25 to 300°C operating with linear heating rates of 5 K s−1. Prior to the TL measurements, samples (pellets of 1 mm thickness with 5 mm in diameter are used for TL measurements) were exposed to
Figure
Comparison of PXRD patterns of Na2SO4, Na2SO4: Li+, and Na2SO4: Li+, Eu3+, Dy3+ with host matrix. (The peaks with asterisks correspond to the mirabilite phase.)
The average crystallite size (
The higher ionic radius of the RE3+ ion can possibly act as an interstitial impurity in the host matrix. The introduction of substitutional dopant metal ions with a higher ionic size would induce cationic vacancies at the surface of thenardite grains, which favours the bond rupture, ionic rearrangement, and structure reorganization for the formation of mirabilite phase. The thenardite to mirabilite phase is generally considered as a nucleation growth process during which the mirabilite nuclei are formed within the thenardite phase. The thenardite grain size decreases with an increase in the Dy3+ concentration (Table
The crystallite size and stress factor of pure, doped, and codoped Na2SO4 samples.
Phosphor | Crystallite size (nm) | FWHM values (rad) |
|
2 |
Stress factor ( |
|||||
---|---|---|---|---|---|---|---|---|---|---|
T | M | T | M | T | M | T | M | T | M | |
Na2SO4 | 55 | — | 0.29 | — | 1.43 | — | 32.4 | — | 1.2 | — |
Na2SO4 : |
22 | — | 0.37 | — | 2.77 | — | 32.3 | — | 5.5 | — |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.2% | 59 | 98 | 0.14 | 0.083 | 2.77 | 3.95 | 32.3 | 22.5 | 2.1 | 1.8 |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.5% | 39 | 82 | 0.21 | 0.100 | 2.77 | 3.95 | 32.3 | 22.5 | 1.5 | 2.2 |
T and M correspond to thenardite and mirabilite content of Na2SO4.
With an increase in the Dy3+ content, the variation in the lattice parameter was reflected in the elongation of the
Comparison of lattice parameters and cell volume of pure, Li-doped, and RE3+-codoped Na2SO4 samples.
Phosphor |
|
|
|
|
||||
---|---|---|---|---|---|---|---|---|
T | M | T | M | T | M | T | M | |
Na2SO4 | 5.80 | — | 12.20 | — | 7.60 | — | 538 | — |
Na2SO4 : |
5.86 | — | 12.32 | — | 7.66 | — | 553 | — |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.2% | 10.7 | 9.51 | 10.43 | 15.6 | 6.08 | 3.452 | 679 | 512 |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.5% | 11.0 | 9.55 | 10.47 | 15.8 | 6.12 | 3.481 | 705 | 525 |
T and M correspond to thenardite and mirabilite content of Na2SO4.
Due to smaller ionic radius, the stress associated with Li+ is dominated by lattice contraction. It is interesting to note that stress factor associated with Li+ doping (lattice contraction) is larger compared to RE3+ doping (lattice expansion).
The surface morphological features of the Na2SO4, Na2SO4: Li0.5%, and Na2SO4: Li0.5%, Eu0.5%, Dy0.5% samples are shown in Figure
SEM image of (a) Na2SO4, (b) Na2SO4: Li, and (c) Na2SO4: Li, Eu, Dy, respectively.
The shape and size of these particles were also determined by TEM (Figures
(a) TEM image and (b) selected area electron diffractions of Na2SO4.
(a) TEM images and (b) selected area electron diffraction of Na2SO4: Li, Eu, Dy.
Results revealed that, in pure Na2SO4, these spots might belong to the
TL property of host sample was investigated at different
TL glow curves of
In Na2SO4: Li0.5%, Li+ quenches the TL intensity of Na2SO4 and shifts the TL glow curves position towards lower temperature (Figure
TL glow curves of
The glow curve of Na2SO4:
TL glow curves of
The trap parameters, such as activation energy
Representative diagram of different quantities used in the glow curve shape method.
In the present study, geometrical form factor value is very close to 0.52, suggesting that the TL emission involves retrapping of charges. Table
Comparison of kinetic parameters for pure, Li-doped, and RE3+-codoped Na2SO4 samples.
Phosphor | Activation energy (eV) | Frequency factor (s−1) |
---|---|---|
Na2SO4 | 0.15 | 4.59 × 109 |
Na2SO4 : |
0.35 | 5.51 × 1013 |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.1% | 0.18 | 1.41 × 1011 |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.2% | 0.19 | 1.20 × 1011 |
Na2SO4 : Li0.5%, Eu0.5%, Dy0.5% | 0.20 | 1.51 × 1012 |
The effect of fading was studied up to 90 days by irradiating samples with a gamma dose of 2 kGy. The extent of fading was maximum for Na2SO4 compared to codoped Na2SO4, suggesting that introduced by addition trap sites favouring the intensification of TL signal (Figure
Fading observed in Na2SO4, Na2SO4: Li0.5%, and Na2SO4: Li0.5% Eu0.5%, Dy0.5%.
The PL emission spectra of host sample consist of a strong peak at 364 nm for pure thenardite and are attributed to band-to-band PL phenomenon due to the band gap excitation (Figure
PL emission spectra of pure Na2SO4
In addition, low intense peak at ~702 nm is attributed to excitonic PL arising from intrinsic defects of thenardite Na2SO4 powders. The emission band with the most intense peak at approximately 702 nm showed a distinct vibronic structure. The vibronic structure characteristic of S2− centre was observed in the red region.
The
The PL emission spectra of Na2SO4: Li0.5% are the same as that of the host sample (Figure
PL emission spectra of Na2SO4: Li
The PL emission spectra of Na2SO4: Li+, Eu3+, Dy3+ for 340 nm excitation showed bands at 481, 490, 575, and 711 nm. The first three peaks are assigned to Dy3+ and the peak arising at 711 nm was assigned to Eu3+ emission (Figure
PL emission spectra of Na2SO4: Li0.5%, Eu0.5%, Dy0.5%
Pure and codoped phosphors have been prepared at RT by slow evaporation technique. PXRD pattern confirmed the phase transformation from stable thenardite to metastable mirabilite after codoping with RE3+ ions. SEM studies showed the presence of uniform distribution of prisms and pyramids. The codopant introduces additional trap sites favouring the intensification of TL signal. Therefore, up to a given irradiation dose, this phosphor has potential candidate for use in radiation dosimetry. The PL intensities of codoped Na2SO4 samples were found to be dependent on the RE3+ concentration.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Y. S. Vidya is thankful to “ISRO-ISEC, advanced devices and radiation cell, Bangalore” for providing facilities for