Tuning of luminescent properties of Zn1-xMgAl10O17:Eux nano phosphor

Accepted Manuscript Tuning of luminescent properties of Zn1-xMgAl10O17:Eux nano phosphor Akshkumar Verma, Sanjay Kumar Pathak, Ashish Verma, G.V. Bramhe PII:

S0925-8388(18)32133-9

DOI:

10.1016/j.jallcom.2018.06.023

Reference:

JALCOM 46359

To appear in:

Journal of Alloys and Compounds

Received Date: 8 January 2018 Revised Date:

1 June 2018

Accepted Date: 3 June 2018

Please cite this article as: A. Verma, S.K. Pathak, A. Verma, G.V. Bramhe, Tuning of luminescent properties of Zn1-xMgAl10O17:Eux nano phosphor, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.06.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Tuning of Luminescent Properties of Zn1-xMgAl10O17:Eux nano phosphor Akshkumar Verma1*, Sanjay Kumar Pathak1, Ashish Verma1, G.V. Bramhe2 1

Department of Physics, Dr. Harisingh Gour Central University Sagar- 470003, M.P., India 2 Department of Physics, Govt. College Sousar, Chhindwara, M.P., India

Abstract The

Zn1-xMgAl10O17:Eux2+

(x=0.01-0.4) nano phosphors were prepared successfully using the

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combustion route by employing urea as a fuel. The structural, microstructural, thermal, optical and luminescent properties were measured on the samples. The nano phosphors were found to be of surface area (~13.92 m2/g) and insulators of the band gap (~5.24eV). Photoluminescence properties on the samples were characterized by excitation (λex.=340nm) and emission (λem. =

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440nm) spectra. The phosphors showed strong blue luminescence (440nm-460nm) as well as showed weak luminescence in 577nm, 589nm, 596nm, and 615nm in the orange-red region,

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which are attributed to 4f65d1→4f7 and 5D0→7Fj (j=0, 1,..) transitions of the Eu (II) and Eu (III) ions respectively. The tuning of this blue luminescent properties as compared to the earlier reported red luminescence has been achieved. Thermo-luminescence glow curve recorded under 10-minute dose of UV (254 nm) radiation have been used to calculate the kinetic parameters and the activation energy of the nano phosphors by using the Chens-equation. The TL emission spectra of the Zn0.99MgAl10O17:Eu0.01 phosphor showed the characteristic Eu2+ emission peaks

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~450nm (blue) originating from the transitions 4f65d1→4f7. The possible mechanism of the PL and TL of blue light emitting phosphor are also discussed.

Keywords: Urea fuel combustion, Blue phosphor, Photoluminescence, Thermo-luminescence, Highlights

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Glow curve and Kinetic parameter.


The Zn1-xMgAl10O17:Eux2+ nano phosphor synthesis by UFC modified route.

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The micro-strain of crystalline composite (Host) depends on dopant (activator) element.
Thermal stability of compound doesn't lose in the range of 250C degrees to 9000C.
The PL and TL intensity decreases from maxima to minima due to the quenching phenomena at higher concentration of dopant ions.
The efficient Eu2+ doped ZnMgAl10O17 phosphor exhibits blue emission spectrum. *

Corresponding author

Email. [email protected]

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1. Introduction Solid state lighting technology has found a lot of recent interest because of their most challenging applications, as this may be a replacement for the conventional incandescent fluorescent lamps

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(CFL) and light emitting diodes (LED) due to its low cost [1,2]. This technology is based on the development of materials with narrow particle size distribution, homogenous, large surface area, high purity, high quantum efficiency, optimum brightness, low energy consumption, long life span, high thermal stability, eco-friendly, lack of pollutants, etc. [1-3]. In recent years, there is an

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urgent demand of novel blue phosphors having low preparative cost, which may help to reduce the cost of LED and CFL lamps for under-developed countries, as blue, red and green phosphors are highly useful for the display technology. The phosphors are luminescent materials that usually

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comprise a transparent matrix activated by rare earth ions [4]. There have been different synthesis techniques for preparing the phosphors, such as electric arc method [5], sol-gel method [6], solidstate reaction method [7], the microwave heating method [8], combustion method [9-11], hydroxide precipitation method [12], etc. Out of all the above methods, the combustion method is very simple, cost-effective, safe, energy saving, takes less time and the samples are prepared

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comparatively at lower temperatures [13-14]. The improved solution combustion synthesis method also synthesizes oxides with regular morphologies of triangles, tubes, wires, and rods [15] and the particle size of the synthesized product depend on the effect of different initiating temperature and mass of urea [16]. In the present communication, the phosphor powders are

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obtained by urea fuel combustion (UFC) reaction of starting materials (usual nitrates), at the relatively lower temperature (5500C).

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Luminescence from phosphor materials doped with rare earth ions is of much interest in recent years [4]. Europium, Cerium, Dysprosium, Samarium etc. rare earth ions have an emission that arises from the electronic transition between levels of the 4fn configuration. These 4f electrons are well shielded from the crystal field of neighboring ions by 5s2 and 5p6 electrons [17]. Some rare earth ions can also give luminescence as a result of transition between 4fn-15d1 excited state and 4fn ground state. The 5d excited states are not strongly shielded from the crystal field by the 5s2 and 5p2 electron and therefore the spectral properties are strongly affected by the host lattice. The spectral widths are typical of (~1000cm-1) because these transitions are not forbidden by the parity selection rule & they have also shorter lifetimes (nanoseconds to microseconds). In recent years, concerted efforts have been made to improve the characteristic of phosphor materials. Strontium aluminate doped with rare earth metal ions were the first studied material in 2

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the late 1930s because of their excellent properties such as high quantum efficiency. In attempts, SrMgAl10O17:Eu2+ a blue emitting phosphor is reported in several recent papers due to the existence of long life phosphorescence phenomena as well as its good stability [18] & application for plasma display panels (PDP) [19-21]. BaMgAl10O17:Eu2+ has been regarded as the preferred blue emitting phosphors for fluorescent lamps, PDP and Hg-free lamps applications because of its high luminescence efficiency and good color parity under ultraviolet (UV) and vacuum ultra-

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violet excitations [17-18]. The Eu doped ZnMgAl10O17 red phosphors, prepared by combustion method are useful for the mercury excited lamp application [4] the detailed description of the method can be found in the original work of Patil and co-worker [22-27]. Since ZnMgAl10O17:Eu2+ phosphor is a known red phosphor [4], In this work, we have undertaken a

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study on this material doped with Eu ion & various characterizations were performed on the samples, in order to investigate the possible improvement of optical properties (from a red

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phosphor to the demanding blue phosphor) by tailoring it’s particle size to a lower value (by varying the experimental conditions during synthesis in comparison to its earlier studies [4]).

2. Experimental details

The chemicals under the study were purchased from Loba chemicals with 99.9% purity. The samples of Zn1-xEuxMgAl10O17 (x=0.01-0.4mol) were synthesized by UFC route. The starting materials were taken as aluminium nitrate [Al(NO3)3.9H2O], Zinc nitrate [Zn(NO3)2.6H2O],

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magnesium nitrate [Mg(NO3)2.6H2O]. Europium nitrate was prepared by dissolving europium oxide [Eu2O3] in 2ml nitric acid. The correct amount of each excess urea [CO(NH2)2] was injected into the precursor solution of these compositions. The appropriate amount of reactants (The amount of metal nitrates (oxidizers) and urea (fuel) were weighed on the analytical balance (WENSAR:

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MAB182) using the total oxidizing and reducing valence’s of the components, which serve as the numerical coefficients so that the equivalence ratio is unity (O/F=1) and the heat liberated during

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combustion is at the maximum. Then weighed quantities of each nitrate and urea are mixed together and crushed with the help of a mortar & pestle for 50 minutes to form a thick paste. The paste in a crucible is introduced into a vertical cylindrical muffle furnace (MICRONIX MIT 961) maintained at 5250C-5500C. Initially, the mixture boils and undergoes dehydration with the evolution of large amount of gases (oxides of carbon, nitrogen and ammonia) & the process becomes exothermic continuously and the spontaneous ignition occurs. The solution underwent smouldering combustion with enormous swelling and producing white foamy with voluminous ash. The flame temperature converting into 16000C-18000C [22-24] that converts the vapour phase oxides into mixed aluminates. The flame persists for (~ 30seconds). The crucible is then taken out of the furnace and the foamy product was milled at room temperature for 30 minutes to form a homogenous white 3

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powder of Zn1-xEuxMgAl10O17. The chemical reaction for the synthesis of the samples is given in equation (1) for clarification and the prepared samples are also given in Table 1. 10Al NO . 9H O + 1 − x ZnNO  . 6H O + Mg NO  . 6H2O + x [Eu O + HNO ] + NH CONH → Zn Eu MgAl! O " + H + N + NH + O + CO + H O ↑ 1

The x-ray diffraction patterns of as-prepared samples were determined by powder X-ray diffractometer (D8 ADVANCED BRUKER) by using Cu-Kα radiation. The morphology, porosity

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particle size and the composition of the products were examined by scanning electron microscopy (FESEM –FEI NOVA NANO SEMTM 450, 30kV) and transmitting electron microscopy (TECNAI G2 T30 (S-TWIN). The BET surface area was measured by BEL SORP MR6 surface area analyzer. Simultaneous DTA/TGA, spectra were recorded using NETZSCH STA 449 F1

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Jupiter. DTA and TGA analyses were carried out using 22mg of the sample at a heating rate of 10oC/minute up to 1000oC, in nitrogen air under a flow of 60cm3 min-1. UV-VIS spectra were recorded using SYSTRONICS UV-2201 Double beam spectrophotometer in the absorbance mode.

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The sample was dispersed in the methanol and then UV-Vis spectra were recorded. Infrared (IR) spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer of SHIMADZU (model no. 8400S) with a resolution of 2cm-1 and in the range 500-3800cm-1. To measure IR spectra, the samples were mixed with KBr (Sigma Aldrich, 99.99%) in 1:5 ratio and then spectra recorded. The Raman spectra of the sample were recorded by RENISHAW Micro Raman

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Spectrometer attached with a He- Ne laser excitation source of 633nm. The photoluminescence spectra measurement were carried out under ultraviolet excitation using 350nm radiation from NdYAG laser and detected by CCD detector ((RF- 5301 PC Shimadzu Model: QE 65000, Ocean Optics, USA) attached with the fiber. The lifetime decay was recorded with a fluorescence

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spectrometer (Hitachi F-4500) with a 100W flash xenon lamp as the excitation source. The thermo-luminescence (TL) glow curves were recorded on the samples [after giving UV (254nm)

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dose for 10 minutes] by using a TL spectrophotometer (THERMO-LUMINESCENCE DOSIMETER READER). TL emission spectra were recorded using interference filters of wavelengths (400nm to 700nm).

3. Results and discussion 3.1. Structural, Micro-structural and Thermal Properties 3.1.1. X-ray Diffraction Analysis The overall structure, phase purity, determine phases and track the structural changes of the synthesized samples were analyzed by X-ray diffractograms. The small amount of doped europium ions has virtually any effect on phase structures. The broadening of the diffraction peaks suggests that the particles are small. The XRD data (Fig. 1 (a) and Table 2) indicates homogeneity and well crystalline hexagonal phase of Zn1-xMgAl10O17:Eux (E1 and E9) samples. 4

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The diffraction peaks are attributed to the [111], [220], [311], [400], [412], [511], [440], [533], and [444] lattice planes of hexagonal phase parameters (a=b=5.63 Å, c=22.37Å, cell volume= 614.045 Å3) by comparing the JCPDS No. = 01-076-2464 [1, 4]. The crystallite size (D) of the samples are estimated by the Scherrer equation (2). () 2

*+,-. is the diameter of the crystallite, λ is the wavelength of x-ray, β is the full width at $%& =

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Where DSE

half maximum of the diffraction peaks, θ is the angle of diffraction, and k is the shape factor (k = 0.9). The effective crystallite (DSE) size was found to be in the range of ~10nm-120nm as shown in Table 2. The Peak size broadening in PXRD pattern gives many information’s. There are many

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factors that cause peak broadening such as instrumental factors, the presence of defects, differences in strain, size of the crystallites. It is easy to calculate the effects of size and strain. Where size broadening is independent of q (K=1/d), strain broadening increases with increasing

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q-values. In most cases, there will be both size and strain broadening. It is possible to separate these by combining the two equations in what is known as the Hall-Williamson method (WH) [28]. In addition, the shifting in lattice parameter creates a micro-strain in the crystal which can be calculated by using the Williamson-Hall method equation (3). * cos. =

() + 2 sin. 3

$

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Thus, the slope of βcosθ/λ verses sinθ/λ plot has given the micro-strain (η) as shown in Fig. 1(b), we have got a straight line with slope η and intercept kλ/ D. The value of η is the micro-strain in the crystallites. The constant k is typically close to unity and ranges from 0.8 to1.39 [29-30].

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The typical micro-strain values of all samples was calculated by equation (3) using Williamson– Hall (W-H) fitting method [31] and positive and negative micro-strain has obtained as shown in Table 2. The positive liner fit (E1, E3, E4, E5 and E6 samples) indicates that the presence of

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defect and stress inside crystal as well as which may be due to crystal planes are not identical. The negative linear fit (E7 and E9) indicates that the instrumental broadening, anisotropy strain, and compressive strain, are present in the crystal. The compressive strain is present in the crystal already reported by Jain et al. 2017[32]. The micro-strain which shows the compressive strain nature of the crystal due to more doping of Eu as shown in Fig. 1(b).Again the effective crystallite (DWH) size has found to be in the range of ~12nm-655nm from equation (3). The effective crystallite size has found more than the crystallite size of Scherer equation due to the present of micro-strain as shown in Table 2. From the XRD analysis, no more appreciable structural change has observed for the ZAM samples and all the samples of ZAM remain symmetrical structure (hexagonal) due to doping of Eu into the matrix. The structure of 5

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ZnMgAl10O17 is a hexagonal structure (β-alumina), space group P63/mmc. In other hand, βalumina [33-34] consists of two spinel blocks (MgAl10O16) separated by one mirror plane [3537], when Eu2+ is substituted into the host lattice, it can have three possible locations: BeeversRoss (BR), anti-Beevers-Ross(a-BR), and mid-oxygen (mO) sites in the mirror plane as shown in Fig. 2. M. Stephan el all were found the majority of Eu ions occupy mO sites in β′′-alumina structure of hexa-aluminate [38]. Recently, nuclear quadrupole calculation has shown that an a-

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BR site is energetically more stable than the other two sites [39]. In ZnMgAl10O17, Mg2+ substitutes half for Al3+ in the widest tetrahedral site, while Zn2+ occupies exclusively slightly offcentered nine-fold coordinate sites around the BR position, in the interblocks planes that form a 2D near–close array with the interblocks oxygen atom O(V)[40].

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3.1.2. Scanning electron microscopy analysis

It is observed from the SEM micrographs (Fig. 3), the obtained structures are about 10nm to

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600nm in diameter compatible with the calculated values of the XRD data. Several micro/nano spherical particles are found within the grain can be easily observable from the high-resolution micrographs. The micrographs also shows the presence of voids or pores [Fig.3 (e & f)] which are formed during the combustion synthesis of the samples as shown in Fig. 3(a, b &c). The nonuniformity of the shape of the samples is attributed to the non-uniform distribution of temperature and mass flow in the combustion flame during synthesis of the samples. From the Variation of

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fuel and fuel-to-oxidant ratio the crystal phase, surface area, particle size, and porosity could also be suitably tailored [41]. We also expect the nano-phosphor prepared by the combustion techniques is thermally stable and optical properties of these phosphor materials may be affected by the particle surface morphology.

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3.1.3. High resolution Transmission electron microscopy & BET surface area analysis Figures 4(a) and 4(b) shows the typical TEM images of one of the samples (E1) under study. It is

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clear that the sample has formed nano-crystalline clusters and the obtained structures are composed of agglomerated particles of size (~ 20nm to 600nm) with a higher extent of nonuniformity in particle shapes (spherical and agglomerated particles ) [42]. It can be seen from TEM micrograph of E1sample that the size of the particles around ~25nm to 120nm which is good agreement with the crystalline size ~26nm to 136nm estimated by Williamson-Hall formula. The non-uniformity of particle shapes & sizes are attributed to the combustion synthesis method [During which a large volume of volatiles release due to very high non-uniform temperature (16000C -18000C) distribution of the flame]. The redox reactions with the nitrates become more exothermic & make them undergo combustion violently. Due to the uncontrolled dynamics of the process, the powder obtained contain pores, irregular shape and sized particles and is an inherent feature of this process. A typical HRTEM image of E1 phosphor sample is 6

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shown in Fig. 4(c). The precise observation of the HRTEM image indicates that the sample exhibits lattice fringes with an estimated interspacing of 0.234nm [311], 0.208nm [400], and 0.160nm [511]. The corresponding selected area electron diffraction [Fig. 4(d)] pattern shows ~68 distinct rings which are the indication of the diffraction pattern from an assembly of nanocrystallites. The first, second, third and fourth rings of the electron diffraction pattern are attributed to the [311], [400], [511] and [440] lattice planes of E1 phosphor with a hexagonal

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phase and are also consistent with the obtained XRD results [Fig.1]. The BET surface area of the E1 sample was found to be ~13.922m2/g. With the help of combustion method, other groups of researchers have prepared phosphor samples and they were found BET surface area ~6.372 m2/g [19], ~10 m2/g [43].

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3.1.4. Fourier Transform Infrared Spectroscopy and Raman spectroscopy analysis

The typical FTIR absorbance spectrum of E1sample in the range of 500–4000cm-1 is shown in

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Fig. 5(a). The strong band around 3500cm-1is assigned to the absorbed water molecule to the sample under observation. The band at 1620cm-1 to 1660cm-1 is assigned to H-O-H stretching vibrations which provide the evidence of the presence of molecular water in the structure [44]. There is no absorbance at around 1387cm-1 [45] which indicates the non-presence of nitrates inside the samples, while in some other studies, nitrates were reported to be present in powders (produced by UFC method and even after their further treatment at 7000C). In the present studies,

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the furnace was maintained at 5500C and combustion occurred to produce an exothermic reaction and rise the reaction temperature up to ~18000C resulting in the removal of nitrates and organic products from the final sample & no post thermal treatment were also required. Some sharp and oscillating peaks were found between 830cm-1 to 500cm-1 are attributed due to the stretching,

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rotating and vibrating modes of ZnMgAl10O17 in hexagonal lattice coordinates. In order to enhance the understanding of the doping effect from the structural point of view, Raman

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scattering study was made on the E1 sample [Fig. 5(b)]. The hexagonal symmetry of the sample is also supported due to the occurrence of vibrational modes (in the range of 2650cm-1 to 2800cm1

) [46].

3.1.5. Simultaneous Thermal Analysis The typical thermal properties of E1 sample is shown in Fig.6. The Eu doped ZnMgAl10O17 was studied by using differential thermo-gravimetric analysis. Heat flow and change in mass were recorded under the nitrogen atmosphere in temperature range 230C to 9000C with heating rate 100C min-1. The DTA analysis reveals an endothermic peak at around 3900C to 4000C, associated with the decomposition of physically adsorbed molecular water to vapour from the sample [4]. The thermogravimetric curve is shown in the Fig. 2 (blue curve). In the thermogravimetric curve 7

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of ZnMgAl10O17 reduces -3.77% weight over 25oC to 900oC. From the room temperature to 40 degree Celsius the minimum -0.4926% weight reduces. Because the thermal process of ZAM is associated with the evolution of the large number of gasses such as CO, CO2, NO2 and water vapour. The initial weight-loss is attributed to the removal of adsorbed water. From the 40°C to 700°C the maximum -3.09% weight reduces of the sample. Because to it points to the decomposition of the polymer network, with the subsequent decomposition of organic

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compounds and the burning of the residual carbon. The narrow and oscillative exothermic peak of DSC curve is observed in the temperature range From 700 °C to 900 °C and again there reduces -0.20% weight due to crystallization phase formation of the sample. The DTA analysis reveals only one significant thermal effect, endothermic peak at around 40oC which is associated

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with the decomposition of physically adsorbed molecular water. The higher thermal stability of the ZnMgAl10O17:Eu sample up to 9000C and hence the prepared nano phosphors are highly

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thermally stable over the normal usable range of temperature according to the work of A. Deshmukh at all for same material [4]. Therefore optical properties of these phosphor materials may be affected by over a usable temperature range. 3.2. Optical and Luminescent Properties 3.2.1. UV-Visible spectra analysis

In order to understand the optical properties of nanocrystals (e.g. the bandgap i.e. the energy

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separation between the filled valence band & the empty conduction band), the optical absorption is an important parameter to be investigated. Optical excitation of electrons across the bandgap is strongly allowed producing an abrupt increase in absorption at the wavelength corresponding to the bandgap energy. This feature in the optical spectrum is known as the optical absorption edge

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and the spectra for the samples (E1, E5, E7, E8, E9) in the range of 190nm to 800nm is shown in Fig. 7. It is observed that no absorption occurs for wavelength λ > 390nm. The absorption is

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found at λ= 237 nm and a diminished absorption is also found at λ= 290 nm. In order to calculate the optical band gap by using the Wood and Tauc relation (Eq.4) [47], a graph is plotted between (hυα)2 versus hυ (photon energy) [Fig. 7 (Inset)]. 5ℎ7 = (ℎ7 − 89 : 4

where α is the absorbance, h is Planck constant, υ is the frequency Eg is the band gap and n is an exponent having values ½, 2, 3/2 or 3 depending on the type of transition such as direct allowed, indirect allowed and directly forbidden respectively. The energy bandgap was determined by extrapolating straight line portion of the curve to the intercept of the photon energy axes and only a slight decrease of bandgap is observed from 5.24 eV to 5.10 eV by doping Eu in ZnMgAl10O17. 3.2.2. Photoluminescence spectra analysis 8

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The PL excitation & emission spectra of all the samples under investigation is shown in Fig. 8(a) and the variation of PL intensity as a function of the doping concentration of Eu2+ ions into the matrix is also shown in Fig.8 (b). The nano phosphors were excited with 340nm radiation and the excitation spectra were collected over a broad band extending from 230nm to 386nm while the emission spectra were collected up to 700nm followed by the excitation spectra. In the excitation spectra, the first peak is observed at 246nm, second highest peak around 265 nm, third peak

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around 286nm and fourth peak around 354 nm and emission spectrum show maximum intensity at 440 nm for all the samples. Maximum intensity in emission spectra is observed for Eu (x=0.01) in the blue region due to the allowed transitions of Eu2+ ion, while the red region emission spectra correspond to the forbidden transitions of Eu3+ ion giving a comparatively weaker intensity. The

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emission spectra reveal strong band (440nm to 460nm) peaked at around 450nm is attributed to 5d→4f transition of Eu2+ ions as shown in Fig. 9 while the possibility of five line emission in the orange-red region with comparatively weaker intensity is attributed to 5D0→7Fj (j = 0, 1, 2, 3, 4)

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transitions of the Eu3+ ions. The peaks observed [Fig. 7(a)] are 577nm, 589nm, 596nm, 615nm are attributed [Fig.8] due to 5D0→7F1, 5D1→7F1, 5D1→7F3, 5D0→7F2 respectively [4]. The process occurs due to the shielding effect of 4f electrons by 5s and 5p electrons in the outermost shells of the europium ion. The strong blue emission properties of this material, deviating from the earlier reported strong red emission [4] are attributed to lowering of particle-size since the blue shift and

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enhanced photo-luminescence behaviour are highly dependent on the size of nanostructures [4851]. It is also reported that the energy state associated with the luminescence centre is influenced by the host lattice material and the degree to which they are influenced depends also on the size and shape of nanostructure of composite particle [52-54]. In this case, we have used UFC route,

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maximum researchers have already reported UFC route gives directly micro-nano particles [2227]. The crystallinity size have found in the order of 12nm to 625nm range (W-H method). The

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ZnMgAl10O17: Eu powder composed of tiny particles have a high surface area (according to BET result), many vacancies in the crystal structure (corresponding to SEM result) and low particle size. It is well known that surface morphology and particle size show impact role for luminescence properties of the material of nano phosphor [55-56]. The Eu2+ ions is particularly unique in giving it’s emission spectra because its broad and intense band luminescence 4f65d1→4f7 strongly depends on the host material, the particle size of the synthesized sample and excitation wavelength ranging from UV to red wavelength of the electromagnetic spectrum. When 4f65d1 band is higher than 6Pj state and only 4f7 (8S7/2→4f65d1) transitions within the Eu2+ ions are observed in excitation and emission because of the large spatial extension of the 5d wave function, the optical spectra due to f–d transition are usually broadened and depends on the surrounding of the Eu2+ ions [1]. According to the Matsuzawa models [57], electrons are excited 9

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in divalent europium ions, since the 5d level of divalent europium lies very close to the conduction band, these excited electrons can easily be released into the conduction band and subsequently caught by a trivalent rare earth co-dopant creating a divalent ion. Thermal energy can then release the trapped electron [58], after which it recombines upon reaching a luminescent centre [53, 59]. Thus the choice of the host is a critical parameter for determining the optical properties of Eu2+ ions. In the present host, the Zn2+

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has two different sites, so that the Eu2+ ions located at the two different Zn2+ sites will have the very similar local environment. The hexagonal structure contains corner- shaping AlO4 and 9coordinated of Zn2+. A small Mg2+ ion is known to be incorporated into inline blocks by replacing trivalent Al3+. Therefore, there are at least two sites available for incorporating Eu2+ in the

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ZnMgAl10O17 lattice. Thus Eu2+ ions mainly occupy Zn2+ sites in the conduction layers and form the corresponding emission centre which peaks at 451 nm. The structure is related to the β–

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alumina structure of NaAl11O17 [60]. In this case, Al3+ ions are replaced with one Mg2+ ion for the reason of charge compensation. The electronic transition of Eu3+ involve only re-distribution of electrons within the inner 4f subshell [1], since the ionic radii for Eu3+ (0.95Å) is much larger than those of Al3+ (0.54Å) and Zn2+ (0.74 Å). The Eu3+ ions may enter into the host lattice to substitute Al3+ or Zn2+ or locate on the surface of the crystal. The second possibility is more feasible and therefore most of the Eu3+ ions are expected to be located on the surface of the

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ZnMgAl10O17:Eu with only a few of them that may enter the host lattice. The peak occurring at 615nm may be reduced in a C/N atmosphere [1]. With the increase of the doping concentration of Eu varying from x = 0.01 to 0.4 into the host, the PL intensity decreases from maximum to

dopant ions [61].

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minimum [Fig.8 (b)] and is attributed to the quenching phenomena at higher concentration of

The CIE-1931, color chromaticity diagram of all phosphors (E1 to E9) is shown in

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Fig.8(c) were found to be at same co-ordinate (X= 0.17, and Y= 0.16) in the blue region of the emission spectra. The excitation intensity and corresponding emission peak showed that these samples will be highly suitable as the blue phosphor for UV excited LEDs [1]. The decay behavior of the E1 phosphor was analyzed with the emission of the wavelength 440nm, excited by a radiation of the wavelength of 340nm [Fig. 8(d)]. In the first part of the decay, the intensity sharply reduces to nearly 1/12th of the initial intensity within few seconds, in the second part, the afterglow emission intensity decreases slowly that persists for several minutes and in the third part, there was almost constant afterglow emission intensity. The decay behavior is fitted [Fig. 8(d)] by an empirical equation (5) [62] stated the initial intensity is much stronger which is due to the higher emission efficiency of E1. 10

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B B B Where I is the emission intensity and I0 is the phosphorescence intensity at any time t after switching off the excitation source, I1, I2 and I3 are weighting constant parameters, τ1, τ2 and τ3 are the average lifetime of excited electron deciding the rate of rapid and slow exponential decay components respectively. The obtained decay curve shows three different values of τ obtained by

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simulating the decay curves as shown in Table 3, indicates that there are three different types of traps and hence three kinds of decay processes. The largest value of τ3 is related to the deepest trap centre and slowest in the decay process. The decay rate consists of a fast and a slow process quickly before 470s but after 500s, it decays slowly the persistence observed in darkness can

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reach several minutes which is the long-lasting phosphorescence phenomena. The mechanism of the long persistence is due to the holes trapped–transported-de-trapped process [63]. The typical decay time analysis and its data for E1 sample are shown in Table 3.

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3.2.3. Thermo-luminescence spectra analysis

The TL spectra (under irradiation for 10 minutes by UV) of the nano phosphors (E1 to E9) were recorded in the temperature range from room temperature to 300°C and at a heating rate of 50C/s [Fig. 10(a)]. The temperature corresponding to maximum TL intensity remains nearly same (~175°C -200°C) for all the samples which indicate the higher extent of homogeneity of the

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samples. Moreover, Fig.10(b) shows the intensity of TL emission decreases with respect to the concentration of Eu2+ ions, varying from x =0.01mol , 0.08mol, 0.2mol, and 0.3mol. Fig. 10(a) shows for higher concentration (x≥0.1), the TL intensity sharply decreases due to the quenching phenomena at higher dopant ions in the host. The E1 Sample (x=0.01) shows maximum TL

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intensity near 1900C. For all samples, the TL intensity was found to be maximum in Eu =0.01mol at peak temperature, where TL intensity decreases with the increase of the concentration of

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europium metal. The traps may either be associated with the particular impurity involved or with the defect produced in the crystal. The TL intensity increases with increasing temperature and attained maximum value near temperature ~1900C and then starts decreasing slowly [Fig. 10(a)]. Due to the rise of temperature, the electrons are released from the trap and recombination takes place in reducing the concentration of the trapped holes and increases the thermo-luminescence intensity. As the electron traps are progressively emitted and the rate of recombination decreases, that leads to lowering of intensity [64]. Phosphors showing broadband emission is attributed to the compensated presence of the divalent & trivalent Eu ions along with the possible vacancies and their recombination phenomena involved in the energy states. The reduction of broadband emission in the presence of monovalent alkali metal is due to the elimination of the vacancy. This would require that the activator ions acquire energy during heating and subsequently de-excited 11

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to their ground state giving rise to the TL emission. The trivalent ion could be reduced to their divalent form by the initial irradiation and during the subsequent heating. If holes are released, that could recombine with the trapped electron at the Eu2+ site resulting in their excitation to Eu3+. The trapping of electron and holes during irradiation and their release and re-combination during the heating step could be independent of the activators ions. The recombination energy could be transferred to the activators by a response of tunnelling process causing its excitation and

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subsequent emission [Fig.11]. We suggest that quenching of color center may take place when the sample is exposed to UV light for the longer period and more concentration of europium in host leading to the decrease of TL intensity. The Maximum intensity of TL was observed for Eu 0.01mol. The glow and kinetic parameters were calculated by Chen’s method, measuring the

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symmetry factor µ g = δ/ω, where δ is half width toward fall off the side of glow curve and ω is full width at half maximum of glow curve. They are calculated as δ = T2-Tm, ω= T2-T1, and B=

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Tm-T1, Tm is peak temperature corresponding to maximum intensity, T1& T2 is temperature on either the side of Tm corresponding to half of the maximum intensity. The trap depth/activation energy was calculated by the Chen’s equation (6) [64] as shown in Table 4. 8E = FE G

K HIJ

E

L – NE 2OPQ 6

Where E(α) is trap depth, K is Boltzmann constant (1.38×10-23 J/k), Tm is peak temperature, µ g is geometrical factor (µ g = δ/ω) and C(α) and bα are constant of Chen’s equation, α is replaced by δ,

Thus

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ω, and B as per the following cases.

FR = 1.51 + 3ST9 − 0.42U 6V

FW = 0.976 + 7.3ST9 − 0.42U 6N

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FY = 2.52 + 10.2ST9 − 0.42U 6+

Similarly for constant bα

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NR = 1.58 + 4.2ST9 − 0.42U 6[) NW = 0 6=

NY = 1.0 6\

Where E(α) and kinetic parameters were calculated from equation (6a), (6b), (6c), (6d), (6e) and (6f) respectively, as shown in Table 4. The activation energy indicates trap depth increases with the increase of temperature. Deeper the trap center, the larger is the trap depth and hence relatively high temperature is required to de-trap the electrons [65]. The thermos-luminescence spectra are equivalent for the three glow curves. The glow curves consist of a superposition of several individual glow peaks with strong overlap. Thermo-luminescence spectra can be divided into two group (1) first group of peaks from 100 to 12

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300K indicating the presence of shallow trap & (2) Second group from 300K to 700K corresponding to deep traps [66]. These results show that the degradation mechanism does not read the creation of different kinds of traps. These defects are the same as the one present before the aging process and due to the synthesis of material. The temperature dependence of the TL intensity is much similar to that of the behavior of photo-luminescence intensities and also they are highly consistent [65].

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Fig. 9 (c) shows that the TL emission spectra of E1 phosphor exhibit a broad emission band cantered with the transition The TL emission spectra of the Zn0.99MgAl10O17:Eu0.01 phosphor showed the characteristic Eu2+ emission peaks ~450nm (blue), originating from the transitions 4f65d1→4f7 of Eu2+. This is similar to the PL emission spectra. TL emission spectra were

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recorded using interference filters of wavelength 400nm to 700nm. For the investigation of decay process of E1 phosphor, it is given UV 10-minute dose of UV (254nm) then the TL decay curve

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recorded. Fig. 9(d) represent decay in TL glow curve of E1 phosphor sample, which consists of an exponential nature. The decay behaviour is fitted [Fig. 9(d)] by an empirical equation (5) stated as the initial intensity is much stronger which is due to the higher emission efficiency of E1. The obtained decay curve shows three different values of τ obtained by simulating the decay curves as shown in Table 3 indicates that there are three different types of traps and hence three kinds of decay processes. The largest value of τ3 is related to the deepest trap centre and slowest

4. Conclusions

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in the decay process.

In this work, hexagonal ZnMgAl10O17:Eu2+ novel efficient nano phosphors were successfully synthesized by UFC method. The crystallite sizes of the synthesized phosphors were of the order

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of ~12nm to 655nm. The hexagonal structure of the phosphors confirmed through SEM, TEM, XRD, FTIR and Raman spectroscopic techniques. The SEM and TEM images indicated the

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phosphor particles were irregular in shape & nanoclusters were formed with the surface area of ~13.92 m2/g. The band gap of the phosphor was found ~5.25eV, which confirmed the nano phosphors to be an insulator. The photo-luminescence intense peak was observed in blue region for all the samples which confirmed that all of them as blue phosphors. The PL emission spectra of the ZnMgAl10O17:Eu phosphor showed the characteristic Eu2+ emission in 440nm-460nm range, and were weak luminescence found in 577nm, 589nm, 596nm, and 615nm in the orangered region due Eu(III) ions. The PL decay curve indicated that phosphor material is more persistence of phosphorescence phenomena of the phosphor. The Thermo-luminescence glow curve spectra of phosphor were observed clearly decrease of TL intensity with an increase of the concentration of Europium in ZnMgAl10O17 matrix. The Kinetic parameters and activation energy of sample were calculated by Chens-equation. The TL emission spectra of the 13

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Zn0.99MgAl10O17:Eu0.01 phosphor has been observed in ~450nm visible region. Therefore according to the all properties of the material may be used in PDP as blue emitting phosphors, mercury excited lamps and also as cheap blue LED phosphor materials, as well as materials used as a TL dosimeter for low dose UV radiation.

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Acknowledgments

Authors are thankful to Govt. P. G. College Chhindwara, M.P., for preparing the samples and Pt. Ravi Shankar University, Raipur for providing the PL and TL experimental facilities. Authors are thankful to Sophisticated Instrument Centre of the University for providing various

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characterization facilities (XRD, SEM, HR-TEM, UV–visible spectrometer).

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Figures and Tables captions Figures Fig. 1. (a)Typical XRD pattern of E1 (x = 0.01), E3 (x= 0.08), E4 (x = 0.10), E5 (x = 0.15), E6 (x= 0.20), E7(x=0.20) and E9(x = 0.25) samples. (b) Typical strain plot of βcosθ/λ vs sinθ/λ of all samples.

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Fig.2. (001) projection of the interblock, showing the possible sites for cations in the Z = ¼ plane. Fig. 3. (a,b & c) Three typical images indicating material formation process, (d, e& f) Three typical SEM images of E1 phosphor sample, (d) This typical SEM image indicating structural morphology, (e & f) Two Typical SEM images of the E1 phosphor sample in higher resolution, indicating the presence of a void, pores, nano-particles &clusters. Fig. 4. (a) & (b) TEM image of E1 sample indicating particle size distribution in different magnifications (c) HRTEM image of E1 sample exhibiting the lattice fringes (d) SAED pattern of E1 sample, labeled with lattice

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planes.

Fig. 5. (a) FT-IR spectra and (b) Raman spectra for E1 sample at room temperature. Fig. 6. Typical DTA, TGA, and DTG curve for E1 sample.

Fig. 7. UV-Visible spectra and the band gap determination of the various samples.

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Fig. 8. (a) PL curve for all samples (x=0.01 to 0.4) at λex. = 340 nm and λem. = 440nm. (b) Effect of contents of doped Eu ion on the PL intensity (c). CIE-1931 Color Chromacity diagram of all the phosphors. (d) Typical decay curve for

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the E1 phosphor sample.

Fig. 9. Energy transfer mechanism due to Eu2+ and Eu3+ ions in the phosphor.

Fig. 10. (a) TL glow curve of the phosphors (E1 to E9) under 10 min. the dose of UV (254nm) irradiation (b) Effect of contents of doped Eu ion on the TL intensity of the phosphor. (d) TL emission spectra of the E1 phosphor for a 10 min, UV (254nm) dose. (c) Typical TL decay curve for the E1 phosphor sample

Fig. 11. Simple two-level model of thermo-luminescence spectra used to show the allowed ionization transition,

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trapping, thermal release, radiative recombination and emission of light.

Tables

Table 1. The various samples prepared with the nominal composition of Zn1-xEuxMgAl10O17. Table 2. Typical XRD data for E1, E3, E4, E5, E6, E7 and E9 samples

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Table 3. Typical PL and TL decay time data for the E1 phosphor.

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Table. 4. Typical Kinetic parameters and Activation energy of E1& E3.

Table 1. The various samples prepared with the nominal composition of Zn1-xEuxMgAl10O17. S.N.

Zn1-xEuxMgAl10O17 (x=0.01-0.4mol.%)

Sample Code

1 2 3 4 5 6 7 8

Zn0.99Eu0.01MgAl10O17 (x=0.01) Zn0.95Eu0.05MgAl10O17 (x=0.05) Zn0.92Eu0.08MgAl10O17 (x=0.08) Zn0.90Eu0.1MgAl10O17 (x=0.10) Zn0.85Eu0.15MgAl10O17 (x=0.15) Zn0.8Eu0.2MgAl10O17 (x=0.20) Zn0.75Eu0.25MgAl10O17 (x=0.25) Zn0.7Eu0.3MgAl10O17 (x=0.30)

E1 E2 E3 E4 E5 E6 E7 E8

9

Zn0.6Eu0.4MgAl10O17 (x=0.40)

E9

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Table 2. Typical XRD data for E1, E3, E4, E5, E6, E7 and E9 samples

E5

Peak6

Peak7

Micro-Strain (η)

2θ (centre) 31.599

37.243 45.290

60.055

60.154

65.99

66.076

0.00461

FWHM

0.346

0.395

0.4405

0.3284

0.220

0.3267

0.271

d (Å)

2.829

2.412

2.007

1.539

1.537

1.411

1.412

DSE (nm)

23.84

21.20

19.54

27.95

41.67

29.005

34.86

27.347 26.053

52.208

136.192 61.028

2θ

31.609

37.255 45.317

60.076

60.16

65.951

66.037

FWHM

0.4206

0.429

0.491

0.588

0.234

0.3125

0.627

d(Å)

2.828

2.411

1.998

DSE (nm)

19.626

1.5388

1.536

1.415

1.413

19.509 17.507

15.581

39.20

30.303

15.087

DWH (nm) 25.949

27.249 25.278

24.177

375.59

122.093

24.130

2θ

31.583

37.203 45.317

60.049

60.13

65.97

66.010

FWHM

0.4609

0.463

0.6325

0.2421

0.3143

0.679

d(Å)

0.283

DSE(nm)

17.911

DWH (nm)

25.987

0.525

0.2414 0.199

0.1539

0.1537

0.1414

0.1414

18.085 16.394

14.5002

37.916

30.133

13.93

28.607 27.447

27.00435

179.556 655.54

26.992

37.124 45.212

59.944

60.160

65.982

65.90

2θ

31.504

FWHM

0.537

0.528

0.648

0.7087

0.2512

0.3165

0.732

d(Å)

0.283

0.241

0.2003

0.1541

0.153

0.141

0.141

15.359

15.846 13.269

12.938

36.547

29.925

12.92

DWH (nm) 20.560

22.831 19.214

21.282

329.063 257.14

22.537

2θ

31.530

36.047 37.1243

59.997

60.076

65.98

FWHM

0.6481

0.3902 0.5982

0.737

0.379

0.345

0.829

0.283

0.2489 0.2419

0.2002

0.1540

0.1538

0.141

12.736

21.407 14.007

11.671

24.193

26.534

11.420

21.600

14.092

11.742

24.594

27.018

11.517

37.23

45.35

60.01

60.11

65.73

65.95

d(Å) DSE (nm)

DWH (nm) 12.795 E7

94.746

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DWH (nm) 30.397

DSE (nm)

E6

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E4

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E3

Peak2 Peak3

EP

E1

Peak1 Xrd& Crystal Paramete rs

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Sam. Code

center FWHM

31.55 0.192

0.258

0.206

45.238

0.08

19

0.2792

0.28

0.2175

0.00632

0.00884

0.00841

0.000187

0.00102

0.283

0.2413 0.199

0.1540

0.1538

0.1419

0.1415

DSE (nm)

42.99

32.414 41.963

114.671

32.870

33.778

43.539

DWH (nm) 47.04

35.085 47.284

198.343

37.399

39.043

52.731

65.92

66.32

2θ

31.786

37.35

45.377

59.963

60.281

0.2831

0.3623 0.1788

0.0398

0.159

0.216

0.335

d(Å)

0.281

0.240

0.154

0.1534

0.1415

0.140

DSE (nm)

29.168

23.142 48.159

230.441

57.775

43.770

28.27

DWH (nm) 30.397

27.347 26.053

52.208

0.1997

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0.000227

136.192 61.028

94.746

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d(Å)

Table 3. Typical PL and TL decay time data for the E1 phosphor.

PL decay TL decay

Time τ1 4.47 min.

Time τ2 7.81min.

3.5s

12.81s

time τ3 11.56min.

I1

I2

I3

Io

15.36

8.134

7.980

15.9

13.5s

2.8

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Type of decay

2.5

1.5

7.346

Table. 4. Typical Kinetic parameter and Activation energy of E1,& E3.

E3

0.447

Cτ

Cδ

Cω

bτ

1.761

2.779

1.695

EP

E1

µg

1.582

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Sample

0.397

1.442

0.812

Tm(0C) 191.83

Eα(electron volt) Eι=0.111

Eav. (eV) 0.199

Eω=0.154 Eδ=0.332 2.297

1.486

197.37

Eι=0.252 Eω=0.167 Eδ=0.150

ALL FIGURES

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Fig. 1. (a)Typical XRD pattern of E1 (x = 0.01), E3 (x= 0.08), E4 (x = 0.10), E5 (x = 0.15), E6 (x= 0.20), E7(x=0.20)

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and E9(x = 0.25) samples. (b) Typical strain plot of βcosθ/λ vs sinθ/λ of all samples.

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Fig.2. (001) projection of the interblock, showing the possible sites for cations in the Z = ¼ plane.

Fig. 3. (a,b & c) Three typical images indicating material formation process, (d, e& f) Three typical SEM images of E1 phosphor sample, (d) This typical SEM image indicating structural morphology, (e & f) Two Typical SEM images of the E1 phosphor sample in higher resolution, indicating the presence of a void, pores, nano-particles &clusters.

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Fig. 4. (a) & (b) TEM image of E1 sample indicating particle size distribution in different magnifications (c) HRTEM

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image of E1 sample exhibiting the lattice fringes (d) SAED pattern of E1 sample, labeled with lattice planes.

Fig. 5. (a) FT-IR spectra and (b) Raman spectra for E1 sample at room temperature.

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Fig. 6. Typical DTA, TGA, and DTG curve for E1 sample.

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Fig. 7. UV-Visible spectra and the band gap determination of the various samples.

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Fig. 8. (a) PL curve for all samples (x=0.01 to 0.4) at λex. = 340 nm and λem. = 440nm. (b) Effect of contents of doped Eu ion on the PL intensity (c). CIE-1931 Color Chromacity diagram of all the phosphors. (d) Typical decay curve for

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the E1 phosphor sample.

Fig. 9. Energy transfer mechanism due to Eu2+ and Eu3+ ions in the phosphor.

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Fig. 10. (a) TL glow curve of the phosphors (E1 to E9) under 10 min. the dose of UV (254nm) irradiation (b) Effect of contents of doped Eu ion on the TL intensity of the phosphor. (d) TL emission spectra of the E1 phosphor for a 10

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min, UV (254nm) dose. (c) Typical TL decay curve for the E1 phosphor sample

Fig. 11. Simple two-level model of thermo-luminescence spectra used to show the allowed ionization transition, trapping, thermal release, radiative recombination and emission of light.

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ACCEPTED MANUSCRIPT Highlights
The Zn1-xMgAl10O17:Eux2+ nano phosphor synthesis by UFC modified route.
The micro-strain of crystalline composite (Host) depends on dopant (activator) element.
Thermal stability of compound doesn't lose in the range of 250C degrees to 9000C. phenomena at higher concentration of dopant ions.

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The PL and TL intensity decreases from maxima to minima due to the quenching

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The efficient Eu2+ doped ZnMgAl10O17 phosphor exhibits blue emission spectrum.

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Novelty statement A novel blue phosphor comprising of zinc magnesium aluminate is prepared successfully by urea fuel combustion route (which is a very simple, cost effective, low energy consumption and take less time method) of nano crystalline size 12nm to 655nm by slightly playing with

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the experimental conditions during synthesis in comparison to the earlier reported red phosphor of zinc magnesium aluminate. Additionally the optical band gap of the nano material and the luminescence behaviour (PL and TL) of the phosphors have been analysed

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with the help of various kinetic parameters and activation energy. The luminescent properties

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have been correlated with structural and micro-structural results of the samples.