Synthesis and photoluminescence properties of Sm3+ doped LaOCl phosphor with reddish orange emission and it’s Judd- Ofelt analysis

Present paper reports the study on luminescence and optical behaviour of samarium doped lanthanum oxychloride (LaOCl:Sm3+) nanophosphor for their potentiality in display devices. The conventional solid state route was employed to synthesize the phosphor La1-xSmxOCl (x = 0, 0.01, 0.03, 0.05, 0.07 and 0.09). Tetragonal phase of the prepared compound was confirmed by PXRD. An agglomerated and spherical like structure were seen of the samples from FE-SEM analysis. The Wood and Tauc relation was employed to find out band gap energy and is around 4.23 eV. At an excitation wavelength of 408 nm, a high intense peak of emission was observed at 608 nm for transitions (4G5/2 → 6H7/2) of Sm3+ ions in the PL spectra. Colour co-ordinates (X̄,Ȳ) were used for the analysis of measured photoluminescence spectra. For the optimized concentration of Sm3+ (La1-xSmxOCl with x = 0.05), the intensity parameters (Ω2, Ω4 and Ω6)were determined. The trend of Ω6 > Ω4 > Ω2 was observed. An emission cross section and Branching ratio are found to be 3.98 × 10−22 cm2 and 59% respectively for 4G5/2 → 6H7/2 transition that makes it suitable for laser designs and application. The obtained results reveal that the LaOCl phosphor doped with samarium can be used on the screens of optical electronic systems and also is suitable for emission of reddish orange colour.


Introduction
Host material plays a significant role in the up conversion method, specifically due to its contribution in luminescence efficiency. An ideal host is expected to have good chemical stability to maintain the physical integrity of the nano materials. Host should have less phonon energy so as to reduce non-radiative processes contributing in enhancing the radiative emission component Wang and Liu 2009. In this regard Lanthanum oxychloride (LaOCl) is found to have high chemical stability, non toxicity, high light absorption efficiency and conduction, making it useful for lighting and displays devices. Moreover among the Lanthanide ions, La 3+ ions have largest ionic radius and can easily substituted by another Ln 3+ ions in the lattice structure [1]. Thus LaOCl serves to be a promising host matrix for doping with various Ln 3+ ions. Further lanthanum oxychloride nanophosphor have gained special attention due to desired luminescence properties and applications like catalyst supports, gas sensors [2,3] and solid state lighting in WLED's etc [4,5]. Controlled synthesis of a distinct nanophosphor plays vital role in the bonding, electronic structure, chemical reactivity and surface energy which intern dependent on their size and morphology [6].
Judd-Ofelt theory was utilized to explain the behaviour of the spectral intensities within the host and hence the spectral intensities characterize the rare earth ions in the host. The magnetic dipole (MD) and electric dipoles (ED) contributes for various optical intensities due to f-f transitions. Within the 4f shell the ED transitions were forbidden because of them having the same parity. The forced ED transitions outside the 4f shell by mixing in opposite parity states from higher lying configurations shell is promoted due to the perturbation caused by noncentrosymmetric crystal field. In J-O theory the mixed parity states were due to the impact of static odd order terms of the crystal field. The importance of the even order terms in energy level arrangement onthe ion be able to explained through a number of JO parameters [7][8][9]. The performances of the luminescent and laser materials can be evaluated where J-O terms (Ω 2 , Ω 4 and Ω 6 ) plays a crucial role which are sensitive to the local environment. Ω 2 can be enriched by covalent bonding and is usually dependent on Nephelauxetic effect whereas Ω 4 and Ω 6 are dependent on the long range effect [10].
Several methods are adopted meant for the synthesis of LaOCl that includes sol-gel, solid-state, combustion, liquid-phase method, mechanochemical grinding, surfactant-assisted solvothermal reaction and solvothermal method [11][12][13][14][15][16]. Amongst various methods of synthesizing due to its large scale production, highly pure products, good compositional control and solvent/surfactant free process solid-state reaction method is used the most.
Various studies have been carried out on Eu 3+ doped phosphors with red emission [17,18] and also Sm 3+ doped phosphors with reddish-orange emission [19][20][21][22]. In this paper, we reported the study of luminescence and Judd-Ofelt analysis of La 1-x Sm x OCl (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09) Nano phosphor synthesized via solid state which was carried out for short duration and at low temperature. The crystallographic parameters, functional groups, morphological and luminescence properties be analyzed via powder x-ray diffraction, FTIR spectroscopy, FE-SEM & TEM and photoluminescence (PL) accordingly. Further from the absorbance data, the Judd-Ofelt parameter, radiative emission rates, radiative lifetime, branching ratio and asymmetry ratios are identified.

Instruments
Investigation of the crystallinity and phase purity of the synthesized sample were carried out by Shimadzu x-ray diffractometer using Cu Kα radiation (1.5406 A°) with nickle filter. From an angle of 2θ=10°-80°, diffracted intensities were recorded. The morphology of the nanophosphor were observed with Hitachi 3000 SEM. TEM analysis (Hitachi H-7100 with an accelerating voltage of 120 kV and solvent as methanol) was used to confirm the size of the particle. Using Perkin Elmer spectrometer (spectrum 1000) with KBr pellets the Fourier transform infrared spectroscopic studies are done by scanning the dry powder of nanophosphors at 4 cm −1 resolution in the range 350-4000 cm −1 . UV-visible spectra were noted by analytic jena spec cord 250+. Photoluminescence (PL) measurements were carried out using Horiba Fluro Log-3 modular Spectroflurometer with nanophosphors excited using linearly polarized, 40 μW, 633 nm or 532 nm lasers using a 100×NA=1.4 oil immersed microscope objective.

Results and discussion
3.1. Powder x-ray diffraction Figure 1 represents the PXRD pattern of La 1-x Sm x OCl (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09) nanophosphor. The formation of single phase LaOCl compound with samarium as dopant is confirmed by the single and sharp peaks of diffraction which are indexed to tetragonal phase of space group p4/nmm [18]. No other impurity peaks were seen confirms the purity of the product. Bragg's equation λ=2dsinθ was employed to calculate the interplanar distance 'd' between the adjacent planes. Making use of the lattice geometry equations the interplanar spacing, lattice constants (a, b, c) and primary cell volume are determined [23] and obtained values were tabulated in table S1 is available online at stacks.iop.org/MRX/7/015003/mmedia.
The calculation of particle size was done using Debye-Scherer equation.
Here 'd' is the size of the particle nm, k is constant=0.94, θ is diffraction angle, 'λ=1.540 A°' represents wavelength for Cu K α radiation, and β is the FWHM (full width at half maximum intensity). It is found that the particle size were in the order of 45-48 nm determined by Williamson and Hall method. The strain (ε) and particle size (d) contributes to the linear combination that gives rise to FWHM s (β) through the W-H relation [18]. Above equation is of the form y=mx+c represents straight line. Figure 2 shows the W-H plot which exhibits a straight line obtained by plotting β q cos versus 4 q sin for Sm 3+ doped LaOCl. The strain factor (ε) is obtained from the straight line slope whereas y-axis intercept l ( ) d 0.9 gives the particle size which is found to be 61-68 nm for La 1-x Sm x OCl (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09) nanophosphor. The values of particle size obtained for La 1-x Sm x OCl (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09) using W-H and Scherer's method are tabulated in table 1.

Electron microscopy (SEM and TEM)
SEM micrographs and the composition of elements are is as shown in figures 3(a) and (b) of La 1-x Sm x OCl (x=0.05) phosphor. It is clear from the images that the morphology of the nanophosphor are agglomerated, which may be due to the method of synthesis.
To understand the particle size distribution La 1-x Sm x OCl (x=0.05) phosphor is examined through TEM and HRTEM. A close view of TEM images in figures 4(a) and (b) reveals so as the phosphor is in the nano system. The lattice fringes as observed in figure 4(b) and suggest 0.34 nm as its interplanar distance, which corresponds to the plane (101) in LaOCl. Figure 4(c) shows the SAED pattern of La 1-x Sm x OCl (x=0.05), which exhibited diffraction circles composed of diffraction dots.  Figure 5 represents the transition spectra of La 1-x Sm x OCl nanophosphor with x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09 from 400-4000 cm −1 . At about 511 cm −1 appeared the vibration peaks of (La-O) metal oxygen bands in the low frequency domain [24]. The band at 1617 cm −1 signifies asymmetric stretching vibrations of C=O carboxyl groups, whereas bands at 3458 and 1391 cm −1 are attributes of stretching bands and bending of O-H group respectively [25]. Figure 6 represents the energy band gap diagram for La 1-x Sm x OCl with x as 5 mol%. Using the data of absorption spectra the optical band gap energy was estimated by means of wood and Tauc relation given by.

UV-Visible spectra
here, α denotes absorption co-efficient, A is proportionality constant, hν is the photon energy, E g is the optical energy band gap and n is influenced by the properties of the transition in semiconductor. For forbidden direct, forbidden indirect, allowed direct, and allowed indirect transitions. 'n' takes the value of 3/2, 3, 1/2 or 2 respectively. As the absorption spectrum is administered by direct electronic transition value of n is ½ for LaOCl nanophosphor [26]. The value of energy band gap is taken from the intercept of tangent to the x-axis and is found to be 4.23 eV for La 1-x Sm x OCl where x=0.05. Figure 7 represents the optical absorption spectrum of LaOCl: 5 mol% Sm 3+ at room temperature from wavelength range 800-1600 nm. The 5 mol% samarium doped LaOCl revealed several bands allotted to f-f transition from ground state to different excited states of Sm 3+ ions in the absorption spectra. It consisted of absorption band positioned at 948, 1089, 1239, 1367, 1464 and 1564 nm assigned to the transitions from 6 H 5/2 to various excited states 6 F 11/2 , 6 F 9/2 , 6 F7 /2 , 6 F 5/2 , 6 F 3/2 and 6 F 1/2 of Sm 3+ ions respectively. Table 2 includes the data of absorption band wavenumbers for 5 mol % Sm 3+ ions doped LaOCl nanophosphor and aqua ions [26] along with nephelauxetic ratio β and bonding parameter δ. Here δ is given by where the where b is found using N the number of levels, u np and u a are the wavenumbers of the corresponding transition in 5 mol % Sm 3+ ions doped LaOCl nanophosphor and aqua ions respectively.  The value of bonding parameter δ being negative or positive represents the ionic and covalent bonding respectively for Sm 3+ -ligands. In the present paper bonding parameter δ is obtained as −1.08 indicating ionic nature of Sm 3+ ligand bond in LaOCl nanophosphor.  3.5. Photo-luminescence, CIE chromaticity diagram and correlated color temperature Figure 8 represents the excitation spectra of Sm 3+ doped LaOCl nanophosphor indicating that the excitation wavelength is 408 nm. Under this excitation the phosphor exhibited emission of reddish orange luminescence. Figure 9 represents the emission spectrum of La 1-x Sm x OCl (x=0.01, 0.03, 0.05, 0.07, 0.09) nanophosphor at 408 nm as its excitation wavelength. Inset of figure 9 indicates the difference in emission intensities with samarium ion concentration. Four emission bands were exhibited corresponding to the transition 4 G 5/2 → 6 H 5/2 , 4 G 5/2 → 6 H 7/2 , 4 G 5/2 → 6 H 9/2 and 4 G 5/2 → 6 H 11/2 at 564, 608, 650 and 708 nm respectively.     The emission peak corresponding to 4 G 5/2 → 6 H 7/2 transition at wavelength 608 nm appears to be maximum than others. The intensity of emission increased with raise in the samarium ion concentration and is seen to be maximum for 5 mol % Sm 3+ concentration after which for higher concentration of Sm 3+ the emission intensity decreased which attributed to the effect of quenching. Quenching effect arises due to the non-radiative energy transfer as an impact of radiation re-absorption, exchange interaction or multipole-multipole interaction.
Firstly, because of the broad overlap of fluorescent spectra of the activator and the sensitizer rate of reabsorption of radiation for transfer of energy occurs [22]. It is clear from the PL spectra of emission and excitation that radiation re-absorption is not responsible for the quenching effect.
Secondly as per Blasse et al [28], R c the critical transfer distance is as follows V=a 2 c is unit cell volume=0.117 (nm) 3 , in a unit cell for LaOCl N=4 the no. of cationic sites vacant, X c =0.05 is the optimum concentration of Sm 3+ ion. Hence the value of critical transfer distance for the present nano phosphor R c is found to be 10.37 A°. R c less than 3-4 A°implies through exchange interaction the nonradiative transfer of energy is possible [29]. Since for LaOCl it is not less than 3-4 A°, the energy transfer is not due to exchange interaction.
Thirdly, the multipolar interaction leads to non-radiative energy transfer if the transfer of energy is relative to (x) θ/3 with x being dopant concentration where x=C/C * , C * is critical transfer concentration of quenching and C is quenching ion concentration and θ=10, 8 or 6 indicates quadruple-quadruple, dipole-quadruple or dipole-dipole interaction respectively [30]. The correlation of emission intensity and activator gives the energy transfer in the host matrix [29] which is written as value is found to be −1.7988 and hence the value of θ is equal to 5.39. As θ value is nearer to 6 confirms that the d-d interaction is liable for quenching of samarium ions in LaOCl host.
The radiative and non-radiative transitions for samarium are as shown in the energy level diagram figure 11. Samarium ions gets elevated to excited states 6 P 3/2 at an excitation of 408 nm and the lower energy states are populated via much multi phonon relaxation process.
The precise emission colour and colour purity for the prepared samples were estimated by Commission International del'Eclairage (CIE) chromaticity diagram. The chromaticity coordinates for La 1-x Sm x OCl (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.09) under an excitation of 408 nm is (0.56, 0.44) that reveals that the emission colors is in the orange-red section as in figure 12 and hence are nearer to the Nichia corporation developed Amber LED NSPAR70BS (0.575, 0.425) [33]. Values of x, y and u', v' and CCT are tabulated in table 1. As the location of luminescent emission barely changes, the CIE chromaticity co-ordinates of the sample was independent of change in concentration of samarium. Thus LaOCl:Sm 3+ phosphor serves to be one component for white emission by connecting it to phosphor exhibiting blue-green. Thus, LaOCl:Sm 3+ finds application in NUV-based WLED's. CCT lower than 5000 K, declares that the current phosphor perhaps will be helpful for solid state lighting and warm white LED application. Figure 7 represents the optical absorption spectra in wavelength series of 900 to 1600 nm consisting of six absorption band with wavelengths 948, 1089, 1239, 1367, 1464 and 1564 nm corresponding to transitions 6 H 5/2 → 6 F 11/2 , 6 H 5/2 → 6 F 9/2 , 6 H 5/2 → 6 F 7/2 , 6 H 5/2 → 6 F 5/2 , 6 H 5/2 → 6 F 3/2 and 6 H 5/2 → 6 F 1/2 respectively for La 1-x Sm x OCl with x=5 mol %. All the transitions are originating from 6 H 5/2 to different higher energy states and are accredited to intra configuration (f-f) transitions.

Absorption spectrum and J-O analysis
Using intermediate-coupling approximation the fundamentals ||U (t) || doubly reduced unit tensor operator are found. For the chosen Sm 3+ bands Carnall et al [27] is taken as reference for the values of unit tensor operators.
The measured values of line strength is measured using the equation, In equation (9), l is average wavelength that corresponds to the particular absorption band of a transition, J is the total angular momentum of initial state, n is the host refractive index ò s l l ( )d is integrated absorption cross section. The absorption cross section is determined using a l =( ) 2.303 A t N with 't' as its thickness of sample=0.1 cm, A=absorbance, and N=6.023×10 20 is concentration of Sm 3+ ion per cm 3 for 5 mol % doped LaOCl [34,35].
By According to Jorgensen and Reisfeld [36], due to covalency between rare earth ions and ligand anions, Ω 2 is strongly affected and is responsive to the local structure of the RE 3+ site where as Ω 4 and Ω 6 are concerned to the rigidity and viscosity of the host medium where the ions are present. The Ω 2 parameter in the present phosphor LaOCl:Sm 3+ is smaller than that of other hosts which can be attributed to higher symmetry of the co-ordination structure surrounding the RE ion. In addition the presence of the Cl −1 ions plays the important role for the reduction of Ω 2 value as the chlorine ions have higher electronegativity (around 3.5) in comparison with the other anion ions, therefore the RE-Cl bonds have the smaller covalency bond than other RE-anion bonds. This is main reason for the reduction of Ω 2 values [37]. In our case since W < W 2 4 we can assume that the field experienced by the Sm 3+ ions in the host matrix (LaOCl) is centrosymmetric in nature. This implies MD transitions are dominant than that of ED transitions. The same is reflected in PL spectrum. In case of PL spectrum, as per the selection rule, 564 nm emission is completely corresponds to MD transition and 650 nm is purely ED in nature. If we observe the spectrum figure 9 we can come to conclusion that the intensity of 564 nm is greater than the intensity of 650 nm. This suggests that the emission corresponding to MD is dominant over ED transitions which are in correlation with the results obtained by JO parameters i.e. (W < W  the ratio Ω 4 /Ω 6 [38]. The present nanophosphor exhibits the highest χ=Ω 4 /Ω 6 value=0.79, indicating the LaOCl:Sm 3+ phosphor display good performance as a host for luminescence activators. These values of J-O analysis were utilized to compute the line strength of the absorption band by equation (8) and data is available in table 3 which also consists of the mean wavelength (l ), reduced matrix elements, band sum (Γ) related to the integrated absorption cross section. For a shift from ground level to higher energy level, The sum of ED and MD probabilities give rise to the radiative transition probability A (J→J′) which is written as is the local field correction for the ED transition. Further S ED can be calculated using equation (9) and S MD can be found by equation, Here, m = p B h 4 mc be the magnetic dipole parity [39]. Table 4 consists of the obtained values of radiative transition rates A ED and A MD . The total radiative transition probability A T is found by the addition of A(J→J′) terms evaluated allover the terminal levels and is represented as, The radiative lifetime τ rad of an excited state J′, being the reciprocal of A T and is given by Sum of the rate of transition probabilities for all the transition results in the total transition rates which is 127.968 for 4 G 5/2 (J=5/2) and hence for this level the radiative lifetime is 7.81 ms.
The ratio of total photon flux from initial J′ to lower manifold J gives the measured branching ratio amongst the manifolds which is given by equation (14) ò ò Table 5 indicates the measured and computed values of branching ratio, rate of transition (A T ), and the percentage difference of 5 mol% Sm 3+ doped LaOCl nanophosphor for the transitions from 4 G 5/2 → 6 H 5/2,7/2, 9/2, 11/2 respectively. The slight difference in the measured and calculated values of branching ratios is because of the environment of ligand to metal charge transfer.    Table 5. Values of transition rates (A T ), Calculated and measured fluorescence branching ratio, measured Line shape function, emission cross section, gain band width and optical gain of different transition of La 1-x Sm x OCl (x=0.05) nanophosphor.
Transition High gain and low threshold laser applications expects for higher stimulated emission cross section and also is used for continuous wave laser action [40]. Table 5 also includes the values of emission cross section, Measured Line shape function, optical gain and gain band width of different transition of La 1-x Sm x OCl (x=0.05) nanophosphor. With the help of equation (16), the room temperature emission cross section of the transition from 4 G 5/2 to 6 H J (J=5/2, 7/2, 9/2, 11/2) of 5 mol% Sm 3+ doped LaOCl nanophosphor are determined 2 rad rad n is the wave number, τ rad is radiative lifetime of upper manifold 4 G 5/2 , λ is the fluorescence peak wavelength, b rad be the fluorescence branching ratio, and g(n ) is line shape function which is resolved by numerical integration of the respective fluorescence spectrum from figure 9.   Table 1 gives the asymmetric ratio as measured for various mol % of La 1-x Sm x OCl (x=0.01, 0.03, 0.05, 0.07 and 0.09). The obtained J-O parameter values indicate that the ions are occupying asymmetric sites and the crystal field is satisfying lack of inversion symmetry, whichis evident from the asymmetric ratio values equation (18) represents the fraction of integrated intensity of the ED (I ED ) to the MD transition (I MD ) called the asymmetric ratio using which we get to know the behaviour of crystalline field around the trivalent 4f ions. In table 6, gives the comparison of the parameters Ω 2 , Ω 4 , Ω 6, branching ratios, quality factor and emission cross section for 4 G 5/2 → 6 H 7/2 in samarium doped matrix [ 19,22,38,41,44]. In the present work, the synthesized nanophosphor follows the trend of Ω 6 >Ω 4 >Ω 2 and exhibits a branching ratio of 59% for the transition 4 G 5/2 → 6 H 7/2 which serves to be a critical parameter for laser design due to its characteristic possibility of stimulated emission. The optical gain (σ e ×τ) and gain bandwidth (σ e ×Δ λeff ) are the factors required to foretell the intensification of the medium where the rare earth ions were located. An optical amplifier should possess greater values of gain bandwidth, emission cross section and optical gain which in our case are found to be 3.457×10 −28 cm 2 , 3.983×10 −22 cm 2 and 31.10×10 −25 cm 2 s respectively for the transition 4 G 5/2 → 6 H 7/2 which are presented in table 5. Also the higher values of χ=0.79 indicates that the present matrix is optically good as active laser media [41].

Conclusion
A series of La 1-x Sm x OCl (x=0, 1, 3, 5, 7 and 9 mol %) phosphors were prepared via conventional solid state route. The XRD pattern shows that the phosphors are indexed with space group P4/nmm (No. 129)to the tetragonal phase. The band gap is obtained as 4.23 eV. An intense orange red emission was monitored at 608 nm Table 6. Comparison of JO parameters (10 −20 cm 2 ), branching ratio, emission cross section and quality factor χ=(Ω 4 /Ω 6 ) of Sm 3+ ions in the nanophosphors with other systems.
Phosphor for an optimized 5 mol% Sm 3+ ion concentration for 408 nm excitation. Optical absorption data for the optimized sample was utilized to determine J-O factors. The obtained J-O parameter value reveals that the ions are occupying asymmetric sites and the crystal field is satisfying lack of inversion symmetry, which is evident from the asymmetric ratio values. The non-radiative energy transfer mechanism is responsible for quenching effect beyond 5 mol% through dipole-dipole interaction. The prepared phosphors were suitable for laser applications due to its high branching ratio of 59% and 3.983×10 −22 cm 2 of emission cross section that corresponds to 4 G 5/2 → 6 H 7/2 . The values obtained for optical gain and gain band width indicate the phosphor exhibits the quality of being a better optical amplifier. The results also concludes that the prepared phosphor LaOCl:Sm 3+ is capable of being a feasible candidate for red element for white LED's in solid state lightning applications.