Investigations of VO2+ doped SrZn2(PO4)2 nanophosphors by solution combustion synthesis

Journal of Alloys and Compounds 787 (2019) 276e283

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Investigations of VO2þ doped SrZn2(PO4)2 nanophosphors by solution combustion synthesis V. Khidhirbrahmendra, Sk. Johny Basha, M. Avinash, R.V.S.S.N. Ravikumar* Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur, 522510, A.P., India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 October 2018 Received in revised form 4 February 2019 Accepted 6 February 2019 Available online 8 February 2019

VO2þ doped SrZn2(PO4)2 nanophosphor is prepared using solution combustion synthesis. The prepared nanophosphor is studied by structural and spectral characterizations XRD, SEM with EDS, TEM, FT-IR, Optical absorption, EPR, and PL. XRD analyses the monoclinic phase of the prepared sample and the crystallite size determined to be in the order below 60 nm. SEM and TEM micrographs show the agglomerated stone-like structures for VO2þ doped SrZn2(PO4)2 nanophosphors. FT-IR spectrum exhibits various vibrational modes for PO3 4 anionic groups and other related groups. Optical absorption spectrum exhibited characteristic VO2þ bands in visible region and the crystal field and tetragonal field parameters are evaluated as Dq ¼ 1461, Ds ¼ 3035 and Dt ¼ 602 cm1. EPR spectrum shows well resolved hyperfine octet structure for VO2þ doped nanophosphor. The spin Hamiltonian parameters (g), hyperfine coupling constants (A) are calculated and are satisfying the condition gk At. From Optical and EPR data, VO2þ ions with the host ligands are identified as tetragonal distortion in octahedral site symmetry. PL spectrum illustrates, emission peaks in blue and green regions and its CIE and CCT parameters are evaluated, which are useful in LEDs and lightening devices. © 2019 Elsevier B.V. All rights reserved.

Keywords: SrZn2(PO4)2 nanophosphor Solution combustion Tetragonal distortion Electron paramagnetic resonance and photoluminescence

1. Introduction Up-to-date inorganic phosphor materials have captivated the world into the field of solid state lighting emission displays, plasma display panels, light emitting diodes, IR quantum counters, scintillation etc. [1,2]. Phosphors are the most important components for the development or fabrication of novel luminescent (lightning) devices [3]. Unlike nitrates and sulfates, phosphates have better stability. Due to greater physical, chemical and thermal stabilities, Phosphate phosphors have multiple advantages like high luminous efficiency, convenient and low-cost fabrication [4]. Among the phosphate groups, Orthophosphates are said to be fascinate materials in luminescent applications because of their good chemical sustainability, less sintering temperature, balanced phonon energies, significant band gap, greater PO3 4 absorption in UV region and outstanding laser damage threshold value [5]. These phosphates require simple raw precursors under simple synthesis environment and show strong luminescence when suitable luminescent center (dopant) is added [6].

* Corresponding author. E-mail address: [email protected] (R.V.S.S.N. Ravikumar). https://doi.org/10.1016/j.jallcom.2019.02.073 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Generally, an inorganic zinc phosphate is used as a host for different luminescent materials including cathode-ray tubes, display devices, fluorescent lamps and also used as anticorrosion layers. Due to its non-toxic nature, it can be used as bioceramic material in the dental field [7e9]. Another orthophosphate Sr3(PO4)2 is the best suitable host material for rare earth ions doped phosphors due to the flexible ionic radius of Sr2þ(1.21 Å) which is similar to the rare earth cations. The luminescence properties of several RE ions doped Sr3(PO4)2 phosphors are extensively studied earlier and observed different color emissions like red, green, blue and white [5,10e14]. In material science and technology, phosphate hosts with divalent or trivalent cations of transition metals and alkali or alkaline earth ions are the prominent compounds. A typical host having formula MZn2(PO4)2, M ¼ Alkali earth element; is of very much interest as lanthanide activator [15,16]. Strontium zinc phosphate is a good host material which is flexible for the incorporation of transition metal and rare earth ions due to sufficient large cationic radii of Zn2þ and Sr2þ sites [17,18]. SrZn2(PO4)2 has been used in the fabrication of novel LED materials for white light emission by doping with suitable transition metal and rare earth ions: SrZn2(PO4)2:Eu2þ, Mn2þ [19], SrZn2(PO4)2:Eu3þ, Tb3þ, Liþ [20], SrZn2(PO4)2:Tb3þ, Mþ (M ¼ Li, Na, and K) [15], SrZn2(PO4)2:Eu2þ [16], SrZn2(PO4)2:Sm3þ,Eu2þ [21], SrZn2(PO4)2:Eu, Mn [18],

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SrZn2(PO4)2: Ce3þ [22], Sr1-xZn2-y (PO4)2: Eu2þx, Mn2þ [17]. This compound is also having an ability to stand in low temperature cofired ceramics and microwave dielectric ceramics [23,24]. Even though rare earth materials have efficient luminescence, rarer availability makes them very expensive and limited. Moreover, synthesis of RE ions doped materials needs very high temperatures. Due to these reasons, transition metals are the better replacement for rare earth ions. Among transition metal ions, Vanadium exhibiting multiple oxidation states depending on host structure. In Vanadyl complexes VO2þ is of d1 configuration and is a useful luminescent center showing blue emission [25]. Till date, strontium zinc phosphate is prepared by traditional solid state reaction method at high temperatures only. There are no reports are available on strontium zinc phosphate phosphor doped with vanadyl ions. Since it is related to orthophosphate group, the phosphor has greater stability and suitable for good luminescent materials. Generally phosphates are prepared at higher temperatures in traditional methods like solid state reaction method. Some of the rare earth doped strontium zinc phosphate phosphors were also prepared in high temperature solid state reaction method. But here VO2þ doped SrZn2(PO4)2 nanophosphor is prepared in simple solution combustion synthesis at a moderate temperature of 550  C. The preparation time also very less about 30 min. The objective of this paper is to synthesize VO2þ doped strontium zinc phosphate nanophosphor by solution combustion technique and discuss the different structural properties like crystalline phase, surface morphology, vibrational modes; and optical properties like site symmetry, bonding nature and luminescent behaviour. 2. Experimental section 2.1. Precursors VO2þ doped SrZn2(PO4)2 nanophosphors were prepared with raw materials: Strontium nitrate (Sr(NO3)2), Zinc nitrate (Zn(NO3)2), Di-ammonium hydrogen phosphate ((NH4)2H(PO4)), Urea (NH2CONH2) and Vanadium pentoxide (V2O5), Nitric acid (HNO3) and deionized water. All the precursors were analytical grade (above 99% pure), purchased from Merck Chemicals, India. 2.2. Preparation of VO2þ doped SrZn2(PO4)2 nanophosphor VO2þ doped SrZn2(PO4)2 nanophosphor was prepared in solution combustion synthesis. 1.058 g of Strontium nitrate (Sr(NO3)2), 2.974 g of Zinc nitrate (Zn(NO3)2), 1.32 g of Diammonium hydrogen phosphate ((NH4)2H(PO4)) and 0.9 g of Urea (NH2CONH2) (fuel) were diluted in the deionized water of 50 ml to make standard solution as per the stoichiometric ratios. Vanadium pentoxide (V2O5) of 0.01 mol % is added in 10 ml of Nitric acid and poured to the above standard solution. Here the ratio (O/F) of oxidizers and fuel need to be unity in order to get the maximum combustion energy. After the dilution, the homogeneous solution was stirred for 30 min then taken into a china dish and heated at 823 K in a preheated muffle furnace. Due to the strong exothermic reaction between fuel and oxidizers, a porous material was obtained which was grounded gently to obtain the final product. Now the prepared phosphor material was taken for several characterizations.

investigate morphological studies with the aid of TEM images by immersing the samples in ethanol. Vibrational spectrum was recorded on SHIMADZU IRAffinity-1S FT-IR instrument. UVeVis absorption spectrum was obtained from JASCO V-670 spectrophotometer in 200e1200 nm range. JEOL JES-TE 100 EPR spectrometer was used to record EPR spectrum in X-band frequencies with 100 kHz field modulation. Horiba Fluromax-4 instrument was used to record Photoluminescence (PL) spectrum with the help of two Xe lamps (450 W continuous lamp and 35 W pulsed lamp) as excitation sources.

3. Results and discussion 3.1. Powder X-ray diffraction analysis X-ray diffraction is a highly standard technique utilized to investigate the crystalline, semi-crystalline or amorphous nature of the materials. So it is termed as the fingerprint of the structure. Fig. 1 indicates the X-ray diffraction pattern of VO2þ doped SrZn2(PO4)2 nanophosphor prepared using solution combustion synthesis at a temperature of 550  C. For VO2þ doped nanophosphor sample, the pattern found with many minor and major peaks, well matched with the monoclinic system of SrZn2(PO4)2 (JCPDS No. 50-0159). These diffraction peaks with respective miller indices and relative intensities along their positions are presented in Table 1. The average particle size of the VO2þ doped nanophosphor is evaluated with Debye-Scherrer's formula [26].

D ¼ ðklÞ=ðbcosqÞ where k ¼ 0.9 (shape factor), l ¼ 1.5406  A (X-rays wavelength), b ¼ FWHM of the most intensive diffraction line and q ¼ diffraction angle. The average crystallite size of VO2þ doped SrZn2(PO4)2 nanophosphor is about 57 nm. From XRD data, lattice parameters of VO2þ doped SrZn2(PO4)2 nanophosphor are evaluated as a ¼ 0.8319, b ¼ 0.9511 and c ¼ 0.9025 nm. The obtained lattice parameter values are in well agreement with the mentioned JCPDS No. 50-0159 data. The slight variations in average particle size and lattice parameters are due to change in ionic radii of VO2þ with respect to Zn2þ. Micro strain and dislocation density are useful parameters which informs the deformation of material from its original structure. Micro strain is a significant parameter to identify variation in displacement of the atoms from its reference position.

2.3. Characterization techniques PXRD pattern was recorded with the help of SHIMADZU XRD6100 diffractometer. SEM images were captured using ZEISS EVO 18. Oxford EDS analyzer with SEM instrument was used to determine the chemical composition. HITACHI H-7600 was used to

277

Fig. 1. XRD spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor.

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Table 1 Diffraction peaks (2q) and the associated planes of VO2þ doped SrZn2(PO4)2 nanophosphor. S. No

Position (2q)

Relative Intensity

Miller indices (h k l)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

14.1149 21.1174 21.3542 22.0237 22.7831 23.3144 23.5008 23.8668 24.6564 25.0144 25.7054 27.2317 28.4854 29.8733 30.1363 31.26 33.4232 33.6592 34.3585 34.5383 34.7381 35.2988 35.5774 37.7935 38.5807 38.808 40.5264 45.2506 48.5957 49.0687 49.2286 51.3245 51.529 58.6988 59.3815 60.0507 63.086

11 13 26 100 38 10 37 40 17 20 49 34 21 42 62 30 68 19 30 28 26 51 15 15 18 22 22 10 10 26 15 18 17 11 15 09 12

1 0 2 1 1 2 1 1 1 2 2 0 2 0 2 2 1 3 2 0 3 0 3 0 3 3 2 2 3 1 4 3 0 1 3 5 5

1 2 0 0 0 1 2 2 1 1 1 2 2 3 1 1 1 1 2 3 1 2 1 4 1 0 3 4 3 5 2 4 1 5 3 0 3

0 1 0 2 2 0 1 1 2 1 1 2 0 1 2 2 3 0 2 2 1 3 1 0 2 2 2 1 2 0 1 1 5 3 4 2 0

Dislocation density is the amount of defects which measures the length of dislocations per unit volume of the crystal. Here these two parameters are in inversely proportion to the crystallite size. Here micro strain and dislocation density of the prepared sample are estimated with the formulae ε ¼ (bcosq)/4 and d ¼ 1/D2 [27] respectively. The calculated ε and d values are 6.0133  104 and 3.0778  1014 lines/m2 for VO2þ doped SrZn2(PO4)2 nanophosphors respectively. The particle size and micro strain are also evaluated by a graphical method suggested by Williamson and Hall [28].

bcosq ¼ ð0:9l=DÞ þ 4εsinq where b, l, D, ε and q have their usual meaning. Fig. 2 illustrates WeH plot of VO2þ doped SrZn2(PO4)2 nanophosphor. The line intersects on Y -axis will contribute crystallite size while slope gives micro strain of the prepared sample. The obtained crystallite size and strain values of VO2þ doped SrZn2(PO4)2 nanophosphor are indexed in Table 2. The crystallite size observed in WeH analysis is slightly higher than Scherrer's condition. This is due to the consideration of the strain component in the WeH equation. 3.2. Morphological studies Fig. 3 demonstrates SEM images of VO2þ doped SrZn2(PO4)2 nanophosphors at different magnifications. The SEM micrograph reveals anisotropic stone like morphology having little agglomeration. This may happen due to narrow spacing among the particles. EDS analysis is carried out to analyze the chemical composition and

Fig. 2. W-H plot of VO2þdoped SrZn2(PO4)2 nanophosphor.

Table 2 Average crystallite size, lattice strain and dislocation density of the VO2þ doped SrZn2(PO4)2 nanophosphors. Method

Crystallite size (nm)

Microstrain (ε x 104)

Dislocation density (d x 1014 lines m2)

Scherrer WeH method

57 58.88

6.0133 2.3558

3.0778 e

constituent elements of the synthesized product. Fig. 4 depicts EDS pattern of synthesized material consisting of only targeting elements (Sr, Zn, P, O and V) in the prepared sample. Structural information and microscopic morphology are explained by TEM analysis. Fig. 5 shows TEM images of VO2þ doped SrZn2(PO4)2 nanophosphors. The TEM images represent stone-like structures.

3.3. FT-IR analysis FT-IR is a better technique used to know the modes of vibrations of the different functional groups involved in a material. Fig. 6 represents the vibrational spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor in 400e4000 cm1 region. The spectrum exhibits different phosphate groups with n2, n4, and n1, n3 modes in 420e650 cm1 and 950-1200 cm1 regions respectively. PeOeH vibrational modes of the compound SrZn2(PO4)2 are observed in the range 1500e2400 cm1 [29]. The peaks in 3000e4000 cm1 region are ascribed due to the bending and stretching vibrational modes of hydroxyl (eOH) groups. In the recorded spectrum the IR bands at 436 cm1 and 525, 561, 652 cm1 are related to symmetric (n2) and asymmetric (n4) bending vibrations of (PO4)3- groups [30]. The peak at 743 cm1 is related to the characteristic PeOeP vibrations of phosphate anion groups of the prepared nanophosphor. The vibrational band at 972 cm1 is attributable to symmetric 2þ stretching modes (n1) of PO3 doped SrZn2(PO4)2 4 group [31]. VO nanophosphors have PO3 asymmetric stretching vibrations (n3) at 4 1015, 1038 and 1122 cm1. In the spectrum bending mode (n2) of hydroxyl group is observed at 1634, 1686 and 1744 cm1 [32]. The vibrational bands at 1536, 2071, 2228, 2302 and 2347 cm1 are related to PeOeH mode of vibrations. All the bands positioned in 3000e4000 cm1 region are connected to vibrational stretching modes of the hydroxyl group [32]. The band positions and the

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Fig. 3. SEM images of VO2þ doped SrZn2(PO4)2 nanophosphor.

Fig. 4. EDS pattern of VO2þ doped SrZn2(PO4)2 nanophosphor.

associated vibrational modes are mentioned in Table 3.

3.4. Optical absorption studies

Fig. 6. FT-IR spectrum of VO2þdoped SrZn2(PO4)2 nanophosphor.

The arrangement of the vanadium in a solid (or) aqueous environment is composed of 5 or 6 oxygen atoms in which one of the oxygens are double covalent bonded with vanadium to form vanadyl ion [33]. Generally, in Vanadyl complexes, 2D is the most common state for the free ionic form of Vanadyl or oxovanadium. In the octahedral crystal field environment, an unpaired d1 electron in vanadyl ions occupies t2g orbital and causes ground state term T2g.

By absorbing energy the electron moves to the upper orbital eg gives rise to the term 2Eg. Actually, in perfect octahedral symmetry, only 2T2g / 2Eg transition is possible. But however, VO2þ never possesses pure octahedral symmetry and moves to the lowered tetragonal symmetry (C4v). In tetragonal symmetry, the ground

Fig. 5. TEM images of VO2þ doped SrZn2(PO4)2 nanophosphor.

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Table 3 Assignments of FT-IR vibrational bands of VO2þ doped SrZn2(PO4)2 nanophosphors. Vibrational frequency (cm1) Band assignment 436 525, 561, 652 743 972 1015, 1038, 1122 1634, 1686, 1744 1536, 2071, 2228, 2302, 2347

Symmetric bending mode (n2) of PO34 Asymmetric bending mode (n4) of PO34 Symmetric stretching mode of PeOeP bridge Symmetric stretching mode (n1) of PO34Asymmetric stretching mode (n3) of PO34 Bending mode of (n2) vibration of hydroxyl ions Vibrational modes of PeOeH bond

state term is a singlet and 2T2g term splits into two sub-levels 2B2g and 2Eg. Moreover, 2Eg splits into 2B1g and 2A1g. In this case three possible transitions are expected, i.e., 2B2g/ 2Eg, 2B2g/ 2B1g and 2 B2g/ 2A1g. The UVeVis absorption spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor is recorded in the wavelength range 200e1000 nm as shown in Fig. 7. From this spectrum, three bands are observed at 825 nm (12117 cm1), 684 nm (14615 cm1) and 421 nm (23746 cm1) respectively. The corresponding assigned transitions in energy increasing form are 2B2g / 2Eg (dxy / dxz, dyz), 2B2g/ 2 B1g (dxy / d2x2-y) and 2B2g/ 2A1g (dxy / d2z ) respectively. The crystal field (Dq) and tetragonal field parameters (Ds and Dt) can be evaluated using the equations [34]:

2

B2g /2 Eg ¼ 3Ds þ 5Dt ¼ 12117cm1

2

B2g /2 E1g ¼ 10Dq ¼ 14615 cm1

2

B2g /2 A1g ¼ 10Dq  4Ds þ 5Dt ¼ 23746 cm1

By solving the above equations we get Dq ¼ 1461, Ds ¼ 3035 and Dt ¼ 602 cm1 which are well agreement with the previous reports [27,35]. From these results, we can assure that the VO2þ ions possess tetragonal compression in octahedral site symmetry in the present host material. The opposite signs of tetragonal field parameters Ds and Dt indicate the octahedral symmetry with tetragonal compression along the axis. Similarly, tetragonal elongation can be expected for the same signs of tetragonal field parameters.

Fig. 7. Optical absorption spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor.

3.5. Electron paramagnetic resonance study Vanadium has two isotopes V50 and V51 in which V51 is the stable isotope having the highest natural abundance of 99.8%. In general, vanadium exists as V4þ and V5þ ions in which V4þ ions are magnetic while V5þ ions are non-magnetic. Since the EPR signal generated for paramagnetic impurities, we can confirm the existence of vanadium ions are as V4þ ions in the prepared materials. Electron paramagnetic resonance spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor is as shown in Fig. 8. The spectrum shows a well resolved hyperfine octet structure due to low concentration and electronic (S ¼ 1/2) and Zeeman interactions (I ¼ 7/ 2) with the allowed selection rules DMs ¼ 0, DMI ¼ ±1 [36]. This hyperfine structure of eight lines is related to eight values of the magnetic quantum numbers ±7/2, ±5/2, ±3/2 and ± 1/2. The sharper lines of the resonance signal are the indication of isolated V4þ ions [37]. Here V4þ ions exist as VO2þ ions in the SrZn2(PO4)2 nanophosphor. Generally VO2þ doped compounds exhibit octahedral symmetry with tetragonal distortion along the axis. The condition required for tetragonally distorted octahedral site symmetry is gk At. Also Dk/ Dt ¼ (ge - gǁ)/(ge - gt) ¼ 3.2 which is greater than unity, reveals that the tetragonal distortion of VO2þ ions in the host lattice [34]. Accordance with optical absorption and EPR results, the following equations are suggested to calculate Fermi contact term k, the hyperfine coupling constant P, molecular orbital coefficients like b21, b22 and g2.

gk ¼ ge 1 

4lb21 b22

! (1)

Dk

and gt ¼ ge 1 

4lg2 b22

!

Dt

(2)

where ge is the free electron g value (ge ¼ 2.0023), l is the spin orbit

Fig. 8. EPR spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor.

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coupling constant (170 cm1). Dǁ and Dt are related to the energy values for 2B2g / 2Eg and 2B2g/ 2B1g transitions respectively. b21 is the degree of in-plane s bonding, b22 is the measure of in-plane p bonding while g2 belongs to out-of-plane p bonding. Here b22 gives the covalence ratio of V]O bond.

bonding is moderate covalent while in plane s bonding has strong covalent nature. From these results it is confirmed that the paramagnetic vanadium ions (V4þ) settled as vanadyl ions (VO2þ) posses tetragonally compressed octahedral symmetry correlated by optical and EPR analysis.

   3  4 Ak ¼ P  b22 e k þ gk  ge þ ðgt  ge Þ 7 7

3.6. Photoluminescence study

and At

  2 2 11 ðgt  ge Þ ¼P b2 e k þ 7 14

(3)

(4)

where Ak and At are the parallel and perpendicular components of the hyperfine coupling interaction respectively. The hyperfine coupling constant P belongs to the radial distribution of the single electron associated with wave function which is P ¼ ge gN be bN < r3 > , where terms in the expression have their usual meanings. The Fermi contact term k is the measure of the degree of deformation of the electron orbitals at vanadium nucleus [35]. By neglecting the second order terms and considering negative values for Ak and At, P value can be calculated from the equation [38].

  7 Ak  At   P¼ 6 þ 32 Dlt

(5)

The actual free ion P value for VO2þ is 160  104 cm1 [39,40]. Here the calculated P value is 125  104 cm1, less than vanadyl free-ion value. This clearly indicates the existence of covalent nature in the vanadyl complexes such that lesser the P value greater the covalency. Further isotropic and anisotropic values of g and A are obtained from equations (6) and (7) as

giso ¼

2gt þ gk

and Aiso ¼

3 2At þ Ak 3

CCT ¼ 437n3 þ 3601n2  6861n þ 5514:31 where n ¼ (x  xe)/(y  ye), (xe, ye) is chromaticity epicenter at (0.3320, 0.1858). The obtained CCT value is 7826 K. These results conclude the prepared material emitting blue color and it can be useful in display panels and other LED display devices.

(6) 4. Conclusion

(7)

The acquired values of giso and Aiso are 1.9592 and 108  104 cm1 respectively. Now from eqs. (3) and (4), we get

A k ¼  iso  ðge  giso Þ P

Fig. 9 shows emission spectrum of VO2þ doped SrZn2(PO4)2 nanophosphor excited at the wavelength of 400 nm. The spectrum contains several bands in visible region in which bands at 426, 454 and 486 nm are observed in blue region while the bands at 500 and 510 nm are located in green color region. The observed bands may be due to structural imperfections such as point defects, surface agglomerations etc. The blue bands may be due to radiative recombination of electrons from the local defect levels with holes at valence band whereas green emission may due to the defects near the surfaces like oxygen vacancies [41]. These defects may be due to zinc, oxygen and other vacancies created during the synthesis of the material [35]. Fig. 10 shows 1931 CIE color chromaticity diagram of VO2þ doped SrZn2(PO4)2 nanophosphor. CIE chromaticity color coordinates are evaluated from emission data using MATLAB program and are (0.2391, 0.2605). These coordinates are located in the blue region represents 4 þ state of vanadium as VO2þ. CCT value is evaluated from McCamy equation.

VO2þ doped SrZn2(PO4)2 nanophosphor is successfully synthesized by solution combustion method. X-ray diffraction and morphological studies reveal the crystalline nature of the prepared sample existed in monoclinic phase in nano-dimensions and its morphology have an anisotropic stone-like structure. Vibrational

(8)

The Fermi contact term k can be determined to be 0.82. Using P and k values in eqs. (3) and (4), b22 can be found and further from (1) and (2), b21 and g2 are determined. Here the slight deviation is observed from b22 represents the existence of covalence nature of V4þ ions with host lattice. The obtained b21, b22 and g2 values are 0.76, 0.92 and 0.30 respectively. b21 is the measure of bonding nature with the condition b21 ¼ 1 for complete ionic whereas b21 ¼ 0.5 for complete covalence. The values of b21 and b22 are in between 0.5 and 1.0 suggests the existence of covalent bonding in in-plane s and p bonds. The out of plane p-bond is represented by g2 value which gives covalent nature for vanadyl oxygen bond and its ligands. b22 value is almost unity represents strong ionic bonding, and also weak p bonding between the vanadyl ions and ligands in the nanophosphor. In this study, the deviated k value represents the weak contribution from 4s orbital of vanadium to vanadyl bond in the prepared nanophosphor. The covalency rates are explained with 1- g2 and 1- b21 gives the information about out of plane p bonding and in-plane s bonding of vanadyl-oxygen and vanadium ions (V4þ) respectively. In this case (1-g2 )¼ 0.70 and (1-b21 )¼ 0.23 shows that out of plane p

Fig. 9. PL spectrum of VO2þdoped SrZn2(PO4)2 nanophosphor.

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Fig. 10. CIE diagram of VO2þ doped SrZn2(PO4)2 nanophosphor.

spectrum exhibited bands related to the presence of symmetric and asymmetric phosphate groups in the prepared sample. Optical absorption and EPR studies conclude the tetragonal compression in octahedral site symmetry for vanadyl ions with respect to surrounding ligands in the host lattice and there is a partial covalent nature. Emission spectrum suggests that the prepared nanophosphor may be useful for light emitting diodes. Acknowledgement This work was supported by UGC, New Delhi, India under the scheme of UGC-BSR Meritorious fellowship (No.F.25-1/201415(BSR)/7-2/2007(BSR), Dt: 05-10-2015). Authors are thankful to UGC-DRS and DST-FIST, New Delhi for financial assistance to the Dept. of Physics, Acharya Nagarjuna University. Authors would like to thank the Head, SAIF, IIT Madras for providing EPR facility. Authors would also like to thank Head, Department of Physics, Kerala University for providing photoluminescence recordings. References [1] B. Ramesh, G.R. Dillip, G.R. Reddy, B.D.P. Raju, S.W. Joo, N.J. Sushma, B. Rambabu, Luminescence properties of CaZn2(PO4)2:Sm3þ phosphor for lighting application, Optik 156 (2018) 906e913. [2] Z. Xia, Z. Xu, M. Chen, Q. Liu, Recent developments in the new inorganic solidstate LED phosphors, Dalton Trans. 45 (2016) 11214e11232. [3] L. Wang, M. Xu, H. Zhao, D. Jia, Luminescence, energy transfer and tunable color of Ce3þ, Dy3þ/Tb3þ doped BaZn2(PO4)2 phosphors, New J. Chem. 40 (2016) 3086e3093. [4] M. Chen, Z. Xia, Q. Liu, Improved optical photoluminescence by charge compensation and luminescence tuning in Ca6Ba(PO4)4O:Ce3þ, Eu2þ phosphors, CrystEngComm 17 (2015) 8632e8638.

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