Influence of modifier oxide on Spectroscopic properties of Ho3+: V4+ co-doped Na2O–SiO2–ZrO2 glasses

Journal of Alloys and Compounds 586 (2014) 159–168

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Influence of modifier oxide on Spectroscopic properties of Ho3+: V4+ co-doped Na2O–SiO2–ZrO2 glasses K. Neeraja, T.G.V.M. Rao, A. Rupesh Kumar, N. Veeraiah, M. Rami Reddy ⇑ Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur, Andhra Pradesh 522 510, India

a r t i c l e

i n f o

Article history: Received 1 October 2013 Accepted 5 October 2013 Available online 18 October 2013 Keywords: Na2O–SiO2–ZrO2 glasses Holmium and vanadyl ions Luminescence emission Energy transfer

a b s t r a c t Na2O–SiO2–ZrO2 glasses co-doped with variable concentrations of Ho3+: V4+ have been synthesized, characterized by different techniques and finally their luminescence characteristics were investigated. The EDS spectra of the glass samples indicate all the elements are intact in the final composition of prepared glass. The infrared and Raman spectral studies are carried out and the existence of conventional structural units are analyzed. The ESR and optical absorption spectra indicated that a considerable proportion of vanadium ions do exist in V4+ state in addition to V5+ state. The absorption and emission spectra of Ho3+ ions were characterized using J–O theory. The radiative transition probabilities and branching ratio were evaluated from luminescence spectra. The analysis these results indicated the highest values of radiative probabilities and branching ratios for the green emission transition viz., 5S2 ? 5I8 transition among various other transitions of Ho3+ ions. However, the presence of higher concentration of V2O5 in the glass matrix seems to be a hindrance for getting the high luminescence efficiency especially in the red region. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Oxide glasses doped with trivalent rare earth ion have been paid much attention due to their potential applications in developing solid state and glass lasers, optical applications such as fibers, amplifiers and visible display devices. Among the rare earth ions Ho3+ are expected to give high luminescence output to view the characteristics of the glasses. These Ho3+ doped glasses which emit emission bands in the ultraviolet, visible region are of great interest and it is used for eye-safe source in atmosphere, wind shear, laser radar, medical and surgery [1–3]. An alkali oxide like sodium is added to glasses, the optical density in the ultraviolet wavelength longer than the absorption edge is directly proportional to the alkali concentration and the intensity of absorption increases. Addition of sodium oxide in to glasses provides suitability for the fabrication of optical wave guide devices [4–7]. Silica has been the most commonly used host glass and having many applications in semiconductor technology, optical devices such as optical data storage, color display, optical communication and other related areas of opto-electronics [8–10]. The incorporating of intermediate oxides like Zirconium in to the glasses leads to an enormous extension of possible vitreous materials with special physical properties. The presence of network modifier effects fundamentally to the glass properties; ⇑ Corresponding author. Tel.: +91 863 2346385 (O), mobile: +91 9866804948. E-mail address: [email protected] (M. Rami Reddy). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.038

consequently the molar volume and the glass transition temperature is lowered due to the reduced degree of cross linking. The addition of alkali and alkaline metal oxide to silicate, zirconate mixed oxides have high alkaline resistance and alkali glass durability of the glass system are used as fibers reinforcing cement [11–13]. Among the various semiconducting transition metal oxide glasses, vanadate glasses are widely used in memory switching devices. Vanadium containing glasses are well known as the n-type semiconductors which show the semiconducting behavior with electrical conductivity of 103 to 105 (X cm1) due to electron hopping between V4+ and V5+ ions. Vanadium ions are mixed with trivalent rare earth ions increase the laser efficiency of the glasses due to the energy transfer process and takes place radiative and non radiative transitions within the glass network [14–18]. The objective of the present study is to investigate how the transition metal oxides (V2O5) influence the optical properties of RE3+ ion and the energy transfer probably takes place between Ho2O3:V2O5 co-doped sodium, silicon, zirconium (NSZ) glasses by using EDS, FT-IR, Raman, EPR, Optical Absorption and Luminescence studies.

2. Experimental technique The following compositions are chosen for the present study. Appropriate amounts of analytical grade reagents of Na2CO3, SiO2, ZrO2, Ho2O3 and V2O5 with 99.9% purity are used to prepare the glasses. Here holmium and vanadium are considered as dopants in the glass network. The melt quenching method is used for the preparation of glass samples; in this, definite amount of powders in mol% were

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thoroughly mixed in an agate mortar and to get homogeneous mixture and it is melted in silica crucible in the temperature range 1350–1450 °C in an automatic temperature controlled furnace about 1 h. The resultant bubble free melt was quickly poured in to preheated brass mould and subsequently annealed at 450 °C temperature. Later the prepared samples were ground and optically polished to final dimensions of 1 cm  1 cm  0.2 cm. The composition of the prepared glasses containing variable amount of contents are given in Table 1. The refractive index of the glass sample is measured by using Abbe’s refractrometer with monobromo naphthalene as the contact layer between the glass and the refractrometer prism. The density of the glasses was determined by the standard Archimedes principle using o-xylene (99.9% pure) as buoyant liquid. The energy dispersive spectroscopy measurements were conducted on a thermo instruments model noran system 6 attached to scanning electron microscope. The electron spin resonance (ESR) spectra of the fine powder of the sample were recorded at room temperature using E11Z Varian X-band (t = 9.5 GHz) ESR spectrometer of 100 kHz field modulation. Infrared transmission spectra are recorded on a JASCOFT-IR -53000 spectrophotometer with resolution of 0.1 cm1 in the spectral range 400–4000 cm1 using KBr pellets (300 mg) containing the pulverized sample (1.5 mg). The Raman spectra (model Nexus 670 Nicolet – Madison –W.I.USA) is recorded on fourier transform Raman spectrometer with resolution of 4 cm1 in the 400–1500 cm1. The optical absorption (UV–Vis) spectra are recorded on JASCO, V-570 spectrophotometer from 200 to 1800 nm with spectral resolution of 0.1 nm. The luminescence Spectra are recorded at room temperature on a photon technology international (PTI) spectroflurometer with excited wavelength 400 nm from 300 to 1200 nm.

3. Results 3.1. Physical properties To know the physical properties of the glasses, the calculated values of density (d) and refractive index along with other physical parameters [19–21] such as vanadium ion concentration (Ni), mean ionic separation (ri), polaron radius (rp), field strength (Fi), electronic polaraizability (a), reflection loss, molar refractivity (RM) and optical dielectric constant (e) are calculated by using conventional formulae and are presented in Table 2. Fig. 1 shows the variation of density and refractive index as a function of V2O5 concentration. Fig. 2 represents the ionic concentration and electronic polaraizability as a function of x mol% of dopant.

around 700 cm1. Another two bands are located at around 850, 880 cm1 and one sharp band with a peak is located at about 1012 cm1. 3.4. Raman spectra Fig. 5 represents the Raman spectra of the undoped and doped Ho3+/V4+: NSZ glasses. Raman spectra analyses the local arrangement of the glasses and also give information about the structural properties that would support the infrared transmission spectra. The Raman spectra of the glasses and band positions are presented in Table 4. The spectra of NSZ glasses have revealed a peak at round 340–365 cm1 and structural vibrations are observed at 800 cm1. In the spectrum contains V2O5, stretching vibrations are observed at around 600 cm1 and another two band is observed at 950 cm1 and 1080 cm1 [23–25]. 3.5. Electron paramagnetic resonance spectra The EPR spectra recorded at room temperature for the present investigated NSZ: Ho3+/V4+ co-doped glasses. No signals are observed for the undoped glasses. When V2O5 enter into the glass matrix, the EPR resonance spectra exhibit eight parallel and eight perpendicular lines arising from the unpaired 3d1 electron of VO2+ ions with 51V (I = 7/2) isotope in an axially symmetric field. The axial spin-Hamiltonian for hyperfine interaction is used to describe the spectra of V4+ ions [26].

h i H ¼ b g k Bz Sz þ g ? ðBx Sx þ By Sy Þ þ Ak Sz Iz þ A? ðSx Ix þ Sy Iy Þ

ð1Þ

Here b denotes Bohr magneton, gk, g\ and Ak, A\ denotes the components of the hyperfine coupling tensor, Bx, By and Bz denotes components of the magnetic field, Sx, Sy, Sz and Ix, Iy, Iz are the spin operator of the electron and the nucleus. The magnetic field positions for the parallel and perpendicular hyperfine peaks are based on the second order perturbation terms are



3.2. Energy dispersive spectroscopy

   A?2 63  m2 4 1Bk ð0Þ " 2 #   Ak þ A2? 63 2  m B ? ðmÞ ¼ B? ð0Þ  mA? 4 4B? ð0Þ2

Bk ðmÞ ¼ Bk ð0Þ  mAk

Results from the energy dispersive spectroscopy (EDS) reveals the chemical makeup of the samples; the analysis indicates the presence of sodium (Na), silicon (Si), zirconium (Zr), samarium(Sm), oxygen (O), carbon(c), and vanadium (V) elements in various phases. Fig. 3 shows the chemical composition of the glasses with 0.2% and 1 mol% of V2O5. The inset figure shows the electronic image spectrum of the glass sample. 3.3. Fourier transforms infrared transmission spectra (FT-IR) Fig. 4 shows the Fourier transforms infrared transmission spectra of undoped and doped Ho3+/V4+: NSZ glasses; which gives the information about the various vibrational modes and also provides different structural units. The observed IR spectral bands are given in Table 3. The spectra exhibit a series of bands [22] one at about 483 cm1, the second band at around 630 cm1 and the third at

ð2Þ ð3Þ

From the above equation m refer to the nuclear spin magnetic quantum number, bearing values ±7/2, ±5/2, ±1/2; Bk(0) = hm/gkb and B\(0) = hm/g\b. From the above parameters, the dipolar hyperfine coupling parameters P = 2dbbN hr3 i and the Fermi contact interaction (K), are evaluated [27] using the expression

Ak ¼ P½ð4=7Þ  K þ ðg k  g e Þ þ ð3=7Þðg ?  g e Þ

ð4Þ

A? ¼ P½ð2=7Þ  K þ ð11=14Þðg ?  g e Þ

ð5Þ

In the equation, ge = 2.0023 and refer to the g factor of the free electron. The term P and K in the above equation result from the

Table 1 Composition of the studied glasses (batch mol%). S. No.

Glass

Na2O (mol%)

SiO2 (mol%)

ZrO2 (mol%)

Ho2O3 (mol%)

V2O5 (mol%)

1 2 3 4 5 6 7

Pure Hov0 Hov1 Hov2 Hov3 Hov4 Hov5

40 40 40 40 40 40 40

55 54 54 54 54 54 54

5 5 4.8 4.6 4.4 4.2 4

– 1 1 1 1 1 1

– – 0.2 0.4 0.6 0.8 1.0

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K. Neeraja et al. / Journal of Alloys and Compounds 586 (2014) 159–168 Table 2 Various physical properties of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

3

Density d (g/cm ) (±0.004) Average molecular weight (M) Ion concentration Ni (1020 ions/cm3) (±0.005) Interionic distance ri (Å) (±0.005) Polaron radius rp (Å) (±0.005) Field strength Fi (1015 cm2) (±0.005) Refractive index n (±0.0001) Reflection loss Molar reflectivity RM (cm3) (±0.005) Electronic polaraizability (ae) (1022 ions/cm3) (±0.005) Optical dielectric constant (e0) (±0.005)

Pure

Hov0

Hov1

Hov2

Hov3

Hov4

Hov5

2.679 63.99 – – – – 1.67 0.033 8.969 – 2.803

2.742 67.17 – – – – 1.66 0.032 9.098 – 2.773

2.780 67.09 0.499 27.15 10.94 2.50 1.66 0.032 8.908 17.66 2.755

2.841 67.09 1.021 21.39 8.61 4.03 1.66 0.032 8.720 8.646 2.760

2.81 66.92 1.517 18.74 7.55 5.25 1.66 0.032 8.792 5.811 2.755

2.80 66.84 2.024 17.03 6.86 6.37 1.66 0.032 8.808 4.366 2.762

2.79 66.59 2.528 15.81 6.37 7.38 1.65 0.032 8.784 3.483 2.752

Fig. 1. Variation of density and refractive index with glass sample of Na2O–SiO2– ZrO2: Ho3+/V4+ co-doped glasses.

Fig. 3. EDS spectra of 1 mol% V2O5 in Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

The molecular bonding coefficient b2 and e2 p are evaluated by correlating the EPR and optical data using [28] the given expressions

Fig. 2. Variation of ionic concentration and electronic polaraizability of Na2O–SiO2– ZrO2: Ho3+/V4+ co-doped glasses.

S-character of the magnetic spin of the vanadium. Generally the s-character is due to the partial unpairing or polarization of the inner s-electron gives the interaction with the unpaired d electrons. The values Dgk/Dg\ are also calculated for the tetragonality of the vanadium site. The covalency parameter for the in-plane r-bonding is evaluated from the expression

A 3 a ¼ k þ ðg k  g ? Þ þ ðg ?  g e Þ þ 0:04 7 0:36 2

ð6Þ

b2 ¼

ðg e  g k ÞDk 8k

ð7Þ

e2 p ¼

ðg e  g ? ÞD? 8k

ð8Þ

In the above equation, k is the free-ion value of spin orbit coupling constants for the vo2+ ions and is taken as 170 cm1. Dk and D\ are the energies of the electronic transitions from 2B2 ? 2Bg and 2 B2 ? 2Eg respectively. The calculated values from these spectra along with the other pertinent data are presented in Table 5. 3.6. Optical absorption spectra The absorption spectra of Ho3+/V4+ ions co-doped NSZ glasses show a large number of bands; the observed bands are assigned on the basis of the reported energy levels [29,30] of Ho3+ ions of

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portion in the lower photon energy region of ln (a) verses hm plot are shown in Fig. 7c. Urbach energy which corresponds to the width of localized states, and used characterizes the degree of disorder in the glass systems. The value of DE in the present work lies in the range 0.26–0.28 eV for all the glasses. The concentration of V2O5 increases with increase in urbach energy. In addition to this, the theoretical optical basicity (Kth) of the glasses can be calculated for the Ho3+/V4+ co-doped NSZ glasses [33,34] by using the formula

Kth ¼

n X Zi ri i¼1

ð10Þ

Zdi

where n denotes the total number of cations present, Zi denotes the oxidation number of the ith cation, ri denotes the ratio of the number of the ith cation to the number of oxides present and Di denote basicity moderating parameters of ith cation. The basicity moderating parameter is calculated from the following equation

di ¼ 1:36ðxi  0:26Þ

ð11Þ

where xi is the pouling electro negativity of the cation. The theoretical and evaluated values of cutoff wavelength, theoretical band gap, direct and indirect band gap and the theoretical optical basicity (Kth) are presented in Table 6. 3.7. Photoluminescence spectra Fig. 4. FT-IR spectra of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

different glass hosts. With the addition of V2O5, the absorption intensity enhances and exhibits more absorption band. The bands are observed at 360,418, 445, 452, 460, 536, 636, 886, 1095 and 1158 nm [31], the bands are related to holmium (Ho3+) ions resulting due to the different transitions are from 5I8 ? 3H6, 5G5, 5F1, 5G6, 3 K8, 5F4, 5F5, 5I5, 5I6. When V2O5 is added to the glass network two more additional bands are observed at 636 and 1095 nm due to the transitions 2B2 ? 2Bg and 2B2 ? 2Eg. Thus, from these optical absorption spectra totally ten bands are observed for Ho3+/V4+ co-doped NSZ glasses as shows in Fig. 6. From the observed edge, we have evaluated the optical band gap (Eo) of these glasses by drawing Tauc plot between (ahm)1/2, (ahm)2 as a function of hm as per the given equation

The Photoluminescence spectra of Ho3+/V4+ co-doped NSZ glasses are recorded at room temperature with excited wavelength 355 nm in region 400–850 nm. When Ho3+ ions are excited at 3H6 level (360 nm), between 3H6 and 5F5 levels, there are several intermediate levels with smaller energy difference, then the radiative and non radiative energy transition takes place and finally depopulated to 5I8 state due to radiative energy transfer between the Ho3+/V4 ions. The emission spectra of the NSZ glasses containing Ho3+ ions exhibit four emission transitions, which are assigned to 5 F3 ? 5I8 (466 nm), 5S2 ? 5I8 (525 nm), 5F4 ? 5I8 (561 nm) and 5 F5 ? 5I8 (665 nm) [35]. By the addition of V2O5 another band is observed at round 783 nm with the transition 2E ? 2T2 [36,37]. Fig. 8 shows the luminescence spectra of Ho3+/V4+ co-doped NSZ glasses. 4. Discussions

aðv Þhv ¼ Cðhv  Eo Þn

ð9Þ

Here C is a constant and the exponent (n) can take values 1/2 and 2 for direct, indirect transitions in glasses respectively [32]. Tauc plots for direct transition in Fig. 7a and indirect transition are shown in Fig. 7b. Extrapolating the linear portion of these plots as (ahm)1/2 = 0, (ahm)2 = 0 gives optical band gap, along with the theoretical optical band gap energy also calculated using equation E = hc/k. Here h is the plank’s constant, c is the velocity of light and k is cutoff wavelength respectively. The value of urbach energy (DE) is calculates by taking the reciprocal of the slope of the linear

Within the physical properties, increase in density is observed with the increasing content of V2O5 in all the glasses. Basically, when V2O5 enters into the glass network in two forms, they acts as network modifier at low content in addition to at high content it acts as network forming group. Due to this, non bridging oxygen content is further increases. The observed parameters density and refractive index of the glasses vary non-linearly with increasing vanadium concentration in x mol%. From these observations, the effect of V2O5 in ionic concentration and electronic polaraizability of Ho3+/V4+: NSZ glasses vary non-linearly. It is observed that both

Table 3 The FT-IR band positions of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples

Band assignments

Pure

Hov0

Hov1

Hov2

Hov3

Hov4

Hov5

482 – 700 – 830 883 1012

482 – 700 – 830 884 1012

482 630 702 821 840 884 1014

483 630 702 821 840 883 1012

483 632 700 823 850 884 1014

483 630 702 823 850 884 1012

483 632 702 823 850 884 1014

Bending and rocking motion of Si–O–Si V–O–V bending vibrations Zr–O–Zr/ZrO4 structural units V–O–V chains Si–O–Si symmetric stretching vibrations Si–O–Zr units Si–O–Si asymmetric stretching vibrations

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Fig. 6. Optical absorption spectra of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

symmetrical band is designated as bending mode of bridging oxygen perpendicular to Si–Si axis within the Si–O–Si plane [38]. A feeble band at about 883 cm1 attributed to the vibrations of Si–O–Zr linkage is also located in the spectrum of these glasses. A band correlated to Si–O–Si bending and rocking modes is also located at about 483 cm1. The spectra also exhibited significant band at about 700 cm1 due to Zr–O–Zr/ZrO4 structural units. The IR spectrum of V2O5 is expected to exhibit vibrational bands at 821 cm1 due to V–O–V chains and 630 cm1 due to V–O–V bending vibrations for the present NSZ glasses. Na2O–SiO2–ZrO2 co-doped Ho3+/V4+ glasses are characterized by the Raman spectra. In these Raman spectra of all glasses are correlated with the IR spectra. We can observe that the presences of Ho3+ ions in the glass matrix as the bands are shifted towards lower frequency. The Raman spectra of all the glasses indicates the vibrational bands at around 340–365 cm1 is due to the Si–O–Si rocking vibrations and the band at around 800 cm1 causes Si–O–Si bending vibrations [39]. With the addition of V2O5 in the composition range 0.2–1.0 mol%, the V–O–V

Fig. 5. Raman spectra of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

the parameters tend to be inversely proportional to increase in x mol% of vanadium concentration. An eventual decrease in interionic separation is noticed when content of V2O5 increases in the glass network. The infrared transmission spectra of Na2O–SiO2–ZrO2 co-doped Ho3+/V4+ glasses contain different structural units. Within the glass region, due to the presence of rare-earth ion Ho2O3 there is no significant difference is observed, but may be shifted to lower frequency. The spectrum exhibited conventional bands due to Si–O–Si (linkage between SiO4 tetrahedral units) asymmetric and symmetric vibrations at about 1012 and 850 cm1 respectively. The

Table 4 Raman band positions of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples

Band assignments

Pure

Hov0

Hov1

Hov2

Hov3

Hov4

Hov5

340 612 815 962 1080

342 612 815 958 1083

352 612 815 946 1083

358 614 817 944 1085

358 614 817 945 1085

364 618 817 943 1087

364 618 817 942 1087

Si–O–Si rocking vibrations V–O–V vibrations/combinations of various vibrations Si–O–Si bending vibrations Si–O–Zr rocking vibrations Si–O–Si stretching vibrations

Table 5 The spin-Hamiltonian parameters and molecular orbitals coefficients for Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples

gII

Hov0 Hov1 Hov2 Hov3 Hov4

1.8949 1.8966 1.8967 1.8968 1.8997

g\

1.9186 1.9188 1.9189 1.9189 1.9193

AII 104 cm1

A\ 104 cm1

b2

147 147 147 147 147

48 48 48 48 48

0.0539 0.0530 0.0530 0.0529 0.0515

e2 p 0.0658 0.0656 0.0656 0.0656 0.0653

Dgk

0.1074 0.1057 0.1056 0.1055 0.1026

Dg\

0.0837 0.0835 0.0834 0.0834 0.083

Dgk/ Dg\

P 104 cm1

K

1.2831 1.2658 1.2661 1.2649 1.2361

120.6 120.3 120.3 121.5 122.8

0.7883 0.7924 0.7915 0.7788 0.7666

a2

cm

AI? 104 cm1

46.4 51.7 51.7 52.3 53.6

47 42 42 42 42

0.5915 0.5897 0.5896 0.5895 0.5864

AIk 104 1

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K. Neeraja et al. / Journal of Alloys and Compounds 586 (2014) 159–168

Fig. 7c. Urbach energy plot of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

Fig. 7. Tauc plots to evaluate (a) direct band gap, (b) indirect band gap of Na2O– SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

vibrational band and combination of another band are observed around 612–625 cm1 [40]. The band at around 940–965 cm1 attributed due to the vibrations of the Si–O–Zr vibrational unit [41]. The band at around 1080 cm1 there is a Si–O–Si stretching vibrations are observed in the Raman Spectra of co-doped Ho3+/ V4+: Na2O–SiO2–ZrO2: glasses. Fig. 9 shows the EPR spectra of Na2O–SiO2–ZrO2 co-doped Ho3+/ V4+ glasses. From this EPR spectra the increased intensity of the signals are observed with an increased the concentration of V2O5, usually vanadium ions seems to exist mainly in V4+ and V5+ state. During the preparation of glasses at high temperature there is a possibility for the following redox equilibrium that takes place 2V5+ + O2 ? 2V4 + 1/2O2". In this region, the presence of the larger concentration of V4+ ions may also be due to exchange coupling between V3+ ions (if any) and V4+ ions. The spectra of V4+ ions are found to exist in either threefold symmetry or fourfold symmetry. This describes the crystal field of V4+ ions in glasses and the V4+ ions in these glass matrix exists in octahedral co-ordination with a tetragonal compression and having C4v symmetry. An octahedral site with a tetragonal compression gives the value of gk > g\ > ge [42,43]. From these observations, it is suggested that the paramagnetic V4+ ion in the glass of vanadyl ion VO2+ is in an octahedral

environment with tetragonal distortion. The acquired hyperfine value of the present study suggests a lesser distortion within the glass matrix. This quantitative analysis of EPR result indicates that, the ratio of Dgk/Dg\ is observed to decrease gradually with ion concentration of V2O5 indicating an increasing degree of distortion of the VO6 octahedron. The molecular orbital coefficient values indicate that the degree of covalence in V–O–r bonds (b2) and p-bonding with the vanadyl oxygen (e2 p ) of all glasses has covalence [44,45]. This EPR study indicates that the tetragonal distortions decrease with increasing V2O5 concentration in the NSZ co-doped Ho3+/V4+ glasses. The covalency parameter (a2) describes the covalency of the r-bonding between vanadium ion and its ligands and is evaluated as 0.59; it indicates the covalency for the bonding. The absorption spectrum of Na2O–SiO2–ZrO2: Ho3+/V4+ glasses, the transition in the absorption spectrum of Ho3+ ions starts from the ground state 5I8 raising to the various excited states. The spectrum consists of V4+ ion belongs to d1 configuration. Vanadyl ion exhibited the three absorption bands on the basis of energy level scheme of VO2+ ions in a ligand field C4v symmetry. The transitions are 2B2 ? 2Bg, 2B2 ? 2Eg and 2B2 ? 2A1, for the present glasses exhibit only first two transitions are observed. The largest intensity of the half width of these bands is observed that indicating the presence of the concentration of VO2+ ions in these glasses. The optical band gap for direct and indirect transitions of the sample is found to decrease with an increasing the concentration of dopant V2O5 due to increase of non-bridging oxygen ions. The calculated theoretical optical basicity is found to decrease with an increasing concentration of dopant indicating an increase in covalent nature of the glasses. Conventional Judd–Ofelt (J–O theory) parameter has been calculated from the absorption spectra of Ho3+ ions. The absorption spectra of rare earth ions are useful to understand the radiative properties. The absorption line arising from 4f M 4f electronic transition can reflect an electric dipole, a magnetic dipole or an electric quadrapole characteristic. The electric dipole transitions between two states within 4f configuration are forbidden, while magnetic dipole and electric quadrapole transitions are allowed. The intensity of the absorption bands can be estimated by using oscillator strength fexp, which is calculated from the absorption spectra by using following equation

fexp ¼ 4:318  109

Z

eðmÞdv dv

ð12Þ

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K. Neeraja et al. / Journal of Alloys and Compounds 586 (2014) 159–168 Table 6 Optical data of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples

Cutoff wavelength

Theoretical band gap

Direct band gap (eV)

Indirect band gap (eV)

Urbach energy (DE) (eV)

Kth

Pure Hov0 Hov1 Hov2 Hov3 Hov4 Hov5

333 335 337 341 344 347 349

3.73 3.70 3.68 3.64 3.61 3.57 3.55

3.72 3.70 3.68 3.64 3.61 3.58 3.55

3.71 3.69 3.66 3.62 3.59 3.57 3.54

0.2693 0.2702 0.2728 0.2759 0.2781 0.2800 0.2822

0.104 0.110 0.088 0.088 0.087 0.087 0.087

Fig. 8. Luminescence spectra of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

where e(m) denotes the molar extinction coefficient at average energy m in cm1. According to the f–f intensity model of the J–O theory, [46] the calculated oscillator strength from initial state to an excited state are described by the expression

f ðwJ; w0 J 0 Þ ¼

  X 8p2mcv ðn2 þ 2Þ2 2 X Xk ðwJkU k kw0 J0 Þ 9n 3hð2J þ 1Þ k¼2;4;6

ð13Þ

where m refer to the mass of the electron, c is the velocity of light in vacuum, h is the plank’s constant, n is the refractive index of refraction of the glass, m is the frequency of the transition wJ ? w0 J0 , Xk (k = 2, 4 and 6) are the J–O intensity parameters and kUkk are the doubly reduced matrix elements of the unit tensor operator of the rank k = 2, 4 and 6 which are evaluated from the intermediate coupling approximation for a transition wJ ? w0 J0 . The experimental oscillator strengths of absorption bands of Ho3+ doped glass are determined from the known values of Ho3+ concentration, sample thickness, peak position and peak areas by using the Eq. (11). By applying least square fitting procedure to determine the J–O intensity parameters X2, X4 and X6 using experimentally measured oscillator strength, the obtained values are presented in Table 7. The J–O intensity parameters determined in the present glass network are found to be in the order X2 > X4 > X6. As shown in Table 8. The rare earth ions that occupy different coordination site with non-centro symmetric potential contribute significantly to X2 [47]. The variations in the sites with non-centro symmetric potential arise due to the influence of long-ranged dielectric constants of media. The covalency and structural changes in the vicinity of Ho3+ ions (short-range effect) lead to changes in X2 value. The value of X4 and X6 are strongly influenced by the vibrational levels associated with the central rare earth ions bound to the ligand atoms. According to the Judd–Ofelt theory the intensity parameters contain two terms: i.e. crystal field parameters, which determines the symmetry and distortion related to the structural changes in the vicinity of rare earth ions. It may be noted that Ho3+ ions mainly acts as modifiers and create more dangling bonds and non-bridging

Fig. 9. EPR spectra of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

oxygens in the glass network. Such that a larger average distance between Si–O–Si, Si–O–Zr chains is expected, causing the average Ho-O distance to increase. Such increase in the bond lengths produces weaker fields around Ho3+ ions, leading to lower value of X2 for Ho2O3 mixed glasses. The second term is the covalency between the rare earth ion and the ligand oxygen atoms. The overall lower values of X2 for Ho2O3 mixed glasses points out that there is a higher degree of disorder in these glasses. The Photoluminescence spectra of the co-doped Ho3+/V4+: NSZ glass system exhibit four emission transitions due to Ho3+ ions in which hypersensitive transition for 5I8 ? 5G6 that obeys the selection rule; DJ < 2, DL < 2 and DS < 0 may be understood due to strong 4f-5d mixing or due to the contribution of odd terms of kUkk. In the present glass system the concentration of Ho3+ was fixed at 1 mol%. We observed an increase in vanadium; the energy transfer takes place from vanadium to holmium. However, it will decrease due to the concentration quenching between the ions [48]. The energy transfer mechanism between Ho3+/V4+ co-doped NSZ glasses with variation of concentration are found cause dipole–dipole interaction. The small energy separation between the two levels of Ho3+ to V4+ indicates that they are thermally coupled to each other and the population ion at the two levels with a fixed concentration will depend on the temperature of the glass. This dependence of temperature is due to energy transfer or by multiphonon relaxation. Fig. 10 represents the energy level scheme for all the observed absorption, excitation and emission transitions of Ho3+/V4+ ions. The possible energy transfer happens from 2 E ? 2T2 level of vanadium ion [49,50] to 5I6 and 5I5 of Ho3+ ions. Hence Ho3+ ion gets excited from 5I6 to 3K8 and 5I5 to 5G5 and di-excited through non radiative decay and there by strengthens the emission transition of Ho3+ ions. Due to the inter conversion of V4+ M V5+ ions, it seems there is a decrease in the redox ratio (V4+/V5+) with increase in the concentration of V2O5 in the glass

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Table 7 Theoretical and experimental oscillator strength of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Transition 5 I8 ?

Hov0 fcal,(106)

3

0.4312 3.4259 32.3201 5.1789 1.4557 1.0258 0.8814 0.2192

H6 5 G5 5 F1 + 5G6 + 3K8 5 F4 5 F5 5 I5 5 I6 Rms deviation

fexp,(10-6) 0.4389 3.4662 32.3269 5.1765 1.4253 1.0235 0.8855

Hov1 fcal,(106) 0.2390 3.2958 35.3698 5.1896 1.4698 1.0329 0.8958 0.1110

fexp,(106) 0.2322 3.2961 35.3635 5.1864 1.4785 1.0366 0.8947

Hov2 fcal,(106) 0.3942 3.5698 34.1236 5.2364 1.5473 1.0358 0.8997 0.2187

fexp,(106) 0.3958 3.5674 34.1257 5.2369 1.5682 1.0368 0.8881

Table 8 J–O intensity parameters of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples

X2 1020 (cm2)

X4 1020 (cm2)

X6 1020 (cm2)

Hov0 Hov1 Hov2 Hov3 Hov4 Hov5

4.92 4.65 4.40 4.32 4.28 4.15

1.42 1.38 1.32 1.28 1.26 1.25

2.62 2.58 2.53 2.46 2.38 2.32

matrix. This is evidenced from the NIR absorption spectra as well as luminescence emission due to V4+ ions. Such decrease of V4+ ion conc. leads to decrease of feeding energy of Ho3+ ions to excited states. As a result there is a possibility for the decrease of luminescence. Though it is not significant such decrease of luminescence due to Ho3+ ions could be seen especially in the red region of the

Hov3 fcal,(106) 0.2245 3.6589 39.2486 5.7885 1.4267 1.0458 0.9658 0.0720

fexp,(106) 0.2214 3.6574 39.2584 5.7485 1.4258 1.0455 0.9458

Hov4 fcal,(106) 0.2715 3.9124 42.1166 6.1825 1.5691 1.0568 0.9685 0.3239

fexp,(106) 0.2752 3.9245 42.1778 6.1856 1.5247 1.0544 0.9658

Hov5 fcal,(106) 0.2652 4.12359 42.3561 6.2558 1.6258 1.0662 0.9775 0.1322

fexp,(106) 0.2623 4.1697 42.3647 6.2357 1.6258 1.0667 0.97581

luminescence spectra (Fig. 8). Hence, V2O5 as a codopant (with Ho3+ ions) in higher concentrations may not be suitable for the getting the high luminescence efficiency especially in the red region. The various radiative properties are calculated from the luminescence spectra are presented in Table 9. The radiative properties of any of Ho3+ ions depends on the number of facts such as network former or modifier of the glasses. The branching ratio br which defines the luminescence efficiency of the transition; among various transitions of the glass network 5S2 ? 5I8 is found to be the highest values among all the glasses. These transitions are considered as a possible laser transition and indicate that the glasses exhibit better lasing action. For better identification of luminescence properties of the prepared glasses, chromaticity coordinates are calculated from the emission spectrum. The CIE system characterizes the color by a two color coordinate x and y which specify the point on the chromaticity diagram as shown in Fig. 11. The two color coordinates x and y are nearly 0.1634, 0.0477 as indicated in Table 10.

Fig. 10. Energy level diagram of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses.

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K. Neeraja et al. / Journal of Alloys and Compounds 586 (2014) 159–168 Table 9 Various radiative properties of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. Glass samples Transitions

Hov0 A (s1)

b%

Hov1 A (s1)

b%

Hov2 A (s1)

b%

Hov3 A (s1)

b%

Hov4 A (s1)

b%

Hov5 A (s1)

b%

5

F3 ? 5I8 5 S2 ? 5I8 5 F4 ? 5I8 5 F5 ? 5I8

750 1125 432 214

29.750 44.625 17.136 8.489

746 1118 436 238

29.393 44.050 17.179 9.377

738 1112 424 226

29.520 44.480 16.960 9.040

734 1108 418 217

29.633 44.732 16.875 8.761

728 1105 412 208

29.678 45.047 16.796 8.479

723 1196 403 298

27.595 45.649 15.382 11.374

AT =

2521 0.396

sR (ms) =

2538 0.394

2500 0.401

2477 0.404

2453 0.408

2620 0.382

as a feeding for the excitation of Ho3+ ions. The absorption and emission spectra of Ho3+ ions were characterized using J–O theory. The radiative transition probabilities and branching ratio were evaluated from luminescence spectra. The analysis indicated the highest values of radiative probabilities and branching ratios for the green emission transition viz., 5S2 ? 5I8 transition among various other transitions of Ho3+ ions. However, the presence of higher concentration of V2O5 in the glass matrix seems to be a hindrance for getting the high luminescence efficiency especially in the red region. Acknowledgement The author is greatly thankful to University Grants Commission, Govt. of India, New Delhi for the sanction of Meritorious Scholarship under UGC-BSR scheme to carry out the present research work.

Fig. 11. The color space chromaticity diagram of Na2O–SiO2–ZrO2: Ho3+/V4+ codoped glasses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 10 The color coordinates of Na2O–SiO2–ZrO2: Ho3+/V4+ co-doped glasses. S. No

1 2 3 4 5 6

Glass samples

Hov0 Hov1 Hov2 Hov3 Hov4 Hov5

Color coordinates x

y

0.0594 0.1579 0.1634 0.1691 0.1583 0.1577

0.2801 0.0438 0.0477 0.0646 0.0484 0.0419

The evaluated CIE color coordinates indicated that this material can be used for blue light emitting devices.

5. Conclusions Na2O–SiO2–ZrO2 glasses co-doped with Ho3+/V4+ ions were synthesized and characterized by EDS, IR, Raman and ESR Spectroscopic techniques. Later an optical absorption and emission spectra of these samples were recorded as a function of V2O5 concentration. From the optical absorption and ESR spectra it is concluded that vanadium ions do exist in V4+ state in addition to V5+ state. The optical absorption spectra and photoluminescence spectra exhibited several conventional bands due to the excitation and de-excitation of Ho3+ ions, respectively. Additionally the bands due to vanadyl ions could also detected in both the spectra. Further, the emission transition 2E ? 2T2 due to vanadyl ions seems to be acting

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