Physical, optical and radiative properties of CaSO4–B2O3–P2O5 glasses doped with Sm3+ ions

Chinese Journal of Physics 56 (2018) 932–943

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Physical, optical and radiative properties of CaSO4–B2O3–P2O5 glasses doped with Sm3+ ions

T

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Y.A. Yamusaa,b, , Rosli Hussina, W.N. Wan Shamsuria a b

Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai, Johor 81310, Malaysia Centre For Energy Research and Training, Ahmadu Bello University, Zaria, Kaduna State 1014, Nigeria

A R T IC LE I N F O

ABS TRA CT

Keywords: Borosulfophosphate glasses FTIR DTA Optical Physical and radiative properties Non-bridging oxygen and samarium ions

Trivalent rare earth ions doped borosulfophosphate glasses are in high demand owing to their several unique attributes that are advantageous for applications in diverse photonic devices. Thus, Sm3+ ion doped calcium sulfoborophosphate glasses with composition of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 (where x = 0.1, 0.3, 0.5, 0.7 and 1.0 mol%) were synthesized using melt-quenching technique. X-ray diffraction confirmed the amorphous nature of the prepared glass samples. Differential thermal analyses show transition peaks for melting temperature, glass transition and crystallization temperature. The glass stability is found in the range 91 °C to 116 °C which shows increased stability with addition of Sm2O3 concentration. The Fourier transform infrared spectral measurements carried out showed the presence of vibration bands due to PeO linkage, BO3, BO4, PO4, PeOeP, OePeO, SeOeB, and BeOeB unit. Glass density showed increase in value from 2.179 to 2.251 g cm−3 with increase in Sm2O3 concentration. The direct, indirect band gap and Urbach energy calculated were found to be within 4.368–4.184 eV, 3.641–3.488 eV and 0.323–0.282 eV energy ranges, respectively. The absorption spectra revealed ten prominent peaks centered at 365, 400, 471, 941, 1075, 1228, 1375, 1477, 1528 and 1597 nm corresponding to 4D3/2,6H5/2→4I11/2,6P3/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6 H15/2 and 6F1/2 transitions respectively. Photoluminescence spectra monitored at the excitation of 398 nm exhibits four emission bands positioned at 559, 596,643 and 709 nm corresponding to 4 G5/2→6H5/2, 6H7/2, 6H9/2 and 6H11/2 transitions respectively. The nephelauxetic parameters calculated showed good influence on the local environment within the samarium ions site and the state of the SmeO bond. The Judd–Ofelt intensity parameters calculated for all glass samples revealed that Ω6 > Ω4 > Ω2. The emission cross-section and the branching ratios values obtained for 4G5/2→6H7/2 transition indicate its suitability for LEDs and solid-state laser application.

1. Introduction Trivalent rare earth (RE) activated glasses are potential luminescent materials for lasers, optical detectors, sensors, light emitting diode and color displays devices applications because of their substantial emissions in the near infrared and visible region. The transitions between the 4f–4f electronic states of the rare earth ions provide emission and excitation spectra of the optical materials in the environmental field of ligand, phonon energy of the host, structure, and symmetry around the rare earth environment [1,2]. The absorption and luminescence spectra of the RE-doped glasses when related to the crystals require greater attention for technological advancement and spectroscopic measurements [3].

⁎

Corresponding author. E-mail address: [email protected] (Y.A. Yamusa).

https://doi.org/10.1016/j.cjph.2018.03.025 Received 15 November 2017; Received in revised form 8 March 2018; Accepted 28 March 2018 Available online 31 March 2018 0577-9073/ © 2018 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.

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More studies on the spectroscopic properties of rare earth doped glasses are still necessary for the development of efficient optical devices or to enhance the performance of the already existing ones for optoelectronic applications. This kind of investigation can provide vital information about parameters such as transition probability, the energy level structure, stimulated emission cross section and lifetime which contribute to the optical gain of the RE-doped optical devices [3].The lasing characteristics and radiative properties of the RE ions which are affected by the local perturbation around the trivalent RE ions are determined by the electronic energy states of the RE ions. For this reason, rare earth ions were doped into different network former matrices which display different laser and luminescence properties [4,5]. In recent past, researchers were focused on the rare earth doped glasses based on concentration and network former dependent spectroscopic properties via optical absorption and spectral luminescence studies to evaluate their suitability for different photonic applications. The doping of rare earth ions into glasses have numerous advantages such as hypersensitive transitions, large rare earth ion doping capability, low cost of production, large heterogeneous bandwidth, low production cost, less preparation time and easy to fabricate into different shapes when compared to crystalline materials [6]. It is necessary to find an appropriate network former for rare earth ion doping due to its vital role in the development of efficient optical devices for the fact that emission and absorption spectral intensities show considerable dependency on the ligand field environment around the rare earth ion site. Among the glass formers, phosphate-based glasses are essential for the design of solid -state ionic and solid-state batteries because they possess unique properties such as glass transition and low melting temperature, high thermal expansion coefficient, high transparency to far infrared and UV radiations and high refractive index, etc. On the other hand, borate-based glasses are appropriate for RE3+ ion doping due to their essential properties such as high rare earth ion solubility, high thermal stability, and high transparency. However, pure phosphate and borate-based glasses are not stable, and their individual properties differ from the properties of combined borophosphate glasses. Phosphate-based glasses are hygroscopic in nature and with less chemical durability which affect its usefulness in the development of efficient optical devices [7,8]. Mixing B2O3 with P2O5 improves the chemical strength of the phosphate glasses by changing the glass network along with the formation of new cross-linked BeOeP bonds [9]. The borophosphate glasses have joint advantages of both the phosphate and borate glasses such as good rare earth ion solubility, good thermal and mechanical stability, excellent optical quality, narrow emission bandwidth and large emission cross section [6]. Among the rare earth ions, samarium ion is a vital activator which exhibits prominent red-orange luminescence in the visible region which is useful in visible solid-state lasers, undersea communications, color displays and high-density optical storage [10]. The Sm3+ ion possesses interesting properties for spectral hole burning studies and is good to analyze and understand the energy transfer between RE3+ and RE3+ ions or RE3+ ions and host matrix through different relaxation process [11]. The Sm3+ ions exhibit four dominant emission bands in the visible spectrum such as 4G5/2→6H5/2, 6H7/2, 6H9/2 and 6H11/2 from its lower emitting metastable state (4G5/2) with higher quantum efficiency [10]. Among the emission bands, 4G5/2→6H9/2 emission is associated with electric dipole allowed transition and is highly affected by the network former matrix and the strength of the transition can be changed by the ligand field around the RE-ion environment [12]. Out of the emission bands, the reddish-orange emission around 602 nm corresponding to 4G5/2→6H7/2 transition of the Sm3+ ion is useful for high-density optical storage and undersea communication applications. Furthermore, this reddish emission band is not influenced by the multi-phonon non-radiative decay since the difference in energy between the emission level and the next lower level (∼7250 cm−1) is considerably higher than the phonon energy of the borate glasses [13]. Swapna et al. studied the laser and spectroscopic properties of lithium fluoroborate doped Sm3+ glasses [10]. Aruna et al. [6], analyzed and reported the emission and absorption spectra of Sm3+ doped borophosphate glasses. The Sm3+ ions in cadmium–aluminum–silicate glasses were investigated by Yang et al. [14]. They explained the suitability for the tunable laser of the novel nearinfrared emissions and near-infrared optoelectronic devices. The investigations of borosulfophosphate glasses doped Sm3+ ions deserve much attention due to their numerous versatility in the field of optoelectronic and laser. This study aimed to synthesize calcium borosulfophosphate glasses doped Sm3+ ions and examines their structural properties by varying the rare earth ion content. The compositional roles of the CaSO4–B2O3–P2O5 glasses doped with samarium ions on their optical and physical properties were assessed. Judd–Ofelt theory has been applied to calculate the radiative transition probability, Fluorescence branching ratio, and emission cross-section. Also, the photoluminescence spectra that are beneficial for fabrication of new solid-state laser devices were analyzed and compared with the existing literature.

2. Materials and method 2.1. Sample preparation Calcium borosulfophosphate sample doped Sm3+ with different compositions of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 (where x = 0.1, 0.3, 0.5, 0.7 and 1.0 in mol%) was prepared by melt quenching method. The imported pure chemicals high purity glass constituents (Sigma Aldrich 99.99%) used for this work were Calcium sulfate (CaSO4), Boric acid (H3BO3), Phosphoric acid (H3PO4), and samarium oxide (Sm2O3) in consignments of about (30 g) were accurately weighed using standard analytical balance. The mixture of the sample was carried in an alumina crucible and then taken to an electric furnace, preheated at 200 °C for 30 min to eliminate the H2O and H2S content. Furthermore, the samples were heated at 1300 °C for 1 h. The melt prepared glass samples were air quenched by transferring it into a preheated stainless steel mold and kept for annealing at a rate of 300 °C for 3 h to eradicate thermal strains and then gradually allowed to cool to room temperature. After that, the glassy samples were polished to their flat surfaces for transparency and further characterization. 933

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2.2. Parameters of the instrumentations The amorphous nature of the prepared glass samples was confirmed by X-ray diffraction studies using a Bruker D8 advance diffractometer employing Cu–Kα radiations (λ = 1.54 Ǻ) functioned at a rate of 100 mA and 40 kv. The diffraction shapes of the prepared samples were used at the scanning rate of 0.05°/s and recorded in the range of 2θ = 0°−100°. Thermal properties of the CaSO4–B2O3–P2O5 glasses doped samarium ion were determined using differential thermal analyzer of Perkin Elmer DTA-7 model. A sample of 10 mg in powder form was heated at 15 °C/min. The machine operated under a dry nitrogen atmosphere with a flow rate of about 200 ml/min. The Hruby parameter H is used to estimate the stability of the prepared glasses from the relation [15].

Hg =

Tc − Tg (1)

Tm − Tc

The spectral range of 400–2400 cm−1 was used to record the Fourier transform infrared transmissions, and Perkin Elmer FTIR 1660 spectrometer was used during the measurement. KBr at ratio 1:100 and comparatively fine glass powder were mixed, and transparent pellets of each sample were formed. Davis and Mott formulated the relation between absorption coefficient (α), and photon energy (hυ) of the incident radiation for direct and indirect optical energy bandgap (Eg), they are found from Tauc plots using

αhυ = B (hυ − Eg )n

(2)

where B is a constant called band talling parameter and n = 2 for direct allowed, n = ½ for indirect allowed, n = 3 for direct forbidden and n = 1/3 for indirect forbidden [16]. The urbach's energy is calculated using

ln α =

hυ −c Eurb

(3)

and can be inscribed in the form of y = mx + c, where m signifies the slope and c is constant (intercept). Therefore, Urbach's energy (Eurb) is deduced as the inverse of the slope from the plot of Inα versus photon energy. The refractive index and molar refraction are essential properties in optical glasses, and they have closed connection with polarization properties. Therefore, the refractive index (n) of glass regarding optical bandgap (Eopt) is found from 1/2

Eopt (n2 − 1) ⎞ =1−⎛ (n2 + 2) ⎝ 20 ⎠ ⎜

⎟

(4)

The connection between refractive index to molar refraction and molar volume of a glass is defined by the equation of the Lorentz–Lorenz and can be expressed as

n2 − 1 ⎞ vm Rm = ⎛ 2 n ⎝ + 2⎠ ⎜

⎟

(5)

where Rm is the molar reflection, n is the linear refractive index, and Vm is the molar volume. This relationship gives the average value of molar refraction for isotopic substances such as glasses and polycrystalline materials [17]. The glass structural material is related to the molar refraction by introducing the Avogadro's number into the equation

3 ⎞ Rm αm = ⎛ ⎝ ΠNA ⎠ ⎜

⎟

(6)

The electronic polarizability is based on the magnitude of electrons responds to an electric field denoted by Lorentz–Lorenz equation with αm in (Ǻ). Therefore, the expression (6) can be re-written as αm = Rm/2.52. Therefore, other important parameters could be evaluated from the value of the refractive index, as follows: dielectric constant: ε = n2, optical dielectric constant £ = n2–1and reflection loss from the glass surface = (n − 1/n + 1)2. The wavelength of 300–1800 nm at room temperature using Shimadzu 3101UV–Vis NIR spectrophotometer was used to determine the absorption spectra, Perkin Elmer LS55 luminescence Spectrophotometer was used to examined and record the excitation and emission spectra at a resolution of ± 0.1 nm. Xenon discharge lamp in the range (200 ˂ λ ˂ 900 nm) source was used for the excitation of the prepared samples, Monk Gillieson monochromator photodiode detector has been used to determine the luminescence signal at the precise excitation wavelength. The measurement of density (ρ) of the prepared glass samples with an error of ± 0.001 g cm−3 was employed by Archimedes principle (Analytical balance of specific density-PrecisaXT220A), The density of each sample is calculated using the expression below [15].

ρ=

a ρ a−b x

(7)

where a is the weight of the glass sample in air, b is the weight of the glass sample in toluene that was used as an immersion fluid and is ρx = 0.866 g cm−3 = toluene). Then, the molar volume Vm was evaluated from the relation below 934

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M (cm3mol−1) ρ

Vm =

(8)

where Vm is the molar volume, M is the molecular weight and ρ is the density. The expressions could calculate the ion concentration and Polaron radius of the dopant (Sm3+) in the glass samples

Ni =

NρX Mav

(9) 1/3

rp (Å) =

1⎛ Π ⎞ 2 ⎝ 6Ni ⎠ ⎜

⎟

(10)

where N is the Avogadro's number, ρ is the density of the glass, X is the mole fraction of dopant in mol%, and Mav is the molecular weight of the sample. Two other related important properties could be evaluated after finding the value of ion concentration according to [18]. These parameters are Field strength = Z / rp2 and Internuclear distance = ri(Ǻ) = (1/Ni)1/3 where Z is the atomic number of the dopant. The experimental oscillator strength of lanthanide ion doped glasses can be determined by measuring the area under the absorption bands and can be calculated using the formula [19].

fexp =

2303mc 2 NA πe 2

∫ ɛ(ν) dν = 4.32 × 10−9 ∫ ɛ(ν) dν

(11)

where m is mass of the electron, c is the speed of light, NA is the Avogadro's number, e is the charge of an electron, and ε(ν) is the molar absorption coefficient of a band at a wavenumber ѵ (cm−1), ε(ν) is obtained from Beer–Lambert's law and can be expressed as:

ɛ(ν ) =

1 I log ⎛ 0 ⎞ cl ⎝I⎠

(12) I log( I0 )

where c is the concentration of rare earth ions in mol/l, I is the thickness of the sample in cm and is the optical density in wavenumber, ѵ (cm−1). The calculated oscillator strengths (fcal) from the ground state (ΨJ) to the excited state (Ψ′J′) can be calculated by Judd–Ofelt theory [20] from the equation below

fcal =

Ωλ =

(n2 + 2)2 8π 2mcv × 3h (2J + 1) 9n

∑

Ωλ ΨJ U λ Ψ′J ′

2

(13)

λ = 2,4,6

3h 9n × 2 × (2J + 1) Tλ 8π 2mc (n + 2)2

(14)

where h is the Planck's constant, J is the total angular momentum of the ground state, n is the refractive index of the samples, ν is the energy of the transition, Ωλ (λ = 2, 4, 6) are the Judd–Ofelt intensity parameter and ‖Uλ‖2 the square doubly reduced matrix element of the unit tensor. The Judd–Ofelt parameters Ωλ (λ = 2, 4, 6) were obtained by using the least square fitting method between the experimental and calculated oscillator strength and can be calculated using Eq. (14). To determine the accuracy of the obtained intensity parameters the root-mean-square deviations (δrms) are calculated using the following relation: 1/2

2 ⎛ ∑ (fcal − fexp ) ⎞ δrms = ⎜ 2 ⎟ ∑ fexp ⎝ ⎠

(15)

where fcal and fexp are the calculated and experimental oscillator strengths respectively, and the summation is taken over all the bands used to evaluate Ωλ. Also using the Judd–Ofelt theory, the following radiative properties are calculated [21]. (a) The spontaneous emission probability, Arad from ground state ΨJ to the excited state Ψ’J’ for an electric dipole transition is given by

Arad =

64π 4v 3n (n2 + 2)2e 2 × 27hc 2 (2J + 1)

∑

Ωλ ΨJ U λ Ψ′J ′

2

(16)

λ = 2,4,6

(b) The total radiative transition probability, AT, obtained by carrying out the summation of all the transitions to the final states is given by

AT =

∑ Arad

(17)

(c) The values of Arad and AT can be used to calculate the fluorescence branching ratio βr given by

βr =

Arad AT

(18) 935

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Fig. 1. XRD Pattern of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses.

(d) The radiative lifetime of an emitting state is related to the total spontaneous emission probabilities for all transition which is represented by;

τrad = (AT )−1

(19)

(e) The induced emission cross–section σ for each transition is given by

σ=

λp4 Arad (20)

8Πcn2Δλ

where λp is peak wavelength, and Δλ is the full width at half maxima of the fluorescent peak for different transitions obtained from the emission spectra. 3. Results and discussion The X-ray diffraction (XRD) patterns have been recorded within the range of 0° ≤ θ ≤ 100°. The X-ray nature of the borosulfophosphate glasses exhibited broad diffusion at lower scattering angles around 5–25°, that shows the characteristics elongated range structural disorder ascertain the amorphous forms of the prepared glasses [22], as shown in Fig. 1 (Table 1). Fig. 2 displays the results of DTA measurements of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. The sharp endothermic peak is corresponding to the melting temperature (Tm) at 674–713 °C, the exothermic peak is corresponding to the crystallization temperature (Tc) at 517–553 °C, and the small peak is corresponding to the glass transition temperature (Tg) at 426–439 °C [23,24]. Both the glass transition temperature and the crystallization temperature increase with an increasing Sm2O3 content. Table 2 displays the values of Tg, Tc, Tm, glass stability (S) and Hruby parameter for the prepared glasses. It was observed that the thermal analysis depicts an increase of glass transition due to the addition of B2O3, This is an indication that combining P2O5 and B2O3 improves the strength of the host. The increase of samarium content yields to an increase in the stability and network rigidity of the prepared glasses. The Hruby parameter can be used to estimate the stability of prepared glasses [15]. Fig. 3 shows the IR spectra of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 system with 0.1 ≤ x ≤ 1.0 (mol%). The band assignments Table 1 Composition of calcium borosulfophosphate glasses doped Sm2O3 in (mol%). Glass code

CaSO4

B2O3

P2O5

Sm2O3

BPCS1 BPCS2 BPCS3 BPCS4 BPCS5

25 25 25 25 25

30 30 30 30 30

44.9 44.7 44.5 44.3 44.0

0.1 0.3 0.5 0.7 1.0

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Fig. 2. DTA curves of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. Table 2 Sm2O3 concentration-dependent thermal properties of the prepared glasses. Glass code

Tg (°C)

Tc (°C)

Tm (°C)

S = Tc − Tg (°C)

H

BPCS1 BPCS2 BPCS3 BPCS4 BPCS5

426 428 428 436 439

517 528 535 547 553

674 682 693 702 713

91 100 107 111 116

0.5796 0.6493 0.6772 0.7161 0.7341

Fig. 3. The IR spectra of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses.

Table 3 IR band assignment and the reported values for 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. 0.1 (mol%)

0.3 (mol%)

0.5 (mol%)

0.7 (mol%)

1.0 (mol%)

Reported

Assignment

509 703 – 903 1052 1286 1645

– – 670 903 1059 1282 1645

509 728 – 903 1056 1293 1645

509 708 – 907 1053 1267 1646

506 706 – 902 1056 1267 1646

500–580 645 −773 670 800–960 1057 1273 1646

Bending vibrations of OePeO Combined BeOeB and PeOeP bonds Bending Vibration of PO4 in Q1 Asymmetric bridging vibra. SeOeB Symmetric stretching of PeOeB links Vibration of PeO linkage Vibrational mode of OH group

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corresponding to their band positions are tabulated in Table 3. The observed band's position between 506–509 cm−1 frequency range goes to the bending vibrations of OePeO bonds [25,26]. When the Sm2O3 concentration is increased, and P2O5 concentration is decreased, the stretching vibrations become broader and less dissimilar at the high-frequency bands. Any bands below 600 cm-1 have significant aids from the modifier cations mostly present in the glass samples [27]. The bands at 670 cm−1 correspond to the bending mode of PO4 in Q1 [28]. The bands at 703–728 cm−1 are assigned to the symmetric stretching vibrations of PeOeP linkages in the host and bending vibrations of BeOeB bonds [5]. The band at 902–907 cm−1 belongs to the asymmetric bridging vibration SeOeB [29]. The bands at 1052–1059 cm−1 have been ascribed to the symmetric stretching of PeOeB links [30]. The bands at 1267–1293 cm−1 belongs to Vibration of PeO linkage [31]. The presence of PeOeP and BeOeB group decreased the BO3 units and increased the BO4 units in the network matrix and increased the NBO's in the glass host [31]. The bands at 1645–1646 cm−1 are due to the vibrational mode of OH group in the glass host [32]. The spectral absorption of the borosulfophosphate glasses doped samarium ions was recorded in the spectral ranges of 300–1800 nm. The spectrum displays ten broadened absorption peaks centered at 365, 400, 471, 941, 1075, 1228, 1375, 1477, 1528 and 1579 nm corresponding to the 4D3/2, 6H5/2→4I11/2, 6P3/2, 6F11/2, 6F9/2,6F7/2,6F5/2, 6F3/2, 6H15/2 and 6F1/2 transitions respectively. Additionally, absorption band monitored at 1375 nm goes to the oversensitive transition (6H5/2⟶6F5/2). Furthermore, it possesses the uppermost intensity when compared to other transitions, and this shows that its follow the selection guidelines |ΔL| ≤ 2 and |ΔJ ≤ 2|i.e., for each rare-earth ion, the spectral intensities and the positions of some absorption transitions are sensitive to the environment around the rare-earth [31]. Consequently, the spectral emission of borosulfophosphate glasses doped with a different concentration of samarium ions has been recorded and obtained by monitoring emission within the range of 540–720 nm due to an excitation wavelength of 398 nm. The spectrum displays four bands at 559 nm, 596 nm, 642 nm and 709 nm corresponding to the ground state transitions 4G5/2 to the excited states of 6H5/2, 6H7/2, 6H9/2 and 6H11/2. It was also observed that the emission intensities increased with the increasing of samarium ions concentrations as displayed in Fig. 4. Fig. 5 shows the excitation spectra and energy level diagram of borosulfophosphate glasses doped samarium ions; the excitation was recorded in the range of 320–500 nm wavelength. The spectrum excitation for 598 nm emission displaced four distinct bands at 358 nm, 398 nm, 436 nm and 468 nm corresponding to the ground state transitions 6H5/2 to the excited states of 4D3/2, 4F7/2, 4G9/2 and 4I11/2 levels respectively. Upon all the various excited bands, the band 6H5/2 ― 4F7/2 at 398 nm is the most intense one which happens to be the excitation wavelength. It was observed that the calcium borosulfophosphate doped samarium glasses could be excited by UV radiation. The physical properties of the prepared borosulfophosphate glasses shown in Table 4. It is precisely observed that the density increases with the increase of samarium ion concentration. A similar pattern is also found for the average molecular weight of the prepared glasses. The increased densities values may be due to an alteration in structural softening, coordination configuration and geometry, and the dimensional spaces of the prepared glass [33]. Meanwhile, the decrease in molar volume could be due to the decrease in the bond length or inter-atomic spacing between the atoms that lead to the compactness of the network structure [34]. The polaron radius and ion concentration are shown in Table 4; polaron radius decreases with increasing of samarium ions concentration while the ion concentration increases with increasing of samarium ions concentration. This apparently may be due to an increase in ionic concentration for samarium ions. Due to that, the field strength around samarium ions became high. Furthermore, Inter-ionic separation has been found to be decreasing with an increased Sm3+ concentration which eventually leads to more compactness of the borosulfophosphate host of the prepared glass system. Table 5 shows the optical parameters of the prepared borosulfophosphate glasses. The optical absorption spectra have been used to ascertain optical and urbach's energy of the prepared glasses doped samarium ions content. The indirect optical energy gap values are found by extrapolating the linear curve area of the plot of (αhʋ)1/2 against photon energy to meet the horizontal axis. The

Fig. 4. The absorption and emission spectra of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. 938

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Fig. 5. The excitation spectra and Energy level diagram of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. Table 4 Physical parameters of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. Physical parameters

Physical parameters for different Sm2O3 content

Average molecular weight Mav (g) Density, ρ (g cm−3) Molar volume, Vm (cm3 mol−1) Dy3+ ion concentration, Ni (×1021 ions. s cm−3) Polaron radius, rp (×10−8 Ǻ) Inter – ionic separation ri (×10−8 Ǻ) Field strength, F (×1016 cm−2)

x = 0.1 mol%

x = 0.3 mol%

x = 0.5 mol %

x = 0.7 mol%

x = 1.0 mol%

105.9422 2.179 48.620 1.239 3.814 10.947 4.263

106.4437 2.196 48.472 3.727 2.643 7.611 8.876

106.9451 2.213 48.326 6.231 2.228 6.424 12.490

107.4465 2.231 48.161 8.753 1.989 5.742 15.672

108.1987 2.251 48.067 12.528 1.765 5.102 19.902

Table 5 Optical parameters of 25CaSO4–30B2O3–(45−x) P2O5–xSm2O3 glasses. Optical parameters

Indirect energy gap (eV) Direct energy gap (eV) Ubach's energy (eV) Refractive index Dielectric constant Optical dielectric constant Reflection losses Molar refractivity (cm3 mol−1) Electron polarizability (Ǻ)

Optical parameters for different Sm2O3 content 0.1 mol%

0.3 mol%

0.5 mol%

0.7 mol%

1.0 mol%

3.641 4.368 0.323 2.243 5.031 4.031 0.147 27.875 11.061

3.599 4.324 0.296 2.252 5.072 4.072 0.148 27.909 11.075

3.583 4.272 0.292 2.255 5.085 4.085 0.149 27.863 11.056

3.581 4.236 0.284 2.256 5.089 4.089 0.149 27.779 11.023

3.488 4.184 0.282 2.277 5.185 4.185 0.152 27.997 11.109

intersection of the photon energy gives the optical energy gap value, and the same trend is done for the linear curve region of the plot of (αhʋ)2 versus photon energy for the direct energy gap. The indirect and direct energy gap values are obtained to be within the range of 3.641–3.488 eV and 4.368–4.184 eV respectively. It could be observed that the indirect and direct energy gap decreased with the increase of samarium ion concentration. Such decrease is attributed to structural changes and the creation of NBO's which leads to the bonding defects in the network matrix of the prepared glasses. The direct and indirect energy gap values obtained were higher than reported by Bulus et al. [16] (Fig. 6). The values of urbach's energy obtained from Table 5 decrease with increment in samarium ion concentration. The glass with lower samarium ion concentration possesses the highest value of urbach's energy; this is an indication of the likelihood of elongated range order locally arising from the bonding defects, these defects yield localized states in the glasses inflicting the decrease in the width of the localized states in the optical energy gap. The urbach's energy also gives information about the degree and disorders in the material (Fig. 7). The refractive index and molar refraction are essential properties in optical glasses, and they have closed connection with polarization properties. It was observed that with increasing samarium ions content the refractive index increases, and the molar refraction increases and then decreases at 0.5 mole%, the electronic polarizability founded to be increasing with the increase of 939

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Fig. 6. Tauc's plot of (αhʋ)1/2 and (αhʋ)2 as a function of photon energy of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses.

Fig. 7. The graph of ln α as a function of photon energy to determine the Eurb of 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses.

samarium ion content as display in Table 5. Hence, from the value of the refractive index above, other essential parameters were calculated such as dielectric constant, optical dielectric constant and reflection loss from the glass surface and all these parameters are found to be increasing with increase in samarium ions content. Table 6 Nephelauxetic ratio of 25CaSO4–30B2O3–(45−x) P2O5–xSm2O3 glasses. Transitions

4

D3/2 H5/2⟶6P3/2 6 H5/2⟶4I11/2 6 H5/2⟶6F11/2 6 H5/2⟶6F9/2 6 H5/2⟶6F7/2 6 H5/2⟶6F5/2 6 H5/2⟶6F3/2 6 H5/2⟶6H15/2 6 H5/2⟶6H1/2 6

β ẟ

Nephelauxetic ratio (β) for different Sm2O3 content

Aqueous ions band position (cm−1) [35]

0.1 mol%

0.3 mol%

0.5 mol%

0.7 mol%

1.0 mol%

0.9880 1.0047 1.0048 1.0097 1.0176 1.0204 1.0211 1.0176 1.0032 0.9820 1.0069

0.9880 1.0047 1.0048 1.0097 1.0176 1.0183 1.0211 1.0194 1.0013 0.9787 1.0064

0.9880 0.9985 1.0108 1.0070 1.0153 1.0204 1.0210 1.0194 1.0032 0.9787 1.0062

0.9880 0.9984 1.0107 1.0127 1.0153 1.0204 1.0171 1.0157 1.0013 0.9820 1.0061

0.9880 0.9984 1.0048 1.0097 1.0153 1.0183 1.0192 1.0214 0.9997 0.9837 1.0059

27,714 24,999 21,096 10,517 9136 7977 7131 6641 6508 6397 1

−0.69

−0.64

−0.62

−0.61

−0.59

0

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Table 7 Experimental and calculated oscillator field strength (×10−6) and root mean square deviation (δrms) for 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 with 0.1 ≤ y ≤ 1.0 mol% glasses. Transition

H5/2→ F3/2 H5/2→6F5/2 6 H5/2→6F7/2 6 H5/2→6F9/2 6 H5/2→6F11/2 6 H5/2→4I11/2 6 H5/2→6P3/2 δrms

6

6

6

0.1 mol%

0.3 mol%

0.5 mol%

0.7 mol%

1.0 mol%

ƒexp

ƒcal

ƒexp

ƒcal

ƒexp

ƒcal

ƒexp

ƒcal

ƒexp

ƒcal

1.2864 2.9642 9.4896 5.1360 1.0069 3.6420 2.7344 ± 0.4023

0.9863 1.9609 6.2630 4.8456 0.8148 0.3410 3.9734

1.6709 3.4353 8.7423 6.0825 1.0339 3.4816 3.8695 ± 0.3552

1.2897 2.2779 6.2863 4.7507 0.7949 0.3322 4.5334

1.3849 3.6182 8.1455 6.4160 1.0406 3.7640 2.8261 ± 0.4046

1.1258 2.1827 6.1160 4.6247 0.7738 0.3234 4.4069

1.9893 4.3631 11.205 9.5413 1.3956 5.6516 6.7115 ± 0.3777

1.5324 2.9072 8.6213 6.5918 1.1057 0.4624 5.8509

1.1289 3.6868 7.7355 6.5377 1.0603 3.8353 2.8797 ± 0.3983

0.9640 2.0677 6.1938 4.7318 0.7935 0.3318 4.2335

Nephelauxetic ratios (β) and the bonding parameters (ẟ) are bonding characteristics, determined from the bond positions (cm−1) of the spectral absorption and the found outcomes are tabulated in Table 6. The nephelauxetic ratios β are calculated using the expression β = (Vc/Va), where Vc is the transition wavenumber (in cm−1) of the samarium ion and Va express the wavenumber in (cm−1) of the exact transition for aquo ion reported by Carnall et al. [35]. The values of β are divided by the total number of energy positions that give the average nephelauxetic ratio (β ). The values of the bonding parameter are computed using the expression ẟ = (1 − β /β ) × 100. The metal-ligand band could be covalent or ionic depending upon the positive or negative values of the ẟ. The negative sign of ẟin this report indicates that the bonding between Sm3+ ions and the surrounding ligands is ionic in nature. The decreasing values of β indicates an increase in covalence character of the glasses [36]. The oscillator strength of Sm3+ion doped CaSO4–B2O3–P2O5 glasses calculated and experimental are displayed in Table 7. The result of the calculated and experimental oscillatory strength has the highest peak value at 6H5/2→6F7/2 transition. This is because of hypersensitivity transition that usually has the maximum intensity in the spectra. The fitting quality is obtained from the root mean square deviation as shown in Table 7 and is like those reported by Vijayakumar et al. [31] and Ahmadi et al. [37]. The deviation sufficiently demonstrates acceptable fitting between experimental and theoretical oscillator strength, which agrees with the Judd–Ofelt theory. This theory is applied in the analysis of experimental absorption spectra. Intensity parameters Ωλ (λ = 2, 4 and 6) for the glasses doped with Sm2O3 are displayed in Table 8. Ωλ values of all the glasses were in the order of Ω6 > Ω4 > Ω2 which indicates similar trend reported by Babu et al. [38]. Among the three J–O parameters, Ω2 is related to the covalency and structural changes near the samarium ion, described as short-range effects, while Ω4 and Ω6 are related to the long-range effects and is strongly influenced by vibrational levels associated with the central rare earth ions bound to the ligand atoms [39]. Radiative transition probabilities (Arad), radiative lifetimes of excited states (τrad), fluorescence branching ratios (βr) and emission cross-section (σ) are estimated when Judd–Ofelt theory is applied further as shown in Table 9. Radiative transition probability (Arad) is higher for the transition 4G5/2→6H7/2 which increases with the elevated concentration of Sm3+. The branching ratio characterizes lasing power of transition. Studies have shown that emission transition having branching ratio higher than 50% are considered suitable for laser emission [40]. 4G5/2→6H7/2 transitions have branching ratio above 50% and are the highest compared to other transitions and are regarded as a lasing transition. Large stimulated emission cross-section is an attractive feature for low-threshold, high gain laser application, that are utilized to obtain continuous wave laser action [37]. Observations show that 4G5/2→6H7/2 transition has the highest emission cross-section value for all the glasses. Also, glass formation with a mixture of SO4 and P2O5 is possible only if the phosphate network contains [POO2/2O]− and [POO1/2O2]−1 structural groups [39]. Glass formation of dithiophosphate species is possible when sulfate and [POO2/2O]− ions are present simultaneously. The concentration of such species relies on the nature of the modifier ion which usually affects rare earth emission probabilities [41]. Furthermore, concentration variations of phosphate structural units, sulfate ions as well as their linkages are expected to alter the crystal field around the lanthanide ions in glass network [39]. 4. Conclusions A newly glassy of calcium borosulfophosphate assorted with different contents of samarium ions have been synthesized by the conventional melt-quenching method. X-ray diffraction determined the amorphous state of the glass samples. It is found that sample Table 8 The Judd–Ofelt intensity parameters Ω2, Ω4 and Ω6 (×10−20) of the 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 with (0.1 ≤ y ≤ 1.0 mol%) glasses. y mol Sm3+ %

Ω2

Ω4

Ω6

Trend

Ω4/Ω6

0.1 0.3 0.5 0.7 1.0

0.39 0.95 0.53 0.82 0.14

2.86 3.24 3.14 4.16 3.05

4.53 4.37 4.24 6.06 4.04

Ω6 > Ω 4 > Ω 2 Ω6 > Ω4 > Ω2 Ω6 > Ω4 > Ω2 Ω6 > Ω4 > Ω2 Ω6 > Ω4 > Ω2

0.63 0.74 0.74 0.68 0.75

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Table 9 Emission band position (λp, nm), radiative transition probability (Arad, s−1), total radiative transition probability (AT), fluorescence branching ratio (βr, %), calculated lifetime (τcal, ms) and emission cross section (σ × 10−22) (cm2) for 25CaSO4–30B2O3–(45−x)P2O5–xSm2O3 glasses. Transition G5/2→ H5/2

4

6

G5/2→6H7/2

4

G5/2→6H9/2

4

G5/2→6H11/2

4

Parameter

0.1 mol%

0.3 mol%

0.5 mol%

0.7 mol%

1.0 mol%

λp Arad βr σ λp Arad βr σ λp Arad βr σ λp Arad βr σ AT τcal (ms)

559 69.73 9.21 1.98 596 446.14 58.98 10.79 642 145.32 19.21 6.55 709 95.21 12.58 2.94 756.4 1.32

559 71.36 9.085 1.95 596 461.73 58.78 10.94 642 152.27 19.38 7.44 709 100.10 12.74 3.09 785.46 1.27

559 74.67 8.49 1.85 596 523.06 59.49 11.84 642 171.26 19.47 7.58 709 110.21 12.53 3.26 879.2 1.13

559 79.18 8.01 1.79 595 572.32 57.90 9.34 642 212.04 21.45 6.38 709 124.77 12.62 3.85 988.31 1.01

559 85.57 7.57 1.76 596 655.04 58.00 9.78 642 242.39 21.46 6.27 709 146.33 12.95 3.73 1129.33 0.88

BPCS2 and BPCS3 have good forming ability and stability among the prepared glass samples. The increase in samarium ion leads to the formation of NBO's by infrared. This investigation found that the optical absorption, density and physical properties of these glasses were strongly influenced by varying the glass composition. The advancing replacement of P2O5 with Sm2O3 concentrations slightly increase the density and decrease the molar volume of the prepared glass samples. The increase in density with Sm2O3 concentrations affirms the creation of rigid and extremely cross-linked network resulting in a firmly packed glass structure. The results of the direct bandgap, indirect bandgap, and the Urbach's energy were obtained to be extremely sensitive and compositionally reliant on the addition of Sm2O3 content. The increase in samarium ions concentration with the increment of non-bridging oxygen could drastically decrease the Urbach's energy and reduced bandgap energy. The refractive index of the synthesized glass increases nonlinearly with increasing content of Sm2O3. The negative values of the bonding parameters authenticated the ionic nature of samarium ions–ligand bond in the network matrix. The validity of the Judd–Ofelt and benefit of fitting procedure have manifested owing to the very low root mean square deviation between the experimental and calculated oscillator strengths. The radiative transitions probabilities, branching ratios were evaluated for various luminescent transitions. The result of the radiative properties, bandgap energy, the nonlinearity graphs presented, excellent structural features shown by Fourier transforms infrared analysis and photoluminescence displayed by the current glasses acclaim their suitability for solid-state lasers, nonlinear optical and LED's applications. Acknowledgments We are thankful to the Ministry of Higher Education Malaysia (MOHE) and UTM for the financial assistance through the fundamental research grant scheme (FRGS), vote number (Q.J130000.2526.16H24). References [1] K. Swapna, S. Mahamuda, A.S. Rao, T. Sasikala, L.R. Moorthy, Visible luminescence characteristics of Sm3+ doped Zinc Alumino Bismuth Borate glasses, J. Lumin. 146 (2014) 288–294. [2] F. Nawaz, M.R. Sahar, S.K. Ghoshal, A. Awang, I. Ahmed, Concentration-dependent structural and spectroscopic properties of Sm3+/Yb3+ co-doped sodium tellurite glass, Physica B 433 (2014) 89–95. [3] R. Vijayakumar, K. Marimuthu, Concentration-dependent spectroscopic properties of Sm3+ doped borophosphate glasses, J. Mol. Struct. 1092 (2015) 166–175. [4] D. Umamaheswari, B.C. Jamalaiah, T. Sasikala, I.G. Kim, L.R. Moorthy, Photoluminescence properties of Sm3+-doped SFB glasses for efficient visible lasers, J. Non Cryst. Solids 358 (4) (2012) 782–787. [5] W.Z. Tawfik, M.M. Mahdy, M.A.K. Elfayoumi, M.M. Elokr, Physical study of Sm3+ doped borochromate glass system, J. Alloys Compd. 509 (41) (2011) 10070–10074. [6] M. Vijayakumar, K. Marimuthu, Structural and luminescence properties of Dy3+ doped oxyfluoro-borophosphate glasses for lasing materials and white LEDs, J. Alloys Compd. 629 (2015) 230–241. [7] N. Kiran, C.R. Kesavulu, A.S. Kumar, J.L. Rao, Spectral studies on Mn2+ ions doped in sodium–lead borophosphate glasses, Physica B 406 (20) (2011) 3816–3820. [8] A. Rohaizada, R. Hussina, N.A.S. Aziza, R. Uninga, N.Z.I. Boharia, Vibrational studies of zinc antimony borophosphate glasses doped rare earth, J. Teknol. 62 (3) (2013). [9] K. Srinivasulu, I. Omkaram, H. Obeid, A. Suresh Kumar, J.L. Rao, Structural investigations on sodium–lead borophosphate glasses doped with vanadyl ions, J. Phys. Chem. A 116 (14) (2012) 3547–3555. [10] K. Swapna, S. Mahamuda, A.S. Rao, S. Shakya, T. Sasikala, D. Haranath, G.V. Prakash, Optical studies of Sm3+ ions doped zinc alumino bismuth borate glasses, Spectrochim. Acta Part A 125 (2014) 53–60. [11] C. Venkateswarlu, M. Seshadri, Y.C. Ratnakaram, Influence of mixed alkalis on spectroscopic parameters of Sm3+, Dy3+ doped chloroborate glasses, Opt. Mater. 33 (6) (2011) 799–806.

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