Transition metal (TM) ions doped oxy-fluoride glasses have been investigated widely because of their important applications in IR transmission components, amplification and 3D display. Especially, the glassy materials are efficient hosts for lasing materials. Compared to all other glass materials, the transparent oxyfluoride glasses containing Cu²⁺ ions have been given a special attention due to their excellent optical and magnetic properties (Bandyopadhyay, 1981; Sands, 1955; Sekhar et al., 2018; Thulasiramudu & Buddhudu, 2006). Among all fluorides, sodium fluoride has high solubility, good chemical stability (Shareefuddin et al., 1996). Al₂O₃ is not a glass former, whereas B₂O₃ readily form glasses by itself. Al₂O₃ does not fulfill the requirements of glass former. It cannot frame a three-dimensional network of Al₂O₃ triangles. But boric oxide has three-fold coordination. The Al³⁺ ion ranges between those particles which prefer to be six-fold or octahedral coordination. These combinations will create considerable changes in the optical and electrical characteristics of the prepared glasses.

Through Electron Paramagnetic Resonance (EPR) spectro scopy one can get information regarding the structural and dynamic properties of materials. TM ions are embedded into the glassy network to get EPR spectra. The EPR spectra inturn reveals about the site symmetry that is present around the TM ions which is quite useful in understanding the basic structure of the glass (Ahmed & Shareefuddin, 2019; Ramadevudu et al., 2000). Many researchers focused mainly on oxide glasses. The work on oxyfluoride glasses is meager and hence we paid attention to study the oxyfluoride glasses behavior through EPR and optical examinations.

Ahmad et al. (2019) studied the effect of Al₂O₃ on strontium borate glasses using FTIR, EPR and Raman experimental techniques and concluded that the addition of Al₂O₃ influences the borate network by creating AlO₄ units. Copper doped alumina bismuth borate glasses were characterised with XRD, optical absorption, EPR and FTIR techniques by Phani et al. (2019). XRD confirmed the amorphous nature; spin Hamiltonian parameters were evaluated from EPR spectra and concluded that the Cu²⁺ ions are in tetragonally distorted octahedral sites with ground state (²B_1g) d_x²-_y². Formation of BO₃, BO₄ and AlO₄ units in the borate network were observed from FTIR spectra. Abd El-Moneim (2019) studied the effect of composition on alkali fluoride and oxyfluoride glasses by correlating some of the physical properties like Poisson's ratio, packing density, etc. Different transition metal ion doped NaF-B₂O₃ glasses were characterised by Murthy et al. (2000) with dielectric measurements. Quantum mechanical tunnelling model is used to explain AC conductivity of NaF-B₂O₃ glasses. Franco et al. (2019) prepared the glasses with composition NaPO₃–Sb₂O₃–CuO and characterised with XRD, EPR and optical absorption to study the structural modifications in the glass network and ligand field near Cu2+ ions. Many other researchers (Bale & Rahman, 2013; Sastry & Rao, 2014; Singh et al., 2013; SivaRamaiah & LakshmanaRao, 2013; Vedeanu et al., 2012) prepared the glass samples with various compositions doped with copper, explaining about environment around transition metal ion using the optical and EPR spectra.

In this paper, we report the effect of incorporating NaF and Al₂O₃ into B₂O₃ network.

Melt-quenching method is employed for the glass fabrication. Table 1 gives the details of the glass composition. Alumino sodium fluoroborate glass (30−x) NaF-xAl₂O₃-69B₂O₃ -1CuO (NFABC) were prepared by conventional melt quenching technique. In this the concentration of Al₂O₃ is varied from 0 to 15 mol% in the place of NaF concentration, which is decreased from 30 to 15 mol% and to this 1 mol% of CuO has been added as paramagnetic probe. In the preparation of these glass samples the analar grade boric acid (H₃BO₃ - 99% purity), sodium fluoride (NaF- 99% purity), aluminum oxide (Al₂O₃ - 99% purity), and copper oxide (CuO- 99% purity) of Aldrich/ Merck make were used as the starting materials. For a given composition, the concentration in mole% of each chemical is calculated and the raw chemicals were weighed in digital monopan balance. All the chemicals forming the glass composition are finely ground to achieve homogeneity. The glass composition is taken in platinum crucible and heated to melt in an electrically heated furnace at 1000 °C. After obtaining homogeneous mixtures the melt were quenched at 200 °C and kept at same temperature to mechanical stresses. The glass compositions with appropriate glass code are given in Table 1.

Table 1. Glass Composition

Philips X-ray diffractometer is used to scan the 2θ range from 10 to 80 degrees. Peak free X-ray diffractograms of NAFBC glass samples confirmed the amorphous nature. Absorption spectra of NAFBC samples recorded at room temperature in the wavelength 300–1200 nm on Shimadzu UV-1800 model spectrophotometer. EPR Spectra of Cu²⁺ ions are recorded using JEOL-1X-EPR operating at X-band with field modulation of 100 kHz. Uncertainties in the measurement of “g” and “A” values were about ±2x10-3 cm^-1 and ±2x10−4 cm⁻¹ , respectively.

3.1. EPR Spectra of Cu²⁺ ions

Cu²⁺ ion is in its oxidation state, and the copper ion will have electronic configuration [Ar] 3d⁹. When the external magnetic field is applied, this degeneracy is removed and the transition occurs between the two energy levels in accordance with the resonance condition, hn = gβH. The nuclear spin (I) and electronic spin (S) of the Cu²⁺ ion are 3/2 and 1/2 respectively. The interaction between the nuclear magnetic moment and electronic magnetic moment (both spin and orbital) results in the hyperfine structure seen in the EPR. Thus, each fine structure splits into (2I+1) components (Abragam & Bleaney, 1970). In the case of Cu²⁺ ion (3d⁹ configuration) the cubic field (six negative charges placed on the axes at equal distance from the metal ion) splits the e_g and t_2g orbitals where e_g orbitals will have more energy than t_2g orbitals (Jorgensen, 1955).

In the case of tetragonal symmetry, where the distance of the negative charges on the z axes from the origin is longer than that in the equatorial plane, the degeneracy of the 3d orbitals of d⁹ will be further removed. The EPR spectrum of Cu²⁺ ion in most of the oxide glasses is distinctive and easily recognized based on the principal 'g' values and four line hyperfine splittings. In majority of oxide glasses containing Cu²⁺ complexes, the coordination of Cu²⁺ ion is that of a tetragonally elongated octahedral.

The EPR spectra of NAFBC glasses are shown in Figure 1. The EPR Spectra of Cu²⁺ ions is recognized in the form of four parallel and four perpendicular hyperfine splitting components. However we noticed only three well resolved parallel components. No perpendicular components are resolved because the spectrum can be recognized on the basis of four hyperfine line splitting due to 63Cu and 65Cu. But the isotropic splitting are not resolved owing to the nearly identical nuclear moments.

Figure 1. EPR Spectra of (30−x) NaF-xAl₂O₃ -69B₂O₃1CuO Glass System

The EPR spectra NAFBC samples are similar to those reported earlier (Zamyatin et al., 2018). For NAFBC glass system an axial spin Hamiltonian is employed (Abragam & Bleaney, 1970).

The spin Hamiltonian parameters are evaluated and are presented in Table 2. The g∥ values varied between 2.3296 for NAFBC-7 sample and 2.3737 for NAFBC-12 and g_⊥ values varied between 2.076 for NAFBC-0 and 2.128 for NAFBC-12. NAFBC-12 has got maximum values. From the Table 2 it was observed that, g_∥ >g_⊥ >g_e (g_e = 2.0023) and A_∥ >A_⊥ and hence it is concluded that d_x²-_y² - orbital (2B_1g state) being the ground state of Cu²⁺ ions and are octahedral sites with tetragonal distortion.

Table 2. Spin Hamiltonian Parameters, Optical Absorption Bands of (30−x) NaF-xAl₂O₃ -69B₂O₃-1CuO Glass Systems

It can be observed from Table 2 that the variation of g and A∥ with Al2 O3 content is found to be non-linear. Variation in g∥ and A∥ values may be associated with the change in the environment around Cu²⁺ ion, i.e., in the ligand field strength at the site of Cu²⁺.

The number of spins (N) and susceptibility (χ) values were evaluated for NAFBC samples and presented in Table 3 (Aschcroft & Mermin, 2001; Ashok et al., 2018; Shareefuddin et al., 1996; Weil & Bolton, 2007).

Table 3. Bonding Parameters, N and ( ) Values of (30−x) NaF-xAl₂O₃-69B₂O₃-1CuO Glass

The variation in N and χ with glass composition is shown in Figure 3. It is observed that both are varying non linearly and showing opposite behaviour. These variations confirms that the glass composition effects the glass network around the transition metal ions.

Figure 2. Variation of gII and AII with Al₂O₃ Content

Figure 3. Variation of N and χ with Al₂O3 Content

Interestingly, the N value with increasing Al₂O₃ concentration in the glass has shown inflections. This variation is due to slight modification of boron network by Al₂O₃ .

3.2 Optical Absorption Spectra

A broad absorption band is observed in near IR region in UV visible spectra of NAFBC glasses (Figure 4) which is the characteristic peak of Cu²⁺ ions. The observed peak positions of the optical absorption spectra of the glasses are listed in Table 2 which are assigned to the ²B_1g → ²B_2g transition of Cu²⁺ ions (Ahmed et al., 2018). With increasing Al₂O₃ content, the absorption peak is found to vary within ±10 nm from 770 nm. The ΔE_xy values are varied between 12820 for NAFBC-10 and 12936 for NAFBC-5.

Figure 4. Optical Spectra of (30−x)NaF-xAl₂O₃-69B₂O₃-1CuO Glass System

The observed optical peak in IR region (ΔE_xy ) used to calculate the bond parameters.

α2 values varied between 0.82 and 0.84, β₁² values varied between 0.76 and 0.86 & β² values varied between 0.91 and 0.93. The calculated values as shown in Table 3 of α² and α₁² indicated in plane σ -bonding as well as in plane π-bonding are moderately ionic and while out of plane π- bonding, β² seems to be ionic (Sekhar et al., 2016). The cupric ion is a network modifier along with NaF and Al₂O₃ in the B₂O₃ glass network.

The competition between the glass former cations and cupric ion in attracting neighboring alone pairs of intervening oxygen ions can be known from β₁² . The value of β₁² strongly depends on network former.

The normalized covalency (Wei et al., 2005) of Cu²⁺ -O in plane bonding σ and π symmetries (resp., t_σ, and t_π) can σ p be expressed interms of bonding coefficients α² and β² .

(30−x)NaF-xAl₂O₃-69B₂O₃ -1CuO (where x=0, 5, 7, 10, 12 & 15 mol%) glasses were prepared using melt quenching method and characterised with XRD, optical absorption and EPR spectroscopic techniques. Non-crystalline nature is confirmed by observing peak free spectrograms of XRD. Optical absorption spectra showed a broad absorption band at Infrared region, which was attributed to ²B_1g → ²B_2g transition. EPR spectra of NAFBC glasses exhibited three parallel lines in low field region and fourth component coincided partially with the perpendicular ones in high field region. The spin Hamiltonian and bonding parameters were calculated by harmonizing the EPR and optical absorption spectra. The estimated g∥ and g⊥ satisfied the relationships g_∥ >g_⊥ >g_e = 2.0023 and A_∥ >A_⊥ which are the characteristic features of Cu²⁺ ions located in a distorted octahedron, elongated along the z-axis. Bonding parameter values revealed that the in plane s-bonding as well as in plane π-bonding are moderately ionic, and while out of plane π-bonding are to be ionic.