Structural, Optical and Electrical Properties of Copper Doped Alkali Borate Glasses

The compositional effect on Physical properties, Optical absorption and Electrical properties of glass composition xK2O (25-x) Li2O-12.5BaO-12.5MgO-49B2O3-1CuO (x =0, 5, 10, 15, 20 and 25mol %) was prepared by the melt-quenching technique. From optical absorption spectra few parameters, like Urbach energy, indirect allowed, reflection loss (R), refractive index (n), molar polarizability(αm), molar refractivity (Rm), and theoretical optical basicity (Λth) values of prepared glass samples were measured. The values of Rm, αm, and Λthwere increase with increasing of x mol%. The Eopt, R, and n values were changed very small with x mol%, which may be due to the nonbridging oxygen’s moments in the glass matrix. The FTIR spectrum of prepared borate glasses indicates that the influence of BO3 and BO4 units of boron atoms. The explicit vibrational peaks were exhibited of Li–O, K–O, Mg–O, and Ba–O bonds in glass network, and A.C conductivity study was devised to know the structural information about the glass network. The drastic change in the ionic conductivity as a function of the alkali content in the glass composition was analyzed by considering both the ionic mobility and the glass structure. Some physical, optical, and electrical values varied non-linearly with the K2O content, which may influence of nonbridging oxygens or mixed alkali effect.


Introduction
The glass is divided into main categories, network formers, network modifiers and transition metal ion species, which falls somewhere between glass modifiers and network formers. Glass modifiers may substitute in a network former and then change the glass structure and glassy state.
Generally, Glasses are amorphous materials, it has been used in different fields like optical, electrical and potential devices and building glass materials. For the glass products, manufacturings, thermal conductivity and solid-state lithium-ion batteries around room temperature [1]. The chemical composition of the glasses plays an important role in determining the properties of the glass. The transition metal ions have more than one valence state in the outermost shell, they show semiconductor properties such as switching, electro-chromatic, fuel cells, solid-state lithium-ion batteries and chemical sensors application [2][3][4][5].In the mixed alkali borate glasses, the glass modifiers are alkali oxides (Li2O and K2O) and glass farmer is borate oxide (B2O3). The BO3 units can be converted to BO4 units with creating bridging oxygen by the addition of glass modifiers in the glass network borate oxide (B2O3). And vice-versa BO4 units also could be transforming into BO3 units and oxygen dangling bonds are produced. This type of phenomena is called non-bridging oxygens (NBOS) effect [6], it occurs in mixed alkali borate glasses (MAE).
In borate mixed alkali oxide glasses if total alkali content constant is kept constant, when two types of alkali oxide are introduced in borate glass former, in which one alkali ion is gradually replaced by another alkali ion, the exhibited non-linear behaviour in some physical properties or breaking the oxygen bonds in B-O or the glass network variation of non-bridging oxygens (NBOS) effect observed. This effect is also known as the mixed alkali effect (MAE) [7]. The ratio between 3 electrical Properties such as density, molar volume, energy band gap, electrical conductivity, ionic diffusion, and dielectric relaxation, etc parameters of mixed alkali borate glasses. Most of the research work on MAE is based on the physical properties of the glasses. However, the influence of MAE is not considerably studied in like Optical absorption, FTIR and Electrical properties. Some research workers studied the MAE in the spectroscopic and electrical properties of glasses [8][9][10][11][12][13][14][15]. The glasses contain alkali ions and substitute transition metal oxides make colored glasses, these are generally acting as optically colorless and ionic conductors. Ultimately these glass modifications lead to the size-dependent structural, optical and electronic properties, these glasses can be designed to be suitable for energy storage and electrical device applications by varying the alkali oxides in the glass host material.
In the present research work, we have studied and analyzed the correlation between the pure and copper doped mixed alkali borate glasses and also, we focus on ionic conductivity studies of copper doped alkali borate glass systems. MAE investigates the non-linear variation in some of the physical, spectroscopic like UV-VIS, FTIR and electrical properties of alkali copper doped borate glasses.

Experimental Method
The current alkali oxide borate glass samples were mixing required amounts of potassium carbonate (K2CO3), lithium oxide (Li2O), magnesium oxide (MgO), and barium oxide (BaO) with boric acid (H3BO3). The current compositions were accommodated as a host and doped with copper oxide (CuO) ions host materials were reported in Table 1. The present glass preparation and X-ray diffractograms were recorded as explained in detail. The influence of the optical absorption spectra of present glass samples was investigated [13].
At room temperature, the reported FTIR spectra of the present glass system were recorded with a heap of Perkin Elmer Frontier FTIR. In this technique, KBr pallet and sample ratio 2:100 was used to record the FTIR spectra with an applied pressure of 7-8 ton using a hydraulic press. The KBr pallet background removal and baseline correction were observed by using Spectrum 10 software.
For electrical measurements, the silver past was coated on two sides of polished glass samples that were used to study electrical properties. The silver coated glass samples were free heat-treated upto150 0 C for electrode stabilization. The Impedance, capacitance, and admittance parameters were recorded from Impedance analyzer with an applied frequency range from 100Hz to 1MHz and temperature variation from 523 to 673K by using a low-frequency impedance analyzer (AUTO LAB PGSTAT). A. C conductivity temperature-dependent at low temperatures and varies strongly at high temperatures and real part of impedance ( )is studied as a function of frequencies with different temperatures. At high temperatures, tends to reach the value of DC conductivity and shows activated temperature dependence for all alkali copper doped borate glasses.

3.1.XRD
All the prepared glass samples of the XRD (Pan Analytic) pattern show the absence of sharp Bragg's free peak to confirmed homogeneous and amorphous nature. Fig.1 shows the XRD pattern of all prepared glass samples at room temperature.

Density and molar volume
At room temperature, the density value was measured by using Archimedes' principle with xylene (99.99%) for all the prepared glasses [16]. The density values were evaluated three times for error elimination. Subsequently, the molar volume values were evaluated from density values. The molar volume mostly associated with the spatial distribution alkali oxygen's in the glass matrix.
Density and molar volume provides a relationship between masses and the volume of different types of structural oxide groups inside the borate glass matrix and key factors to study the physical properties of alkali borate host materials. Similarly, the molar volume can be describing the glass network structure and arrangement of the building units as it deals with the spatial structure of the oxygen network. The small change in the density and molar volume values such as around 2.75 g/cc& 26.765 cc/mol, but 2.812 g/cc is highest density value for Li2O content and 2.744 g/cc is low-density value for K2O content and molar volume values increases with the K2O content in the alkali borate host materials. The measured values of density and molar volume values were given in Table-2. Fig.2 plots show the variation of ρ and Vm as a function of K2O content, which manifests the mixed alkali effect and it's maybe also noted that ionic radius of alkali modifiers in the present glass system [17,18].

3.3.Optical basicity
The theoretical optical basicity ( ℎ ) of all the glass samples values were evaluated by using the following equation [18] Here the alkali oxide theoretical optical basicity value taken and explained about theoretical values [16]. The copper oxide theoretical optical basicity value is taken from [19]. The mixed alkali borate glass system of theoretical optical basicity ( ℎ ) values are increased with a decrease of the Pauling electronegativity of the modifiers also increasing fromLi2O→ Na2O→K2O content, these values are reported in Table 2. The theoretical optical basicity ( ℎ ) values are increases with increasingK2O content in the alkali borate host materials, it was suggested that the local states (coordination and valence) of transition metal ions in the glass matrix and finally the glass makes color itself.  Table.2.   [20]. The nonlinear variation of bandgap and Urbach energy values are shown in Fig.6, which manifests the mixed alkali effect and non-bridging oxygens of the present glass system.

Refractive index (n) & Dielectric constant (ε)
Refractive index and dielectric constant values are the most powerful important parameters related to glass structure modification. The refractive index and dielectric constant values of mixed alkali-alkaline earth oxides borate glass samples containing Cu 2+ ions were calculated from optical band gap energy using eq. and explained details [2] and are reported in Table.2.From the  [21]. This may be expected due to its structure of glass and chemical composition of alkali oxides in the borate network.
Where n is the refractive index and Vm is the molar volume of the present glass system, The molar polarizability ( ) as a function of molar refraction can be calculated for each glass samples using relation The evaluated values of present glasses of molar refraction and molar polarizability were reported in Table.2. The and values increase with the increase of K2O content mol%. Therefore the increase in molar volume, Theoretical optical basicity and Broad Optical absorption band with an increase of K2O content, these suggested that this will be due to transition metal ions of local states or NBO in the glass matrix.metallization is defined from the Herzfeld theory, it means the metallization is the ratio of is called polarizability per unit volume [26], based on metallization ( M ) condition criteria, it is = 1 − . The materials act as metallic nature, if only > 1and the materials act as insulator nature if only < 1 for an expectation of prepared present glass samples are acting as an insulator [27]. Small changes in the metallization values were observed. These metallization criterion values are given in Table 2. The metallization nature is more for Li2O content compared to K2O content, i.e metal oxides radius is increased metallization is decreased. Copper doped metallization values decrease compared to pure borate glass systems, which means that the width of both valence and conduction bands become closer, resulting in a decrease of bandgap and an increase of the metallic nature of the glass. Fig.7 influence the variation of refractive index and metallization as a role of molar polarizability, the non-linear variation suggest that which may be due to the NBO or MAE.

FTIR spectra
The influence of active FTIR spectra recorded at room temperature with wavenumber range 400 -1 to 1600 cm -1 . Fig.8 suggested the FT-IR transition spectra of mixed alkali oxide borate glass of glass-forming structural units for quality identification in glass materials; it can be explained in detail [2]. TheFT-IR transition spectrum can be divided into 3 parts in which one at400 -800 cm  Table.3. In the present article, we discussed sharp, deep bands at 416 &462cm −1 are due to

3.8.Electrical studies
A.C conductivity study is the most important and powerful technique to study the electrical and impedance properties of glass materials. Electrical studies give the conductivity nature of host material and investigation of conductivity belongs to the motion of charged particles. The appearance of conductivity will be a reasonable response of activated hopping manner of mobile charges, therefore it can occur moment of mass and charge in the material. This may be due to rapid polarization processes occurring in all the glass at low frequency (below 10K Hz). These types of investigation can be possibly due to the decrease of space charge in the material of impedance pattern. All the curves are merging with one another at higher frequencies (above 10K Hz) [35]. The measured values of Z' are reported in Table.4 at 100Hz, 1K Hz, 100K Hz and at 300 o C temperature for mixed alkali oxide borate glass samples. It is found that impedance decrease with an increase in the value of frequency. At higher temperature, the energy distribution in the glass network become more uniform and the variation of the magnitude of impedance values at higher frequencies become very less. This indicates that the impedances to converge at higher frequencies and also reveals that at higher frequencies conductivity and impedance become independent of temperature.
A.C conductivity study was devised to know the structural information about glass network.A.C conductivity is the dependence frequency is used to study the characterization of convenient formalism to investigate in a material with help of power-law relation proposed by Jonscher [36].
where is total conductivity, is frequency independence (dc) conductivity, the coefficient A and exponents are corresponding temperatures and material-dependent parameters respectively. The term indicates comprises the ac conductivity and characterization of all dispersion phenomena [37][38][39].
In a material the exponents can vary with corresponding temperature and more variation with material to another material,i.e., 0 ≤ s ≤ 1. Fig. 10shows the frequency-dependent conductivity plots with a variation of frequency from 100Hz-1MHz at different temperatures (from 250 -400 0 C) for the present glass system. The ac conductivity of present glass systems shows similar frequency dependence. From fig.9 we suggest that the at low-frequency range ac conductivity is almost constant, then after abruptly increases in the high-frequency region.It is concluded that the electrical conduction mechanism is the same as that process of the dielectric polarization in the low-frequency region.
Present all the glass samples show the conductivity as increasing trend with increasing temperature. The conductivity increases may attribute to a number of bridging oxygen increase in atoms and mobile charge carriers in the glass matrix. The observed conductivity spectra can be divided into two distinct regions. The first dispersive region at high-frequency range (above 10K Hz) manifests the back-and-forth motion of the ions. The second dispersive region at low-frequency range (below 10K Hz) due to the long-range transportation, it signifying the D.C conductivity.
At the same frequency, the temperature is increasing, at which the conductivity becomes prominently transform to higher frequencies or high conductivity due to space charge polarization [40,41]. At high-frequency regions, increasing the kinetic energy of the ions and as well as vibrational frequency increases. In the low-frequency region accumulation of space charge carrier or mobile ion effect due to the electrode, polarization was observed at two different temperatures. Therefore finally, we conclude that the frequency decreased as the total conductivity of all the glasses decreases at constant temperature, and conductivity increases as the temperature increases for all the glass materials. The recorded electrical/ionic conductivity values at different frequencies with temperature variation weregiven in Table.5 for KLMBBC glass systems. FromTable.5it was observed that the ionic conductivity increases with an increase in temperature and at a higher temperature (400 0 C) the mobile ions are more active and low resistivity in alkali oxide borate glass network. The conductivity also depends on the density and ionic radius of alkali oxides. Mixed alkali borate glasses act as semiconductors at higher temperatures. High conductivity for Li2O compared to another alkali oxide, may be used for battery application [38]. The similar electrical results are reported for alkali ions conducting glasses used for different applications [42]. In amorphous material at low-frequency region enhancement of AC conductivity may be due to the interfacial or space charge polarization and it also disorder distribution of relaxation time occurs [43]. The conductivity values decrease with increasing ionic radius from Li + to K + ions, due to the increase of non-bridging oxygen atoms. Thus suggesting that the conductivity values were related to the density, glass transition temperature and band gaps. Fig. 11 shows the variation of versus 10 3 /T at 10k Hz frequency for present glass systems. It is observed that the conductivity of glasses increases with the increase in temperature according to the Arrhenius relation [44].
where∆ is the activation energy for the AC conduction mechanism, "k" is the Boltzmann constant, T is the temperature and 0 is the pre-exponential factor. From the investigated present glass system, it is found that the slope variation was high at a higher temperature, which indicates that at higher temperature region samples show high conductivity and also having less activation energy.  Table 6. Fig. 10 gives a negative slope indicates that ionic conductivity in these samples is satisfied by the theory of the electronic structure of amorphous materials. Variation in the electrical resistivity with temperature by several orders of magnitude was observed [45]. It is proposed to discuss the changes in activation energy with the changes in the molar ratio of modifiers. This model assumes that the ion transport is considered to be "site preferred" which takes place by hopping from one site to another rather than the migration through specific pathways.
The mobility of the ions is considered to be essentially dependent on the mechanism of the hopping process and overall glass composition [46].

Conclusions
Resultants The non-linear behaviour may be due to the mixed alkali effect.