Electronic and optical properties of Na2O doped P2O5-TeO2− CoO-ZnO glasses

The phospho-tellurite glasses, (P2O5)0.25-(TeO2)0.5-x-(CoO)0.1-(ZnO)0.15-(Na2O)x ; x = 0.15, 0.20, 0.25, 0.30 and 0.35 were prepared by melt quenching and their non-crystalline nature confirmed through XRD. Density is found decreased and molar volume increased with increase of Na2O concentration and they behaved opposite to each other. The physical parameters such as transition metal ion density, mean distance between TMIs, small polaron radius, oxygen packing density and field strength were estimated. DC conductivity was measured by following two probe method in the temperature range 303K – 633K. High temperature conductivity is explained by Mott’s small polaron hopping (SPH) model and low temperature conductivity by Mott’s VRH model. Both conductivity and activation energy are found to be increasing with Na2O content. Density of states at Fermi level are found to be of the order of 1021 to 1024 eV−1 cm−3. UV-Vis spectra were recorded for the wavelength range 200-800 nm. Optical band gap (direct and indirect) except for x=0.20 is found increased, urbach energy and refractive indexes are found decreased with Na2O concentration. Various optical parameters such as molar polarizibility, metallization criterion, reflection loss, electronic polarizability, optical transmission coefficient, dielectric constant and optical dielectric constant have been determined.


Introduction
P2O5-TeO2 glasses are the potential candidates for use in non-linear optical devices as they posses high refractive index and thermal expansion, good transmission in the infrared and ultraviolet regions.TeO2 is a glass forming material made up of TeO3, TeO4 and TeO3+1 as its structural components.Alkali oxides in phosphate glasses serve as modifiers by breaking P-O-P linkage and adding NBOs in the structure [1,2].Phosphate glasses have low chemical stability and that can be improved by adding network modifiers [3].Because of their low mass, small size and strong electro-positivity, alkali ions like Na + are the advantageous ones for electrochemical applications [4].Ionic conduction is caused by the movement of positive ions, whereas polaronic conduction is caused by the hopping of electrons.Those glasses that exhibit both electronic and ionic conduction may be utilized as potential cathode materials in solid state batteries, optical switching and memory switching devices, whereas those that only exhibit ionic conduction can be used as solid electrolytes in electrochemical devices i.e. sodiumion batteries [5].ZnO can be considered as a glass modifier as it improves mechanical properties and glass forming ability and reduce melting temperature [2].With a broad band gap of 3.37 eV and a significant exciton binding energy of 60 meV, ZnO is made use in a wide range of opto-electronic devices [6].
Since, phospho-tellurite glasses doped with both alkali and transition metal ions have not been investigated previously for electrical conduction mechanisms and optical studies, we prepared phospho-tellurite glasses doped with Na2O, ZnO and CoO.The phospho-tellurite glasses are normally more durabile than pure phosphate glasses and have attractive applications.This article provides investigated results on structural, physical, electrical and optical properties of the multi component phospho-tellurite glasses.

Experimental details
The phospho-tellurite glasses in the composition of, (P2O5)0.25-(TeO2)0.5-x-(CoO)0.1-(ZnO)0.15-(Na2O)x; x = 0.15, 0.20, 0.25, 0.30 and 0.35 were made by melt quenching and coded as PTZCN1, PTZCN2, PTZCN3, PTZCN4 and PZTCN5 respectively.All the starting chemicals were of Himedia make and analytical reagent grade.The chemicals were mixed in the defined quantities and heated in a furnace at 1303 K using silica crucibles.After that homogeneous melt was formed, it was quickly quenched between two stainless steel plates.The samples were subjected for annealing process at 573 K to wipe out the internal strains so that samples can have a good mechanical stability.The samples were cut into pieces to the desired size and shape for electrical and optical measurements.
To confirm the phase of the samples, they were subjected to powder XRD studies in a Smartlab SE Rigaku x-ray diffractometer, in which Cu-Kα radiation (λ= 1.54 Å) was used and scanned over 2θ range of 5° to 90°.The Archimedes principle was followed to measure density (ρ) of the glasses with a precision of 0.01 mg in a VIBBRA make HT Analytical Balance and Xylene as an immersion liquid (ρ = 0.8650 gm/cm 3 ) [2].The molar volume Vm was calculate as Vm=M/ρ where, M the molecular weight of the sample which was worked out as M = (0.25)M(P2O5) + (0.5-x) M(TeO2) + (0.1) M(CoO) + (0.15) M(ZnO) + (x) M(Na2O).The DC conductivity was measured by following the two point method in the temperature range 303 to 633K.For electrical measurement, the two opposite large surfaces of the samples were coated with silver paste.Error on the measured conductivity was within 5%.Detailed procedure of conductivity measurements can be found in reference [15].The well polished glasses were subjected to UV absorption measurements in a Jasco Spectrophotometer (Model name V-770) in the wavelength range of 200-800 nm.

X-Ray Diffraction
XRD spectrum of the glass samples are depicted in figure 1 shows no discrete peaks indicating amorphous nature.This demonstrates lack of periodicity of the three-dimensional network and the absence of long-range atomic order.A small broad hump is observed in the 2θ range of 24 to 36° which reveals the presence of short range order.Similar results were reported for phospho-tellurite [16] and phosphate glasses [17].

Density (ρ) and molar volume (Vm)
The variations in ρ and Vm with Na2O concentration are shown in figure 2. As the Na2O concentration increases, the density decreases.This may be as a result of relatively lower molecular weight of Na2O (105.99 g/mol) replacing the TeO2 of larger molecular weight (159.60 g/mol).Thus, the replacement of TeO2 by Na2O, glass became low dense.The ρ decreases from 4.466 to 3.834 g/cm 3 with increasing mole fractions of Na2O from 0.15 to 0.35.The molar volume is inversely proportional to density.The Vm of the PTZCN glasses increased from 33.352-36.054cm 3 /mole with increasing Na2O content.The density and molar volume values of the current glasses are comparable to the quoted values for lithium phospho-tellurite glasses [1].The other physical parameters connected to density have been calculated.For example, transition metal ion density, N = 2[{ρ(mCoO + mZnO) / (MCoO + MZnO)}NA] [15], Where  N 2 O is the Na2O concentration and  Na 2 O is the molecular weight of the Na2O and NA is the Avagadro number, Field strength (F)= Z/(rp) 2 [18], where Z is the atomic weight of sodium.The obtained values of these parameters are listed in table 1. TMI density (N) and field strength (F) increases due to increasing Na2O content and decreasing TeO2 concentration.Mean distance between TMIs, R=(1/N) 1/3 [5], small polaron radius rp=(1/2)(/6N) 1/3 [18], oxygen packing density (OPD)= 1000 C   [18].The decrease in R, rp and OPD can be attributing to the decrease in density with incorporation of Na2O [14,19].

DC Conductivity
The measured conductivity is found to vary between 10 -3 -10 -5 Ω -1 m -1 .Increase of conductivity with temperature reveals semiconductor behavior.The conductivity, σ is also increasing with increasing Na2O content (figure 3).It can be seen that the conductivity was almost constant up to 473K and started increasing after wards.Similar trend of conductivity variation was reported for CoO and Li2O doped boro-phosphate [15] and alkali doped boro-tellurite glasses [20].
Where, W is the activation energy of dc conductivity.The pre-exponential factor is expressed as Where, ʋ0 is the optical phonon frequency (ʋ0=θDKB/h) [22].The plots appeared linear in the region of T > θD/2 and non-linear in region of T < θD/2.For temperature T > θD/2 [where θD is the Debye temperature (Table 2)], all the samples showed a linear temperature dependence.That is why, linear lines were fit to the data above θD/2 and slopes were collected.Below θD, data deviate from linearity, it reveals that the activation energy in this region is temperature dependent.Activation energy estimated from the slopes of the linear fits for T > θD are tabulated in table 2.
Figure 5 shows the plots of W and σ (T= 608 K) against Na2O concentration.It can be observed that W increased with increasing concentration of Na2O up to 0.3 and thereafter it decreases.The conductivity also increases with increasing mole fraction of Na2O content.The increase in σ is due to the predominance of ionic transport.In the present glasses, both ionic predominance and hindered polaronic motion together present.The activation energy, W was found increased from 0.4372 to 0.9760 eV.σ values at 608K were lying in the range from 5.8021 x 10 -5 to 5.0600 x 10 -3  -1 m -1 .These values of σ and W are nearer to the quoted values for alkali doped boro-tellurite [21] and alkali doped Boro-phosphate [23].TD and θD decreases with increasing Na2O content and those values are also found nearer to the boro-tellurite glasses [24].Making use of Mott's VRH model [25], conductivity data below TD has been analyzed.Conductivity is expressed as, Where, A=[e 2 /2(8π Where, N(EF) is the density of states at Fermi level.Value of A and B were acquired from the slopes of linear fits to ln(σ) versus (T −1/4 ) plots (Figure 6).N(EF) values were estimated from B and the obtained values are recorded in table 2. They are in the range 10 21 -10 24 eV -1 cm -3 .ʋ0 values obtained are nearer to the vanadium boro-phosphate glasses [26].The present N(EF) values are found to be nearer to the published values for oxide glasses containing alkali and TMI [ 15,21,22,23,24].

UV-Visible absorption
Figure 7 represents UV-Vis absorption spectrum of the present glasses.It shows an absorption band in UV region at 310 nm for N2, N3, N4, N5 glasses and broad band centered at 580 nm for all the glasses.The absence of absorption peak at 310 nm for N1 glass may be due to its strong amorphous nature.The absorption bands in the UV-Visible region have been reported for the Phosphate-tellurite glasses (298 nm and 532 nm) and manganese doped alkali phosphate glasses (275 nm and 530 nm) [27,28].Absorption spectra have been utilized for determining optical band gap and Urbach energy of the glasses.The absorption coefficient α(ʋ) can be expressed as [19,29], Where, A is the absorption intensity and t the thickness of the glass.
The optical absorption spectra play a prime role in the understanding of electronic structure and optical transitions occurring in the glasses.The band gap of the amorphous substances may be calculated by applying Davis and Mott relation for direct and indirect permitted transitions.Direct and indirect transitions occur when electromagnetic waves come into contact with valence band electrons which were excited to the conduction band across the fundamental band gap.The absorption coefficient, α(ʋ) can be expressed as [2,19,30], Where, B is the constant, hʋ the photon energy and Eg the optical band gap energy.The index n designates the numbers 1/2 and 2 signify the direct and indirect allowed transitions respectively.
Tauc's plots of (αhʋ) 2 vs (hʋ) for direct and (αhʋ) 1/2 vs (hʋ) for indirect allowed transitions are made and shown in figure 8.The Eg values were acquired by extrapolating the linear dependence of (αhʋ) 2 and (αhʋ) 1/2 with hʋ on to hʋ-axis.The obtained direct and indirect band gap values are listed in table 3. Which are in the range 2.690 -3.187 eV (direct) and 2.356 -2.840 eV (indirect).The direct and indirect Eg values are found to decrease up to 0.25 mole fractions of Na2O and thereafter it increased.Due to the increased NBOs and loose packing of the glass network with increasing Na2O content, the bond strength is reduces and Eg increases [30].
Urbach energy of localized states (ΔE) has been estimated from the slope of the linear fits of the graphs of ln(α) versus (hα), depicted in figure 9.The relation between Urbach energy (ΔE) and absorption coefficient α(ʋ) is given as [14], α (υ) = B exp (hυ / ΔE) (8) Where, B is the constant.The obtained band gap values (direct and indirect) and Urbach energy (ΔE) are tabulated in table 3. The trend of change in ΔE with Na2O is noted to be opposite to that band gap energy behavior.The influence of Na2O is to loosen the network's packing by producing a Te-O-Na, resulting in increase of NBOs.This decreases network connectivity and bond strength and that is why ΔE decreases [30].Urbach energy is in the range 0.595 -0.341 eV, which is nearer to the range quoted for Dy 3+ doped boro-telluro-phosphate glasses [31].
Table 3 also presents various other optical parameters such as refractive index (n), molar refraction (Rm), reflection Loss (RL), metallization criterion (M), electronic polarizability (αm), optical transmission coefficient (T), dielectric constant (ε) and optical dielectric constant (ε0), which were estimated using formulae given in reference [14,19,30].Metallization criterion (M) of the glasses increases with increasing concentration of Na2O except for N2 glass.If Rm/Vm< 1, then M > 0, this means samples are insulating in nature.The present M values are found positive revealing insulating nature of these glasses.Similar conclusion was drawn in [14,30].RL and n decreased with increasing sodium content for N3-N5 glasses.These outcomes are in close agreement with the alkali doped phospho-tellurite glasses [14,30].Topt values increases which is same as reported behavior in the reference [19].Higher values of Rm and αm indicate nonlinear nature of the glasses.The ε and ε0 values obtained are larger than the reported values for the sodium doped phospho-tellurite glasses [14].

Conclusions
The glasses, P2O5-TeO2-CoO-ZnO-Na2O were synthesized by melt quenching and their physical, electrical and optical characteristics were measured.Amorphous phase of samples has been confirmed by XRD.The density decreased from 4.4667 to 3.8346 g/cm 3 and molar volume increased from 33.352 -36.054 cm 3 /mol for increase of Na2O mole fractions from 0.15 to 0.35.Temperature dependence of conductivity reveals semiconducting character.For higher temperature conductivity, Mott's SPH model holds.Addition of Na2O leads to increase in W and σ.Mott's VRH model describes low temperature conductivity of the glasses.The density of states at Fermi level estimated from MVRH model fits are found to be in the range 10 21 to 10 24 eV -1 cm -3 .Optical band (direct and indirect) gap except for x=0.20 is increased, urbatch energy and refractive index decreased with increasing Na2O concentration.Various other optical parameters such as metallization criterion (M) and optical transmission coefficient (Topt), refractive index (n), reflection loss (RL), dielectric constant (ε) and optical dielectric constant (ε0) values were determined.Molar refraction (Rm) and electronic polarizability (αm) revealed nonlinear nature of the glasses.

Figure 1 .
Figure 1.The XRD spectra for all PTZCN glasses.

Figure 2 .
Figure 2. Variation of ρ and Vm versus Na2O content for PTZCN glasses.

Figure 3 .
Figure 3. Variation of conductivity, σ as a function of temperature, T.

Figure 5 .
Figure 5. Plots of W and σ at 608 K against Na2O content.

Table 1 .
Physical parameters of PTZCN glasses.

Table 2 .
W, σ at 608 K and extracted parameters from Mott's SPH and VRH model.