Physical and electrical properties of Sm2O3 doped Boro-Zinc-Vanadate glasses

Sm2O3 doped boro-zinc-vanadate glass systems were synthesised by following the melt quenching method. XRD patterns indicated largely non-crystalline nature with few nano-crystallites. Room temperature density was measured. Molar volume and various polaron parameters were estimated. Density and molar volume are found to vary non-linearly with samarium concentration. Conductivity has been measured by two probe technique for temperature range 303K - 573K. High temperature conductivity obeyed the small polaron hopping (SPH) theory. Activation energy for conduction in the temperature regime of small polaron theory is found to vary from 0.249 eV to 0.368 eV non-linearly with Sm2O3 concentration. The conductivity data at low temperature deviated has been looked into using Mott’s VRH model and the density of states at Fermi level were determined. Shimakawa’s multiphonon tunnelling model has also been applied to the low temperature conductivity and found linearity between logarithmic conductivity, ln(σ) and logarithmic temperature ln(T) as predicted by the model. The temperature exponent values obtained from Shimakawa’s model fit are found to be in good agreement with literature. Therefore, it is concluded that at low temperature, carrier multiphonon tunnelling is the charge transport mechanism in the present glasses.


Introduction
Many researchers have discussed importance of transition metal oxides (TMO) as modifiers in the glass formation.One of the most conventional TMOs is vanadium pentoxide (V2O5), which is a conditioned glass former and has good electrical properties [1][2].Almost all V2O5 doped glasses demonstrated semiconducting characteristics due to hopping of electrons from a low valence state (V 4+ ) to the high valence state (V 5+ ) [3][4].These glasses are known as n-type semiconducting glasses and are useful in opto-electronic, memory switching, solid state devices and lasers [1,3].Boric oxide (B2O3), a glass former has been incorporated into various glasses as a flux material to attain low melting temperatures, thermal stability, etc. [5].Addition of ZnO enhances glass forming, chemical resistance and reduces crystallization rate.It also acts as a modifier and as well as network former [6,7].Samarium oxide (Sm2O3) is a rare earth oxide used for enhancing physical, electrical, dielectric and optical properties of glasses [8,9].A limited literature exists on Sm2O3 and TMI doped borate glasses.Available results indicated significant changes in electrical properties with increasing rare-earth ion concentrations caused by fluctuations in the mobility of rare-earth ions and their blocking effect to polaronic and ionic motions [10][11][12][13][14].There are reports suggesting that addition of Sm2O3 improves glass performance in solid-state devices [15].The technological applications of zinc vanadate glasses as cathode materials in batteries, electrical and optical memory switching devices makes them fascinating.Few researchers recently studied physical, structural and electrical properties of ternary zinc borovanadate glasses and reported that their electrical properties got enhanced [16][17][18].
Understanding the influence of rare earth and transition metal ions on glasses enables to develop suitable glass compositions for desired applications with required electrical properties and chemical durability [10,14].The zinc-boro vanadate glasses containing samarium oxide (Sm2O3) have not been previously received much attention.The present study was aimed at investigating the role of Sm2O3 on density, structure and conductivity of zinc-boro vanadate glasses.These glasses provide a unique opportunity to learn about the conduction process in these scientifically important set of glasses.Activation energy for electrical conductivity, phonon frequency and Debye's temperature were also determined.Density of states at Fermi level in the light of Mott's VRH model and temperature exponent in Shimakawa's model at low temperature have been determined.

Experimental
Samarium doped zinc borovanadate glasses were produced by melt quenching route.Chemicals of AR grade (V2O5, ZnO, H3BO3 and Sm2O3) in fixed concentrations were mixed in an agate mortar and taken in silica crucibles and melted at 1323K in a furnace and quenched between SS plates.The samples so obtained were annealed at 523K for 24 hours to relieve the thermal strains caused by the quenching process.The composition of the samples is fixed to be, (ZnO)0.3-(V2O5)0.3-(B2O3)0.4-x-(Sm2O3)x where x = 0.002, 0.005, 0.007, 0.01, 0.02 mole % labeled as ZVBS1, ZVBS2, ZVBS3, ZVBS4 and ZVBS5 respectively.
The XRD patterns were recorded in a Smartlab X-ray diffractometer having CuKα radiation of wavelength 1.541 Å, in the 2θ range 5 -90.Density, ρ, was measured as per Archimedes principle using, a VIBRA makes HT analytical balance of 0.1mg precession and Xylene for immersing the samples.Samples were cut to 3mm x 3mm x 4mm size and their two large surfaces were silver painted.The electrical resistance measurements were carried out for the temperature range 303-573K by adopting the two point method.Before measuring the current and voltage, the ohmic contacts have been confirmed.The method described in [13] has been followed to determine conductivity.

X-ray diffraction studies
The recorded XRD patterns of Sm2O3-doped boro-zinc-vanadate glass nanocomposites are presented in figure1.The patterns exhibited a single small peak.The diffraction peak for all the samples is appearing exactly at 28°.This crystalline peak is indexed by comparing with JCPDS card.As per JCPDS card no.00-009-0142, the space lattice of VO2 is monoclinic with a = 5.743 Å, b = 4.517 Å, and c = 5.375 Å.The Miller indices (h k l) of this diffraction peak corresponds (0 1 1) [19,20].The average grain size (D) due to the observed peak has been determined using the Scherrer formula [10], Where, β is the full width at half maxima (FWHM) of the peak,  the wavelength of X-rays (1.54Å) and θ the Bragg diffraction angle.The average crystallite sizes are found to be 1.995 nm, 0.036 nm, 1.901 nm, 1.895 nm and 2.671 nm for samples of x = 0.002, 0.005, 0.007, 0.01 and 0.02 respectively.Since the grain sizes determined are of few nanometers, the current samples are called glass nanocomposites.The ZVBS2 sample is observed to be less crystalline compared to other samples as, the peak corresponding to it is of less intensity.

Density and Molar volume
Density, ρ is a significant physical parameter that can be utilised to understand network packing.It also explains the degree of structural compactness.Density of the present glass nanocomposites was measured by following Archimedes method and using the following relation [5], ρ = ρb× Where, ρb is the density of the buoyant (Xylene = 0.865gm/cm 3 ), Wa and Wb are weight of the samples in air and buoyant respectively.The experiments were repeated four times for reproducibility and the error on the measured density has been estimated to be ±0.005g/cm 3 .It is noticed that density and molar volume behave in opposite fashion to each other with Sm2O3 content and they vary in a zig-zag way.In fact, in the present glasses high dense Sm2O3 (8.35gm/cm 3 ) is substituted for low dense B2O3 (2.46gm/cm 3 ).Therefore, the samples are expected to show increasing trend of density with Sm2O3 content.Variations in both density and VM with Sm2O3 are depicted in figure 2 and recorded in Table 1.Density showed increasing trend with Sm2O3 content except for ZVBS2 sample for which density is bit high.It may be due to its less crystalline in nature.
Using the measured density and composition, the molar volume, VM has been estimated as per equation [6], Where, xi is the mole fraction, Mi the molecular weight of the i th component and ρ the density of glass sample.Increase of ρ and decrease of VM with increase of Sm2O3 content except for ZVBS2 sample indicates that glass network is getting tight packed with Sm2O3 concentration.Such changes in ρ and VM have been noticed in the previous works of samarium doped zinc-tellurite glasses [21].The concentration of TMI (N), internuclear distance (R) and polaron radius (rp) are determined using the following mentioned expressions [15], (5) Polaron radius rp = The obtained values of N, R and rp are tabulated in Table 1.Both R and rp are decreased with increase of Sm 3+ ion concentration (N) and this result agree with reports on Sm2O3 doped lithium-borate glasses [14] and lithium borosilicate glasses [15].Concentration of TMI (N) in the present case is increased as Sm2O3 content is increased.Sm2O3 content is increased at the expense of B2O3 and this should lead to decrease in the concentration of TMI (N).Instead, N is increasing with Sm2O3.This may be due to dynamical changes in network with Sm2O3 content.This agrees with reports on Sm2O3-ZnO-TeO2 [21] and Co3O4-V2O5-B2O3 [22] glasses.
Figure2.Compositional dependence of density, D and molar volume, Vm.

DC electrical conductivity
As depicted in figure 3, the electrical conductivity, σ, was found to vary between 0.5x10 -4 and 3.2x10 -4 (Ωm) -1 for the measured temperature range.Due to positive change in σ with temperature, these samples may be treated as semiconducting glasses.It can be observed that conductivity was very minimal for temperature upto 400K and started increasing then onwards.The conductivity of the present glasses is more than reported values for Sm2O3-B2O3-V2O5 -Li2O glasses [8] and Sm2O3- B2O3-Al2O3 -Na2O-CuCl2 glasses [9] by a few orders of magnitude.However, it is less than that reported for V2O5-SeO2-ZnO glasses [2] and V2O5-ZnO and glasses [4].The SPH model of Mott has been used to know changes occurring in σ with temperature.In Mott's SPH model [23,24] the conduction is predicted to be caused by phonon-assisted hopping of (small) polarons between localised states.It predicts conductivity due to nearest-neighbour hopping mostly in the non-adiabatic regime for temperature above TD to be, σ = (σ0/T) exp (-W/KB) (7) Where, W being the activation energy required for conduction and σ0 the pre-exponential quantity.
The graphs of ln(σT) versus (1/T) are plotted in figure 4. It is observed that ln(σT) decreases linearly with (T) -1 up to TD and then deviates from this behavior at lower temperature.The temperature at which ln(σT) departs from linear behavior with (T) -1 is related to Debye's temperature, θD as θD =2TD.
So, the linear lines were fit to data above TD.The W value was estimated using the slope and is found in the range 0.2489 -0.3675eV.The W values so obtained are found comparable with literature value for Fe2O3-Sm2O3-ZnO-P2O5 glasses [10].
As per reference [2], hν0 = kBθD where, ν0 is the optical phonon frequency, h the planck's constant and kB Boltzmann constant.The calculated θD and ν0 values are listed in Table 2.With increasing Sm2O3 concentration, the Debye temperature (θD) and optical phonon frequency (ν0) gradually increases.The increased Sm2O3 content results in opening up of the glass network structure, which in term increases thermal vibrations due to a rise in optical phonon frequency and hence θD.Variation of θD with Sm2O3 content is shown in figure 5.
Figure 6 shows observed changes in σ at 563K and W with mole fractions of Sm2O3.A close observation of conductivity and W variations with Sm2O3 content revealed that they are decreasing with increase of Sm2O3 up to 0.005 mole fractions and increased for higher amounts of Sm2O3 and then decrease.Both σ and W show the same trends with increment in Sm2O3 content.This indicates that the structure of the present nano-composites is changing such that an obstacle to polaronic hopping develops in increasing order with increment in Sm2O3.Obtained results are useful to gauge possible mechanism of conductivity.In general, the increase in conductivity due to the presence of Sm2O3 can be attributed to the successive jumping of Sm +3 ion from one non-bridging oxygen (NBO) to another [23].Mott's VRH model was also used for analyzing conductivity for below TD.This model predicts positive variation of conductivity with n temperature as follows [24], Where, A = [e 2 /2(8π)  7. The computed values of N(EF) are of the order of 10 23 -10 24 eV -1 cm -3 are listed in (Table 3).These N(EF) values are in close agreement with the values mentioned in [23,24].These N(EF) are higher than those obtained for La2O-V2O5-P2O5 glasses [25] and La2O and CeO2 doped vanadotellurite glasses [26] and, are smaller than reported values for V2O5-SeO2-ZnO [2] and Fe2O3-Sm2O3-ZnO-P2O5 [10] glasses.
Shimakawa and Miyake [24,27] suggested a different model for explaining conductivity at low temperatures in glass nanocomposites based on hopping of localized carriers due to multi phonon tunneling with weak electron-lattice coupling.It involves absorption and/or emission of phonons to overcome the site separation energy.
According to this model, conductivity, σ is given as [27], Where, the constant σ0 depends on composition of the glass and exponent p. Fig. 8 shows Shimakawa's plot of ln(σ) versus ln(T) and linear fits gave p values in the range from 4.034 to 9.307 for ZVBS glasses.A linear relationship between ln(σ) and ln(T) is clearly is in figure 8.A comparable values of p are reported in [27,28] for different V2O5 based glasses.The temperature exponent, p values obtained are nearer to reported values for two TMI doped tellurite glasses [24].It is possible to draw the conclusion that the carrier transport below TD may involve large polarons (electrons which are weakly coupled) tunnelling between the nanocrystallites through multiplephonons.

Figure 3 .
Figure 3. Variation of dc conductivity as a function of temperatures.

Figure 6 .
Figure 6.Variation of σ at 563K and activation energy (W) with mole fractions of Sm2O3.

Table 2 :
Conductivity σ at 563K and Debye's temperature, θD and activation energy, W from SPH fits.