Dielectric relaxation studies in Sm2O3 doped boro-zinc-vanadate glasses

In this work, the conventional melting procedure was adopted to synthesize glasses with a composition of (ZnO)0.3 – (V2O5)0.3 – (B2O3)0.4-x – (Sm2O3)x, x = 0.002, 0.005, 0.007, 0.01 and 0.02 mol%. The dielectric properties of these glasses were measured. The dielectric constant (εʹ) and dielectric loss (εʹʹ) of glasses are found to vary in the range from 1.9177x102 to 3.9456x103 and from 1.5751 to 6.9095x105 respectively for the temperature range 343 K – 573 K and frequency range 50 Hz to 10 MHz. With increase of frequency, both dielectric constant and loss decreased and increased with increase of temperature. The analysis of dielectric constant and dielectric loss confirmed that the phenomenon of dielectric relaxation is mainly due to the frequency-dependent polarization mechanism. Electric modulus and impedence spectroscopy revealed non-Debye type and a single phase relaxation process. Activation energy for dc conductivity and dielectric relaxation are found to be of same size indicating that the potential barrier encountered by the charge carriers is same in both the processes. The master curves suggested that temperature-independent relaxation is occurring in the present glasses. The high values of measured dielectric parameters suggest that the present glasses are suitable in semiconducting and energy storage applications.


Introduction
Zincvanadate glasses are very significant as they are widely used in many applications such as electrochromic devices, electrical memory switching devices and solid state electrolytes etc., [1][2].They posses low melting point and good thermal stability etc.Although these glasses are dielectric materials, the presence of transition metal ions in them produces semiconductor properties [3][4][5].By incorporating rare earth (RE) elements into glasses, it is possible to develop a variety of electro-optic and optical devices.Rare earth ions do not contribute to conductivity.However, their addition can change the network which in turn affects conduction and dielectric properties [6][7].
There are several reports on the investigation of electrical and dielectric properties of glasses containing rare-earth ions [8][9][10][11][12].They claimed that these glasses exhibit ideal debye type behavior as per cole-cole model and the decrease in dielectric loss observed at high temperatures is due to reorientation of dipoles along the direction of the electric field.1300 (2024) 012044 IOP Publishing doi:10.1088/1757-899X/1300/1/012044 2 Electrical conductivity and dielectric properties of Sm2O3 doped glasses namely, Li2CO3-B2O3-ZnF2 [5], B2O3-CuCl2-Na2O-Al2O3 [6], Li2O-B2O3 [7] and P2O5-NaF-AlF3 [13] have been reported.These reports showed decreasing tend of dielectric parameters with increase of rare earth ions content suggesting increasing degree of electrical rigidity in the glass network.
There have been electrical and dielectric investigations conducted on vanadate doped glasses, V2O5-ZnO [1-2, 14], B2O3-V2O5 [15][16][17][18] and B2O3-V2O5-ZnO [19].They observed that high concentration of the transition metal cations produces electronic conduction and the ionic contribution is blocked, exhibited single relaxation process.The dielectric parameters reflected possibility of their utility in various electronic and energy storage devices.No reports on the investigation of electrical and dielectric properties of Sm2O3 doped ZnO-B2O3-V2O5 glasses.
The current study focuses on the dielectric properties of boro-zinc-vanadate glasses containing different concentration of Sm2O3.The aim is to measure dielectric properties of these glasses, analyse data and understand dielectric relaxation mechanism.These will also enable one to understand the effect of rare earth oxide on conductivity and dielectric behavior.

Dielectric measurements
The prepared glasses were polished and their two large parallel faces were silver painted and placed between two electrodes of the sample holder.The contacts and continuity were checked before the measurement.The capacitance, C and dissipation factor, tanδ were measured in an Impedence analyzer (Wayne Kerr 6500B) for the frequency range 50 Hz -10 MHz and the temperature range 343 K -573 K.
The dielectric constant (εʹ) and loss tangent (εʹʹ), real and imaginary parts of electric moduli and impedance were estimated using following mentioned expressions [9], (3) Where, ε0 is the permittivity of the free space, ω = 2πf the angular frequency, A the cross-sectional area and, d the thickness of the sample.

Dielectric properties
Dielectrics are the materials, under normal conditions, will not have free electric charges.Yet they respond to the applied electric field considerably.The dielectric constant of a material indicates the number of electrostatic lines of flux concentrated in it.The dielectric loss defines the materials capacity to support the electrostatic field with least dissipation of energy in the form of heat [20].The dielectric constant (εʹ) and dielectric loss (εʹʹ) of the present glasses are found to vary in the range from 1.9177x10 2 to 3.9456x10 3 and from 1.5751 to 6.9095x10 5 respectively over the measured temperature and frequency range.The variation of εʹ and εʹʹ as a function of frequency at different temperatures for ZVBS1 glass are shown in Figure 1 (a) & (b) respectively.It can be observed that both εʹ and εʹʹ decreased with frequency and increased with temperature.The accumulation of charge occurs in the glass at low frequency prevents the flow of charges.This in turn increases polarisation and that is why εʹ and εʹʹ have large values.Increased frequency of periodic electric field reduces polarisation and lowers εʹ and εʹʹ.Moreover, according to recent studies, the nonbridging oxygens (NBOs) in the glass have a strong correlation with dielectric parameters.More number of NBOs produces more polarization which results high values of dielectric constants [17].Variation in εʹ and εʹʹ of remaining glasses followed the same trend as that of ZVBS1 glass.Figure 2 (a) & (b) depicts frequency dependence of εʹ and εʹʹ for all the ZVBS glasses at 573 K.With the addition of Sm2O3 (at the expense of B2O3), the values of εʹ and εʹʹ are decreased.ZVBS1 glass has the highest polarizability compared to the remaining glasses.The εʹ and εʹʹ of the MoO3-V2O5-BaO-B2O3-Bi2O3 (MVBBB) glasses were found to be varying from 37 to 112 and from 25 to 134 respectively.They reported that the MVBBB (x =0) has highest values of εʹ and εʹʹ due to high concentration of NBOs which lead to highest polarizability among all other glasses.Based on εʹ and εʹʹ, they proposed these glasses for use in energy storage applications.The εʹ and εʹʹ of the present glasses are found to be varying from 1.9177x10 2 to 3.9456x10 3 and from 1.5751 to 6.9095x10 5 respectively.As compared to MVBBB glasses the εʹ and εʹʹ of present glasses are high which leads to high polarizability.Therefore, the present glasses can be useful for use in energy storage applications.

Electric Modulus (M)
The frequency dependence of Mʹ and Mʹʹ for different temperatures for ZVBS1 glass is presented in Figure 3.It can be noticed that at low frequencies, Mʹ is extremely small.The distribution of relaxation processes accounts for the rise in Mʹ with frequency.The ions move freely within the potential well only on the higher frequency side of the Mʹmax region.On the lower frequency side of the Mʹmax region, ion drifts over extended distances [22].
At lower frequency and at higher temperatures, the permittivity increases significantly.It may have been caused by the combination of electrode polarization, transmission of charge and space-charge polarization [21].As observed in Figure 3, at all temperatures, Mʹʹ exhibits humps and they correspond to the movement of charge carriers causing dc conductivity.It is clearly visible that the major relaxation peak of Mʹʹ, which characterises the ion transport, is observed and shifted towards higher frequencies as the temperature increased [10].There is no change in the peak intensity as its hopping frequency significantly increases.This shows that the mobility of charge carriers has clear effect on temperature [16].In addition, it can be noted that the modulus graphs have almost the same shapes (at all temperatures), indicating that the distribution of the relaxation time is unaffected by temperature [18].The remaining glasses of the present series were found to exhibit identical changes in Mʹ and Mʹʹ with temperature and frequency as that of ZVBS1 glass.

Impedence Spectroscopy (Z)
The impedence can be used to describe non-Debye type relaxation behavior associated with the material [18].It also gives information about bulk resistance (Rb).The Nyquists cole-cole plots of Zʹ versus Zʹʹ at different temperature for ZVBS1 glass are shown in Figure 4.It can be seen that at all the temperatures, a single semicircle is traced indicating single phase relaxation process, which can be attributed to the bulk contribution to electrical conduction.As the temperature increases, the semicircle's intercept shifts towards a lower Zʹ which suggests that the sample's bulk resistance is decreasing.So, the electrical relaxation in ZVBS1 glass is a pure bulk phenomenon.This reveals the existence of single phase with non-Debye type relaxation in all the current glasses [18,22].The remaining glasses in the current series displayed changes in Zʹ and Zʹʹ same as that of ZVBS1 glass.This means single phase relaxation process is occurring in all the glasses.
The bulk dc resistance, Rb was obtained from the intercept of the semi circles on Zʹ axis.Using Rb the dc conductivity, σdc was determined as per the equation [23] Where, A is the area and d is the thickness of the sample.
It is observed that σdc is increased with temperatures.The dc conductivity behavior with temperature is analysed using Arrhenius expression [18], σdc =  0  ( Where, σ0 is the pre-exponential factor and Edc the activation energy for conduction.Figure 5 presents the Arrhenius plots of dc conductivity.The activation energy Edc was calculated using the slopes of linear lines fit to the data and shown as solid lines in the figure.Obtained Edc values are listed in Table 1.The Edc is decreasing with increasing Sm2O3 content and are comparable with reported values for Sm2O3 doped lithium borate glasses [7] and Li2O doped barium vanadate glasses [23].

Relaxation mechanism
The hopping of charge carriers can be used to understand dielectric relaxation.Small polarons with the same mobility as ions can be found in transition metal oxide doped glasses.The frequency (fp) corresponding to Mʹʹmax or Zʹʹmax in Figures 3 & 4 has been extracted for different temperature.By assuming thermal activation of fp in the Arrhenius fashion, the activation energy (Ep) for fp was calculated using following relation [24], Figure 6 shows the plots of ln(fp) against (1000/T) for all the ZVBS glasses.By fitting linear lines to the data, activation energy, Ep was determined and are tabulated in Table 1.The relaxation time, τ was determined by applying the condition, ωpτp = 1 where, ωp = 2πfp [24].According to the theory that relaxation is also thermally activated process associated with certain activation energy and is expressed again in the Arrhenius form as [22], τ =  0  (   As per Eq. ( 10), the sketches of ln(Mʹʹ) and ln(Zʹʹ) as a function of (1000/T) are made in Figures 7  and 8, respectively.Linear lines were fit to the data and activation energy   ʹʹ and   ʹʹ were estimated and recorded in Table 1.These activation energies are found to be in agreement with one another and are decreasing with increasing Sm2O3 content.This suggests that the hindrance developed for dipole orientations are increasing with increase of temperature.The present relaxation activation energy and dc activation energy are slightly higher than those reported for Fe2O3 doped lithum borovanadate glasses [16] and Li2O doped barium vanadate glasses [23].The range of activation energy for DC conduction, the electric modulus and impedence have been found to be almost same.This implies that the potential barrier for conduction and dielectric relaxation will be the same for charge carriers [7,24].

Master curves
The frequency dependent modulus for each glass at different temperatures can be scaled to a single master curve.It implies that the observed process is modulated by a single physical phenomenon and it can only be differentiated by thermodynamic scale [23].The normalized (M''/M''max) plot of the modulus data for the glass ZVBS1 at different temperatures is displayed in Figure 9.The perfect overlapping of master curves (Normalised plots) suggests that temperature-independent relaxation process occur in glass ZVBS1.This reveals that the relaxation process occuring in the present glasses is of single phase.The master curves of the remaining glasses of the present series appeared same.Identical results have been reported for ternary vanadate glasses [23,24].

Conclusions
A series of Sm2O3 doped boro-zinc-vanadate glasses were studied for dielectric properties over the frequency range of 50 Hz to 10 MHz and the temperature range of 343 K-573 K. Different polarisation phenomenon have been used to explain observed variations in dielectric parameters.The dielectric constant ɛʹ and dielectric loss εʹʹ are found to decrease with increasing frequency and decrease with the addition of Sm2O3.The high values of ɛʹ measured for the present glasses enable us to propose these glasses for energy storage applications.Modulus graphs have the same shape at all temperatures, indicating that the distribution of the relaxation time is independent of temperature.Single semi-circles were seen in the cole-cole plots, indicating a single phase relaxation process of non-Debye type.The relaxation activation energy Eτ, obtained from the temperature-dependent loss peaks, are in agreement with dc activation energy Edc.These activation energies are decreasing with increasing concentrations of Sm2O3.It is learnt that the relaxation processes occuring in the current glasses are temperature-independent.It is for the the first time Sm2O3 doped boro-zinc-vanadate glasses were thoroughly investigated for dielectric relaxation.

Figure 1 .
Figure 1.Variation of (a) εʹ and (b) εʹʹwith frequency at different temperatures for ZVBS1 glass.

Figure 2 .
Figure 2. Variation of (a) εʹ and (b) εʹʹ as a function of frequency for ZVBS glasses at 573 K temperature.

Figure 3 .
Figure 3. Plot of Mʹ and Mʹʹ versus ln(f) with temperature for ZVBS1 glass.

Figure 9 .
Figure 9. Normalized plots of electric moduli for ZVBS1 glass at four different temperatures.