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Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses
NOC-18266; No of Pages 7 Journal of Non-Crystalline Solids xxx (2016) xxx–xxx
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Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses I. Jlassi a,b,⁎, N. Sdiri a, H. Elhouichet a,b a b
Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National de Recherches en Sciences des Matériaux, B.P. 95, Hammam-Lif 2050, Tunisia Département de Physique, Faculté des Sciences de Tunis, Université de Tunis ElManar, Campus Universitaire Farhat Hachad ElManar 2092, Tunisia
a r t i c l e
i n f o
Article history: Received 24 February 2017 Received in revised form 16 March 2017 Accepted 29 March 2017 Available online xxxx Keywords: Infrared spectroscopy Dielectric properties Electrical properties Phosphate glasses MgO
a b s t r a c t The electrical and dielectrical properties of Li2O-P2O5-Al2O3 doped with MgO glasses were measured using complex impedance spectroscopy. IR study reveals the influence of gradual increase in MgO content on the glass structure. The Nyquist diagrams were investigated in terms of equivalent circuits due to resistors and constant phase elements (CPE). Constant-phase elements (CPE) were used in equivalent electrical circuits for the fitting of experimental impedance data. Complex impedance analysis showed the behavior of a dielectric relaxation non-Debye type. The ac conductivity increases with temperature following the Arrhenius law, with single apparent activation energy for conduction process. The variation of conductivity and high temperature activation energy with composition revealed the possibility of ionic contribution to the conductivity and a transition from predominantly polaronic conductive regime to ionic conductive regime around 1.5 mol% of MgO in lithium phosphate glasses. The frequency dependence of the electric conductivity follows a simple power law behavior, according to relation σac(ω) = σ(0) + Aωs, where s b 1. It was evident that the electrical transport process in the system was due to the hopping mechanism. The frequency and temperature dependence of the electrical modulus as well as dielectric loss parameters have exhibited a relaxation character. © 2016 Published by Elsevier B.V.
1. Introduction Lithium phosphate glasses were gained great attention during the last decades due to their wide range of compositional and structural possibilities. The low melting and glass transition temperatures, the high electrical conductivity and the high thermal expansion coefficient of these glasses enable their use in laser host matrices, lithium microbatteries or in electro-optical systems [1–3]. Recently, a large number of phosphate glasses have been studied and their structure, properties, conductivities and dielectric behavior have been ascertained [4,5]. The study of electrical conductivity of phosphate glasses has been subject of extensive investigation due to their potential applications in solid state batteries [6,7]. Among all glasses, lithium phosphate glasses are widely studied because of their potential application in solid state batteries [8]. Since lithium is light and highly electropositive, lithium-based glasses find potential applications in high energy density solid state batteries [9]. With the addition of Li2O, the conductivity of these glasses has been found to increase. When more than one glass former is used, there is an increase in conductivity due to mixed former effect [10,11]. Further ⁎ Corresponding author. E-mail address: [email protected] (I. Jlassi).
addition of lithium halides increases the conductivity of these glasses [12,13]. It has been reported that adding Al2O3 to phosphate glass improves their physical properties and its chemical stability [14]. On the other hand, Al2O3 increases the cross-linking between PO4 tetrahedra in the glass which results in moisture free and thermally stable glass with low thermal coefficient of expansion that are used for ion exchange planar waveguide devices [15]. However, for the study, the glass systems are considered as ionic glasses due to the existence of alkaline earth ions (Mg2 +) with no more than one valence state. Hence, electrons jumping from low valency state to other higher valency state are not taking place. Glass is a complex material and until today there has been no theory of ionic transport which is accepted widely. Thus, ionic diffusion in ionic conducting glass material has been an issue of interest because of its importance in technological uses. Therefore, a better understanding of its electrical conductivity is required [16]. The study on dielectric properties such as dielectric constant, loss and ac conductivity of phosphate glasses over a wide range of frequency and temperature is expected not only to reveal comprehensive information regarding the nature and origin of the loss occurring in these materials as well as conduction mechanism but also to provide information on the structural aspect of the glasses [17,18].
http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042 0022-3093/© 2016 Published by Elsevier B.V.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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In this contribution we present the results of electrical and dielectric investigation of a Lithium phosphate glasses as function of temperatures. Ac electrical measurements show that the values of conductivity are higher than those studied at room temperature in previous work [19]. The aim of the present study is to investigate the structural, electrical and dielectric properties in lithium phosphate glass with various concentrations of MgO in which the function of frequency is (40 Hz–6 MHz) and the temperature (533 to 568 K) using FTIR and impedance spectroscopy. 2. Experimental Lithium phosphate glasses doped MgO were prepared by the melt quenching technique. High purity NH4H2PO4 (99.9%), Li2CO3 (99.99%), Al2O3 (99.99%) and MgO (99.9%) were used as starting materials. The compositions (in mol.%) of glasses used in the present study are (66-2x/3) P2O5–(33-x/3)Li2O–(1)Al2O3–(x) MgO (referred as PLA0, PLA1, PLA2 and PLA3, for x = 0, 0.5, 1 and 1.5 mol%, respectively). The ratio 1:2 of Li2O to P2O5 was kept constant for all glasses. 1 mol% of Al2O3 was added to the batches to prevent accidental devitrification during the cooling and annealing of the samples because lithium glasses are very sensitive to variations in temperature and Al2O3 stabilizes the preparation process. About 6 g of the batch composition was thoroughly ground in an agate mortar and this homogeneous mixture is taken into a platinum crucible and heated in an electric furnace at 950 °C for 2 h. The melt was quickly quenched to room temperature by pouring on a preheated stainless steel plate and kept for annealing at 250 °C for 3 h. The glass samples were covered with another stainless steel plate after that slowly cooled down to room temperature. The annealing process sought to minimize the internal mechanical stress and obtain glasses with good mechanical stability. The sample has the form of a disk with (d = 1 ± 0.1) mm diameter and (e = 0.75 ± 0.1) mm thickness. The amorphous nature of these glass samples was established and confirmed using X-ray diffraction (XRD) [19]. Infrared spectra of all glasses were collected in the range of 400–4000 cm − 1 using a Perkin-Elmer (FTIR 2000) spectrometer at 4 cm − 1 resolution. The IR spectra absorption measurements were done using the KBr pellet technique. The spectrum of pure KBr was subtracted from each glass spectrum to correct the background. Electrical conductivity measurements were performed as a function of both temperature (533 to 568 K) and frequency (40 Hz–6 MHz) using an Agilent 4294A impedance analyzer.
Fig. 1. IR spectra of P2O5-Li2O-Al2O3 glasses doped MgO at room temperature.
band near 895 cm− 1 has been assigned to P\\O\\ groups, the phosphate-non-bridging oxygen portion of PO4 tetrahedra in a chain structure [28]. The absorption bands near 895 and 737 cm−1 are assigned to the asymmetric stretching and the symmetric stretching modes of the in-chain P\\O\\P linkages, (P\\O\\P) as and (P\\O\\P)str, respectively [29,30]. At low frequency, the spectra show a strong broad absorption band near 522 cm−1 which can be composed of two component bands near 450–500 cm−1 and 530–550 cm−1 assigned to deformation modes of PO34 − groups [31] and the bending mode of O\\P\\O linkages [32] respectively. On the other hand, the intensity bands around ~1078 cm−1 and near 895 cm−1 increase with MgO amount. The result reveals an increase of non-bridging oxygen (NBO) group, which implies an increase in the chemical durability of the glass network [33–35]. All these facts are consistent with the results of Raman spectra studied in previous work [19,36]. 3.2. Impedance analysis Fig. 2 shows complex impedance cole-cole plots for PLA0 (case 0 mol% MgO) glasses at different temperatures. A theoretical curve fitting and experimental data are measured. A good agreement between
3. Results and discussion 3.1. FTIR analyses Fig. 1 shows the FTIR spectra in the frequency range between 400 and 2000 cm−1 of five samples. They are similar to those reported for other phosphate glasses [20]. The band at ~ 1634 cm−1 reflected the bending vibrations of P\\OH bonds [21]. The band at ~ 1734 cm−1 in the glass may be assigned to water-bending mode [21,22]. Moreover, the absorption bands near 1369 cm−1 has been assigned to the asymmetric stretching vibration of the P_O bonds [23]. The bands near 1242 cm−1 is assigned to asymmetric stretching mode of the two non-bridging oxygen atoms bonded to phosphorus atoms, the O–P–O or (PO2)as units, in the phosphate tetrahedral [24,25] and the stretching mode of P_O double bonds [25,26]. These two bands, (PO2)as and P_O, are overlapped to form the broaden bands in the spectra. The band at around 1147 cm−1 is assigned to the PO2 symmetric stretching mode, (PO2)str [25]. The band around ~1078 cm−1 is asymmetric stretching vibration mode of P\\O\\P non-bridging oxygen group. The band at ~ 900 cm− 1 is asymmetric stretching vibration of P\\O\\P [27]. The
Fig. 2. Experimental impedance diagrams of PLA0 glasses at different temperatures with inset the corresponding equivalent circuit.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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the experimental and theoretical curves was attained. For all compositions, a slight degree of decentralization can be detected at impedance. The semicircular arcs are depressed and their centers lie below the ReZaxis (Z′), which suggests that the relaxation is of non-Debye type in all the cases [37,38]. The deviation from a Debye profile in the present system may be attributed to the formation of the nanopolar clusters [37]. In order to analyze the impedance spectrum, it is useful to have an equivalent circuit model that provides a realistic representation of the electrical properties of the respective regions. So each semicircle is represented by a parallel RC element [39,40]. The equivalent electrical circuit of the samples is well described by the parallel of capacitance and resistance. Because of the detected impedance decentralization, the capacitor may be replaced by a constant phase element (CPE). The total impedance of the circuit is given by: Z ¼ Re Z þ Im Z ¼ Z 0 þ jZ ″ ¼
1 1 þ Rb Z CPE
ð1Þ
where the impedance of the CPE is defined via: [41,42]. Z CPE ¼
1 A0 ð j ωÞn
ð2Þ
where j is the imaginary unit (j2 = − 1) and ω the angular frequency (ω = 2πf, f is the frequency), A0 is a constant independent of frequency and in F cm− 2 Sn − 1 [43], and n is a dimensionless parameter ranging between zero and unity and determining the degree of deviation from an exact semicircle [44]. The bulk resistance R b is the cross of axis with the impedance curve. This latter determines the degree of deviation from an exact semicircle. When n = 1, Eq. (2) yields the impedance of a capacitor, where A0 = C. These semicircles have been fitted by the software ORIGIN6.0 based on the following relationships. nπ Rb 1 þ Rb A0 ωn cos 2 Z ¼ nπ þ ðRb A0 ωn Þ 1 þ 2Rb A0 ωn cos 2
ð3Þ
nπ R2b A0 ωn sin Z ¼− nπ 2 þ ðRb A0 ωn Þ2 1 þ 2Rb A0 ωn cos 2
ð4Þ
0
00
In the same context of glasses studies, several others [45–47] observed the identical equivalent circuit. The proposed equivalent circuit is composed by the parallel combination of a single R-CPE circuit as shown in Inset of Fig. 2. The extract parameters for the circuit elements are collected in Table 1. From Fig. 2, the maximum Imaginary impedance spectra Z″ shifts to high frequency, with increase in temperature, in which the resistance of the sample decreases, hence the electrical conductivity increases [44,48]. On the other hand, the real part of impedance Z′ decreases with temperature. This decrease may be associated with the charge carrier hopping. The curves show that Z″ values reach a maximum peak Z″max and the value of Z″max shifts to higher frequencies with temperature. The asymmetric variation in the broadness
3
of Z″ peaks suggested an electrical process with a spread of relaxation time [49]. 3.3. Conductivity analysis 3.3.1. Dc electrical conductivity Conductivity is a sensitive structure property because the structure decides both the potential barriers for the transport of mobile ions as well as the mobile ion concentration. The structure is modified by both the increasing temperature and the concentration. The effect of temperature on conductivity is plotted as in Fig. 3 which shows that conductivity varies with temperature according to Arrhenius equation: Ea σ dc T ¼ σ 0 exp − kT
ð5Þ
where Ea is the activation energy, σ0 is pre-exponential factor, kB is the Boltzmann constant and T is the temperature. Arrhenius plots of the variation of dc conductivity for the prepared glass samples with temperature (533–568 K) are shown in Fig. 3. The values of activation energy Ea can be deduced from the slopes of the lines fitting the curves σdc T versus1000 T . According to the plot, it is observed that the conductivity of the glass sample was found to increase linearly with increasing temperature indicating temperature dependence activation energy (Ea). Activation energy as a function of MgO is shown in Fig. 4. It can be seen from the figure that the activation energy increases with the increase of MgO up to 1 mol% and decreases with the increase of MgO content. Fig. 4 shows the non-linear behavior of conductivity. The result may be explained by a competition between electronic and ionic conduction type. The electronic conductivity was predominant up to 1 mol% of MgO and ionic conductivity was predominant thereafter. In fact, polarons were formed from holes in the valence band where charge carriers inducing strongly localized lattice distortions form “small” polarons conduction [50,51]. The weak value of activation energy for 1.5 mol% MgO content may be explained by the dominance of the ionic conduction. Indeed, The FTIR spectroscopy shows that NBO increased with the increase in the rate of MgO since the intensity bands around ~ 1078 cm−1 and near 895 cm− 1 increases. The change of conduction mechanism has been detected in zinc vanadophosphate and silver vanadium tellurite glasses [52,53]. The ln(σdc) as a function of mole fraction of MgO at temperature 563 K is plotted in Fig. 4. The conductivity of passing through a deep minimum at approximately 1 mol%. This figure suggests a kind of transition from predominantly electronic conductivity to ionic
Table 1 The best fitting values of equivalent circuit elements in Fig. 2 in different temperatures. Temperature (K)
Rb (kΩ)
A (10−10 F cm2 sn
533 538 543 548 553 558 563 568
151 89 73 63 52 30 24 21
1.64 1.82 1.72 1.70 1.66 2.59 1.35 1.24
− 1
)
n 0.816 0.816 0.821 0.823 0.826 0.805 0.846 0.852
Fig. 3. Arrhenius relation of Ln(σdc T) versus 1000/T for all samples.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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Fig. 4. Variation of Ln(σdc) and activation energy for PLA0, PLA1, PLA2 and PLA3 glasses.
Fig. 5. Frequency dependence of the conductivity, σac(T), at temperatures shown for the PLA glass doped 1 mol% MgO.
conductivity. It becomes clear from this figure that the minimum in conductivity observed in Fig. 4 is accompanied by an inverse behavior of Ea. In Table 2, the values of the activation energy (Ea) have been presented at different concentration of MgO and compared with other glasses. It is clearly seen that conductivity activation energy of the PLA is lower than those of the sodium mixed phosphates glasses NPP5 (Ea = 1.54 eV) [54], Lithium phosphate glasses (1.64 eV) [55] and Lithium Bismuthate glasses ((75-x) Bi2O3-10ZnO-15B2O3-(x) Li2O, x = 5, 10, 15, 20 mol%)(1.29–1.35 eV) [56]. Besides, the obtained values of Ea are much higher than those reported in other papers for 10MgO–90P2O5 glasses (0.108 eV) [57], Borophosphate glasses PBF4 (0.54 eV) [58] and lithium niobium silicate glasses SLN D [59]. However, these values are in the same range (1.04–1.16 eV) as those reported for MgO doped ZnO-P2O5 [60].
Fig. 5 shows the frequency dependence of the ac conductivity for the PLA2 glass at different temperatures. The spectra display the typical shape found for electronically conducting glasses [66]. At low frequency, the curves show a plateau region, which corresponds to the frequency independent dc conductivity. Thus, dc conductivity is a constant value. The ac electric conductivity shows a dispersion that shifts to higher frequencies with temperature. At the high frequency end the curves overlap, indicating the possibility of the presence of space charges [67]. This assumption is reasonable since the space charge effect vanishes at higher temperature and frequency. To determine the predominant conduction mechanism of the ac conductivity for the sample, one can suggest the appropriate model for the conduction mechanism in the light of the different theoretical models correlating the conduction mechanism of ac conductivity with s(T) behavior. From the theoretical fit, we can obtain the parameters σdc and s and evaluate their variation with temperature and doping. From Inset of Fig. 6, the σac values were found to augment with increasing temperature in the range 533–568 K. At higher temperature, Barton [68] and Namikawa [69] consider that the behavior showed that σac has high values due to increase in the number of charges, which makes hopping increase. The effects of polarization of electrodes are found at low frequency, while but they are absent in the dispersive region at high frequencies [70]. Low values of the ac conductivity at low frequency are
3.3.2. AC electrical conductivity In order to evaluate non-linearities conductivity, it is useful to present the ac conductivity for all the compositions of PLA glass system. The frequency variation of ac conductivity, at various temperatures is shown in Fig. 5. The nature and the mechanism of the conductivity dispersion in solids are generally analyzed using Jonscher's power law [61,62]: σ ac ¼ σ dc þ A ωs
ð6Þ
where σdc is the dc conductivity in particular range of temperature and frequency, A is the temperature dependent parameter and s is the temperature dependent exponent in the range of 0 ≤ s ≤ 1 [63,64]. The exponent s represents the degree of interaction between mobile ions with the lattices around them, while the prefactor exponent A determines the strength of polarizability [65]. Table 2 Values of Ea, ε′ and tan δ for different compositions of glasses at room temperature and in the relaxation frequency. Sample code
PLA0 PLA1 PLA2 PLA3 Li2O-P2O5 [55] PBF4 [58] SLN D [59] CZ13 [60]
Composition (mol%) P2O5
Li2O
Al2O3
MgO
66 65.65 65.33 65 60
33 32.85 32.67 32.5 40
1 1 1 1 –
0 0.5 1 1.5 –
Ea (eV)
ε′
tan δ
0.83 1.15 1.16 0.88 1.64 0.54 0.43 1.13
5.37 5.49 2.51 5.74 – 10 62 2.52
0.38 0.7 1.27 0.73 – 0.76 0.56 0.78
Fig. 6. Variation of the exponent parameter s with temperature for PLA glasses doped MgO. Inset shows the variation of σdc with temperature for PLA1 glasses.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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observed due to the accumulation of ions. This is explained by the low frequency electric field zone. In high or medium frequency region, the power law is observed and conductivity sharply increases with frequency [71,72]. The exponent s can be expected if the polarizability of involved material depends on the energy barrier for a simple hopping process between two sites [73]. Some reports, involving hopping models, indicate that s varies in the range from 0.6 to 1 and indicate that it depends on temperature and frequencies [74]. In order to identify the conduction mechanism in our glasses systems, the exponent s is plotted as a function of temperature as depicted in Fig. 6. From Fig. 6, it was found that exponent “s”, calculated at higher frequencies, slightly decreases from 0.84 to 0.51 with increasing temperature from 533 K to 568 K. At 533 K, the exponent “s” increases compared with values given at room temperature [19]. Indeed, the thermal agitation increases, which induces a disorientation of the electric dipoles thus the polaronic conduction decreases in favor of the ionic conduction. To explain the temperature dependence of “s”, a theoretical model for ac conductivity has been predicted such as correlated barrier hopping (CBH) model proposed by Elliot et al. [75]. In our work, it is obtained that the exponent “s” parameter, which is calculated at higher frequencies, decreases with rising temperature, confirming the predicted values as CBH model that is the suitable model to characterize the electrical conduction mechanism of our samples. According to this model, the exponent s parameter is written as follows: s ¼ 1−
6K B T WM
ð7Þ
where kB is Boltzmann constant, WM is optical band gap and T the absolute temperature. 3.4. Dielectric properties 3.4.1. Permittivity and loss studies The complex permittivity formalism has been employed to reveal significant information about the chemical and physical behavior of the electrical and dielectric properties, it is expressed as [76,77]: 0
00
ε ðωÞ ¼ ε ðωÞ−j ε ðωÞ ¼
1 0 00 j ω C 0 Z þ jZ
ð8Þ
Fig. 7. The frequency dependence curves at T = 553 K of dielectric constant ε′(ω) (a) and loss factor (b) for PLA glasses doped MgO. Inset shows the variation of ε′(ω) and tan(δ) at different temperatures case of 3 mol% MgO.
00
0
ε ¼−
Z 0 ω C 0 Z 2 þ jZ 00 2
ð9Þ
0
00
ε ¼−
Z 0 ω C 0 Z 2 þ jZ 00 2
ð10Þ
where ε′ and ε″ are the real part known as dielectric constant and the imaginary part known as dielectric loss of the complex permittivity respectively. They depend on frequency. C 0 ¼ ε0eA is the capacitance of the empty cell (ε0 is the permittivity of the vacuum (8.854 · 10−12 F m−1), A is the cross-sectional area of the flat surface of the pellet (78.5 · 10−6 m2) and e is its thickness (0.75 mm). Z′ is the real part of impedance and Z″ is its imaginary part. From values of the dielectric constant, we can determine the loss factor tangent given by [78]: tan δ ¼
Z Z
0
00
00
¼
ε ε0
ð11Þ
The frequency dependence of the dielectric constant (ε′) and loss factor (tan δ) are studied for the PLA glasses doped MgO and shown in Figs. 7(a) and (b). The temperature dependence of ε′ and tan δ (case of PLA3: 1.5 mol% MgO) as a function of logarithmic frequency at several temperatures is depicted in Inset of Fig. 7(a) and (b). It is observed that
ε′ and tan δ increase with the increase in temperature at lower frequencies which is normal behavior in oxide glasses and decreases with the increase in frequency. Similar results are observed for other glass samples. This behavior can be attributed to the polarization effects of the long range hopping of mobile ions with respect to the immobile glass matrix [79]. This increase in ε' and tan δ, in lower frequency and higher temperature region, may be due to the application of the field, which assists electron hopping between two different sites in the glasses. At high temperatures, the jump frequency of the charge carriers becomes large and comparable with the frequency of the applied field. Accordingly, at low frequency, the charge carriers hop easily out of the sites with high free energy barriers. This leads to a net polarization and gives an increase in the dielectric constant and loss factor [80]. In high frequency region, both the dielectric constant and factor loss approach a constant values which results from rapid polarization processes occurring in the glasses under applied field [81]. However, at high frequency, the charge carriers will no longer be able to rotate sufficiently rapid, so their oscillation will begin to lay behind this field resulting in a decrease of dielectric constant and loss factor. At low temperatures, jump frequency of the charge carries becomes smaller than the frequency of the applied field. The low values of dielectric constant and loss factor, at higher frequency, of the samples are important for
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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extending the material applications towards photonic and electro-optic and fundamental use for nonlinear optical materials [82,83]. In Table 2, the dielectric constant (ε′) and the loss factor tan δ are compared in different oxide glasses at room temperature and in the relaxation frequency. 3.4.2. The electrical modulus formalism Electrical modulus data analysis, the complex permittivity can be converted to the complex modulus, 0
00
M ðωÞ ¼ M þ j M ¼
1 ε
ð12Þ
where M′ is the real part of the electric modulus and M″ is the imaginary part of electric modulus, via [84]. 0
00
ð13Þ
00
0
ð14Þ
M ¼ ω C0 Z
M ¼ ω C0 Z
where Z′ and Z″ are the real and imaginary parts of the impedance, respectively. Fig. 8 shows real part of the electric modulus as a function of frequency at several temperatures. At all temperature investigated, M ′(ω) reaches the constant value at high frequencies and it tends to zero at low frequencies suggesting negligible or absent electrode polarization phenomenon [85]. Fig. 9 shows the imaginary part of the electric modulus M″(ω) as a function of frequency at several temperatures. The maximum in the M ″ peak shifts to higher frequency with increasing temperature. The shift in M″ peak maximum corresponds to the conductivity relaxation. This behavior suggests that the dielectric relaxation is thermally activated in which hopping mechanism of carrier charge dominates intrinsically [86]. At frequency above peak maximum M″, the carriers are spatially confined to potential wells, being mobile on short distances making only localized motion within the wells. The M″(ω) parameter shows a slightly asymmetric peak at each temperature, as can be seen in Fig. 8. The frequency range below the peak frequency fp determines the range in which charge carriers are mobile over short distances [87]. Frequency corresponds to peak frequency denoted as fp and M″ associated with it is denoted as M″max. At each temperature the left hand side of relaxation peak indicates the range with which ions drift to long, while the right-hand side indicates that the ions are spatially confined to potential well and free to move within the wells. Frequency fp which corresponds to M″max defined condition ωcτ = 1 where relaxation timeτ ¼ 2π1f . Fig. 10 shows the variation of relaxation time with p
Fig. 8. Variation of electric modulus with frequency and temperature for 0.5 mol% MgO.
Fig. 9. The frequency dependence of M″ at different temperatures for the sample PLA1.
temperature for 0.5 mol% MgO as representative, which follows the relation [87]: τ ¼ τ 0 exp
Eaτ kT
ð15Þ
where τ0 is the pre-exponential factor and Eaτ is the activation energy for the conductivity relaxation. The calculated value of Eaτ, deduced from the slope of the line of Fig. 10 by using Eq. (15), is estimated at 1.08 eV. It is observed that Eaτ and Ea for dc conductivity are quite similar, which indicates that ions have to overcome the same barrier during conduction and relaxation [80]. The similar values of activation energies indicate that the distribution of random energy barriers in glass for charge carriers hopping is isotropic. Moreover, the barrier height differences between long and short distances that charge carriers' travel are not pronounced [88]. In this respect Kang and Choi [89] showed that a relationship exists between the dielectric relaxation and the mode of electrical conductivity through the relaxing species in the region of high temperatures. 4. Conclusions P2O5–Li2O–Al2O3 glass system doped with MgO was prepared via a melt quenching technique. IR spectroscopy, has showed the dependence of the borate structural units on the MgO content. For our system, both PO3− and P\\O\\ units were detected. Nyquist plots show a non4
Fig. 10. Variation of relaxation Ln(τ) with 1000/T for the sample of PLA1 glass system.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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Debye relaxation and the equivalent circuit is a parallel resistor, capacitor and constant phase element. These trends of variations in conductivity and activation energy with MgO content reveal the possibility of ionic contribution to the electrical conductivity and the transition of conduction mechanism from predominantly electronic to ionic regimes in our glasses. The dielectric constant decreases with increasing frequency. The increase of the dielectric constant with the decrease in frequency may be attributed to the presence of dipoles. The decrease in dielectric constant with MgO count is due to the decrease in space charge carriers or interfacial polarization. The non-Debye type in the modulus plot expresses the distribution of the relaxation time for mobile ions resulting from the hop in the random energy barriers. These glass systems could be a suitable host for the optical amplifiers from the point of view of good ac electric conductivity and high dielectric constant. These features allow our new glass to be a good candidate in the applications of nonlinear optics. References [1] Shixun Dai, Akira Sugiyama, Lili Hu, Zhuping Liu, Guosong Huang, Zhonghong Jiang, J. Non-Cryst. Solids 311 (2002) 138–144. [2] L.M. Sharaf El-Deen, M.S. Al Salhi, M.M. Elkholy, J. Non-Cryst. Solids 354 (2008) 3762–3766. [3] O. Cozar, D.A. Magdas, I. Ardelean, J. Non-Cryst. Solids 354 (2008) 1032–1035. [4] M. Nagarjuna, P. Raghava Rao, Y. Gandhi, V. Ravikumar, N. Veeraiah, Physica B 405 (2010) 668–677. [5] P. Srinivasa Rao, S. Bala Murali Krishna, S. Yusub, P. Ramesh Babu, C.H. Tirupataiah, D. Krishna Rao, J. Mol. Struct. 1036 (2013) 452–463. [6] J. Saienga, S.W. Martin, J. Non-Cryst. Solids 354 (2008) 1475–1486. [7] J. Kolář, T. Wágner, V. Zima, Š. Stehlík, B. Frumarová, L. Beneš, M. Vlček, M. Frumar, S.O. Kasap, J. Non-Cryst. Solids 357 (2011) 2223–2227. [8] F. Berkemeier, M. Shoar Abouzari, G. Schmitz, Appl. Phys. Lett. 90 (2007) 113110. [9] P. Knauth, Solid State Ionics 180 (2009) 911–916. [10] A.C.M. Rodrigues, R. Keding, C. Russel, J. Non-Cryst. Solids 273 (2000) 53. [11] C.-H. Lee, K.H. Joo, J.H. Kim, S.G. Woo, H.-J. Sohn, T. Kang, Y. Park, J.Y. Oh, Solid State Ionics 149 (2002) 59. [12] J. Swenson, R.L. McGreevy, L. Borjesson, J.D. Wicks, Solid State Ionics 105 (1998) 55. [13] A. Hall, S. Adams, J. Swenson, J. Non-Cryst. Solids 352 (2006) 5164. [14] V.B. Sreedhar, C. Basavapoornima, C.K. Jayasankar, J. Rare Earths 32 (10) (2014) 918. [15] Karine Seneschal, Frédéric Smektala, Shibin Jiang, Tao Luo, Bruno Bureau, Jacques Lucas, Nasser Peyghambarian, J. Non-Cryst. Solids 324 (2003) 179–186. [16] R.H. Doremus, Glass Science, John Wiley, New York, 1973 146. [17] G. MuraliKrishna, N. Veeraiah, N. Venkatramaiah, R. Venkatesan, J. Alloys Compd. 450 (2008) 486–493. [18] G. Sahaya Baskaran, M.V. Ramana Reddy, D. Krishna Rao, N. Veeraiah, Solid State Commun. 145 (2008) 401–406. [19] I. Jlassi, N. Sdiri, H. Elhouichet, M. Ferid, J. Alloys Compd. 645 (2015) 125–130. [20] S.W. Yung, S.M. Hsu, C.C. Chang, K.L. Hsu, T.S. Chin, H.I. Hsiang, Y.S. Lai, J. Non-Cryst. Solids 357 (2011) 1328–1334. [21] F.H. ElBatal, S. Ibrahim, A.M. Abdelghany, J. Mol. Struct. 1030 (2012) 107–112. [22] Y.M. Lai, X.F. Liang, S.Y. Yang, J.X. Wang, L.H. Cao, B. Dai, J. Mol. Struct. 992 (2011) 84–88. [23] Yuanming Lai, Xiaofeng Liang, Shiyuan Yang, Pei Liu, Yiming Zeng, Changyi Hu, J. Alloys Compd. 617 (2014) 597–601. [24] R.K. Brow, D.R. Tallant, S.T. Myers, C.C. Phifer, J. Non-Cryst. Solids 191 (1995) 45–55. [25] K. Meyer, J. Non-Cryst. Solids 209 (1997) 227–239. [26] R.K. Brow, D.R. Tallant, J.J. Hugdens, S.W. Martin, A.D. Irwin, J. Non-Cryst. Solids 177 (1994) 221–228. [27] Liyan Zhang, Hongtao Sun, Shiqing Xu, Junjie Zhang, Lili Hu, Physica B 367 (2005) 1–5. [28] R.F. Bartholomew, J. Non-Cryst. Solids 7 (1972) 221–235. [29] H.S. Liu, T.S. Chin, S.W. Yung, Mater. Chem. Phys. 50 (1997) 1–10. [30] J.O. Buyn, B.H. Kim, K.S. Hong, H.J. Jung, S.W. Lee, A.A. Izyneev, J. Non-Cryst. Solids 190 (1995) 288–295. [31] P.Y. Shih, Mater. Chem. Phys. 80 (2003) 299–304. [32] P. Pascuta, G. Borodi, A. Popa, V. Dan, E. Culea, Mater. Chem. Phys. 123 (2010) 767–771. [33] B.C. Sales, J.U. Otaigbe, G.H. Beall, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 226 (1998) 287–293. [34] T. Minami, J.D. Mackenzie, J. Am. Ceram. Soc. 60 (1977) 232–235. [35] B.C. Sales, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 263&264 (2000) 155–166.
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Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses I. Jlassi a,b,⁎, N. Sdiri a, H. Elhouichet a,b a b
Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National de Recherches en Sciences des Matériaux, B.P. 95, Hammam-Lif 2050, Tunisia Département de Physique, Faculté des Sciences de Tunis, Université de Tunis ElManar, Campus Universitaire Farhat Hachad ElManar 2092, Tunisia
a r t i c l e
i n f o
Article history: Received 24 February 2017 Received in revised form 16 March 2017 Accepted 29 March 2017 Available online xxxx Keywords: Infrared spectroscopy Dielectric properties Electrical properties Phosphate glasses MgO
a b s t r a c t The electrical and dielectrical properties of Li2O-P2O5-Al2O3 doped with MgO glasses were measured using complex impedance spectroscopy. IR study reveals the influence of gradual increase in MgO content on the glass structure. The Nyquist diagrams were investigated in terms of equivalent circuits due to resistors and constant phase elements (CPE). Constant-phase elements (CPE) were used in equivalent electrical circuits for the fitting of experimental impedance data. Complex impedance analysis showed the behavior of a dielectric relaxation non-Debye type. The ac conductivity increases with temperature following the Arrhenius law, with single apparent activation energy for conduction process. The variation of conductivity and high temperature activation energy with composition revealed the possibility of ionic contribution to the conductivity and a transition from predominantly polaronic conductive regime to ionic conductive regime around 1.5 mol% of MgO in lithium phosphate glasses. The frequency dependence of the electric conductivity follows a simple power law behavior, according to relation σac(ω) = σ(0) + Aωs, where s b 1. It was evident that the electrical transport process in the system was due to the hopping mechanism. The frequency and temperature dependence of the electrical modulus as well as dielectric loss parameters have exhibited a relaxation character. © 2016 Published by Elsevier B.V.
1. Introduction Lithium phosphate glasses were gained great attention during the last decades due to their wide range of compositional and structural possibilities. The low melting and glass transition temperatures, the high electrical conductivity and the high thermal expansion coefficient of these glasses enable their use in laser host matrices, lithium microbatteries or in electro-optical systems [1–3]. Recently, a large number of phosphate glasses have been studied and their structure, properties, conductivities and dielectric behavior have been ascertained [4,5]. The study of electrical conductivity of phosphate glasses has been subject of extensive investigation due to their potential applications in solid state batteries [6,7]. Among all glasses, lithium phosphate glasses are widely studied because of their potential application in solid state batteries [8]. Since lithium is light and highly electropositive, lithium-based glasses find potential applications in high energy density solid state batteries [9]. With the addition of Li2O, the conductivity of these glasses has been found to increase. When more than one glass former is used, there is an increase in conductivity due to mixed former effect [10,11]. Further ⁎ Corresponding author. E-mail address: [email protected] (I. Jlassi).
addition of lithium halides increases the conductivity of these glasses [12,13]. It has been reported that adding Al2O3 to phosphate glass improves their physical properties and its chemical stability [14]. On the other hand, Al2O3 increases the cross-linking between PO4 tetrahedra in the glass which results in moisture free and thermally stable glass with low thermal coefficient of expansion that are used for ion exchange planar waveguide devices [15]. However, for the study, the glass systems are considered as ionic glasses due to the existence of alkaline earth ions (Mg2 +) with no more than one valence state. Hence, electrons jumping from low valency state to other higher valency state are not taking place. Glass is a complex material and until today there has been no theory of ionic transport which is accepted widely. Thus, ionic diffusion in ionic conducting glass material has been an issue of interest because of its importance in technological uses. Therefore, a better understanding of its electrical conductivity is required [16]. The study on dielectric properties such as dielectric constant, loss and ac conductivity of phosphate glasses over a wide range of frequency and temperature is expected not only to reveal comprehensive information regarding the nature and origin of the loss occurring in these materials as well as conduction mechanism but also to provide information on the structural aspect of the glasses [17,18].
http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042 0022-3093/© 2016 Published by Elsevier B.V.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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In this contribution we present the results of electrical and dielectric investigation of a Lithium phosphate glasses as function of temperatures. Ac electrical measurements show that the values of conductivity are higher than those studied at room temperature in previous work [19]. The aim of the present study is to investigate the structural, electrical and dielectric properties in lithium phosphate glass with various concentrations of MgO in which the function of frequency is (40 Hz–6 MHz) and the temperature (533 to 568 K) using FTIR and impedance spectroscopy. 2. Experimental Lithium phosphate glasses doped MgO were prepared by the melt quenching technique. High purity NH4H2PO4 (99.9%), Li2CO3 (99.99%), Al2O3 (99.99%) and MgO (99.9%) were used as starting materials. The compositions (in mol.%) of glasses used in the present study are (66-2x/3) P2O5–(33-x/3)Li2O–(1)Al2O3–(x) MgO (referred as PLA0, PLA1, PLA2 and PLA3, for x = 0, 0.5, 1 and 1.5 mol%, respectively). The ratio 1:2 of Li2O to P2O5 was kept constant for all glasses. 1 mol% of Al2O3 was added to the batches to prevent accidental devitrification during the cooling and annealing of the samples because lithium glasses are very sensitive to variations in temperature and Al2O3 stabilizes the preparation process. About 6 g of the batch composition was thoroughly ground in an agate mortar and this homogeneous mixture is taken into a platinum crucible and heated in an electric furnace at 950 °C for 2 h. The melt was quickly quenched to room temperature by pouring on a preheated stainless steel plate and kept for annealing at 250 °C for 3 h. The glass samples were covered with another stainless steel plate after that slowly cooled down to room temperature. The annealing process sought to minimize the internal mechanical stress and obtain glasses with good mechanical stability. The sample has the form of a disk with (d = 1 ± 0.1) mm diameter and (e = 0.75 ± 0.1) mm thickness. The amorphous nature of these glass samples was established and confirmed using X-ray diffraction (XRD) [19]. Infrared spectra of all glasses were collected in the range of 400–4000 cm − 1 using a Perkin-Elmer (FTIR 2000) spectrometer at 4 cm − 1 resolution. The IR spectra absorption measurements were done using the KBr pellet technique. The spectrum of pure KBr was subtracted from each glass spectrum to correct the background. Electrical conductivity measurements were performed as a function of both temperature (533 to 568 K) and frequency (40 Hz–6 MHz) using an Agilent 4294A impedance analyzer.
Fig. 1. IR spectra of P2O5-Li2O-Al2O3 glasses doped MgO at room temperature.
band near 895 cm− 1 has been assigned to P\\O\\ groups, the phosphate-non-bridging oxygen portion of PO4 tetrahedra in a chain structure [28]. The absorption bands near 895 and 737 cm−1 are assigned to the asymmetric stretching and the symmetric stretching modes of the in-chain P\\O\\P linkages, (P\\O\\P) as and (P\\O\\P)str, respectively [29,30]. At low frequency, the spectra show a strong broad absorption band near 522 cm−1 which can be composed of two component bands near 450–500 cm−1 and 530–550 cm−1 assigned to deformation modes of PO34 − groups [31] and the bending mode of O\\P\\O linkages [32] respectively. On the other hand, the intensity bands around ~1078 cm−1 and near 895 cm−1 increase with MgO amount. The result reveals an increase of non-bridging oxygen (NBO) group, which implies an increase in the chemical durability of the glass network [33–35]. All these facts are consistent with the results of Raman spectra studied in previous work [19,36]. 3.2. Impedance analysis Fig. 2 shows complex impedance cole-cole plots for PLA0 (case 0 mol% MgO) glasses at different temperatures. A theoretical curve fitting and experimental data are measured. A good agreement between
3. Results and discussion 3.1. FTIR analyses Fig. 1 shows the FTIR spectra in the frequency range between 400 and 2000 cm−1 of five samples. They are similar to those reported for other phosphate glasses [20]. The band at ~ 1634 cm−1 reflected the bending vibrations of P\\OH bonds [21]. The band at ~ 1734 cm−1 in the glass may be assigned to water-bending mode [21,22]. Moreover, the absorption bands near 1369 cm−1 has been assigned to the asymmetric stretching vibration of the P_O bonds [23]. The bands near 1242 cm−1 is assigned to asymmetric stretching mode of the two non-bridging oxygen atoms bonded to phosphorus atoms, the O–P–O or (PO2)as units, in the phosphate tetrahedral [24,25] and the stretching mode of P_O double bonds [25,26]. These two bands, (PO2)as and P_O, are overlapped to form the broaden bands in the spectra. The band at around 1147 cm−1 is assigned to the PO2 symmetric stretching mode, (PO2)str [25]. The band around ~1078 cm−1 is asymmetric stretching vibration mode of P\\O\\P non-bridging oxygen group. The band at ~ 900 cm− 1 is asymmetric stretching vibration of P\\O\\P [27]. The
Fig. 2. Experimental impedance diagrams of PLA0 glasses at different temperatures with inset the corresponding equivalent circuit.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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the experimental and theoretical curves was attained. For all compositions, a slight degree of decentralization can be detected at impedance. The semicircular arcs are depressed and their centers lie below the ReZaxis (Z′), which suggests that the relaxation is of non-Debye type in all the cases [37,38]. The deviation from a Debye profile in the present system may be attributed to the formation of the nanopolar clusters [37]. In order to analyze the impedance spectrum, it is useful to have an equivalent circuit model that provides a realistic representation of the electrical properties of the respective regions. So each semicircle is represented by a parallel RC element [39,40]. The equivalent electrical circuit of the samples is well described by the parallel of capacitance and resistance. Because of the detected impedance decentralization, the capacitor may be replaced by a constant phase element (CPE). The total impedance of the circuit is given by: Z ¼ Re Z þ Im Z ¼ Z 0 þ jZ ″ ¼
1 1 þ Rb Z CPE
ð1Þ
where the impedance of the CPE is defined via: [41,42]. Z CPE ¼
1 A0 ð j ωÞn
ð2Þ
where j is the imaginary unit (j2 = − 1) and ω the angular frequency (ω = 2πf, f is the frequency), A0 is a constant independent of frequency and in F cm− 2 Sn − 1 [43], and n is a dimensionless parameter ranging between zero and unity and determining the degree of deviation from an exact semicircle [44]. The bulk resistance R b is the cross of axis with the impedance curve. This latter determines the degree of deviation from an exact semicircle. When n = 1, Eq. (2) yields the impedance of a capacitor, where A0 = C. These semicircles have been fitted by the software ORIGIN6.0 based on the following relationships. nπ Rb 1 þ Rb A0 ωn cos 2 Z ¼ nπ þ ðRb A0 ωn Þ 1 þ 2Rb A0 ωn cos 2
ð3Þ
nπ R2b A0 ωn sin Z ¼− nπ 2 þ ðRb A0 ωn Þ2 1 þ 2Rb A0 ωn cos 2
ð4Þ
0
00
In the same context of glasses studies, several others [45–47] observed the identical equivalent circuit. The proposed equivalent circuit is composed by the parallel combination of a single R-CPE circuit as shown in Inset of Fig. 2. The extract parameters for the circuit elements are collected in Table 1. From Fig. 2, the maximum Imaginary impedance spectra Z″ shifts to high frequency, with increase in temperature, in which the resistance of the sample decreases, hence the electrical conductivity increases [44,48]. On the other hand, the real part of impedance Z′ decreases with temperature. This decrease may be associated with the charge carrier hopping. The curves show that Z″ values reach a maximum peak Z″max and the value of Z″max shifts to higher frequencies with temperature. The asymmetric variation in the broadness
3
of Z″ peaks suggested an electrical process with a spread of relaxation time [49]. 3.3. Conductivity analysis 3.3.1. Dc electrical conductivity Conductivity is a sensitive structure property because the structure decides both the potential barriers for the transport of mobile ions as well as the mobile ion concentration. The structure is modified by both the increasing temperature and the concentration. The effect of temperature on conductivity is plotted as in Fig. 3 which shows that conductivity varies with temperature according to Arrhenius equation: Ea σ dc T ¼ σ 0 exp − kT
ð5Þ
where Ea is the activation energy, σ0 is pre-exponential factor, kB is the Boltzmann constant and T is the temperature. Arrhenius plots of the variation of dc conductivity for the prepared glass samples with temperature (533–568 K) are shown in Fig. 3. The values of activation energy Ea can be deduced from the slopes of the lines fitting the curves σdc T versus1000 T . According to the plot, it is observed that the conductivity of the glass sample was found to increase linearly with increasing temperature indicating temperature dependence activation energy (Ea). Activation energy as a function of MgO is shown in Fig. 4. It can be seen from the figure that the activation energy increases with the increase of MgO up to 1 mol% and decreases with the increase of MgO content. Fig. 4 shows the non-linear behavior of conductivity. The result may be explained by a competition between electronic and ionic conduction type. The electronic conductivity was predominant up to 1 mol% of MgO and ionic conductivity was predominant thereafter. In fact, polarons were formed from holes in the valence band where charge carriers inducing strongly localized lattice distortions form “small” polarons conduction [50,51]. The weak value of activation energy for 1.5 mol% MgO content may be explained by the dominance of the ionic conduction. Indeed, The FTIR spectroscopy shows that NBO increased with the increase in the rate of MgO since the intensity bands around ~ 1078 cm−1 and near 895 cm− 1 increases. The change of conduction mechanism has been detected in zinc vanadophosphate and silver vanadium tellurite glasses [52,53]. The ln(σdc) as a function of mole fraction of MgO at temperature 563 K is plotted in Fig. 4. The conductivity of passing through a deep minimum at approximately 1 mol%. This figure suggests a kind of transition from predominantly electronic conductivity to ionic
Table 1 The best fitting values of equivalent circuit elements in Fig. 2 in different temperatures. Temperature (K)
Rb (kΩ)
A (10−10 F cm2 sn
533 538 543 548 553 558 563 568
151 89 73 63 52 30 24 21
1.64 1.82 1.72 1.70 1.66 2.59 1.35 1.24
− 1
)
n 0.816 0.816 0.821 0.823 0.826 0.805 0.846 0.852
Fig. 3. Arrhenius relation of Ln(σdc T) versus 1000/T for all samples.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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Fig. 4. Variation of Ln(σdc) and activation energy for PLA0, PLA1, PLA2 and PLA3 glasses.
Fig. 5. Frequency dependence of the conductivity, σac(T), at temperatures shown for the PLA glass doped 1 mol% MgO.
conductivity. It becomes clear from this figure that the minimum in conductivity observed in Fig. 4 is accompanied by an inverse behavior of Ea. In Table 2, the values of the activation energy (Ea) have been presented at different concentration of MgO and compared with other glasses. It is clearly seen that conductivity activation energy of the PLA is lower than those of the sodium mixed phosphates glasses NPP5 (Ea = 1.54 eV) [54], Lithium phosphate glasses (1.64 eV) [55] and Lithium Bismuthate glasses ((75-x) Bi2O3-10ZnO-15B2O3-(x) Li2O, x = 5, 10, 15, 20 mol%)(1.29–1.35 eV) [56]. Besides, the obtained values of Ea are much higher than those reported in other papers for 10MgO–90P2O5 glasses (0.108 eV) [57], Borophosphate glasses PBF4 (0.54 eV) [58] and lithium niobium silicate glasses SLN D [59]. However, these values are in the same range (1.04–1.16 eV) as those reported for MgO doped ZnO-P2O5 [60].
Fig. 5 shows the frequency dependence of the ac conductivity for the PLA2 glass at different temperatures. The spectra display the typical shape found for electronically conducting glasses [66]. At low frequency, the curves show a plateau region, which corresponds to the frequency independent dc conductivity. Thus, dc conductivity is a constant value. The ac electric conductivity shows a dispersion that shifts to higher frequencies with temperature. At the high frequency end the curves overlap, indicating the possibility of the presence of space charges [67]. This assumption is reasonable since the space charge effect vanishes at higher temperature and frequency. To determine the predominant conduction mechanism of the ac conductivity for the sample, one can suggest the appropriate model for the conduction mechanism in the light of the different theoretical models correlating the conduction mechanism of ac conductivity with s(T) behavior. From the theoretical fit, we can obtain the parameters σdc and s and evaluate their variation with temperature and doping. From Inset of Fig. 6, the σac values were found to augment with increasing temperature in the range 533–568 K. At higher temperature, Barton [68] and Namikawa [69] consider that the behavior showed that σac has high values due to increase in the number of charges, which makes hopping increase. The effects of polarization of electrodes are found at low frequency, while but they are absent in the dispersive region at high frequencies [70]. Low values of the ac conductivity at low frequency are
3.3.2. AC electrical conductivity In order to evaluate non-linearities conductivity, it is useful to present the ac conductivity for all the compositions of PLA glass system. The frequency variation of ac conductivity, at various temperatures is shown in Fig. 5. The nature and the mechanism of the conductivity dispersion in solids are generally analyzed using Jonscher's power law [61,62]: σ ac ¼ σ dc þ A ωs
ð6Þ
where σdc is the dc conductivity in particular range of temperature and frequency, A is the temperature dependent parameter and s is the temperature dependent exponent in the range of 0 ≤ s ≤ 1 [63,64]. The exponent s represents the degree of interaction between mobile ions with the lattices around them, while the prefactor exponent A determines the strength of polarizability [65]. Table 2 Values of Ea, ε′ and tan δ for different compositions of glasses at room temperature and in the relaxation frequency. Sample code
PLA0 PLA1 PLA2 PLA3 Li2O-P2O5 [55] PBF4 [58] SLN D [59] CZ13 [60]
Composition (mol%) P2O5
Li2O
Al2O3
MgO
66 65.65 65.33 65 60
33 32.85 32.67 32.5 40
1 1 1 1 –
0 0.5 1 1.5 –
Ea (eV)
ε′
tan δ
0.83 1.15 1.16 0.88 1.64 0.54 0.43 1.13
5.37 5.49 2.51 5.74 – 10 62 2.52
0.38 0.7 1.27 0.73 – 0.76 0.56 0.78
Fig. 6. Variation of the exponent parameter s with temperature for PLA glasses doped MgO. Inset shows the variation of σdc with temperature for PLA1 glasses.
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5
observed due to the accumulation of ions. This is explained by the low frequency electric field zone. In high or medium frequency region, the power law is observed and conductivity sharply increases with frequency [71,72]. The exponent s can be expected if the polarizability of involved material depends on the energy barrier for a simple hopping process between two sites [73]. Some reports, involving hopping models, indicate that s varies in the range from 0.6 to 1 and indicate that it depends on temperature and frequencies [74]. In order to identify the conduction mechanism in our glasses systems, the exponent s is plotted as a function of temperature as depicted in Fig. 6. From Fig. 6, it was found that exponent “s”, calculated at higher frequencies, slightly decreases from 0.84 to 0.51 with increasing temperature from 533 K to 568 K. At 533 K, the exponent “s” increases compared with values given at room temperature [19]. Indeed, the thermal agitation increases, which induces a disorientation of the electric dipoles thus the polaronic conduction decreases in favor of the ionic conduction. To explain the temperature dependence of “s”, a theoretical model for ac conductivity has been predicted such as correlated barrier hopping (CBH) model proposed by Elliot et al. [75]. In our work, it is obtained that the exponent “s” parameter, which is calculated at higher frequencies, decreases with rising temperature, confirming the predicted values as CBH model that is the suitable model to characterize the electrical conduction mechanism of our samples. According to this model, the exponent s parameter is written as follows: s ¼ 1−
6K B T WM
ð7Þ
where kB is Boltzmann constant, WM is optical band gap and T the absolute temperature. 3.4. Dielectric properties 3.4.1. Permittivity and loss studies The complex permittivity formalism has been employed to reveal significant information about the chemical and physical behavior of the electrical and dielectric properties, it is expressed as [76,77]: 0
00
ε ðωÞ ¼ ε ðωÞ−j ε ðωÞ ¼
1 0 00 j ω C 0 Z þ jZ
ð8Þ
Fig. 7. The frequency dependence curves at T = 553 K of dielectric constant ε′(ω) (a) and loss factor (b) for PLA glasses doped MgO. Inset shows the variation of ε′(ω) and tan(δ) at different temperatures case of 3 mol% MgO.
00
0
ε ¼−
Z 0 ω C 0 Z 2 þ jZ 00 2
ð9Þ
0
00
ε ¼−
Z 0 ω C 0 Z 2 þ jZ 00 2
ð10Þ
where ε′ and ε″ are the real part known as dielectric constant and the imaginary part known as dielectric loss of the complex permittivity respectively. They depend on frequency. C 0 ¼ ε0eA is the capacitance of the empty cell (ε0 is the permittivity of the vacuum (8.854 · 10−12 F m−1), A is the cross-sectional area of the flat surface of the pellet (78.5 · 10−6 m2) and e is its thickness (0.75 mm). Z′ is the real part of impedance and Z″ is its imaginary part. From values of the dielectric constant, we can determine the loss factor tangent given by [78]: tan δ ¼
Z Z
0
00
00
¼
ε ε0
ð11Þ
The frequency dependence of the dielectric constant (ε′) and loss factor (tan δ) are studied for the PLA glasses doped MgO and shown in Figs. 7(a) and (b). The temperature dependence of ε′ and tan δ (case of PLA3: 1.5 mol% MgO) as a function of logarithmic frequency at several temperatures is depicted in Inset of Fig. 7(a) and (b). It is observed that
ε′ and tan δ increase with the increase in temperature at lower frequencies which is normal behavior in oxide glasses and decreases with the increase in frequency. Similar results are observed for other glass samples. This behavior can be attributed to the polarization effects of the long range hopping of mobile ions with respect to the immobile glass matrix [79]. This increase in ε' and tan δ, in lower frequency and higher temperature region, may be due to the application of the field, which assists electron hopping between two different sites in the glasses. At high temperatures, the jump frequency of the charge carriers becomes large and comparable with the frequency of the applied field. Accordingly, at low frequency, the charge carriers hop easily out of the sites with high free energy barriers. This leads to a net polarization and gives an increase in the dielectric constant and loss factor [80]. In high frequency region, both the dielectric constant and factor loss approach a constant values which results from rapid polarization processes occurring in the glasses under applied field [81]. However, at high frequency, the charge carriers will no longer be able to rotate sufficiently rapid, so their oscillation will begin to lay behind this field resulting in a decrease of dielectric constant and loss factor. At low temperatures, jump frequency of the charge carries becomes smaller than the frequency of the applied field. The low values of dielectric constant and loss factor, at higher frequency, of the samples are important for
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
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I. Jlassi et al. / Journal of Non-Crystalline Solids xxx (2016) xxx–xxx
extending the material applications towards photonic and electro-optic and fundamental use for nonlinear optical materials [82,83]. In Table 2, the dielectric constant (ε′) and the loss factor tan δ are compared in different oxide glasses at room temperature and in the relaxation frequency. 3.4.2. The electrical modulus formalism Electrical modulus data analysis, the complex permittivity can be converted to the complex modulus, 0
00
M ðωÞ ¼ M þ j M ¼
1 ε
ð12Þ
where M′ is the real part of the electric modulus and M″ is the imaginary part of electric modulus, via [84]. 0
00
ð13Þ
00
0
ð14Þ
M ¼ ω C0 Z
M ¼ ω C0 Z
where Z′ and Z″ are the real and imaginary parts of the impedance, respectively. Fig. 8 shows real part of the electric modulus as a function of frequency at several temperatures. At all temperature investigated, M ′(ω) reaches the constant value at high frequencies and it tends to zero at low frequencies suggesting negligible or absent electrode polarization phenomenon [85]. Fig. 9 shows the imaginary part of the electric modulus M″(ω) as a function of frequency at several temperatures. The maximum in the M ″ peak shifts to higher frequency with increasing temperature. The shift in M″ peak maximum corresponds to the conductivity relaxation. This behavior suggests that the dielectric relaxation is thermally activated in which hopping mechanism of carrier charge dominates intrinsically [86]. At frequency above peak maximum M″, the carriers are spatially confined to potential wells, being mobile on short distances making only localized motion within the wells. The M″(ω) parameter shows a slightly asymmetric peak at each temperature, as can be seen in Fig. 8. The frequency range below the peak frequency fp determines the range in which charge carriers are mobile over short distances [87]. Frequency corresponds to peak frequency denoted as fp and M″ associated with it is denoted as M″max. At each temperature the left hand side of relaxation peak indicates the range with which ions drift to long, while the right-hand side indicates that the ions are spatially confined to potential well and free to move within the wells. Frequency fp which corresponds to M″max defined condition ωcτ = 1 where relaxation timeτ ¼ 2π1f . Fig. 10 shows the variation of relaxation time with p
Fig. 8. Variation of electric modulus with frequency and temperature for 0.5 mol% MgO.
Fig. 9. The frequency dependence of M″ at different temperatures for the sample PLA1.
temperature for 0.5 mol% MgO as representative, which follows the relation [87]: τ ¼ τ 0 exp
Eaτ kT
ð15Þ
where τ0 is the pre-exponential factor and Eaτ is the activation energy for the conductivity relaxation. The calculated value of Eaτ, deduced from the slope of the line of Fig. 10 by using Eq. (15), is estimated at 1.08 eV. It is observed that Eaτ and Ea for dc conductivity are quite similar, which indicates that ions have to overcome the same barrier during conduction and relaxation [80]. The similar values of activation energies indicate that the distribution of random energy barriers in glass for charge carriers hopping is isotropic. Moreover, the barrier height differences between long and short distances that charge carriers' travel are not pronounced [88]. In this respect Kang and Choi [89] showed that a relationship exists between the dielectric relaxation and the mode of electrical conductivity through the relaxing species in the region of high temperatures. 4. Conclusions P2O5–Li2O–Al2O3 glass system doped with MgO was prepared via a melt quenching technique. IR spectroscopy, has showed the dependence of the borate structural units on the MgO content. For our system, both PO3− and P\\O\\ units were detected. Nyquist plots show a non4
Fig. 10. Variation of relaxation Ln(τ) with 1000/T for the sample of PLA1 glass system.
Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042
I. Jlassi et al. / Journal of Non-Crystalline Solids xxx (2016) xxx–xxx
Debye relaxation and the equivalent circuit is a parallel resistor, capacitor and constant phase element. These trends of variations in conductivity and activation energy with MgO content reveal the possibility of ionic contribution to the electrical conductivity and the transition of conduction mechanism from predominantly electronic to ionic regimes in our glasses. The dielectric constant decreases with increasing frequency. The increase of the dielectric constant with the decrease in frequency may be attributed to the presence of dipoles. The decrease in dielectric constant with MgO count is due to the decrease in space charge carriers or interfacial polarization. The non-Debye type in the modulus plot expresses the distribution of the relaxation time for mobile ions resulting from the hop in the random energy barriers. These glass systems could be a suitable host for the optical amplifiers from the point of view of good ac electric conductivity and high dielectric constant. These features allow our new glass to be a good candidate in the applications of nonlinear optics. References [1] Shixun Dai, Akira Sugiyama, Lili Hu, Zhuping Liu, Guosong Huang, Zhonghong Jiang, J. Non-Cryst. Solids 311 (2002) 138–144. [2] L.M. Sharaf El-Deen, M.S. Al Salhi, M.M. Elkholy, J. Non-Cryst. Solids 354 (2008) 3762–3766. [3] O. Cozar, D.A. Magdas, I. Ardelean, J. Non-Cryst. Solids 354 (2008) 1032–1035. [4] M. Nagarjuna, P. Raghava Rao, Y. Gandhi, V. Ravikumar, N. Veeraiah, Physica B 405 (2010) 668–677. [5] P. Srinivasa Rao, S. Bala Murali Krishna, S. Yusub, P. Ramesh Babu, C.H. Tirupataiah, D. Krishna Rao, J. Mol. Struct. 1036 (2013) 452–463. [6] J. Saienga, S.W. Martin, J. Non-Cryst. Solids 354 (2008) 1475–1486. [7] J. Kolář, T. Wágner, V. Zima, Š. Stehlík, B. Frumarová, L. Beneš, M. Vlček, M. Frumar, S.O. Kasap, J. Non-Cryst. Solids 357 (2011) 2223–2227. [8] F. Berkemeier, M. Shoar Abouzari, G. Schmitz, Appl. Phys. Lett. 90 (2007) 113110. [9] P. Knauth, Solid State Ionics 180 (2009) 911–916. [10] A.C.M. Rodrigues, R. Keding, C. Russel, J. Non-Cryst. Solids 273 (2000) 53. [11] C.-H. Lee, K.H. Joo, J.H. Kim, S.G. Woo, H.-J. Sohn, T. Kang, Y. Park, J.Y. Oh, Solid State Ionics 149 (2002) 59. [12] J. Swenson, R.L. McGreevy, L. Borjesson, J.D. Wicks, Solid State Ionics 105 (1998) 55. [13] A. Hall, S. Adams, J. Swenson, J. Non-Cryst. Solids 352 (2006) 5164. [14] V.B. Sreedhar, C. Basavapoornima, C.K. Jayasankar, J. Rare Earths 32 (10) (2014) 918. [15] Karine Seneschal, Frédéric Smektala, Shibin Jiang, Tao Luo, Bruno Bureau, Jacques Lucas, Nasser Peyghambarian, J. Non-Cryst. Solids 324 (2003) 179–186. [16] R.H. Doremus, Glass Science, John Wiley, New York, 1973 146. [17] G. MuraliKrishna, N. Veeraiah, N. Venkatramaiah, R. Venkatesan, J. Alloys Compd. 450 (2008) 486–493. [18] G. Sahaya Baskaran, M.V. Ramana Reddy, D. Krishna Rao, N. Veeraiah, Solid State Commun. 145 (2008) 401–406. [19] I. Jlassi, N. Sdiri, H. Elhouichet, M. Ferid, J. Alloys Compd. 645 (2015) 125–130. [20] S.W. Yung, S.M. Hsu, C.C. Chang, K.L. Hsu, T.S. Chin, H.I. Hsiang, Y.S. Lai, J. Non-Cryst. Solids 357 (2011) 1328–1334. [21] F.H. ElBatal, S. Ibrahim, A.M. Abdelghany, J. Mol. Struct. 1030 (2012) 107–112. [22] Y.M. Lai, X.F. Liang, S.Y. Yang, J.X. Wang, L.H. Cao, B. Dai, J. Mol. Struct. 992 (2011) 84–88. [23] Yuanming Lai, Xiaofeng Liang, Shiyuan Yang, Pei Liu, Yiming Zeng, Changyi Hu, J. Alloys Compd. 617 (2014) 597–601. [24] R.K. Brow, D.R. Tallant, S.T. Myers, C.C. Phifer, J. Non-Cryst. Solids 191 (1995) 45–55. [25] K. Meyer, J. Non-Cryst. Solids 209 (1997) 227–239. [26] R.K. Brow, D.R. Tallant, J.J. Hugdens, S.W. Martin, A.D. Irwin, J. Non-Cryst. Solids 177 (1994) 221–228. [27] Liyan Zhang, Hongtao Sun, Shiqing Xu, Junjie Zhang, Lili Hu, Physica B 367 (2005) 1–5. [28] R.F. Bartholomew, J. Non-Cryst. Solids 7 (1972) 221–235. [29] H.S. Liu, T.S. Chin, S.W. Yung, Mater. Chem. Phys. 50 (1997) 1–10. [30] J.O. Buyn, B.H. Kim, K.S. Hong, H.J. Jung, S.W. Lee, A.A. Izyneev, J. Non-Cryst. Solids 190 (1995) 288–295. [31] P.Y. Shih, Mater. Chem. Phys. 80 (2003) 299–304. [32] P. Pascuta, G. Borodi, A. Popa, V. Dan, E. Culea, Mater. Chem. Phys. 123 (2010) 767–771. [33] B.C. Sales, J.U. Otaigbe, G.H. Beall, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 226 (1998) 287–293. [34] T. Minami, J.D. Mackenzie, J. Am. Ceram. Soc. 60 (1977) 232–235. [35] B.C. Sales, L.A. Boatner, J.O. Ramey, J. Non-Cryst. Solids 263&264 (2000) 155–166.
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Please cite this article as: I. Jlassi, et al., Electrical conductivity and dielectric properties of MgO doped lithium phosphate glasses, J. Non-Cryst. Solids (2016), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.042