Electronic conductivity in zinc iron phosphate glasses

Available online at www.sciencedirect.com

Journal of Non-Crystalline Solids 353 (2007) 4395–4399 www.elsevier.com/locate/jnoncrysol

Electronic conductivity in zinc iron phosphate glasses V. Licˇina

a,*

, A. Mogusˇ-Milankovic´ a, S.T. Reis b, D.E. Day

b

Rud-er Bosˇkovic´ Institute, NMR Center, Bijenicˇka c. 54, 10000 Zagreb, Croatia University of Missouri-Rolla, Materials Research Center, Rolla, MO 65401, USA

a b

Available online 25 September 2007

Abstract The electrical properties of (40x)ZnO–xFe2O3–60P2O5 (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe2O3 content and Fe(II)/Fetot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe2O3 content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe2O3 content from 10 to 30 mol%.  2007 Elsevier B.V. All rights reserved. PACS: 72.80.Ng; 78.30.j Keywords: Conductivity; Raman spectroscopy; Phosphates

1. Introduction In the recent years interest for preparation, theoretical and experimental research of phosphate glasses has been increased. Because of high thermal expansion coefficient, low melting temperature and high ultra-violet transmission, phosphate glasses show several advantages over conventional silicate and borate glasses [1,2]. On the other hand, a limitation of their usefulness is a relatively poor chemical durability. However, it was shown that chemical durability of phosphate glasses can be enhanced by the addition of oxides such as Al2O3 and especially Fe2O3 [3,4]. High chemical durability of iron phosphate glasses is attributed to the replacement of P–O–P bonds by more moisture resistant P–O–Fe bonds [5]. As a result, iron

*

Corresponding author. Tel.: +385 1 4561 111; fax: +385 1 4680 085. E-mail address: [email protected] (V. Licˇina).

0022-3093/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.04.045

phosphate glasses are of the great interest for several technological and biological applications [6]. Binary iron phosphate glasses are electronically conducting glasses with polaronic conducting mechanism [7– 9]. In these glasses iron exists in two valence states and electrical conduction occurs by hopping of polaron from Fe(II) to Fe(III) sites. In the process of hopping, the electron disorders its surrounding, by moving neighboring atoms from their equilibrium positions causing structural defect in the glass network named small polaron. In other words, small polarons are charge carriers trapped by self-induced lattice distortion. Transport of polarons occurs via phononassisted hopping. The binary zinc phosphate glasses are interesting because of their unusual changes in the correlation between structure and physical properties at the metaphosphate composition (ZnO/P2O5 = 1). The anomalous property behavior of these glasses has been attributed to a transition in average Zn–O coordination number from 6 to 4 [1,10].

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Zinc iron phosphate glasses have a chemical durability comparable to soda lime and borosilicate, which makes them potential matrix host for the vitrification of radioactive waste [4,11,12]. In this paper the electrical properties of iron phosphate glasses containing different amounts of ZnO have been investigated by impedance spectroscopy. The purpose was to study conduction mechanism of the different glass compositions and the effect of ZnO on electrical properties of zinc iron phosphate glasses. Several efforts have been made in scaling conductivity spectra into single master curve [13– 15]. The intention was to discuss the procedure of scaling ac data at various temperatures and to find the relation between structural and electrical properties of zinc iron phosphate glass. 2. Experimental 2.1. Preparation of glass samples The ZnO–Fe2O3–P2O5 glasses were prepared from appropriate mixture of reagent grade NH4H2PO4, Fe2O3, ZnO and melted at 1373 K for 2 h in air in high purity alumina crucibles. The melt was quenched in air by pouring it into a steel mould to form rectangular bars (1 · 1 · 5 cm3), which were annealed for 3 h at 723 K. The density of the glasses was measured at room temperature by Archimedes method in water as buoyancy liquid and data are listed in Table 1. The estimated error is ±0.02 g cm3. 2.2. Mo¨ssbauer and Raman spectra The Mo¨ssbauer spectra were measured at room temperature in spectrometer (ASA 600), which utilized a room temperature 50 mCi cobalt-57 source embedded in a rhodium matrix. The spectrometer was calibrated at 296 K with a-iron foil. The Mo¨ssbauer absorbers of approximate thickness of 140 mg cm2 were prepared using 125 lm powders. The Mo¨ssbauer spectra were fitted with broadened paramagnetic Lorentzian doublets [11]. The Raman spectra of glasses were obtained using 200 mW of 514.5 nm light from a coherent argon-ion laser model (Innova 100) and were recorded with computerized triple monochromator Dilor model Z 24. A 90 scattering

geometry was used with the sample oriented at a nearglancing angle. The recorded Raman spectra were deconvoluted using a symmetric Gaussian function. The structural units in phosphate network were classified according to their connectivities by Qn notation, where n represents the number of bridging oxygen atoms per PO4 tetrahedron (n = 0–3). 2.3. Electrical measurements Samples for electrical properties measurements were cut from annealed bars and polished with 600-grit-polish paper. Gold electrodes, 6 or 7 mm in diameter, were coated onto both sides of 1 mm thick discs cut from the glass bars using Sputter coater SC7620. The samples were stored in a desiccator until the electrical conductivity measurements were performed. Electrical properties were obtained by measuring complex impedance using a home made impedance analyzer over a wide frequency range from 0.01 Hz to 4 MHz and in temperature range from 303 to 473 K. The temperature was controlled to ±1 K. The conductivity r(x) was calculated from the impedance measurements using the equation r(x) = G(x)d/S, where G(x) is the conductance and S and d are the sample area and thickness [4]. 3. Results 3.1. Raman spectra Raman spectra for the ZnO–Fe2O3–P2O5 glasses are shown in Fig. 1. The deconvolution of the Raman spectrum for the Zn3 glass in the spectral region 600– 1500 cm1 is shown in Fig. 1(a). According to the literature [6,16], the band assignment for the glass containing 30 mol% ZnO are characteristic for metaphosphate structure with the most prominent bands at 1205 and 705 cm1 related to the symmetric stretching mode of non-bridging (PO2)sym and bridging (P–O–P)sym oxygen atoms in Q2 units. Some barely detectable bands at 1080 and 748 cm1 are attributed to the non bridging, (PO2)sym and bridging oxygen atoms (P–O–P)sym in pyrophosphate Q1 phosphate tetrahedra. The barely detectable shoulder at 940 cm1 suggests the very small presence of orthophosphate Q0 phosphate units. The band at 1284 cm1 is related to the phosphorus oxygen double

Table 1 Composition and selected properties of the zinc iron phosphate glasses Sample code

Zn1 Zn2 Zn3 a b

Glass composition (mol%) ZnO

Fe2O3

P2O5

30 20 10

10 20 30

60 60 60

Fe(II)/Fetota (±0.3%)

D (g cm3) (±0.02 g cm3)

˚) R (Fe–Fe) (A (±0.5%)

rdcb (X m)1 at 473 K (±0.5%)

Edc (kJ mol1) (±0.5%)

6 12 17

2.94 2.97 3.01

4.76 3.76 3.27

4.7 · 109 1.7 · 107 1,4 · 106

68.2 61.9 58.3

Fe(II)/Fetot ratio was calculated from the Mo¨ssbauer spectra. Obtained from conductivity plateau.

V. Licˇina et al. / Journal of Non-Crystalline Solids 353 (2007) 4395–4399

Relative intensity

10 mol% of ZnO is shown in Fig. 2. The conductivity spectra display the typical shape found for electronically conducting glasses [17]. At low frequency region the conductivity is independent of frequency and identical to dc conductivity, rdc, of glasses. However, at higher frequency the conductivity obeys a power-low, rac(x) = Axs, where A is a temperature-dependent constant and s is an exponent [18]. Thus, the total conductivity can be given as r(x) = rdc + Axs. The same shape of the conductivity spectra has been obtained for all glasses in the present study. Since the shape of conductivity, r(x), does not depend on temperature it is possible to superimpose these curves using the so-called Summerfield scaling. In this scaling approach, the conductivity axis is normalized by rdc and the frequency axis is normalized by product rdcT as proposed by Roling [13]. The master curves of zinc iron phosphate glasses with different glass compositions are shown in Fig. 3.

Zn3

1600

1400

1200

1000

800

600

-1

Raman shift (cm ) 1080 616 512

1205 940

705 748

-2

Zn3

Zn3

-3 -4 Zn2

-1

log (σ(ω) / (Ω m) )

Relative intensity

1284

4397

Zn1 1600 1400 1200 1000

800

600

400

200

-5 473 K

-6

423 K 393 K

-7

363 K

-8

0

333 K -9

-1

Raman shift (cm )

bond. As indicated by the literature [1] stretching mode of P@O band can be seen at 1390 cm1 in Q3 units. However, the Q3 units were not found in these glasses. So, the presence of the mentioned band, displaced at 1284 cm1, can be explained by decrease in the average p-character of P@O bond in PO4 units caused by depolymerization of the phosphate network. The band that appears at low frequency (512 cm1) may be attributed to the overlapping vibrations of zinc oxygen tetrahedral and PO3 4 groups. The replacement of ZnO with Fe2O3 leads to significant changes in Raman spectra inducing the change of structure from metaphosphate to pyrophosphate. Bands characteristic for pyrophosphate structure become larger, while a typical metaphosphate bands decrease. With increasing Fe2O3 content, from 10 mol% to 30 mol%, the spectra show a shoulder around 616 cm1. This shoulder may be due to vibrations of some iron-oxide bonds. 3.2. Electrical conductivity The frequency dependence of the total conductivity, r(x), at various temperatures for the glass containing

-2

-1

0

1

2 3 log (f / Hz)

4

5

6

7

Fig. 2. Frequency dependence of the conductivity, r(x), at temperatures shown for the glass containing 30 mol% of Fe2O3.

10

Zn1

Zn1 Zn2 Zn3

9 8

Zn2 Zn3

7 log (σ (ω) / σdc)

Fig. 1. The Raman spectra of the zinc iron phosphate glasses.

303 K

-10

6 5 4 3 2 1 0 0

2

4

6 8 10 12 -1 log ((f / σdc T) / Hz Ω m K )

14

16

Fig. 3. Summerfield scaling of conductivity for zinc iron phosphate glasses.

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However, it should be noted that the position of master curves differs on the X axis, i.e. in higher frequency region. The temperature dependence of the electronic conductivity for amorphous semiconductor containing transition metal ions, such as Fe(II) and Fe(III), is usually expressed by Austin–Mott equation [9,17]: r¼

mel e2 Cð1  CÞ expð2aRÞ expðW =kT Þ; kTR

ð1Þ

where mel is the electronic frequency, C is the fraction of reduced transition metal ion, Fe(II)/Fetot, R is the average spacing between transition metal ions, a is the tunnelling factor (rate of the wave function decay), e is the electronic charge, k is the Boltzmann constant, T is the absolute temperature and W is the activation energy for the hopping conduction. Eq. (1) describes a non-adiabatic regime of small polaron hopping. In Mott’s theory, the mechanism of electron transport in amorphous semiconductors depends on temperature. In the high temperature region (T > H/2), where H is the Debye temperature, the conduction mechanism is considered as phonon-assisted hopping of small polaron (SPH) between localized sites [8]. In this temperature region the jump of an electron (polaron) occurs between nearest neighbors with activation energy W = WH + Wd/2. WH is the polaron hopping energy and Wd is the energy difference between two neighboring ions. Since W and R in Eq. (1) are constant, the dependence of the dc conductivity on the temperature can be compared with Arrhenius equation rdc = r0exp(Edc/kT), where rdc is the dc conductivity, r0 is the pre-exponential factor, Edc is the activation energy for the dc conductivity, k is the Boltzmann constant and T is the temperature (K). According to this equation, activation energy for dc conductivity, Edc, was calculated from the slope of log rdc vs. 1/T curve. In Table 1 are listed values for the dc conductivity, rdc, at 473 K obtained from conductivity plateau and activation energy for dc conductivity, Edc. With increasing iron-oxide content from 10 to 30 mol% the rdc increases from 4.7 · 109 to 1.4 · 106 (X m)1, whereas Edc decreases from 68.2 to 58.3 kJ mol1. Going further in the interpretation of results, the dependence of rdc and Fe(II)/Fetot, calculated from Mo¨ssbauer spectra, upon the distance between Fe ions, RFe–Fe is presented in Table 1 as well. The average distance between the iron ions, RFe–Fe, was calculated using the relation:  RFe–Fe ¼

4pN 3

13 ;

ð2Þ

where N is the concentration of total iron ions (both Fe(II) and Fe(III) ions) per unit volume, calculated from the glass composition and density. It is shown that the rdc increases with decreasing distance between Fe ions.

4. Discussion Several factors affect the dc conductivity of these glasses. The increase of rdc from 4.7 · 109 to 1.4 · 106 (X m)1 with increasing Fe2O3 content from 10 to 30 mol% indicates that electrical conductivity in these glasses is dependent upon Fe2O3 content. Also, the concentration of Fe(II) ions, determined from the Mo¨ssbauer spectra for each glass, strongly affects the dc conductivity. Since the ratio Fe(II)/Fetot has the lowest value, 6%, for Zn1 glass, consequently, the concentration of polarons is the smallest. The increase in Fe(II) ion concentration from 6% to 17%, for Zn1 and Zn3 glasses, respectively, results in an increase in dc conductivity and a decrease in activation energy. These results agree with the studies [4,7,9,17], where ratio Fe(II)/Fetot, Fe2O3 content and distance between iron ions were found to be the important factors for electrical conductivity of these glasses. Since the dc conductivity is associated with the migration of polaron between Fe(II) and Fe(III) ions with the distribution in hopping distance it seems that increase in Fe2O3 content from 10 to 30 mol% decreases the distance between Fe ions, RFe–Fe. Such decrease in RFe–Fe enhances the probability of electron hopping and increases the possibility of polaron formation [17]. Going further in the interpretation of results, it is well known that electrical conductivity in semi-conducting materials is thermally activated process. In other words, electrical conductivity increases with increasing the temperature. The temperature-dependent conductivity spectra of a given glass can then be superimposed onto one master curve upon the application of one of several known scaling approaches [19–22]. By using the Summerfield scaling for electrical conductivity we obtained single master curve for each glass. These results are implying that the only effect of the temperature is to speed up or slow down the hopping process, leaving the conducting mechanism and number of mobile charge carriers unchanged [23]. On the other hand, one master curve for all three compositions was not observed. The rise of r(x)/rdc from the plateau becomes slower with decreasing polaron concentration, which creates small deviation in the crossover regime from dc to dispersive conductivity. Sidebottom [24] showed that the ac conductivity for alkali phosphate glasses is influenced by the glass structure. It was proposed that oxide glasses possess some maximal dimensionality of conduction space associated with a random disordered glassy network [24]. So, it is possible that the change from metaphosphate to pyrophosphate structure with increasing Fe2O3 content, Fig. 1, influences the mobility of the charge carriers and causes the deviation in the scaled data. Thus, the hopping process is affected by structural changes of glasses. Considering small polarons as charge carriers trapped by self-induced lattice distortion it seems that the conductivity process is strongly coupled with the structural changes.

V. Licˇina et al. / Journal of Non-Crystalline Solids 353 (2007) 4395–4399

Therefore, this leads to the conclusion that the dissimilarities in the scaled data of the glasses with different compositions, seen at high frequencies, are related to the structural changes in the glasses. 5. Conclusion The changes in the electrical conductivity of the zinc iron phosphate glasses show that the electrical conductivity is dependent upon Fe2O3 content and strongly controlled with polaron concentration which depends on the Fe(II)/ Fetot ratio. By applying Summerfield scaling procedure the master curve of conductivity data was obtained for each glass. On the other hand, deviation from one master curve for glasses with different compositions was observed at high frequency region. Possible explanation for the divergences in the scaled data for these glasses can be found in the conversion of the metaphosphate to pyrophosphate structure by changing the glass composition, as it was observed by Raman spectroscopy. The electrical conductivity, i.e. transport of the polarons, is strongly affected by the structural changes. This means that the mobility of the charge carriers is various in different glass structures. Acknowledgments The work was financially supported by the Croatian Ministry of Science, Education and Sports from the project ‘‘Influence of structure on electrical properties of (bioactive) glasses and ceramics’’ (098-0982929-2916). The authors acknowledge Dr K. Furic´ (Rud-er Bosˇkovic´ Institute, Molecular Physics Laboratory) for performing Raman spectroscopy measurements. Kindest thanks to Dr A. Sˇantic´ (Rud-er Bosˇkovic´ Institute, NMR Center) for unselfish help.

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References [1] R.K. Brow, J. Non-Cryst. Solids 263&264 (2000) 1. [2] Y.M. Moustafa, A. El-Adawy, Phys. Status Solidi (a) 179 (2000) 83. [3] A. Mogusˇ-Milankovic´, A. Gajovic´, A. Sˇantic´, D.E. Day, J. NonCryst. Solids 289 (2001) 204. [4] A. Mogusˇ-Milankovic´, A. Sˇantic´, S.T. Reis, K. Furic´, D.E. Day, J. Non-Cryst. Solids 351 (2005) 3246. [5] X. Yu, D.E. Day, G.J. Long, R.K. Brow, J. Non-Cryst. Solids 215 (1997) 21. [6] T. Jermoumi, M. Hafid, N. Niegisch, M. Mennig, A. Sabir, N. Toreis, Mater. Res. Bull. 37 (2002) 49. [7] L. Murawski, C.H. Chung, J.D. Mackenzie, J. Non-Cryst. Solids 32 (1979) 91. [8] N.F. Mott, J. Non-Cryst. Solids 1 (1968) 1. [9] L. Murawski, J. Mater. Sci. 17 (1982) 2155. [10] K. Suzuya, K. Itoh, A. Kajinami, C.K. Loong, J. Non-Cryst. Solids 345&346 (2004) 80. [11] S.T. Reis, M. Karabulut, D.E. Day, J. Non-Cryst. Solids 292 (2001) 150. [12] A. Sˇantic´, A. Mogusˇ-Milankovic´, K. Furic´, V. Bermanec, C.W. Kim, D.E. Day, J. Non-Cryst. Solids 353 (2007) 1070. [13] B. Roling, A. Happe, K. Funke, M.D. Ingram, Phys. Rev. Lett. 78 (1997) 2160. [14] D.L. Sidebottom, P.F. Green, R.K. Brow, Phys. Rev. Lett. 74 (1995) 5068. [15] L. Murawski, R.J. Barczynski, Solid State Ionics 176 (2005) 2145. [16] B. Tischendorf, J.O. Otaigbe, J.W. Wiench, M. Pruski, B.C. Sales, J. Non-Cryst. Solids 282 (2001) 147. [17] A. Mogusˇ-Milankovic´, A. Sˇantic´, V. Licˇina, D.E. Day, J. Non-Cryst. Solids 351 (2005) 3235. [18] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983, p. 89. [19] B. Roling, C. Martiny, Phys. Rev. Lett. 85 (2000) 1274. [20] B. Roling, C. Martiny, K. Funke, J. Non-Cryst. Solids 249 (1999) 201. [21] D.L. Sidebottom, Phys. Rev. Lett. 82 (1999) 3653. [22] T.B. Schroder, J.C. Dyre, Phys. Rev. Lett. 84 (2000) 310. [23] K. Funke, R.D. Banhatti, Solid State Ionics 177 (2006) 1551. [24] D.L. Sidebottom, J. Phys.: Condens. Matter 15 (2003) S1585.