Ion conducting phosphate glassy materials

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Progress in Crystal Growth and Characterization of Materials 55 (2009) 47e62 www.elsevier.com/locate/pcrysgrow

Ion conducting phosphate glassy materials S.S. Das a,*, N.P. Singh b, P.K. Srivastava b a

Chemistry Department, D.D.U. Gorakhpur University, Gorakhpur 273 009, India b Chemistry Department, U.P. Autonomous College, Varanasi, India

Abstract Fast ion conducting (FIC) phosphate glasses have become very important due to a wide range of applications in solid-state devices. We present an overview on silver based fast ion conducting phosphate glasses. Silver phosphate glasses containing chlorides of some metals viz; Li, Na, Mg, Pb and Cu [Ag2Oe P2O5exMCly, where x ¼ 0, 1, 5, 10 and 15 wt% and y ¼ 1 when M ¼ Li or Na and y ¼ 2 when M ¼ Mg, Pb or Cu] have been synthesized by melt quenching technique. Studies on these glassy materials characterized by X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetric techniques and ion transport measurements are presented. The FT-IR studies support the formation of PeOeM linkages. The values of glass transition temperature (Tg) of the glassy materials containing lithium or sodium chloride have been found to decrease with increasing dopant concentrations indicating expansion of the glassy network. On the other hand, the Tg values increase with increasing magnesium, lead or copper chloride concentrations in silver phosphate glasses. This indicates an increase in crosselink density and enhanced chemical durability of these glassy materials. Ion transport studies suggest that the values of electrical conductivities of the metal chloride doped glassy materials are higher than those of the undoped ones and, at a particular dopant concentration, the following trend is observed.

sðeLiClÞ  sðeNaClÞ > sðeMgCl2 Þ > sðePbCl2 Þ > sðeCuCl2 Þ These results are supported by the experimental results of FT-IR spectral and thermal studies. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Silver ion conduction; Phosphate glass; Ion conducting glasses; Electrical conductivity; Lithium phosphate glass

* Corresponding author. E-mail address: [email protected] (S.S. Das). 0960-8974/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2009.07.001

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1. Introduction In recent years, studies on fast ion conducting (FIC) glasses [1] have gained considerable attention due to their potential applications in various solid state devices. These glasses have several distinct advantages over other solid electrolytes. They have higher conductivity, better isotropic properties, absence of grain boundaries and good workability. They can be prepared over a wide composition range and hence their properties could easily be controlled. Besides these, the glasses are inert to atmosphere, possess good thermal stability and high ionic conductivity [2,3]. The electronic conductivity in glasses is negligible and as a result electronic leakage does not occur in electrochemical devices on using glass membrane as an ionic separator. Glassy electrolytes can be vacuum deposited, sputtered on various electrode supports in the form of thin films or pelleted from the powdered material as thick layers in order to suit the shape needed for the appropriate type of battery technology. Very good electrodeeelectrolyte contacts can be obtained by using glasses with low glass transition temperature. They may thus be used as solid electrolytes in solid state batteries and electrochemical devices. FIC glasses are classified into two main groups viz; (i) cation conducting and (ii) anion conducting. However, the cation conducting glasses have been given greater importance and a number of Li þ, Naþ, Cuþ and Agþ conducting glasses have been investigated. Anion conducting glasses (mainly F ion conducting glasses) are of potential interest because of their optical properties. The cation conducting glasses could further be sub classified into two groups depending whether the glass formers and modifiers are oxide, or sulphides. The most widely studied oxide glasses are mainly silicates, phosphates, borates or vanadates glasses with SiO2, P2O5, B2O3 or V2O5 respectively as glass formers [4]. Among different oxide glasses, ion conducting phosphate glasses [5e8] are practically interesting due to their several unusual properties like high thermal expansion coefficient and low glass transition and softening temperatures [9]. Phosphate glasses have several technological applications. They can be used for making solid state electrolytes in solid state batteries, glass ceramics, amorphous semi conductors, laser glasses and optoelectronic devices [10e13]. They exhibit some of the highest conductivities ever reported [14,15] and have been studied more than any other class of ionically conducting glasses due to their simple composition and strong glass forming character. The ionic conductivity in phosphate glasses were investigated first in the simple binary phosphate glasses having composition xM2O þ (1  x) P2O5 where M may be an alkali (lithium/sodium), silver or copper. The ionic conductivities of various binary phosphate glasses having compositions xLi2O þ (1  x) P2O5, xNa2O þ (1  x) P2O5 and xAg2O þ (1  x) P2O5 systems were studied by Bartholomew [16]. Binary magnesium phosphate glasses were also studied [17]. Numerous attempts were then made to increase the conductivity of these glasses by the addition of halides of the metal (MX), glass modifiers (M2O) or an oxy salts of the metal such as M2SO4 as dopants [18e21] and the doped glasses were named as ternary phosphate glasses. It has been found that the ionic conductivity of these (doped) ternary phosphate glasses increases greatly on the addition of dopant salts. The overall trends in Liþ and Naþ ion ternary conducting glasses were found to be nearly similar [2,22,23]. In the last three decades several attempts [24e26] have been made to develop Liþ and Naþ fast ion conducting glasses because of their application in high energy density batteries [23,27e33]. It is reported that Liþ borates doped with lithium halides [28,29,34] or silicates with Li2SO4 [30,31] are best ion conductors. Their ionic conductivities have been found in the range 103e102 S cm1 at 300  C which is

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slightly higher than the Naþ ion conductors. Naþ ion conducting glasses are important because of their use in sodiumesulphur batteries [22,35e38]. In Agþ ion conducting phosphate glasses, the ionic conductivity exhibited by AgI þ Ag2MO4 (where M is As or P) glassy systems [39] were comparable to RbAg4I5, one of the best known crystalline ionic conductors. After then conductivities of AgI þ Ag2O þ P2O5 glasses [40] were found to be as high as 2  102 S cm1 at 25  C. Agþ ion conductivity in PbI doped silver phosphate glasses are also reported in the literature [41]. Silver ion conducting glasses are also interesting from an academic point of view and can be used as model systems for the development of solid state batteries [42]. The ionic conductivities of Cuþ ion conducting phosphate glasses [43,44] were found to be comparable to those of Agþ ion conducting phosphate glasses. In the family of ternary phosphate glass, lead phosphate glasses were also prepared, characterized and studied [45]. The concentration dependent electrical conductivity of phosphate glasses containing zinc oxide and cadmium oxide have also been studied [46]. Ternary and quaternary mixed glass former phosphate glasses in which a co-glass former such as B2O3, Al2O3, SiO2, Ta2O5, V2O5, etc. were studied by several workers [47e53]. It has been reported that the conductivities could be increased to a large extent by the addition of another glass former into the phosphate glass matrix. In the recent years, a large number of phosphate glasses such as lanthanumealuminume phosphate glasses [54], ironesodium phosphate glasses [55,56], zinc phosphate glass with Mn(II) and Co(II) ions [57], barium phosphate glasses doped with transition metal ions of Co, Fe and Mn [58], erbium doped sodium phosphate glasses [59], electrical properties of silver iodide doped silver phosphate glass [60], electrical properties of zinc iron phosphate glasses [61], vanado phosphate glasses with sodium and sodium potassium [62], lithium iron phosphate glasses [63], Mg2þ and Pb2þ ion conducting glasses [17,64] have been studied and their structure, properties, conductivities and dielectric behaviour have been ascertained. Literature reveals that the main emphasis has been given to alkali ion (Liþ or Naþ), silver ion (Agþ) and copper ion (Cuþ) based ionically conducting phosphate glasses by the electrochemical research communities [8,63,65e67]. But, the Naþ and Agþ ion conducting phosphate glasses have been studied in detail [65,66,68,69] and given greater attention in the preparation of sodium and silver based glass electrolyte in solid-state batteries [10,70e72]. Among the Naþ and Agþ fast ion conducting glasses, the Agþ ion conducting phosphate glassy materials [10,66,69] are more interesting and important as they can be easily prepared in different forms and show high ionic conductivity and high stability at ambient temperatures. It has been reported that ionic conductivities and many other dynamic properties change when another mobile ion is added in the matrix of a particular ion conducting glass [73]. The addition of alkali, alkaline earth and transition metal ions into the glass matrix makes the doped phosphate glasses relatively more stable and durable [62,74]. Also the addition of lead ions in many oxide glasses plays a unique role in the modification of glass network structure and its properties [75]. While looking for a good and chemically durable fast ion conducting glassy systems, it was considered worthwhile to add some alkali (Li and Na), alkaline earth (Mg), transition metal (Cu) and Pb ions into the matrix of silver ion conducting phosphate glass and study their ionic transport behaviour in a systematic manner. Addition of Li, Na, Mg, Pb or Cu ion to silver phosphate glass is expected to modify its electrical conductivity in such as way as to make it extremely useful in the fabrication of solid state batteries.

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The present paper reports an overview on the synthesis, characterization and ion transport studies on silver phosphate glass doped with different concentrations of chlorides of Li, Na, Mg, Pb and Cu. The control sample of undoped silver ion conducting silver phosphate glass has been also prepared to study the effect of the dopant metal cations on its properties and electrical conductivities values. These glasses were characterized by FT-IR spectra and DSC extensively. 2. Experimental methods for preparation and characterization 2.1. Preparation of glasses All the chemicals used were of analytical reagent grade. AgNO3, NH4H2PO4 and chlorides of Li, Na, Mg, Pb and Cu (Merck) were used as such without any further purification. Silver phosphate glasses both undoped and doped with chlorides of Li, Na, Mg, Pb and Cu were prepared by employing the melt quenching technique in a way similar to that described by Das et al. earlier [65,66]. In the case of silver phosphate glass, a mixture of AgNO3 with NH4H2PO4 in 1:1 molar ratio was heated in a platinum dish in a muffle furnace (pre heated to the desired temperature in the range 700e800  C). The melt was then quenched over an ice cooled stainless steel plate to obtain glassy samples. For preparing metal (Li, Na, Mg, Pb and Cu) chloride doped silver phosphate glasses, appropriate amounts of metal chlorides in terms of weight percents such as 1, 5, 10 and 15% were added to 1:1 molar ratio of AgNO3 and NH4H2PO4. In all the cases, the mixtures were first heated slowly in an oven to 150e200  C taking care that the material did not spurt out in the initial stage, when the frothing took place due to brisk evolution of ammonia, oxides of nitrogen and water vapour. When the frothing subsided, the platinum dishes were placed in the muffle furnace for a period of 4 h before quenching. Mixtures of both undoped and metal chloride doped glasses were heated in a muffle furnace side by side in two platinum dishes and their melts were quenched almost simultaneously so that similar conditions of preparation could be maintained. After drying in an oven at 100  C, the glassy samples were stored in glass tubes kept in desiccators. 2.2. X-ray diffraction studies X-ray diffraction studies have been used extensively to ascertain the amorphous nature of glassy materials. In our studies, the X-ray diffractogram of the powdered samples were recorded on a Philips PW 3020 X-ray powder diffractometer using Cu Ka radiation. 2.3. Spectral studies FT-IR has been a powerful tool to characterize phosphate glasses. We used a FT-IR spectra SHIMADZU 8201 PC FT-IR spectrometer in the range 4000e450 cm1 using KBr pellets for phosphate glasses. 2.4. Thermal studies Differential scanning calorimetry (DSC) has been used for thermal characterization. The DSC experiments on all synthesized glassy samples were carried out with the help of a Mettler Toledo Star DSC instrument with a heating rate of 5  C per minute in nitrogen atmosphere.

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2.5. Transference number measurements The transport properties were measured to estimate the suitability of materials for use in various devices. Wagner’s polarization method [76] was used by our group to determine the ionic transference number of all the samples by employing the cell configuration Agþ/glasses/ C. Silver paste (Eltecks India) was coated on one face of the glass pellet to act as a reversible electrode. The other face of the pellet was then coated with graphite paste, which serves as the irreversible electrode. A constant dc polarization voltage was applied across the sample pellets. The current was monitored as a function of time after the application of polarizing voltage and the experiments were performed in a way similar to that reported earlier [66]. The transference number, tion, was then determined with the help of Eq. (1); tion ¼

IT  Ie IT

ð1Þ

where IT is the initial current and Ie is the residual current after polarization. The transference number values are given in Table 2. 2.6. Electrical conductivity measurements Electrical conductivity is a very important property of phosphate glasses for their use in solid state devices. For determining the values of the electrical conductivities of all the glassy samples, we made measurements at room temperature with the help of a HIOKI 3520-01 LCR Tester at different frequencies viz. 400 Hz, 1 KHz, 2 KHz, 4 KHz, 10 KHz, 20 KHz, 40 KHz, 100 KHz and 150 KHz in a manner similar to that reported earlier [66]. A pellet of the sample with the help of a die was prepared by giving a pressure of 5 t by a hydraulic press machine. The thickness of the pellet was typically measured with the help of a screw gauge. Silver epoxy was pasted on both sides of the pellet and the pellet was allowed to dry completely. Before the electrical impedance measurements the variation of conductivity with temperature was studied in the range 303e473 K for all the glassy systems at a frequency of 100 KHz. The temperature was monitored by a temperature monitor model e SE-202 (Sonit). The electrical conductivities of the glasses were obtained with the help of Eq. (2); s¼

Gl A

ð2Þ

where G is the conductance, l is the thickness and A is the area of cross-section of the sample pellet. 3. Results and discussion The X-ray diffraction patterns of all these phosphate glasses show broad and diffused humps which indicate the amorphous nature of all the synthesized glassy materials. Sometimes fast cooling is also required to achieve homogeneous materials. The FT-IR spectral studies of the doped and undoped silver phosphate glasses yielded nearly similar spectra. Fig. 1 shows a typical spectra of Ag2OeP2O5 glassy systems doped with different metal chlorides in the frequency range 2000e450 cm1. The comparison of spectral bands suggests that all the characteristic bands found in the spectra of the undoped silver

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Fig. 1. FT-IR spectra of metal chloride doped silver phosphate glasses in the range 450e1500 cm1 (a) Ag2OeP2O5, (b) Ag2OeP2O5e(15%)LiCl, (c) Ag2OeP2O5e(15%) NaCl, (d) Ag2OeP2O5e(5%) MgCl2, (e) Ag2OeP2O5e(5%) PbCl2 and (f) Ag2OeP2O5e(5%) CuCl2.

phosphate glass are also present in the metal chloride doped glasses with very slight shift (5e25 cm1) in their peak frequencies. The corresponding band assignments of the IR spectra are listed in Table 1. The absorption bands in all the IR spectra of the glasses appearing in the range 1633e 1599 cm1 may be attributed to the d (H2O) in e plane bending vibrational modes of physisorbed water molecules [77,78]. The absorption bands appearing in the range 1347e 1351 cm1 in all the metal chloride doped glasses may be attributed to the stretching mode of P]O double bonds [79,80]. However, in the undoped glass, a weak shoulder is present at w1390 cm1. The strong intense bands in the region 1250e1278 cm1 can be ascribed to the asymmetric stretching of PO2 terminal groups, yasy(PO2) [80,81]. The weak bands appearing at 1122e1145 cm1 are assigned to the PO2 symmetric stretching mode, ysym(PO2) of the two non-bridging oxygens [81,82]. The absorption bands near 1100 cm1 and 1000 cm1 have been assigned to PeO groups, the phosphate non-bridging oxygen portion of PO4 tetrahedra in

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Table 1 Characteristic IR spectral band assignments for the undoped and metal chloride doped silver phosphate glasses in the range 2000e450 cm1. Assignments

Ag2OeP2O5

e(15%) LiCl

e(15%) NaCl

e(5%) MgCl2

e(5%) PbCl2

e(5%) CuCl2

d (OePeO) d (O]PeO) ysym (POP) ysym (POP) yasy (POP) y (PeO) y (PeO) ysym (PO2) yasy (PO2) y (P]O) PeOeH

480 530 720 765 910 970 1086 1143 1250 1390 1633

490 526 714 770 895 965 1080 1140 1260 1351 1599

460 538 690 768 888 970 1103 1130 1278 1347 1599

495 530 714 766 897 1000 1090 1122 1263 1350 1597

480 527 700 767 900 1010 1088 1135 1266 1348 1599

485 528 696 760 901 1020 1090 1145 1265 1348 1600

a chain structure [59]. In the present investigation such bands have been found in the range 1080e1103 cm1 and 965e1020 cm1 respectively confirming the presence of PeO groups in all the glassy systems. The absorption bands appearing in the range 888e910 cm1 could be assigned to the asymmetric stretching modes of the in e chain PeOeP linkages, yasy(PeOeP), while the two bands observed in the range 760e770 cm1 and 690e720 cm1 suggest the presence of symmetric stretching modes of the linear PeOeP groups, ysym (PeOeP) [78,79]. The two weak bands appearing in the range 526e538 cm1 and 460e495 cm1 have been assigned as the bending vibrations of d (O]PeO) and d (OePeO) respectively [59,78]. On the basis of FT-IR spectral studies, it may be inferred that the network of the phosphate glass consists of PO4 tetrahedra with three bridging (PeOeP) and one non-bridging (P]O) oxygen atoms. On the addition of Ag2O into the PO4 network of the phosphate glass, the three dimensional network is converted into linear phosphate chains [78]. The PeOeP bridging oxygens are replaced by PeOe Agþ linkages. When metal chloride is added to the undoped silver phosphate glass, PeOeM bonds (where M ¼ Li, Na, Mg, Pb or Cu) are formed in the glass structure which replace the PeOe Agþ bonds [77]. In the case of monovalent lithium and sodium chloride doped silver phosphate glasses, the metal ions (Liþ/Naþ) get attached to the negative ends of the PeO of the phosphate chain. On the other hand, in the case of divalent metal chlorides, Mg2þ, Pb2þ and Cu2þ ions serve as ionic cross - links between the nonbridging oxygens of two phosphate chains of the silver phosphate glass. Glass transition temperature (Tg) values for glasses are usually evaluated by differential scanning calorimetry (DSC). Some results are reported in Table 2. The results show that the Tg values of silver phosphate glasses containing monovalent lithium or sodium chlorides decrease as the concentration of the dopant chloride increases. Whereas, the Tg values show a systematic increase with increasing concentrations in the glasses which contain divalent magnesium, lead or copper chlorides, the following sequence in Tg values is observed. Tg ðeLiClÞ < Tg ðeNaClÞ < Tg ðAg2 OeP2 O5 Þ < Tg ðeMgCl2 Þ < Tg ðePbCl2 Þ < Tg ðeCuCl2 Þ The Tg value of phosphate glass is related to its chain length, cross-link density and bonding strength of the phosphate chains [59]. FT-IR studies have confirmed the formation of PeOeM linkages between two chains of the phosphate glasses in which divalent metal chlorides are

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Table 2 Percent ionic transference number (%ti), conductivity (s), glass transition temperature (Tg) and activation energy (Ea)values for the undoped and metal chloride doped silver phosphate glasses. S. No.

1

Phosphate glasses

Ag2OeP2O5 (undoped)

Conductivity (s) (S/cm)

Activation energy (Ea) (eV)

At room temperature (30  C)

At high temperature (200  C)

Glass transition temperature (Tg) ( C)

95.4

8.51  107

2.69  104

208.8

0.68

6

4

% Ionic transference number (% ti)

2 3 4 5

Ag2OeP2O5e(1%) LiCl e(5%) LiCl e(10%) LiCl e(15%) LiCl

93.3 95.5 96.3 98.3

7.41  10 3.63  105 6.91  105 8.91  10L5

7.39  10 1.86  103 2.57  103 4.16  10L3

204.8 199.2 194.8 184.2

0.51 0.49 0.49 0.47

6 7 8 9

Ag2OeP2O5e(1%) NaCl e(5%) NaCl e(10%) NaCl e(15%) NaCl

93.1 94.4 97.1 98.0

5.01  106 9.33  106 3.38  105 6.45  10L5

3.16  104 6.02  104 2.39  103 4.16  10L3

208.2 201.6 196.1 185.9

0.53 0.50 0.50 0.49

10 11 12 13

Ag2OeP2O5e(1%) MgCl2 e(5%) MgCl2 e(10%) MgCl2 e(15%) MgCl2

93.2 94.1 92.4 91.7

4.67  106 8.12  10L6 3.20  106 2.21  106

6.02  104 8.70  10L4 3.38  104 2.81  104

209.5 213.6 220.7 231.8

0.59 0.57 0.60 0.61

14 15 16 17

Ag2OeP2O5e(1%) PbCl2 e(5%) PbCl2 e(10%) PbCl2 e(15%) PbCl2

92.4 93.8 90.6 91.6

4.01  106 7.41  10L6 2.75  106 1.09  106

5.12  104 7.76  10L4 4.07  104 2.81  104

210.1 215.6 222.8 234.5

0.62 0.61 0.62 0.63

18 19 20 21

Ag2OeP2O5e(1%) CuCl2 e(5%) CuCl2 e(10%) CuCl2 e(15%) CuCl2

92.8 93.3 90.7 92.5

2.63  106 3.80  10L6 1.62  106 9.12  107

5.01  104 6.76  10L4 3.80  104 2.75  104

212.2 221.8 230.1 240.6

0.65 0.64 0.66 0.68

added as dopants. This reflects an increase in the cross-link strength of the glass network as dopant metal ions are introduced in the undoped silver phosphate glass. However, in the case of silver phosphate glass doped with monovalent metal chlorides, the decrease in Tg with increasing concentrations of lithium or sodium chloride could be attributed to loosening of the glass structure [83,84]. The monovalent Liþ or Naþ ions will not increase the cross-link density by interconnecting two linear chains of silver phosphate glass. The total ionic transference number (ti) values, as evaluated by using Eq. (1), are given in Table 2. The % ti values of all the glassy samples range between 90.6 and 98.3. The values indicate that charge transport in the prepared glasses is predominantly ionic in nature [85]. The electrical conductivities of the glasses were measured at different frequencies viz: 400 Hz, 1 KHz, 2 KHz, 4 KHz, 10 KHz, 20 KHz, 40 KHz, 100 KHz and 150 KHz at room temperature. The plot of log s versus log f at 303 K is shown in Fig. 2. Conductivities are found to be almost frequency independent upto 100 KHz, beyond which the values deviate from linearity. Therefore, the electrical conductivity values of all the glasses measured from room temperature to 473 K and calculated with the help of Eq. (2), are reported at 100 KHz frequency in Table 2. The electrical conductivity of undoped Ag2OeP2O5 glass is less than all the metal chloride doped glassy systems. The room temperature s values for (15 wt%) LiCl and (15 wt%) NaCl doped glasses are maximum. In the case of Mg, Pb and Cu chloride containing glasses, the s values are maximum for (5 wt%) metal chloride doped glasses. On further

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-3.5

(f) -4

(e)

(d) (c)

-5

(b)

log

(S cm–1)

-4.5

-5.5

(a) -6

-6.5 2

2.5

3

3.5

4

4.5

5

5.5

log f (Hz) Fig. 2. log s vs log f plots for (a) Ag2OeP2O5, (b) Ag2OeP2O5e(5%)CuCl2, (c) Ag2OeP2O5e(5%) PbCl2, (d) Ag2Oe P2O5e(5%) MgCl2, (e) Ag2OeP2O5e(15%) NaCl and (f) Ag2OeP2O5e(15%) LiCl glassy systems at 303 K.

addition of dopant chlorides the conductivity decreases. The variation of conductivity (log s) with concentration (wt %) of doped chlorides is given in Fig. 3 which suggests that the trend in s values is sðeLiClÞ  sðeNaClÞ > sðeMgCl2 Þ > sðePbCl2 Þ > sðeCuCl2 Þ > sðAg2 OeP2 O5 Þ The possible explanation for this observation may be given in the following manner. The conductivity of undoped silver phosphate glass at the room temperature is 8.51  107 S cm1. On doping this silver phosphate glass with lithium and sodium chlorides, the conductivity increases enormously with increasing concentrations of the dopants. The increase in the conductivity with increasing concentration of LiCl or NaCl is essentially due to the increase in the number of Agþ ions in the glass matrix. Liþ and Naþ having higher electrode potential than Agþ [86] may easily replace Agþ ions from the main chain of the silver phosphate glass leading to an increase in the numbers of Agþ ions. These replaced Agþ ions along with Cl ions remain free and cause expansion of the glassy network. This leads to a more opened up structure suitable for greater Agþ ion conduction [87]. The slightly higher s values in Li chloride containing silver phosphate glasses could be due to higher electrode potential of Liþ which may replace more Agþ ions in comparison to Naþ. The electrical conductivity values (Table 2) of the divalent magnesium, lead and copper chloride doped silver phosphate glasses show an increase in s values upto 5 wt % of the dopant chlorides and then decreases as the wt % of the dopant increases. This may be explained in the following manner. The addition of Mg2þ, Pb2þor Cu2þ ions in Ag2OeP2O5 glass can increase the conductivity of the glass to a very little extent due to very low mobilities of these divalent cations [1]. As such the basis of increase in the s values of these glasses could be due to the

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-4

(a) (b)

-4.5

log

(S cm–1)

-5

-5.5

(c)

(d) (e)

-6

-6.5 0

2

4

6

Concentration (weight

8

10

12

14

16

) of dopant metal chlorides

Fig. 3. Composition dependence of conductivity (log s) in (a) Ag2OeP2O5eLiCl, (b) Ag2OeP2O5eNaCl, (c) Ag2Oe P2O5eMgCl2, (d) Ag2OeP2O5ePbCl2 and (e) Ag2OeP2O5eCuCl2 glassy systems.

reaction between the dopant chlorides (MCl2) and Ag2O forming AgCl during glass preparation. AgCl may dissociate to provide mobile Agþ ions in the glass matrix, thus, it may be inferred that the mobile Agþ ions are mainly responsible for an increases in the conductivity. Further, the results (Table 2) show as the concentrations of divalent dopant chlorides (MgCl2 PbCl2, CuCl2) increase from 5 wt %, the s values start decreasing. This could be explained due to the formation of more cross - links in the glass structure. FT-IR results suggest the formation of PeOeM linkages between two chains of the undoped silver phosphate glass. At lower concentrations of the dopant chlorides (upto 5 wt %), the number of cross - links in the glassy network are less due to which the mobility of replaced Agþ ions is not affected and hence s values are higher. But at higher dopant concentrations, the number of cross-links are expected to increase which makes the glassy structure close packed and unsuitable for easy migration of Agþ ions. The mobility of Agþ ions thus decreases which eventually leads to lower conductivity values. The higher values of Tg (Table 2) in magnesium, lead and copper chloride doped silver phosphate glasses support this explanation. In order to evaluate the activation energy of conduction in these glasses, the conductivities of all the undoped and the doped glasses were measured from room temperature (303 K) to higher temperature (473 K) at a frequency of 100 KHz. The s values at 473 K are reported in Table 2. Plots of log s vs reciprocal temperature (1000/T ) for different glassy systems are shown in

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Fig. 4.1. Variation of log s vs 103/T for [a] Ag2OeP2O5, [b] Ag2OeP2O5eLiCl and [c] Ag2OeP2O5eNaCl glassy materials.

Fig. 4.2. Variation of log s vs 103/T for [a] Ag2OeP2O5eMgCl2, [b] Ag2OeP2O5ePbCl2 and [c] Ag2OeP2O5eCuCl2 glassy materials.

Fig. 4.1 and 4.2. The values of activation energy for conduction (Ea) were calculated from the linear regions of the plots by using Eq. (3). s ¼ s0 expð  Ea =kTÞ

ð3Þ

where Ea is the activation energy, s0 the pre exponential factor and k, the Boltzman constant. The calculated Ea values are reported in Table 2. The variation of activation energy, Ea with dopant concentration (wt %) is shown in Fig. 5. This indicates that the Ea value of the undoped Ag2OeP2O5 glass is maximum. Where as, the Ea values of Ag2OeP2O5e(15 wt %) LiCl,

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0.7

(e)

0.65

Activation energy (Ea)

(d) (c) 0.6

0.55

0.5

(b) (a) 0.45 0

2

4

6

Concentration (weight

8

10

12

14

16

) of dopant metal chlorides

Fig. 5. Composition dependence of activation energy (Ea) in (a) Ag2OeP2O5eLiCl, (b) Ag2OeP2O5eNaCl, (c) Ag2Oe P2O5eMgCl2, (d) Ag2OeP2O5ePbCl2 and (e) Ag2OeP2O5eCuCl2 glassy systems.

Ag2OeP2O5e(15 wt %) NaCl, Ag2OeP2O5e(5 wt %) MgCl2, Ag2OeP2O5e(5 wt %) PbCl2 and Ag2OeP2O5e(5 wt %) CuCl2 glassy materials are seen to be minimum in their respective series of glasses. These glasses also exhibit the highest values of conductivities both at room temperature and higher temperatures. The increase in ionic conductivity on metal chloride doping is almost entirely due to the fact that the activation energy for conduction (Ea) decreases in all the glassy systems [88]. Expansion of glass skeleton with introduction of dopant ions in voids in the structure forming narrow pathways leads to lowering of the activation energy. This in turn, facilitates the easy migration of the mobile Agþ ions and thus ionic conductivity is promoted on doping the silver phosphate glass with metal chlorides [88]. Various models, such as Weak Electrolyte [89], Strong Electrolyte [89] and Cluster Bypass [90] models have been used to explain the increase in conductivity of phosphate glasses with increase in temperature. According to the weak electrolyte model, the concentration of the mobile charge carriers increases with increase in temperature. In the strong electrolyte model, it has been proposed that loosening of the glass structure occurs with increase in temperature. Due to this the mobility of the charge carriers rather than their concentration increases. This eventually leads to an increase in the conductivity at higher temperatures. In the Cluster Bypass model, constitution of preferred pathway as well as partial pathways for the migration of mobile ions takes place. It is proposed that the formation of non-bridging oxygen functional

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groups open up some of the partial pathways at higher temperatures and due to this the mobility of the free ions increases with temperature. In silver phosphate glasses doped with metal chlorides, two different types of cations are present in the glassy matrix. One is the highly mobile Agþ ion (replaced from the main chain by the dopant cations) which is associated with the chloride (Cl) environment and the other the immobile dopant metal ion (Mþ or M2þ) associated with the oxygens of the glass network. Thus, the increase in the conductivity with increase in temperature is mainly due to the increase in the mobility of the Agþ ions. This can easily be explained with the help of Cluster Bypass model [90]. At higher temperatures, formation of non-bridging oxygens provides more openings (or channels) which eventually increases the mobility of free Agþ ions. Due to this reason the conductivities of all the synthesized metal chloride doped glassy materials increase with increasing temperature. FT-IR studies on these glasses support the formation of non-bridging oxygens in the glassy network. 4. Summary Silver phosphate glassy materials containing Li, Na, Mg, Pb and Cu chlorides as dopants have been synthesized and investigated extensively. From the results of ion transport studies, it can be concluded that the electrical conductivity (s) values of silver phosphate glasses increase on the addition of metal chlorides. In the case of monovalent lithium and sodium chlorides, s values increase enormously with increasing dopant concentrations and the Ag2OeP2O5e (15 wt%) Li/NaCl glassy materials exhibit maximum electrical conductivity both at all temperatures. On the other hand, in the case of bivalent magnesium, lead and copper chlorides, the conductivity increases only upto 5 wt% of dopant chlorides and then starts decreasing. The increase in conductivity in the doped glasses is mainly due to Agþ ion conduction. FT-IR studies confirm the formation of PeOeM linkages when metal chlorides are added to the silver phosphate glass matrix. The addition of lithium or sodium chloride loosens the glass structure while the divalent magnesium, lead or copper chloride increases the cross - link density of the glassy network leading to a close packed structure. This has been adequately supported by DSC studies on these glasses. The increase in conductivity with increase in temperature has been explained with the help of Cluster Bypass model. The increase in the mobility of Agþ ions results due to the formation of new non-bridging oxygens providing more opened-up channels for Agþ conductions at higher temperatures. 5. Perspectives Extensive studies clearly indicate that phosphate glass belong to an important family of solid electrolytes which could be prepared at a low temperature with simple compositions and strong glass forming character. The large conductivity variation in these glasses has been correlated with glass structures of different glassy systems. Studies show that the ionic mobilities and conductivities depend upon the structural disorder of the glassy systems. Initially, the relatively poor chemical durability of phosphate glasses made them unsuitable for practical applications, But the properties of phosphate glasses were modified to exhibit better ionic conductivity and chemical durability by the addition generally of halides or oxides of the alkali, alkaline earth and transition metals into the glassy network in such a way that made them quite suitable for various electrochemical application. Phosphate glasses show several advantages over conventional silicate and borate glasses and thus possess several technological and biological

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applications. Recently zinc phosphate glasses are reported to be used for manufacturing glasse polymer composites, optical waveguides and solid state laser sources. Phosphate glasses with transition metals exhibit unique memorizing and photochemical properties. They have potential application in solid state batteries and sensors, solid state lasers, luminescent solar energy concentrators (LCS) and optical fibers for communications devices. Iron phosphate glasses have been used as a potential host material for the vetrification of radio-active wastes. Thus, there is a constant demand of investigating new fast ion conducting phosphate glassy systems and tailor their physical and chemical properties in such a way so as to meet the specific requirements for various technological application.

Acknowledgements The authors are thankful to Prof. K.D.S. Yadav, Head, Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur and Prof. R.C. Agrawal, Department of Physics, Pt. Ravi Shankar Shukla University, Raipur for providing necessary laboratory facilities and help in carrying out the present work. Thanks are also due to Prof. N.B. Singh, Former Head, Department of Chemistry D.D.U. Gorakhpur University, Gorakhpur and Prof. Suresh Chandra, Department of Physics, BHU, Varanasi, for helpful discussions.

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