Spectroscopic and electrical studies of silver sulfophosphate glasses

JOUeNAL or ~ ' ~ I [ 1 ~ ~0IlI~

Journal of Non-Crystalline Solids 160 (1993) 73-81 North-Holland

Spectroscopic and electrical studies of silver sulfophosphate glasses B.V.R. Chowdari a K.F. Mok b, J.M. Xie b and R. Gopalakrishnan a Department of Physics and b Department of Chemistry, National University of Singapore, Kent Ridge, 0511, Singapore Received 12 November 1992 Revised manuscript received 20 January 1993

New conducting glasses have been synthesized in the xS:(1 - x)mgPO 3 system. X-ray diffraction and differential scanning calorimetric studies have confirmed that these glasses can be formed in the range x = 0.0 to 0.35 sulfur content. The glass transition temperature has been found to increase with increase of sulfur content. The dc conductivity, derived from complex impedance measurements, increases with increasing sulfur content and reaches a value of 1.32 × 10-6 (fl cm)-1 at room temperature with an activation energy of 0.58 eV in the sample for which x = 0.30. This conductivity is approximately five times higher than that of AgPO 3 glass. Various techniques such as X-ray photoelectron spectroscopy, FT-IR spectroscopy and far-infrared spectroscopy have provided evidence for sulfur participation in the phosphate network by substituting for the oxygen in the non-bridging position. The results demonstrate the formation of [PO4_xS x] species and weaker cation-network interactions. The conduction mechanism is discussed in relation to the structure.

I. Introduction Fast-ion conducting glasses have been investigated in view of their potential application as electrochemical devices. Among them, phosphate glasses are structurally interesting because they accept a wide range of anion substitution, e.g., N and F in NaPO 3 glass. Silver phosphate glasses have been studied owing to their ease of formation, environmental stability and interesting structures [1-5]. Addition of silver halide to these glasses enhances their conductivity and decreases their glass transition temperature, Tg, without significantly altering the phosphate network [6-8]. Unlike the AgI:AgPO 3 system, recent investigation of the Ag2S:AgPO 3 system has indicated the existence of differing structures due to the changes in the length of the phosphate chains [9]

and possible incorporation of sulfur in the phosphate network. However, a detailed structural investigation was not undertaken. In order to investigate the direct influence of the sulfur on the phosphate network, glasses in the xS:(1 - x ) A g P O 3 system have been prepared and measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fouriertransform infrared absorption spectroscopy (FTIR), far-infrared absorption spectroscopy (FIR), X-ray photoelectron spectroscopy (XPS) and electrical conductivity studies are presented and discussed.

2. Experimental procedure 2.1. Glass preparation

Correspondence to: Dr B.V.R. Chowdari, Department of Physics, National University of Singapore, Blk S12, Faculty of Science, Lower Kent Ridge Road, Kent Ridge, 0511 Singapore.Tel: + 65 772 2956. Telefax: + 65 777 6126.

A mixture of reagent-grade AgNO 3 and NH4H2PO 4 in appropriate proportions was preheated in an open silica crucibl~ placed in an electric furnace at 300°C for 30 min to prevent

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

74

B.V.R. Chowdari et al. / Silver sulfophosphate glasses

excessive boiling and consequent spillage. The mixture was then melted at about 600°C for 30 min (with stirring to ensure homogeneity). The liquid was then passed through a rotating twin roller in order to have a faster quenching rate. AgPO 3 glass flakes thus obtained were pulverized in a ball mill and the powder was stored in aluminum foil for future use. Powder samples of these AgPO 3 specimens were intimately mixed with sulfur in appropriate proportions in a mortar, and melted at 600-690°C for 15-45 min in a covered silica crucible. The resulting liquid was poured into a stainless steel mould to obtain disc-shaped samples about 10 mm in diameter and 1 mm in thickness. The samples were annealed for 2 h at a temperature 30°C less than Tg. The annealed samples were covered in aluminum foil and stored in a desiccator to prevent exposure to moisture and decomposition by photochemical processes.

IR (400-4000 cm -1) and FIR (200-400 cm -1) absorption spectra were obtained on powder specimens dispersed in a powder KBr or CsI disk, using a P e r k i n - E l m e r 1600 series FT-IR and P e r k i n - E l m e r 598 infrared spectrophotometer. XPS studies were carried out using a VG Escalab MkII spectrometer with Mg anode (K~, 1253.6 eV). The hemispherical analyzer was scanned in constant-analyzer-energy mode at a pass energy of 20 eV. All data were collected and processed using a IBM PC and VGS 2000 software. The spectra were referenced to the C ls peak ( E b = 284.6_+ 0.l eV) resulting from the adventitious hydrocarbon present on the sample surface.

2.2. Techniques

In the xS:(1 - x ) A g P O 3 pseudo-binary system, the glass-forming domain is observed in the range of x = 0 to x = 0.35. Homogeneous glasses in which x > 0.35 are difficult to obtain because sulfur is then not totally dissolved in the matrix and many dark spots appeared in quenched solids. The glass-forming region is confirmed by X R D and DSC results. It is observed that Tg increases systematically with the increase of sulfur content as shown in fig. 1. A similar glass-forming region in the x A g z S : ( 1 - x ) A g P O 3 system for x = 0 to x = 0.35 has been reported [9].

X R D spectra were taken for all the samples using a Phillips X-ray generator (PW 1729) and a diffractometer control (PW1710). The samples were further analyzed using the DSC technique to determine Tg. Specimens encapsulated in aluminum pans were scanned in the temperature range 50-500°C with a heating rate of 20°C/min using a Shimadzu differential scanning calorimeter DSC-50 with a TA-50 personal analysis software system. Gold electrodes ( ~ 100 nm thick) were deposited on both sides of glass samples, which were polished using fine-grain sandpaper and diamond paste, using the thermal evaporation technique. Conductivity measurements were performed by the complex impedance method using a Solartron 1255 frequency response analyzer and a Solartron 1286 electrochemical interface coupled to an IBM compatible microcomputer for data acquisition and analysis. All measurements were carried out in an electrical furnace under vacuum which was controlled by a Eurotherm temperature controller with an accuracy of + I°C. Measurements were carried out in the range of 10 Hz to 1 MHz and 50-150°C.

3. Results

3.1. Glass formation and transition temperature

3.2. XPS studies Core-level binding energies (BE) of Ag 3d5/2, S 2P3/2, P 2p and O ls determined from the respective XPS spectra are given in table 1. The O ls spectrum has two distinctive components at about 531 eV (denoted as O Is(l)) and 533 eV (denoted as O ls(2)). In the sulfophosphate glass system, it can be seen clearly that the core-level BE of Ag 3d5/2 and O ls have not shifted appreciably with reference to the corresponding values for AgPO 3. However, the binding energy of P 2p has a gradual shift towards higher values with increase in sulfur content. It can be seen from fig.

B.V.R. Chowdari et al. / Silver sulfophosphate glasses

75

300 xS:(1-x)AgP03

b

x =0 2 0 ! ~

250

c.,

200

O

O

O O

150

O

100(~.0

O

I

01.1

I

i

0.2 0.3 x (mot % )

0.4

0.5

x :0,25/~

a

L

6 >.

x = 0.25

"E

__t-/ g £ LU

I

-0.2o x :o15

~

(111

528 E £ ~

I

532

528 532 536 BINDING ENERGY{eV)

536

Fig. 2. The experimental and deconvoluted Oaussian peaks of O ls XPS spectra for various glasses in the x S:(1 - x ) A g P O 3 system.

x : 0.05

0

I

100

*

/

I

200

I

300

t

400

500

Temperature ('C) Fig. 1. (a) DSC thermograms for various glasses in the xS: (1 - x ) A g P O 3 system; (b) variation of Tg with sulfur concentration.

2 that there are two chemical environments for O ls with different BE, and their relative intensities vary with increase in sulfur content. It is found that the area ratio of the lower BE peak (O ls(1)) to the higher BE peak (O ls(2)) decreases with increase of sulfur content.

Table 1 Binding energies, O l s ( 1 ) / O ls(2) area ratio and S / A g atomic ratio. Binding energies are in eV with an uncertainty of _+0.1 eV x

Binding energy (eV)

Area ratio

Atomic ratio

(%)

Ag3ds/2

O ls(1)

O ls(2)

S 2p

P 2p

O l s ( 1 ) / O ls(2)

S / A g (at.%)

0.00 0.05 0.10 0.15 0.20 0.25 0.30

368.1 368.3 368.2 368.6 368.4 368.5 368.5

531.2 531.4 531.3 531.6 531.5 531.7 531.6

532.9 533.1 533.0 533.2 533.2 533.2 533.1

162.5 162.5 162.8 162.7 162.6 162.6

134.1 134.3 134.3 134.6 134.6 134.6 134.8

1.196 1.094 1.091 0.872 0.850 0.825 0.428

0.04-8 0.068 0.1 ] 9

B.V.R. Chowdari et al. / Silver sulfophosphate glasses

76

3.3. IR and FIR studies

xS: (1 - x)AgP0 3 x =0.25

A spectroscopic investigation of the glass system x S : ( 1 - x ) A g P O 3 has been undertaken by using I R and F I R as these techniques are sensitive to structural variation and cation-network interaction, respectively. As shown in figs. 3 and 4, systematic changes in I R and F I R spectra are observed with increase in sulfur content. Based on published data on the A g 2 S : A g P O 3 and A g I : A g P O 3 systems [9,10], the observed vibration bands have been assigned. The doublet bands in the region 1268 to 1298 cm -1 and 1071 to 1089 cm -1 appear to become single bands for 0.2 < x < 0.25. The intensity of the bands in the region 1071 to 1089 cm -1 is found to be lower relative to that of the bands in the region 1268 to 1290 cm - t , as x is increased. The band observed at about 1035 c m - 1 showed a decrease in intensity with the addition of sulfur. Within the 878 to 951 cm -1 region, a weak band at 890 c m - 1 has been observed to increase with increase of sulfur content. The intensity of the band at about 795 c m decreased gradually with increase in sulfur content and it almost disappeared for x >i 0.25. The broad band observed at 518 to 531 cm-~ may be assigned to bending vibration [9,10]. All these bands together with their assignments are listed in table 2. As shown in fig. 4, the F I R absorption

A t3

I--

1400

1000 Wavenumber (cm -1 )

6O0

Fig. 3. IR absorption spectra for various glasses in the xS: ( 1 - x)AgPO 3 system.

i

xS : ( 1 - x ) A g P 0 3

. . . . . .

:3

x =0.00

. . .

o

'..

a.

x : 0.05

\

..Q

\

200

.

..

•

215

.

.

.

.

x

--

x=

-

J~'~ 230

-~¢~ 245

0.10 0.20

_:-~.~. t . . - ' 260

275

Wavenumbef ( c r n ' l )

Fig. 4. FIR absorption spectra for various glasses in the x S : ( 1 - x)AgPO 3 system.

B.V.R. Chowdari et aL / Silver sulfophosphate glasses

xS:(1-x)Ag~

77

3

6.24 15.8 Kl-lz 4.68 ..,,~ o ,'-... a

x:0.20 3.12

1.56

0.0 0.0

I 2.4

4.8

7-2

9.6

12

z 71o% Fig. 5. Complex impedance data for x S : ( t - x ) A g P O 3 system for x = 0.2 and 0.3. The solid curves represent the one obtained by fitting the experimental datapoints ([]) into semicircular arcs.

edge in the 200 to 275 cm - t region shifted to lower frequency upon sulfur addition.

the complex-impedance technique. Typical complex-impedance data measured over a wide frequency range of 10 Hz to 1 M H z are shown in fig. 5. The value of the intercept made by extrapolation of the semicircular arc to the real axis was used to calculate ~r. The t e m p e r a t u r e depen-

3. 4. Electrical conductivity measurements The electrical conductivity was measured by

k-,. I I

'

-4.85

-6.50

2.3

xs :c, x IAg o3 ~

~,

---,~ ~ .

-.%. ~

t

t

2.58

2.81

3.04

• r~

x=O.2O x =0.15

O

x= 0.10

3.27

3.50

IO00/T ( K -1)

Fig. 6. T h e temperature dependence of the conductivity for various compositions in x S:(1 - x ) A g P O 3 system.

B.V.R. Chowdari et aL / Silver sulfophosphate glasses

78 Table 2 IR data for x S : ( 1 - x ) A g P O 3 Assignment

Table 3 Arrhenius parameters in the xS : ( 1 - x ) A g P O 3 system Band ( c m - 1)

Ref.

P= 0

1290-1268

[9-10]

Uas,PO 3

II - P- 0

1089-1071

[10]

vs,PO 2 Vas,POP

-PO 2POP/P-S

3~,~as,PO 2

-PO 2-

1035 878-951, 795 518-531

[10] [10] [11] [10]

I

V~s,P02

-

-

I

I

O'RT X 10-7 (D c m ) - 1

Eac (eV)

log ~0

0.0 0.10 0.15 0.20 0.30

2.91__+0.2 3.73_+0.3 4.06+0.2 11.1 _+0.2 13.2 __+0.1

0.63_+0.030 0.63+0.004 0.61__+0.030 0.58_+0.040 0.57_+0.018

3.39_+0.09 3.44_+0.14 3.14_+0.08 3.19_+0.11 3.18_+0.05

for the conductivity, or = t r 0 exp(-Eac/kT). Figure 7 shows log or and the activation energy, Eac , as a function of the sulfur content. The data of the conductivity at room temperature, the activation energy and the pre-exponential factor for different compositions are given in table 3.

dence of the conductivity for various compositions has been measured and is shown in fig. 6. The data are well fitted by an Arrhenius relation

-2.0

X

I

1.00

[

i

xS:(1-x)AgP03

• Conductivity o Eoc

-3.5

0.88 e T = 140"C

-

5.0

~o

•

T = 120 *C

•

= 100 *C

•

= 80"C

•

: 5 0 °C

0.77

w

-- 6.5

bJ

_______._ Y

0.66

~o Eac - 8.0(~.0

I

0.1

012 0~.3 X (mol %)

.4

0.50.55

Fig. 7. Compositional dependence of log tr at different temperatures and Eac. T h e lines are drawn as a guide for the eyes.

Table 4 T h e BE(eV) of P 2p, S 2p, Ag 3d5/2 and BS (single bond strength, k c a l / m o l ) in different compounds. Binding energies are in eV with an uncertainty of +0.1 eV Material

Bond

P 2p

S 2p

Ag 3d

PEO5 PzS5 (C6HsO)3PS Ag2S Ag20 S8 Ag 2 SO4 xS :(1 - x ) A g P O 3

P-O P-S P-S/P-O Ag-S Ag-O S-S

135.8 134.2 134.7 -

163.0 160.8 164.2 168.6 162.5-162.8

368.0 368.4 368.3 368.2-368.6

P-S/P-O

134.3-134.8

BS

Ref.

142.6 106.0 51.9 53.7 -

[7] [18,19] [23] [20,21] [19] [ 19,22]

B.V.tL Chowdariet al. / Silversulfophosphate glasses 4. Discussion

4.1. X-ray photoelectron spectroscopy In m e t a p h o s p h a t e glasses, R P O 3 (R = Li, Na, K, Ag, etc.), long chains of PO 4 tetrahedra are joined together with each PO 4 tetrahedron containing bridging oxygens ( - O - ) and non-bridging oxygens ( - O e, = O). The negative charge on the non-bridging oxygen is compensated by a nearby positive ion (R +). This positive ion also serves as a link between the PO 4 tetrahedra chains [3]. Table 4 gives the binding energy of P 2p and S 2p core electrons for glasses studied in this work and some other related compounds. As is evident from the table, the P 2p BE lies between the corresponding values for P205 and P285, which we suggest may be due, in the present system, to contributions from both P - S and P - O bonds. As can be seen from table 4, the greater the single bond strength the larger the BE. Since some of the P - O bonds are considered to be replaced by P - S bonds and also the electronegativity of sulfur is less than that of oxygen, the value of the single bond strength and hence the BE of P 2p in the present system is expected to be between the corresponding values for PzS5 and P20 5. The observed BE of P 2p core electrons is in fairly good agreement with the corresponding values for P - S / P - O in (C6HsO)3PS. The BE of S 2p obtained in the present investigation is less than that for elemental sulfur (164.2 eV), which we suggest is due to the incorporation of sulfur in the network. This hypothesis is further supported by the close agreement between the BE values for the S 2p core electron in the present system and (C6HsO)3PS. As seen from table 4, on the basis of a comparison of BE for S 2p in AgzS, AgzSO 4 and the present system, we suggest that the sulfur in the present system is closer to the environment in Ag2S than in AgzSO 4. On the basis of the earlier data [7,8], the observed two peaks in the O ls spectra labelled as O ls(1) and O ls(2) may now be assigned to non-bridging oxygen (NBO) and bridging oxygen (BO) respectively. As can be seen from table 1, the area ratio of O ls(1) to O ls(2) decreases with increase in

79

sulfur content, which we suggest is attributable to a decrease in the number of NBO. Together with the conclusion based on P 2p spectra, we conclude that the non-bridging oxygen sites are occupied by sulfur ions. 4.2. Infrared and far-infrared spectra The band at 878 to 951 cm -~ was assigned to vas POP vibration. Within this region, a weak band at 890 cm-1 was observed to increase with the increase of sulfur content. According to Mielke et al. [11], a P - S vibration appears at 891 c m - I and hence the weak band at 890 cm-~ is assigned to a P - S vibration consistent with the XPS results. The band in the region 1268 to 1290 cm-1 has been attributed to the P = O vibration (vasPO2). This band is well resolved feature in the spectra of all metaphosphate chain structures in which double-bonded oxygens exist. It was observed that the intensity of this band is unchanged, within errors of measurement, as the sulfur concentration is increased. The band at 1071 to 1089 cm -~, which has been assigned to Vas PO3, decreased in intensity and shifted in frequency relative to the next band at 1268 to 1290 cm-1. Since it has already been concluded from XPS studies that the NBO are decreasing in number with increase in sulfur content, we assign the band at 1071 to 1089 cm -~, which is also decreasing with increase in sulfur content, to the single-bonded NBO in PO 3. We suggest that some of the P - O bonds are replaced by P - S bonds, leading to the formation of PO4_xS x species. This conclusion is further supported by the fact that the band at about 1035 cm -1, which was assigned to vs vibration of - P O 2 - , decreased in intensity with increase in sulfur content. Thus, all the observed trends of /"as PO2, /"as PO3, Us PO2, /"as POP, us POP vibration are clearly suggestive of sulfur participation in the network, in agreement with XPS results. As the present sulfophosphate glasses have been synthesized in ambient conditions at high temperature, it is possible that the oxygens that are replaced by sulfur may have transformed into

80

B.V.R. Chowdari et al. / Silver sulfophosphate glasses

molecules of oxygen, contributing to charge neutrality, that finally escape to the atmosphere. The oxyanion substitution by sulfur and fluorine have also been confirmed by optical absorption and XPS studies [24,1].

~3. Conduc6vi@ As shown in table 3 and fig. 7, the conductivity, ~r, at a given temperature increases while Eac decreases with increase of sulfur content. The variation of conductivity with composition appears to be a result of the variation of the activation energy alone, as the pre-exponential factor as given in table 3, which is a measure of charge carrier concentration, is constant within errors of measurement. In order to account for the dependence of the conductivity on composition, the model proposed by Anderson and Stuart [12] has been considered, since the weak electrolyte theory [13,14] requires a compositional dependence of ~r on the pre-exponential factor. Anderson and Stuart have considered Eac as the energy required to overcome electrostatic forces, E b, plus the energy required to open a 'doorway' in the structure large enough for an ion to pass through, E s. Hence, Eac -- E b + E s = flZZoeZ/y(r + ro)

+ 4aVGrD(r -- rD) 2, where z0, r 0 are the charge and radius of anions, /3 is a 'lattice' parameter depending on the distance between neighboring sites, r is the cation radius, r o is the radius of 'normal' doorways, G is the elastic modulus and y is a 'covalency parameter'. However, Almeida and Mackenzie [15] have reported that the electrostatic energy term was generally dominant over the strain energy term ( E s << Eb). Hence it may be reasonable to assume that the electrostatic term (E b) alone may account for the variation in activation energy in the present system. As the radius of the sulfur ion, rs, is greater than the radius of oxygen ion, r o, and the electronegativity of sulfur is smaller than that of

oxygen, the terms r + r 0 and y are expected to be higher and thus decrease the activation energy with an increase in sulfur content. If we consider E s also (although it is not mainly responsible for the variation of Eac), the easier polarization of the sulfur ions, resulting in decreasing the strain energy, would also give rise to a smaller value of E s.

4.4. Correlation between structure, or and Tg XPS and IR studies have shown that, with the addition of sulfur to AgPO 3, substitution of sulfur for non-bridging oxygen in the oxygen network and the formation of [PO4_xS x] species occurs. As shown in fig. 4, the absorption band edge shifted to lower wavenumber with increase in sulfur content. Kamitsos and Karakassides [16] has shown that the FIR band shifted to higher frequencies due to the increase in the charge density at the anionic site; conversely, if the anionic site has a lower charge density, the cationnetwork interaction is weaker and the FIR absorption band should shift to lower frequency. Thus, based on our FIR results, we suggest a weaker interaction between silver and the network in the present system consistent with the conductivity results. Generally, Tg of an oxide glass increases with the bond strength, the degree of cross-linking, and the density [17]. It is noted that Tg increases with increasing sulfur concentration. Hence we suggest that the substitution of sulfur for NBO has resulted in an increase in cross-linking between various phosphorus chains.

5. Conclusions The results of FTIR, FIR and XPS studies are consistent with the hypothesis that sulfur participates in the phosphate network by substituting for oxygen in the non-bridging position. Weaker cation-network interactions result in an increase in conductivity with increase in sulfur content. The authors would like to thank Professor K.L. Tan for his cooperation and support in carrying out this work.

B.V.R. Chowdari et al. / Silver sulfophosphate glasses

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81

[13] D. Ravaine and J.L. Souquet, Phys. Chem. Glasses 18 (2) (1977) 27 [14] D. Ravaine and J.L. Souquet, Phys. Chem. Glasses 19 (5) (1978) 115. [15] R.M. Almeida and J.D. Mackenzie, J. Mater. Sci. 17 (1982) 2533. [16] E.I. Kamitsos and M.A. Karakassides, Solid State Ionics 28&30 (1988) 783. [17] N.H. Ray, J. Non-Cryst. Solids 15 (1974) 423. [18] C.D. Wagner, Discuss. Faraday Soc. 60 (1975) 291 [19] C.D. Wagner, W.M. Riggs, C.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of XPS (Physical Electronics Div., Eeden Prairie, MN, 1979). [20] R. Romand, M. Roubin and J.P. Deloume, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 229. [21] G. Sehoen, Acta. Chem. Scand. 27 (1973) 2623. [22] N.H. Turner, J.S. Murday and D.E. Ramaker, Anal. Chem. 52 (1980) 84. [23] W.E. Morgan, W.J. Stec, R.G. Albridge and J.R. Vanwazer, Inorg. Chem. 10 (1971) 926. [24] A.A. Ahmed, T. EI-Shamy and N. Sharaf. J. Am. Ceram. Soc. 63 (1980) 537.