phys. stat. sol. (a) 168, 367 (1998)

Subject classification: 66.30.Hs; S11

Ionic Conductivity of Aliovalent Cation-Doped Ag2SO4

A Closer Look

K. Singh (a) and S. S. Bhoga (b)

(a) Department of Physics, Nagpur University, Nagpur-440010, India

(b) Department of Physics, Hislop College, Nagpur-440001, India

(Received February 3, 1998; in revised form May 18, 1998)

The ionic conductivity of (1±±x)Ag2SO4 : (x)MSO4 (where M = Mg, Ca, Ba, Mn and x = 0.0101 to0.10) is investigated in the temperature range from 450 to 250 �C. The solid solubility limits up to5.27 mol% of MSO4 in Ag2SO4 are set with the help of X-ray powder diffraction technique. Thepartial replacement of Ag+ by cations of larger ionic size such as Ba2+ increases the conductivity,whereas conductivity fall is recorded upon the substitution of Mg2+ and Ca2+. The results are ex-plained on the basis of a concept of lattice distortion taking place in the Ag2SO4 matrix. Interest-ingly, within the solid solubility limit despite having a dopant of smaller ionic size, the ionic con-ductivity is found to increase with MnSO4 concentration. It is likely that among variouscontributing factors, the electronic configuration too plays a vital role in deciding the conductivityin this system.

1. Introduction

A special advantage of sulphate based solid electrolytes is that many mono- and di-valent cations are mobile in their high-temperature modifications [1]. Remarkably,Ag2SO4 is a potential candidate for electrochemical SO2/SO3 gas sensor application [2].It undergoes a phase transition from the high-temperature hexagonal a-phase to thelow-temperature orthorhombic b-phase at 416 �C. According to Hofer et al. [3] aliova-lent cation doping in Na2SO4 gives enhancement in the conductivity irrespective ofionic size while our earlier investigations on Li2SO4 and Ag2SO4 systems reveal that, inaddition to the valence, ionic size and the electronic configuration of the dopant ionsplay important roles in ion migration through the solids at low temperatures [4 to 6].Unfortunately, there is an obvious lack of a well defined theory. These factors haveprompted us to undertake a systematic study to further support our view on the influ-ence of ionic size, the valence and the electronic configuration of the aliovalent dopantcation. The partial replacement of Ag+ by various divalent cations is made with a viewto distorting the host lattice while creating extrinsic vacancies on the basis of the formu-la Ag2±±2xMx&xSO4.

2. Experimental

The initial ingredients, Ag2SO4 and MSO4 (where, M = Mg, Ca, Ba and Mn), withpurity greater than 99% were procured from Aldrich Chemicals (USA). Appropriatemole fractions of the above chemicals in (1±±x)Ag2SO4 : (x)MSO4 (where x = 0.0101 to0.10) were prepared by slow cooling the melt at a predetermined cooling rate of 20 K/min.

K. Singh and S. S. Bhoga: Ionic Conductivity of Aliovalent Cation-Doped Ag2SO4 367

The prepared samples were characterized by X-ray powder diffraction as discussed else-where [6].

For electrical conductivity measurements, the samples were obtained in the form ofcircular discs of about 9 mm diameter and 1.5 mm thickness by pressing the powder ina stainless steel die-punch and hydraulic press. A good ohmic contact was ensured byusing quality silver paint (Eleteck, India) followed by baking at 200 �C for 2 h. Theabove process was meticulously carried out in a dark room so as to avoid photodecom-position of Ag2SO4. The pellets were sintered at 500 oC for 24 h before spring-loadingthem between silver electrodes of a sample holder. Prior to impedance measurements,the sample was again heated to 440 �C for 2 h in order to homogenize the chargecarriers. Real and imaginary parts of impedance were measured as a parametric func-tion of frequency and temperature in the range from 5 Hz to 13 MHz and from 450 to250 �C, respectively, using a computer controlled HP 4192A lf impedance analyzer. HP16048 test leads were used for electrical connections from the sample to the analyzer toavoid any otherwise parasitic impedance due to improper connecting cables. The tem-perature of the furnace was controlled to � 1 K with the help of Eurotherm 810 PIDtemperature controller.

3. Results and Discussion

The solid solubility is found at least up to 5.27 mol% for Ba2+, Mg2+ and Mn2+ addedto Ag2SO4 (Table 1). This is confirmed by the X-ray powder diffraction patterns of allthe samples under study in 0.9473Ag2SO4 :0.0527 MSO4, which indicated no line corre-sponding to the added phases, whereas all the experimental lines were found in closeagreement with the JCPDS data for Ag2SO4. A comparison of lattice cell constants aswell as lattice cell volume of host and doped systems (Table 2), obtained using a com-puter software, shows that the host lattice of Ag2SO4 undergoes an appreciable contrac-tion upon the substitution of Mg2+ and Ca2+ and a negligible one in case of Mn2+,whereas, it expands upon Ba2+ addition within the limits of solid solution.

The bulk conductivity of each sample is obtained by following a complex impedanceanalysis [5]. The plots of log (sT) versus 1000/T, for orthorhombic (stable below

368 K. Singh and S. S. Bhoga

Ta b l e 1A comparison of experimental XRD data with JCPDS for 0.9473Ag2SO4:0.0527MSO4

(M = Ca, Sr, Ba and Mn)

Ag2SO4 M = Ca Sr Ba Mn JCPD

d(AÊ ) I/I0 d(AÊ ) I/I0 d(AÊ ) I/I0 d(AÊ ) I/I0 d(AÊ ) I/I0 d(AÊ ) I/I0 phase [hkl]

4.687 8 4.70 10 4.730 8 4.702 7 4.710 10 4.699 10 Ag2SO4 [111]3.985 18 3.99 24 4.020 21 3.991 21 4.001 28 3.994 25 Ag2SO4 [220]3.172 92 3.18 91 3.190 90 3.179 65 3.179 61 3.177 70 Ag2SO4 [040]2.870 100 2.87 100 2.874 100 2.875 100 2.879 100 2.873 100 Ag2SO4 [311]2.642 87 2.64 50 2.657 80 2.647 74 2.645 69 2.644 90 Ag2SO4 [022]2.527 19 2.53 19 2.537 15 2.533 17 2.530 15 2.530 17 Ag2SO4 [202]2.413 30 2.42 20 2.419 24 2.420 27 2.419 24 2.421 30 Ag2SO4 [331]1.924 18 1.920 28 1.928 25 1.925 27 1.925 29 1.926 30 Ag2SO4 [351]

I/I0 is the relative intensity.

416 �C) and hexagonal (stable above 416 �C) modifications of Ag2SO4 (Fig. 1), obeythe Arrhenius law

�sT� � �sT�0 exp �ÿEa=kT� ; �1�where Ea = Ef + Em, Ef and Em being the energies of defect formation and defectmigration, respectively, in the entire range of measurements. Thus, the conductivity isgoverned by the thermally activated process, i.e., by thermally activated defects n givenby

n � exp �ÿEf=kT� :Similarly, the plots of log (sT) versus 1000/T for the system (1±±x)Ag2SO4 : (x)MSO4

(M = Ca, Ba, and Mn) also obey the Arrhenius law. Figs. 2a and b display the conduct-ivity behaviour of all the samples under study in both hexagonal and orthorhombic

Ionic Conductivity of Aliovalent Cation-Doped Ag2SO4 369

Ta b l e 2Comparison of lattice cell constants and cell volume of 5 mol% divalent cation dopedAg2SO4.

cell JCPDS AgP AgMg AgCa AgMn AgBaconstants

a (AÊ ) 10.2699 10.247 10.255 10.268 10.268 10.287b (AÊ ) 12.7069 12.688 12.669 12.679 12.677 12.764c (AÊ ) 05.8181 05.748 05.743 05.737 05.745 05.765volume (AÊ 3) 759.25 747.32 746.26 746.8 747.59 756.96

AgP: pure Ag2SO4; AgMg: 95Ag2SO4:5MgSO4; AgCa: 95Ag2SO4:5CaSO4;AgMn: 95Ag2SO4:5MnSO4; AgBa: 95Ag2SO4 :5BaSO4.

Fig. 1. Variation of log (sT) with 103/T in (1±±x)Ag2SO4 : (x)MnSO4

modifications of Ag2SO4, respectively. It can be seen from these figures that as theimpurity concentration increases the conductivity also increases and exhibits a maxi-mum in both structural modifications for (1±±x)Ag2SO4 : (x)MSO4 (M = Mn and Ba)systems. These results are similar to those of earlier reported results in case ofLi2SO4 : CaSO4, AgBr :AgI and Na2SO4 :Na2WO4 [4, 7, 8]. On the other hand, theconductivity behaviour in (1±±x)Ag2SO4 : (x)MgSO4 and (1±±x)Ag2SO4 : (x)CaSO4

stands out in contrast to the above results (Figs. 2a and b). The maximum conductiv-ity is found at about x = 0.0757. The conductivity behaviour could be explained asfollows:

The replacement of host monovalent Ag+ by the divalent guest M2+ gives rise toadditional vacancies for charge compensation in the host matrix of Ag2SO4. It is re-

370 K. Singh and S. S. Bhoga

Fig. 2. Variation of log (s) with MSO4 (M = Mg, Ca, Ba and Mn) concentration. a) Hexagonal (at450 �C) and b) orthorhombic (at 300 �C) phases

ported that in the high-temperature phase, such extrinsic vacancies contribute mainly tothe conductivity, whereas the effect of ionic size is insignificant following aliovalentdopant substitution [3]. The hexagonal phase being a more open structure, the Ag+ ionssqueeze through the lattice with a high elementary hopping probability [9, 10] due toavailability of additional vacancies created by M2+ substitution. The increase in dopantconcentration enhances the vacancies in Ag2SO4 lattice giving rise to additional ionmigration paths which in turn increases the conductivity. Beyond a critical concentra-tion in all these systems, the conductivity due to Ag+ is reduced (Fig. 2a) followingvacancy interactions [3].

A comparison of the conductivity isotherms in Fig. 2a clearly reveals higher con-ductivity in (1±±x)Ag2SO4:(x)MSO4, M = Mn2+, Mg2+ and Ca2+ than that in(1±±x)Ag2SO4:(x)BaSO4. This may be on account of the fact that Ba2+ has the largestionic radius among all these (rBa = 1.35 AÊ , rMg = 0.65 AÊ , rMn = 0.8 AÊ and rCa = 0.99 AÊ )which obstructs the Ag+ mobility/conductivity. This is in good agreement with the ear-lier finding of monovalent cation-doped Li2SO4 and Ag2SO4 [11, 12].

In the low-temperature modification, the conductivity for the systems (1±±x)Ag2SO4 :(x)MgSO4 and (1±±x)Ag2SO4 : (x)CaSO4 is seen decreasing as a function of addedconcentration in sharp contrast to that in systems (1±±x)Ag2SO4 : (x)BaSO4 and(1±±x)Ag2SO4 : (x)MnSO4 (Fig. 2b). The results in the domain of the low-temperaturemodification of Ag2SO4 could be understood as follows.

In the low-temperature region, the bulk conductivity is chiefly governed by the ionicsize and the valence of the guest substituting cation [6]. Thus this is termed as an im-purity controlled or extrinsic region. In case of both MgSO4 and CaSO4 added toAg2SO4 systems, the decrease in conductivity cannot be understood by the foregoingclassical theory of aliovalent doping only [3]. An extension of this theory, which apartfrom the concept of charge neutrality, takes into account the effect of lattice distortionto explain the above behaviour of conductivity [10, 13]. The partial replacement of thehost monovalent Ag+ by the divalent dopant M2+ gives rise to additional extrinsic va-cancies in the vicinity of the site of the guest ion in the host lattice. In addition to this,the Ag2SO4 lattice undergoes a strain, due to host and guest cation size mismatching,locally up to a few atomic distances in the proximity of the guest M2+. The magnitudeof this localized strain is governed by the extent of rg/rh as well as the electronic config-uration of the dopant ion (here, rg and rh are the ionic radii of the guest and host,respectively). Thus, the substitution of a smaller Mg2+ or Ca2+ facilitates an appreciablecontraction of Ag2SO4 lattice, whereas, upon substitution of the bigger Ba2+, the hostlattice undergoes an expansion. This is substantiated by the experimental results (Ta-ble 1). Consequently, depending on the type of distortion caused in the Ag2SO4 matrixthe mobile Ag+ ions would hop and migrate thus contributing to the conductivity. Thisis further supported in Fig. 2b, where the lattice contraction and expansion reduces andenhances the conductivity, respectively. These results are in good agreement with ourearlier studies of monovalent cation-doped systems [6, 10].

In sharp contrast to these results, the conductivity is seen rising as a function ofMnSO4 concentration in the low-temperature region (Fig. 2b). The conductivity beha-viour in the (1±±x)Ag2SO4 : (x)MnSO4 system is anomalous, particularly in the light ofthe fact that Mn2+ has a smaller ionic radius (rMn= 0.8 AÊ ) than that of Ag+. The resultspertaining to this system could be explained qualitatively on the basis of the electronicstructure of the dopant as follows.

Ionic Conductivity of Aliovalent Cation-Doped Ag2SO4 371

It is worthwhile to note that the manganese element along with iron, cobalt, nickeland zinc belongs to the d-block and is grouped in the 3d class of elements which aretransition metals. On the other hand, alkaline earth divalent cationic species are identi-fied with the s-block. Accordingly, each of these cations belonging to d- and s-blocksdistinctly has got its own particular electronic cloud and the corresponding distinct po-tential contours while it occupies a lattice site in Ag2SO4 matrix by way of solid solu-tion. It was suggested that the quadrupolar polarizability of mobile metal ions shouldreduce the energy barrier associated with the motion along mobile ion pathways fromthe minima at the site of high symmetry to the energy barrier at the site of lowersymmetry [14]. This implies that for `isostructural' materials mobile ions with high quad-rupolar polarizability should be relatively more mobile than ions with lower quadrupo-lar polarizability [15]. The Ag+ ion which is indeed a mobile ion may get more polar-ized when made to pass through the potential contours surrounding the ª3d-blockºdopant cation such as Mn2+. This will impart a higher mobility to the Ag+ in spite ofthe lattice contraction taking place on account of the fractional rg/rh ratio. However, asseen in Table 2 the lattice contraction is very small. Interestingly, then the mobile Ag+

will contribute largely to the conductivity owing to number of extrinsic vacancies avail-able and polarization because of substitution of Mn2+ ions in Ag2SO4 matrix. Similarresults are reported in the K2SO4 system [16].

4. Conclusion

From the present study it is concluded that the ionic size might be the predominantfactor contributing to the conductivity features of Mg2+, Ca2+ or Ba2+ doped Ag2SO4,but on the other hand, the characteristic features of Mn2+ doped Ag2SO4 could belinked to the presence of an asymmetric cloud of d-electrons.

Acknowledgement The authors are thankful to DST, New Delhi (India) for providingfinancial assistance to carry out this work.

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372 K. Singh and S. S. Bhoga: Ionic Conductivity of Aliovalent Cation-Doped Ag2SO4