Electrical conductivity studies in sulphate glasses and the mixed alkali effect t

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1 Prec. ndian Acad. Sci. (Chem. Sci.), Vol. 1, Number 3, June 182, pp Printed in ndia. Electrical conductivity studies in sulphate lasses and the mixed alkali effect t H G K SUNDAR and K J RAO* Solid State and Structural Chemistry Unit, ndian nstitute of Science, Baaalore , ndia MS received 31 December 181 Abstract. Electrical conductivities of alkali sulphate-zinc sulphate lasses have been measured. The variation of conductivity with compositon confirms the presence of the mixed alkali effect. The oriin of mixed alkali effect has been explained on the basis of structural considerations reported earlier by us. Keywords. onic lasses ; conductivity; mixed alkali effect, 1. ntroduction n an earlier paper (Rao and Sundar 180) on conductivity of sulphate lasses, we reported the mixed alkali effect in K~SO,-~Na,SO,-ZnSO4 lasses. We had attributed the effect to a structural oriin usin a viable structural model for sulphate lasses. Microscopic strain fields develop around the No+ ior.s and it could account for the observed conductivity minimum. We therefore considered it imperative to examine how other alkali ions behave in mixed alkali compositions of sulphate lasses. n this paper, the observation of the mixed alkali effect for two other alkali pairs is reported and the eneral conductivity expression suested earlier is found to be adequate to account for the mixed alkali effect. 2. Experimental Glasses were prepared usin ZnSO4 ' 7H~O, K~SO4 (BDH) and Rb~SO4 or Cs2SO, (Merck) samples with a minimum purity of ~o as batch materials. Batches weihin 5- were melted in platinum crucibles in an electric furnace and samples suitable for c onductivity measurements were obtaiv.ed as described earlier (Naras:;mham and Rao 178, Sundar and Roe 180). The conductivity cell and the measurin techniqu~ have also been discussed earlier (Narasimham et al 1" Communication No. 160 from the Solid State and Strnclural Chemistry Unit. *To whom all correspondence should be addressed. 207

2 208 H G K Sundar and K Y Rao 17). Tl~e resistances were calculated by measurin the current (with a 6 C Keithley elecetromter) throuh the sample for a known applied voltae (5 30. Ttxe current was passed only for very brief periods (less than 30 see at a time) in order to avoid polarizatior~. n the present studies also conductivities were measured beyond the lass transition and upto temperatures where the samples either deform under the sprin loaded electrodes or crystallised, thus renderin further measurem*mts inaccurate. All the lasses were annealed at a temperature K below T a before final measurements of (r were made. 3. Results and discussion The conventional plots of lo # vs T -1 are iven for various lasses in tiure 1. For clarity actual points have not been shown in tile conductivity plots. The lasses exhibit an inflection point which corresponds to the lass transition tempefuture. Variation of condttctivity (a), activation enery (B=) and the limitin conductivity (%) are plotted separately for Rb and Cs lasses in fiures 2 and 3. (11, L., (1) -. 7 TEe 11 "; ~_c d b e C" "G b 12 i "t2.,, ' Q o" 8 lo tl "s 0 -i Fiuro 1. Variation of conductivity with tompemturc. 50% ZrtSO~ (a) 45% K,SO4 5% Rb~SO4 (b) 35% K=SO4 15% Rb~SO4 (C.) 25% K:O 4 25~/~ Rb:O 4 (d) 20% Ka$O4 30% b~so4 (o) 15% K#O4 35% Rb,SO4 t 60~ Z~tSO4 (a) 3570 K=SO4 5Vo Cs=SO4 (b) 30% KsSO4 % C~SO4 (c) 25% K=SO4 15~ Cs#O4 (d) 20~ K~$O4 20~ (-~s~so4 (c) ~st. K~SO4 2S% C,,SO4 (f) 5 ~ K, SO4 35% Cs=SO4

3 Electrical conductivity studies in sulphate lasses 2O The relevant quantities for the pure K2SOcZnSO~ lasses were taken from earlier work (Narasimham et al 17). The percentaes of ZnSO4 in K-,Rb-~Zn and K-Cs-Zn lasses were 50% and 60% respectively since these formed very stable lasses. The activation eneries and lo no, values for Zn (Kb, K)SO4 lass exhibit no variation in the entire composition rane. The behaviour of these quantities in Zn (K, Cs)SO4 lasses exhibited considerable scatter but with an avera,: widish had little variation. The conductivity variations in fiures 2 and 3 clearly establish the occurrence of the mixed alkali effect in ionic sulphate lasses. n the structural model of sulphate lasses considered in our earlier study K + ions are co-ordinated to four sulphate ions. The occupation of K + ion sites by smaller Na+ ions causes a small deree of inward pull of the surroundin ions (al (b)~ <c) V _o ;-'~ ~ _ ~o o ~176 1,2 1.C i.0 Rb/Rb K Fiure 2. Variation of (a) lo (r, (b)lo (r0 and (c) activation enery duo to mixed alkali effect in 50 ZnSO4- xk~so4" (50- x) R.b~SO4 lasses. OO0 (a): (b) Col -.0 B b -.0 o bo $1.2 O ,6 o -o o ~ , Cs/Cs'- K Fiure 3. Variation of (a) lo (r, (b) lo a. and (el activation enery due to mixed alkali effect in 60 ZrtSO4. xk2so4" (40-- x) CSaSO4 lasses.

4 2 H G ~ Sundar and K J Rao Fiure 4. Schematic of possible rearranement of siructure when a K+ ion is substituted by larer Cs+ ion. Note that diffusion outwards is diff~ult when the larer ion is present. and hence creates a local strain Reld. This strain Reldhas the effect of lockin up the K + ions in the neihbourhood which, we assumed, would therefore not contribute to conductivity. Since the substitution of ~a+ ions in eneral leads to a decrease in volume, there is an averae enhancement of activation barriers. The contribution of Zn ~+ ions to conductivity was nelected throuhout. Similar considerations may be applied to the other alkali pairs thouh we do not have a clear idea of the nature of the co-ordination of Cs+ or Rb + ions. Substitution of K + ions by larer ions may not, in eneral, affect the activation enery since the anions can rearrane with no net strain when pushed outwards. We assume therefore, that the activation enerizs are unmodified. However, the K+ ions in the immediate neihbourhood of larer (Rb + or Cs+) ions are still locl~d up as schematically represent.~d in fiure 4 in two dimensions. The fact that activation eneries and the prc-exponcntional terms are not much affected is borne out in fiures 2 and 3. Hence, the conductivity variation is only due to the lockin up of a fraction of dissimilar alkali-ion pairs which ar~ nearest ncihbours. We can therefore repre. sent the total conductivity ~, of a xznso4 (-x)[f(rb, Cs)sSO4 (-f) KsSO,] lass by the expression, #, = (1 - x) A~.,+ ' exp _ R-T~ J + 2 (1 - x) (1 - f) Az+ exp L R T J + 2 (1 - x)farb+/c,+ exp[-- E~l'%'cs+'l RT J L.. a',,~.j ()

5 Electrical conductivity studies in sulphate lasses 211 A's and E's represent the respective pre-exponentional (frequency) factor and activation barriers. The last term represents the contribution from locked-up K + ions whose concentration is overned by the joint probability of the presence of K+ and the other alkali ion as immediate neihbours. Assumin that ARb+/cs+ " exp RT J A~+ exp in view of fiaro 2. Thus the value of fcorrespondin to the minimum conductivity may be obtained by differentiatin equation (1) with respect to f, RT -~7-0 =exp {-2(l-x) A+2(1-x)A - 4A (1 - x) z (1-2f)} = 0 or 2(1 - x) (1-2f)- O; or f-8 The value off obtained is in areement with the conductivity behaviour in fiures 2 and 3. The unique siz. ratio ot K + to O ~- is the factor which, while causin sinificant strain contributions when Ha+ is present, does not affect the activation barriers when larer alkali ions are present. We therefore feel that in ionic lasses the mixed alkali effect is primarily a structural consequence and conductivity is dominantly affected by packin considerations, perhaps unlike in other network and covalently bonded lasses (sard!6, Day 176) Acknowledements The authors are thankful to Professor C ]q R Rao for his kind encouraement. We are also thankful to the Department of Sciepce and Technoloy, Government of ndia, for financial assistance. References Day D E 176 J. Non-Cryst. Solids saxd J O 16 J. Non-Cryst. Solids Narasimhazn P S L and Rao K J 178 J. Non-Cryst. Solids Narasimham P S L, Sudha 1V[ahadevan and Rao K J 17 Proc. ndian Acad. Sci Rao K J and Sundar H G K 180 Phys. Chem. Glasses Sundar H G K and Rao K J 180 J. Chem. Soc. Faraday Trans