Ionic transport in the Na2SO4Na2WO4 and Na2SO4M2(SO4)3 (M=La, Dy, Sm, In) systems

Solid State Ionic.s23 (1987) 151-163 North-Holland, Amsterdam

IONIC TRANSPORT IN THE Na2SO4-Na2WO 4 AND Na2SO4-M2(SO4) 3 (M = La, Dy, Sin, In) SYSTEMS G. PRAKASH * and K. SHAHI # Department of Materials Sciences, Indian Institute of Technology, Kanpur 208 016, India

Received 30 September 1986; accepted for publication 7 October 1986

Premelted, predried Na2SO4, premelted Na2WO4, Na2 SO4-Na2WO4 composites and N a 2 S O 4 - M 2 ( S O 4 ) 3 (M = La, Dy, Sin, In) have been studied by means of X-ray diffraction, DTA and electrical conductivity measurements. The high temperature, highly conducting Na2SO4 phase I has been successfullystabilised at room temperature; the Na2 SO4 containing 4 mole% La2 (SO4)s exhibits the highest conductivity (o) and lowest activation energy (Ea) (o = 1.08 X 10-3 I2-1 cm-1 at 290°C and Ea = 0.50 eV) and therefore this system appears promising for further development..

1. Introduction

2. Experimental details

Solid electrolytes [1-4] characterized by very high ionic conductivity and relatively small electronic conductivity have attracted a great deal of attention because of their potential applications in batteries, sensors, etc. Our main interest lies in the development of a high conductivity material for low temperature applications. Recently, there have been attempts to make use of Na2SO 4 as solid electrolyte for a SOx detector [ 5 - 8 ] . It has been shown that Na2SO 4 has five polymorphs, and several investigations have been carried out to study the crystal structure of these phases [9, 10]. The phase transition [11-13] and the electrical conductivity studies at elevated temperature have been reported in air or SOx gases [5,14]. Following the work of Keester et al. [15], several attempts have been made to enhance the electrical conductivity and to stabilize the Na2SO4.-I phase at room temperature [ 1 6 - 1 8 ] . In this work, we report on the Na2SO 4 Na2WO 4 and the Na2SO4-M2(SO4) 3 (M = La, Dy, Sm, In) systems and investigate the stabilization of Na2SO4-I at room temperature.

The starting materials used in this work are Na2SO4, Na2WO4.2H20, In2(SO4) 3, La2(SO4)3, Sm2(S04) 3 and DY2(SO4) 3. Anhydrous Na2SO 4 was obtained from Sarabhai Chemicals (India), Na2WO4 • 2 H 2 0 from BDH Chemicals (England), In2(804)3 from SISCO (India) and the other M2(SO4) 3 (M = La, Dy, Sin) from Indian Rare Earths Limited.

* Present address: Tara Energy Research Insitute, 7, Jot Bagh, New Delhi 110 003, India. # Author to whom all correspondence may be addressed.

0 167-2738/87/$ 03.50 © Elsevier Science Publishers B.V. (North.Holland Physics PublishingDerision)

2.1. Na2SO 4 - pure

Na2SO 4 was heated to about 200°C in an oven for 4 h to eliminate any water content and then weighed to make the batches in the case of the binary system. The Na 2 SO4 sample was prepared by grinding in a pestle and mortar assembly for about 20 rain and a part of this powder was melted in a porcelain crucible with the help of a resistance heated box-type furnace. Once Na2SO 4 melted (at "~884°C) completely, it was allowed to stand there for about 15 rain before being poured onto a clean aluminium plate. The resolidified Na2SO 4 was then finely ground and used for making pellets. A similar treatment was followed for the Na2WO4 sample (m.p. 697°C).

152

G. Prakash, K. Shahi/lonic transport in Na2SO4-Na2 W04 and Na2804-M2 (804)s systems

2.2. Na2SO4-Na2W04 system

2.5. Preparation of pellets

The various composition§ of the Na2SO 4 + x m/o Na2WO 4 system (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100) were weighed using a Sartorius (Model No. 2006 MP) digital electronic balance tO an accuracy of 0.0001 gm. Each weighed composition was carefully transferred to the agate mortar, thoroughly mixed, and ground before transferring it to the porcelain crucible for melting. This was followed by quenching it in air as described earlier for pure Na2SO4 . Once again the resolidified samples were well powdered and then pelletized in a manner discussed later.

The fine powder of each compositon was then transferred to a stainless-steel die. After levelling the powder by means of a die.piston, the whole assembly was placed in a hand-operatedhydraulic press. For pelletization of Na 2 SO4-Na2wO 4 compositions, a pressure of 4 tons/cm 2 was applied while in case of other binary systems, a slightly higher pressure (6 tons/cm 2) was used. The piston diameter fixed the pellet diameter at 0.975 cm, while the thickness of the pellets usually ranged between 0.6 and 0.9 cm.

2. 3. Na2SO 4-In2(S04) 3 systems The sample preparation in the case of the Na2SO4 In2(SO4) 3 system had to be varied to an extent. The first of the series to be probed, the 5 m/o In2(SO4) 3 sample, showed some sign of decomposition on melting. The material became somewhat yellowish and did not recover its original colour. The next batch of the same composition was therefore heated to a temperature lower than the melting point. The resulting material was not only less yellow than the previous one, but also gave good conductivity results. This encouraged us to probe further in this direction. Hence four samples of 5 m/o In2(SO4) 3 were prepared by heating at four different temperatures (below melting point) and for different durations, and then allowed to cool to room temperature by pulling the crucible out of the furnace. The powder was reground and then pelletized. The surprising results obtained with the 6 m/o In2(SO4)3 encouraged us to carry out similar tests with the 10 m[o In2(SO4) 3 sample. This exercise resulted in optimization of the heating temperature arid duration of heating for high conductivity.

2.4. Na2SO4~M2(S04)3 system In case of the Na2SO4-M2(SO4) 3 systems (M = La, Dy, Sm), a similar treatment was followed as for the Na2SO4-In2(SO4) 3 to see if the two methods of preparation (i.e. melting and heating to a temperature below the melting point) made any difference.. ,

2. 6. Measurements In this work, extensive electrical conductivity measurements have been carried out. The temperature inside the furnace was controlled to a precision of 1° using an Eurotherm controller. A General Radio 1608 A impedance bridge was used for measuring the conductance. The fiat surfaces of the cylindrical pellets were polished to obtain parallel and smooth surfaces so that the platinum foils placed over the thicker platinum back up electrodes adhere well to the specimen. The tip of the thermocouple was placed close to the sample to efficiently control and measure the temperature. The samples were annealed at 200 and 300°C in case of the Na2SO4-Na2WO 4 and Na2SO4-M2(SO4) 3 (M = La, In, Sm and Dy) systems respectively. The temperature was then raised to the highest temperature and the elctrical conductivity measurements were made during the cooling cycle. In order to substantiate the conductivity results, it is necessary to identify the various phases since Na2SO4 exhibits a number of them. The results have limited role since the X-ray diffraction (XRD) was carried out only at room temperature. The XRD was recorded using a Rich Siefert (ISO-DEBYEFLEX 2002) counter diffractometer employing a filtered Cu Kct radiation. Differential thermal analysis (DTA) was done using a Mom Derivatograph with a heating rate of 10°C/min, using ceramic crucibles and P t [ P t - 1 0 % Rh thermocouples.

G. Prakash, K. Shahi/Ionic transport in Na2SO4-Na2 WO4 and Na2SO4-M2fS04) 3 systems

3. Results and discussion 3.1. X-ray diffraction (XRD) Considerable work has been carried out on the polymorphism of Na2SO4; the various phases and tranformations have been reported by Murray and Secco [19]. The room temperature phase V is orthorhombic in nature and ofter referred to as thenardite. The prominent phase transition at 241°C results in the hexagonal phase I, which cannot be quenched at room temperature. Our studies reveal that the sample which was dried above 200°C is orthorhombic (phase III). The same sample after some days shows only lines corresponding to phase V indicating that a slow III -~ V transition takes place. This kind of transformation to thenardite has been reported earlier by Kracek [20]. The premelted Na2SO4 clearly indicated phase V only which is consistent with the already reported data that the high temperature, hexagonal phase I cannot be quenched to room temperature. Unlike Na2SO4, Na2WO 4 does not have too many solid-solid transitions. The only known and identified phase has the ideal cubic spinel type structure with a = 9.1297 A. However, there is at least one report by Bottelberghs [21] which claims the existence of another phase, i.e./5-Na2WO 4 over a very narrow temperature range (589-590°C). Our own studies are consistent with this report as discussed later in the conductivity results. The Na2SO4 + 10 m/o Na2WO 4 exhibits two phases; that of Na 2 WO4 and phase V of Na 2 SO4 which is the stable room temperature orthorhombic phase. In addition to these, certain strong lines were observed in the pattern which could neither be attributed to Na2SO4 nor to Na2WO 4 and for a while we thought that some intermediate compound was being formed. However, a continued search to identify this unknown phase revealed the presence of Na2WO4 • 2H20 which happened to be the starting material. This implies that during the Na2SO4-Na2WO 4 . 2H20 composite preparation, the water content goes away and the XRD pattern obtained immediately after preparation shows lines corresponding to Na2SO4 and Na2WO4 only. However, the composite regains the water of stored at room temperature for sometime, and therefore lines

153

corresponding to Na2SO4, Na2WO4 and Na2WO4 • 2H2 0 are observed. The intermediate compositions (e.g., 40, 50 and 60 m/o Na2SO4) strongly suggest that Na 2 WO4 •2H 2 0 takes over as the prominent phase, though the other phases are still present. The XRD patterns of the Na2SO4 +x m/o La2(SO4)3 (x = 2, 4 and 6) indicate that a sufficient amount of the high temperature phase can be quenched in these solid solutions. The earlier results indicate solid solution formation in this system [15]. The XRD analyses of the two compositions in the Na2SO4-Sm2(SO4) 3 system were carried out, and the results show that the sample containing 2 m/o Sm2(SO4) 3 exhibits phase V, the stable room temperature orthohombic phase, and the metastable phase III of Na2SO4. The pattern of Na2SO4 + 4 m/o Sm2(SO4) 3 shows that Na2SO4-I is stabilized only to a limited extent. In the case of the Na2SO4 + 4 m/o DY2(SO4) 3 sample, the XRD pattern indicates lines corresponding to Na2SO4-I only. The DTA trace, however, shows a peak at 240°C correspondint to the V ~ I phase transition. It is possible that a small fraction of phase V is present together with phase I, and that the XRD fails to detect the presence of former. 3.2. Differential thermal analysis (DTA ) Besides determining the transition temperatures, DTA has been used to study the phase behaviour in multicomponent systems such as solid solubility, two phase mixtures, etc. Na2SO4 has a number of phase transitions and the most prominent one is the V -~ I transition occuring at about 241°C. The DTA reveals that the transition in our sample occurs at 240°C. There are no other peaks, and this is in agreement with the reported work in literature. Na2WO 4 is known to exhibit a solid-solid transition at ~589°C where the room temperature 7-phase transforms to/3-phase which is claimed to be stable over a very narrow temperature range (589-590°C) and more conducting [21 ]. However, our DTA shows four peaks; two strong peaks at 583 and 696°C corresponding to 7-~3 and melting transitions respectively, and two rather weak peaks at 530 and 650°C which were not observed before. Due to unavailability of high temperature X.ray diffraction facility, these thermal events (at 530 and

154

G. Prakash, I(,. Shahi/Ionic transport in Na2SO4-Na 2 WO4 and Na2SO4-M2(S04) a systems

650°C) could not be further verified. But these peaks, however weak, are present not only in pure Na2WO 4 but also in composites containing different amounts of Na 2WO4 . The DTA of Na2SO4-La2(SO4) 3 system was carried out mainly to verify the results of XRD analysis which had earlier indicated that the phase I of Na2SO4 gets stabilised to room temperature. The DTA results are shown in fig. 1 for pure Na2SO4, Na2SO4 - 2 m/o La2(SO4) 3 and Na2SO 4 - 4 m/o La2(SO4)3. It is observed that the V ~ I transition in Na2SO 4 at 240°C is not present in Na2SO 4 doped with 2 and 4 m/o La2 (504) 3 which supports the conclusion drawn from XRD analysis. It should be mentioned however, that there is a very minor kink at ~320°C in the DTA trace of Na 2 SO4 + 2 m/o La2(SO4) 3 which probably is insignificant in view of the fact that no such kinks are present in pure Na2SO 4 or Na2SO4 + 4 m/o La2(SO4) 3. The DTA of Na2SO 4 + 6 m/o La2(SO4) 3 also does not show any peak, and therefore it is con-

cluded that the addition of La2(SO4) 3 stabilizes the high temperature phase I of Na2SO 4 at room temperature. Both DTA and XRD of Na 2 SO4 containing 2 and 4 m/o Sm2(SO4) 3 consistently showed that the phase V ~ I transition ofNa2SO4 was still present. However, the XRD corresponding to 4 m/o Sm2(SO4) 3 indicated the presence of both phases (V + I) of Na2SO4 at room temperature suggesting a partial stabilization of phase I at this temperature. This was further supported by DTA which showed a peak at ~250°C. The DTA trace of the Na2SO4 + 4 m/o DY2(SO4)3 composition showed a peak at 240°C, corresponding to the phase V ~ I transition of Na2SO 4. The XRD on the other hand, showed lines corresponding to phase I of Na2SO 4 only suggesting that the phase I had been fully stabilized at room temperature. It would seem that the DTA is sensitive to the traces of any Na2SO4-V phase which might be present and could not be detected in the XRD pattern.

3.3. Electrical conductivity Pre- melted

No2 SOz.

AT I 240

[a) I

I

100

I

I

200 300 L,O0 Temperature ,°C

I

I

500

600

Na 2 S0~ +2 ~I~ L ~ (S 01,]3 - - V

AT

I

320

{i) RT

I

I

I

100

200

300

I L,00

I 500

Temperoturel~C N o 2 S O ~ 4 ~/o Loz(SO~) s AT (j) RT

I 100

I 200

I 300

/~00

500

Temper oture, oC

Fig. 1. The DTA results for pure Na2SO4, Na2SO4-2 m/o La2(SO4)s and Na2SO4-4 m/o La2(SO4)s.

The electrical conductivity was measured at a fixed frequency of 1 KHz. The literature data indicates that Na 2 SO4 exhibits negligible electronic conductivity which is remarkable, and the transference number measurements show that almost all the curr6nt is carried by Na+ ions. The results of electrical conductivity measurements have first been discussed for the starting materials (Na 2 SO4 and Na2WO4), and then the two component systems have been dealt with. There is considerable information available on Na2SO4 primarily because of its interesting superionic transition at 240°C. The binary sulphate systems like Na2SO4-Li2SO4, Na2SO4-ZnSO4, Na2SO4-Y2(SO4)3 etc., have also been probed to quite an extent. A report by Keester et al. [15] that Na2SO4-I forms extensive solid solutions with other sulphates, i.e. MeSO4 (Me = Ni,Co,Mg,Cu,Zn,Mn,Cd, Ca,Ba,Sr,Pb) and Me2(SO4)3 (Me = Fe,In,Y,Gd,La) giving rise to massive defect structures (upto 15% cation vacancies), has thrown open a very broad range of systems which can be examined in a bid to stabilize the highly conducting, high temperature phase of Na2SO4 at or around room temperature.

G. Prakash, K. Shahi/Ionie transport in Na2SO4-Na 2 W04 and Na2SO4-M2(S04} a systems

3.3.1. Na2SO4 560

The electrical conductivity results of the present investigation along with the earlier studies on Na2SO 4 are summarized in table I. Hofer et al. [17] state that the details of the defect structure are not clearly understood. The report of Jacob and Rao [5] indicate the same. It is evident form table 1 that the various results are not consistent. Despite the efforts made by some workers, the defect structure in Na2SO 4 is not yet known. In fact the very first question as to whether Schottky or Frenkel type of defects prevail in Na2SO 4 has still not been answered satisfactorily. However, considering the fact that the Na + ion is much smaller than the SO~-2 anion and further that the former is monovalent and the latter divalent, it can be safely assumed that Na 2 SO4 is likely to exhibit cationic Frenkel defect just as PbF 2 and CaF 2 are known to exhibit anionic Frenkel defect. The fact that Na2S exhibits cationic Frenkel disorder [22] lends further support to the above proposition for Na 2 SO4 . There are no other sodium salts involving divalent anions with an established defect structure to further substantiate the above proposition for Na2SO 4. There discussion, it is therefore presumed that Na 2 8 0 4 exhibits cationic Frenkel defect. The electrical conductivity, o, of pure Na2SO 4 has been measured for two types of samples; (i)Na 2 SO4 dried at 200°C before pelletization and (ii) Na2SO 4 melted and resolidified before pelletization. The results are shown in fig. 2. It is noted immediately that

/#-.1 I

155

Temperature in *C 352 282 227 1

f

I

181 I

•

Premelted No,SO 4

•

Predried NQ2SOt,

-d

_:.

C ~ -z ~ _~ b -5 -~ -6

-7

-8

1,0

1.2

I

1.4

I

I

1.6 1.8 103/T (K -1)

I

2.0

a

T* C (D =1 em -1)

Ea (eV)

Temp. range (0 C)

Refs.

1.5 × 10 ..2 6.93 x 10 - s 1.58 X 10 - s 2 X 10 -a 4.0 X 10 -6 1.8 X 10 -4 6.93 X 10 - s 1.21 X 10 -a

800 400 360 700 352 527 400 500

1.36 0A4 0.84 0.93 0.51 0.50

640-800 250-727 250-800 250-560 240-460 260-575

[71 [34]

b) premelted Na2 SO4.

2.4.

Fig, 2. Electrical conductivity (o) of pure Na2SO 4 for two types o f samples: 0) Na2SO4 dried at 200°C before pelletization and (fi) Na2SO4 melted and resolidified before pelletization.

Table 1 The electrical conductivity results o f Na 2 SO4 alongwith the earlier studies.

a) Predried Na2 SO4;

i

2.2

[351 [81 [171 this work a) this work b)

156

G. Prakash, K. Shahi[Ionic transport in Na2SO4-Na2 WO4 and Na2SO4-M2(S04) a systems

the conductivity of pre-melted Na 2 SO4 is higher than that of a pre-dried sample. This may be due to the fact that a fraction of lattice defects produced at higher temperatures on melting is retained on cooling. Similar results have been reported for Li2SO4 by Deshpande and Singh [23]. In the log o versus 103/Tplot for pure Na2SO 4 shown in fig. 2, the high-temperature region above 240°C corresponds to the hexagonal phase I of Na2SO 4 which we call a-phase. The conductivities and the activation energies of a-Na2SO 4 (stable over 240°C) are summarized in table 1. It is noted in table 1 that there is a considerable scatter in the reported values of activation energy. From fig. 2 and also table 1, it is almost clear that a-Na2SO 4 exhibits extrinsic and intrinsic regions of ionic conduction which is a common feature of normal ionic solids rather than fast ionic conductors. The intrinsic regionseems to set in around 460,480°C except for the pre-melted sample in cooling cycle. The result of Saito and Maruyama [7] who have reported the conductivity of Na 2 SO4 at high temperatures (640-800°C) is consistent with the above view. Thus their activation energy of 1.34 eV must be identified with the one in the intrinsic region. All other studies including ours are mainly confined to the extrinsic region. Despite the large scatter in the reported values of activation energy, a qualitative agreement can now be seen. The activation energies of 0.84 and 0.93 eV reported respectively by Jacob and Rao [35] and Hofer et al. [17] are higher than ours (0.50 eV) because they seem to have included some data of intrinsic region in evaluation of E a for extrinsic region. E a = 0,.44 eV reported by Adachi et al. [34] is in disagreement with the other three investigations. Thus we can conclude that the activation energy (Ea) of extrinsic and intrinsic regions are 0.51 and 1.34 eV respectively. Therefore Ea(extrinsic ) = h i or h v and Ea(intrinsic ) = 1/2 hf + h i or h v , where hf and h m are enthalpies of formation and migration of defects and the subscripts i and v refer to interstitials and vacancies respectively. In order to separate hf and h m we need to know whether interstitials or vacancies are more mobile. This information about Na2SO 4 is not available. However, it is generally found that the cation interstitials are more mobile

than vacancies, for example in AgX (X = C1,Br,I). If we assume this to be the case with Na2SO 4, we find h i = 0.51 eV and hf = 1.66 eV which appear reasonable in view of the values o f h f = 1.77 eV and h~n = 0.76 eV for NaS, The conductivity-temperature dependence in extrinsic range for t~-Na2SO4 can be given by o = 0.43 exp(-0.51 e V / k T ) ,

240-460°C.

(1)

3. 3.2. Na 21¢04 A literature survey reveals that not much work has been done on Na2WO4. Its structure is reported to be cubic spinel type upto 588°C, where it undergoes a phase transition. The ~-phase is stable only over a degree or so and then a / 3 - a transition occurs. The ~phase is the most conducting one and efforts have been made to decrease the t~-~ transition temperature and to retain the/3-phase over a temperature range. Bottelberghs [21] has reported that the conductivity of the order of 10 -2 [2 -1 cm -1 is exhibited by Na2WO4 at 589°C and also that the ~/~/3 transition temperature could be lowered. A metastable ~-phase region has also been reported therein. The defect structure of the phases have however not been examined. Our results are in agreement with those of Bottelberghs [21] in that we do observe the/3-phase which is stable over a range of I°C only (589°C) and where the conductivity was found to be maximum. The logo versus 103/T plot shown in fig. 3 reveals this feature. The activation energy prior to transition is worked to be 0.96 eV. Arguing along the lines of Na2SO 4, the unequal sizes and vacancies of the Na + and WO~-2 ions suggest that Na2WO 4 must also exhibit cationic Frenkel defect. There are no reports however on the formation and migration energies of the defects. The conductivity of Na2WO4 is less than that of Na 2 SO4 at lower temperatures suggesting higher activation energies of formation and migration o f N a ÷ ions in Na2WO4 than in Na2SO 4. The h i = 0.96 eV derived from the conductivity-temperature dependence of Na2WO4 o = 26.93 exp(-0.96 e V [ k T ) ,

327-580°C.

(21

Eq. (2) does support this observation. Bottelberghs [21] has reported that the highly con ducting/3-phase can be made stable in the range of

G. Prakash, K. Shahi/lonic transport in Na 2SO4-Na2 W04 and Na2SO4-M 2 (S04) 3 systems

-1 727

560 [

/,/.1 I

Temperoture in °C 352 282 22,7 I

I

•

181 I

lZ

Prernelted Na2WO,.

-2

-3 .-,.

'E u

+4

6- 5

-6

-7

157

AgI-SiO 2 [24-27] dispelled these ideas and pointed out that the enhanced electrical transport in multiphase mixtures was a rather general phenomenon, and not merely limited to f'me AI203 particles. The enhanced electrical conductivity in homovalently doped solid solutions has also generated interest. The classical theories of ionic conductivity do not postulate that the size of the dopant makes any difference, the charge alone being responsible for whatever happens. Recent investigations on AgI-AgBr, KI-KBr etc., [27-29] have proved that the effect of size may be significant in certain cases. The experimental activities in these areas have been backed by theoretical models proposed to explain the enhanced electrical conductivity in the mixed crystals as well as in multiphase mixtures [30--32]. The conductivity versus composition curves at two different temperatures, i.e. at 500°C and 327°C are shown in fig. 4. The conductivity of premelted Na 2 SO4 itself is quite high at 500°C and hence the effect of addition of Na2WO 4 is not pronounced. However, -2

-8

1.0

I

I

1.2

1./.

l

I

1.6 1.8 103/T {K "l)

I

J

2.0

2.2

2./. I

II

-3

Fig. 3. The log o versus 10a/T plot of premelted Na2 WO4.

545-575°C with a limited doping of about 8 m/o Na 2 SO4 and other dopants. Further addition is believed to cause the/3-phase to disappear.

3. 3. 3. Na2SO4-Na 2 WO4 composites

'E -~ u T E

~o 6

a

SO0O c

~-s

In order to discuss the conductivity behaviour of Na2SO4-Na2WO 4 composites, we encounter two different compositon regions: (i) in which the two salts form solid solution and (ii) two-phase mixture. Both these areas i.e. fast ion transport in mixed crystals or solids solutions and two phase or multiphase mixtures have only recently emerged as areas of considerable interest. The earlier studies involved the dispersion of Free (micron size) A1203 powder in LiI, AgI and other ionic conductors, and all the reports pointed out that the enhancement in conductivity was mainly due to something very unique with AI2 03 particles alone. However, the studies on AgI-AgBr, AgI-flyash,

327°C -8

A

-7

I

1

0

20

I

I

1

/..0 80 80 Mole % Na~,WO~.

i

100

Fig. 4. The conductivity versus composition curves at two different temperatures at 500°C and 327°C of Na2 SO4Na2 WO4.

G. Prakash, K. Shahi/Ionic transport in Na2SO4-Na2 W04 and Na2SO4-Mz (S04)3 systems

158

the plot exhibits a maximum (I) at 10 m]o Na2WO4 and the region below this composition range can be termed as the solid solution region where Na2SO4 possibly dissolves all of Na2WO4 at this temperature (500°C). Unfortunately, we could not carry out the XRD work at 500°C to confirm the above point. Starting now from the other side (100% Na2WO4), it appears that at 500°C, Na2WO4 can dissolve upto 30 m]o Na2 SO4 and our results indicate that the conductivity of Na2WO 4 increases with the increasing concentration of dissolved N a 2 S O 4 passing through a maximum around 30 m/o Na2SO 4 and then the o decreases. This maximum (marked II in fig. 4 is more pronounced than the maximum marked I). In the two-phase region, the results are surprising and interesting too. All the previous investigations [25-27] suggest that the o of the two phase mixtures is usually larger than that of either of the pure components, leading to only one maximum in the o-corn-

727

560

~41

I

I

TemperQture in *C 352 282 227 I

I

1

181

position plot. Our results on Na 2 SO4-Na2WO 4 (fig. 4) are consistent with previous studies [27,28] as far as the solid solution regions are concerned, but very different for two-phase mixture region. We f'md that the conductivity begins to decrease as soon as the solid solution range is over and the two-phase region begins. The result is that the o-composition curve at 500°C exhibits two maxima (at 10 and 70 m/o Na2WO4) and consequently a minimum (at 50 m/o Na2WO4). The conductivity as a funciton of temperature for Na2SO4 + x m/o Na2WO 4 is shown in two subsequent figures for the purpose of clarification. Fig. 5 shows the results for x = 0, 20, 40, 50, 80, 100 and fig. 6 for x = 10, 30, 50, 70, 90. The conduction data, i.e. the pre-exponential factor and the activation energy E a for all the compositions are summarized in table 2. Figs. 5 and 6 indicate that the low temperature conductivity of all the composites lie between that of Na2SO 4 and Na2WO 4, This is more clearly observed in the conductivity versus composition curve at 327°C (fig. 4). Further, the higher temperature region indi-

144

I

Na~SO~ : Na2WO~ •

~

-2

o

100 80

•

60

• v

40

v

20 0

-3

0 20 40 60 80 100

-1

727 I

560 '1

441 i

T e m p e r o t u r e in °C 352 282 227 i I i

181 I

144 i

Na2SO~ : Na2WO~

-2

tx o •

90 70 SO

10 30 50

• •

30 10

70 90

-3

%,

'E - /

'rE _~ 7 b -5 o~ -E -6

-7

-8-

1.0

I

1.2

1

1.4

1

I

1.6 1.8 103/T (K -I)

I

2.0

!

2.2

Fig. 5. The conductivity as f u n c t i o n o f temperature for Na2SO4 + x m]o Na2WO#.

2.4

I 1.0

] 1.2

I 1.4

I I 1.6 1.8 103/T (K " l )

! 20

I 2.2

I 2./.

Fig. 6, The conductivity as a f u n c t i o n o f temperature for Na2 SO4 + x m / o Na2 S 0 4 .

(7. Prakash, K. 8hahi/Ionic transport in Na2SO4-Na2 I¢04 and Na2SO4-M2(S04) a systems

159

Table 2 Activation energies and pre-exponential factors for Na2SO4-Na2 WO4 compositions. Material

o

Na2 SO4 (predried) Na2 SO4 (premelted)

6.93 x 7.33 x 1.21 x 1.50 x 1.42 x 4.31 x 1.24 x 2.42 x 5.73 x 4.03 x 2.32 x 1.26 x 1.62 x 6.86 x 2.11 x

Na2W04 (premelted) Na2S04 + 10 m/o Na2W04 Na2 SO4 + 20 m/o Na~WO4 Na2SO4 + 30 m/o Na2WO4 Na2SO4 + 40 m/o Na2W04 Na2S04 + 50 m/o Na2W04 Na2S04 + 60 m/o Na2WO4 Na2 SO4 + 70 m/o Na:~WO4 Na2S04 + 80 m/o Na2WO4 Na,~SOl + 90 m/o Na2WO4

10-s 10-7 10-a 10-s 10-a 10-s 10-a 10-s 10-4 10-4 10-s 10-s 10-s 10.-6 10-7

T° C (s2-1 cm-1)

Ea (eV)

Temperature range (°C)

oo (s2-1 cm-1)

400 205 500 500 500 327 500 327 500 500 327 327 327 327 327

0.51 1.69 0.50 0.96 0.59 0.53 0.86 0.65 0.84 0.78 0.48 0.80 0.77 0.82 0.99

240-460 180-225 260-575 327-580 255-575 227-400 400-600 190-360 360-600 327-600 200-395 190-427 190-400 227-427 300-460

0.43 5.56 x 1012 2.34 26.93 9.44 1.25 5.39 x 10 2 6.86 1.63 x 102 49.2 0.26 68.97 44.53 56.95 41-73

cates that some of the composites exhibit a higher conductivity than Na2SO4 . This may be due to the solubility of Na2WO 4 in Na2SO4, thereby leading to lattice loosening [28] and hence excess of defects and therefore a higher conductivity. The other sample that exhibits a higher conductivity than Na2SO4 at higher temperatures is Na2WO 4 which does so because of the phase transition at 589°C. Our finding is that the phase transition in Na2WO4 is still present even when it contains 10 m/o Na2SO4, and that the transition temperature drops down to 5550C from 589°C. The dissolved Na2SO4 not only decreases the transition temperature but also introduces defects that go on to enhance the electrical conductivity. The conductivity-composition curve does show this.

3.3.4. Na2SO4-La2(S04) 3 system The Na2SO4-La2(S04) 3 system,,',mvolves dop~g of Na2SO4 by aliovalent La+3 ions of wrong charge . Each La+3 ion which goes into the Na2SO4 lattice and replaces the host Na + ion introduces two Na+ ion vacancies in order to preserve the electrical neutrality of the crystal and thereby enhances the o. The present investigation was promped by the report of Keester et al. [15] that in ot-Na2SO4 (the high temperature modification) the Na+ can be replaced

to a large extent (upto ~ 15%) by divalent and trivalent ions such as Ni, Mg, Cu, Co, Zn, Mn, Cd, Ba, Ca, Sr, Pb, Fe, In, Y, Gd and La, and thus extensive solid solutions are formed which can also be quenched to room temperature. Following this report Hofer et al. [17,18] examined some of these materials (Na2 SO4 NiS04, Na2SO4-Y2(S04)3, Na2SO4-SrSO4 and Na2 SO4-ZnSO4) for fast ion transport. The highest conductivity they reported was 1.5 X 10 -2 ~2-1 cm -1 at 500°C and the least activation energy reported was 0.7 eV. Our XRD results indicated solubility of La2(SO4) 3 in Na2 SO4 at least to the extent of 6 m/o. These resuits together with the absence of any peak in DTA imply that the quenched solid solution at room temperature is the high temperature hexagonal a-phase of Na2SO4. Fig. 7 shows the conductivity versus composition curves at four different temperatures (180,227, 427 and 600°C. The curve for 180°C shows a maximum at 2 m/o La2(SO4) 3 since the effect of"wrong charge" dominates at lower concentrations and lower temperatures. For 2 m/o La2(SO4)3, the conductivity is more than thousand times higher than that of pure and premelted Na2SO4 at 180°C. The conductivity decreases with further increase in La2(SO4) 3 concentration probably because of the formation of impurity-vacancy complexes at higher levels of dopant

160

G. Prakash, K. Shahi/lontc transport in Na2SO4-Na 2 W04 and Na2SO4-M2(S04) 3 systems -1

-2

3.3.5. N a 2 S O 3 - S m 2 ( S 0 4 ) 3 system

•

/

~-

600°C

~427°C

-3

~

'E -4 t~ T E

227°C

/

",, 18o°c

/

b -5

-6

-7

-8

I

O

I

I

2 .Z, mole % Lo2 (SOz.)3

The conductivity results in this system are expected to be similar to that of La2(SO4) 3 doped Na2SO 4. The Sm 3+ being a trivalent cation is expected to generate two Na + ion vacancies, just as in case of La 3+. Fig. 8 shows the log o versus 103/T plot for 2 and 4 m/o Sm2(SO4) 3 . The conductivity of the Na2SO 4 + 2 m/o Sm2(SO4) 3 composition is about 800 times more than that for premelted Na2SO 4 at 180°C. This enhancement is possible only due to the large number of vacancies produced by the Sm 3+ ion doping. Further, the conductivity data recorded down at 100°C does not indicate a change in the slope of log o versus 103/T plot around the V-* I phase transition temperature of Na2SO 4, implying that the conductivity arises from the phase.I contribution, though phase-V also exists in this composition as reflected by XRD. The change in the slope at about 290°C would probably suggest a change in the conduction mechanism.

1

6

560 I

-1

4,41

Temperoture in °C 352 282 227

181

i

L

No2SO~

Fig. 7. Conductivity versus composition curves of (Na2SO4 + x m/o La2SO4)3 at four different temperatures (180, 227, 427 and 600* C).

144 i

112 i

:Sm2(SOz.)3

• o

I00 98

0 2

A

96

4

% -3

concentration. The concentration of such impurityvacancy pairs is of course temperature dependent. The higher the temperature, the lower the concentration of these pairs due to thermal agitation. At 427°C, it can be seen that the conductivity increases upto 2 m/o La2(SO4) 3 and then remains almost constant for higher concentrations of La2(SO4) 3. This can be explained on the basis of the fact that the concentration of the impurity-vacancy clusters is reduced due to the thermal effects and more vacancies are released which contribute to the conductivity. The effect of temperature is more clearly evident at 600°C, where the maximum itself shifts to 4 m/o La2(SO4) 3 as the optimum number of mobile vacancies are available there. With 6 m/o La2(SO4) 3 , once again the impurity-vacancy complexes account for the lesser conductivity.

"7 E 5 -5

-6

\

-7

-8

10

I

1.2

I

1.L

I

1.6

I

I

1.8 2.0 103/T (K -1)

I

2.2

I

2.4

I

2.6

Fig. 8. The conductivity versus temperature plots for two different combinations of Na2 SO4 + Sin2 (SO4)3.

G. Prakash,K. Shahi/Ionic transportin Na2SO4-Na2W04 and Na2SO4-M2 (SO4)a systems 3. 3. 6. Na2SO4-Dy2(S04) 3 system In this system, only one composition, Na2SO4+ 4 m/o DY2(SO4) 3 could be examined. It is seen that in this system, the activation energy in the low temperature region (0.75 eV) is quite high and hence the conductivity is low. The Na2SO4-I phase stabilization at room temperature, indicated by XRD and confirmed by DTA is responsible for conductivity enhancement. The activation energy of 0.75 eV cannot be associated with h v alone.

3.3. 7. Na2SO4-1n2(SO4b system Considerable effort was put in to characterize this system unsuccessfully, as the results were quite a variance for different preparational conditions. The XRD and DTA for this system had already indicated that this system is rather unstable with increasing content of Ir12(SO4)3 . The conductivity results futher strengthened this view because the results were quite difficult to reproduce. This system therefore, will not be discussed in any detail except that we would like to mention that the samples prepared by heating the mixture at 780°C for 8 h exhibited the highest conductivity. The conductivities were lower for samples either prepared by melting the mixture or heating it above or below 780°C.

4. Comparative studies and conclusion

The five systems that have been studied can be

categorized i'n two groups based on the nature of the dopant used. The first group includes the system in which the dopant is homovalent, i.e., the constituent ions of the dopant have the same valency as the host ions e.g., Na2 SO4 - N a 2 WO4 system. The second group includes systems where the dopant is an aliovalent ion e.g. Na 2 SO4-La2(SO4)3 , Na2SO4-Sm2(SO4) 3 and Na2SO4-DY2(SO4)3. It should be interesting to compare the results obtained for these systems. The conduction data for Na2 SO4 - N a 2 WO4 is already presented in table 2 while those for Na2SO4-La2(SO4) 3 , Na2SO4-Sm2(SO4) 3 and Na2SO4-Dy2(SO4) 3 are summarized in table 3. The XRD results indicate that the Na2 SO4 - N a 2 WO4 system has limited (up to 10 m/o) solid solubility of one component into the other, and that a two phase region exists over a wide composition range for 10 m/o to 80 m/o Na2WO4 at room temperature. The various phases identified include Na 2 SO4 (III and V), Na2WO4 and Na2WO4 • 2H20. The DTA results show that an addition of 20 m/o Na2WO4, or more, can completely supress or eliminate the V ~ I phase transition of Na2SO4. Similarly, the 3'-/~ transition (~590°C) of Na2WO4 disappears by an addition of 10 m/o Na2SO4 or more. This is supported by electrical conductivity measurements as well. As for conductivity enhancement, the Na 2 SO4 + 70 m/o Na 2 WO4 composition exhibits 75 times higher conductivity than pure Na2SO4. The two phase region is poorly conducting, contrary to all previous reports. We plan to pursue this work further. The effect of wrong size dopant, i.e. WO~-2 for SO~-2 has been seen only at higher temperatures and higher concentration of the dopant.

Table 3 Activation energies and pre-exponential factors for Na2SO4-M2 (SO4)3 (M = La, Sm, Dy) systems. Materials

Na2SO4 + 2 m/o La2(S04)3 Na2SO4 +4 m/o La2(SO4)3 Na2SO4 + 6 m/o La2(SO4)3 Na2SO4 + 2 m/o SM2(SO4)a Na2SO4 +4 m/o SM2(SOa)a Na2SO4 + m/o Dy2(SO4)a

o

3.07 X 10-4 7.36 X 10-a 2.15 X 10-4 7.73 X 10-a 2.82 X 10-4 7.26 X 10-a 7.37 X 10-s 8.76 X 10-s 9.07 X 10-a

161

T°C (s2-1 cm-1)

Ea (eV)

Temperature range (°C)

oo

227 427 227 427 227 427 227 227 427

0.56 0.39 0.50 0.67 0.62 0.42 0.76 0.75 0.36

120-360 360-600 120-550 140-427 100-290 290-500 100-327 180-390 390-600

1.49 X 102 4.37 21.62 5.08 X 102 4.76 X 102 7.13 3.45 X 10a 3.28 X 10a 3.67

(s2-1 era-l)

G. Prakash, K. Shahi/Ionic transport in Na2SO4-Na2 W04 and Na2SO4-M2(SO4)s systems

162

The Na2 S04-M2(S04) 3 systems (M = La, Dy, Sm) show partial or complete stabilizationof Na2SO4-I phase at room temperature as reflectedby XRD, D T A and electricalconductivity results.The conductivity enhancement is maximum in case of the Na2SO 4La2(SO4)3 system. Na2SO4-2 m/o La2(SO4) 3 composition shows a conductivity three orders of magnitude higher than that of Na2SO 4 at 180°C, and an activation energy of 0.56 eV which corresponds to the migration energy of vacancies (h vm)" Further, the conduction data for pure Na2 SO4 leads to hf = 1.66 eV and h i = 0.51 eV. This is of course, on the assumption that Na + interstitials are more mobile than Na + ion vacancies. An examination of conductivity enhancements versus the size of various trivalent ions (La 3+ = 1.20 A, Sm 3+ = 1.10 A, Dy 3+ = 1.05 A and In 3+ = 0.93 A), taken from the compilation of Shannon and Prewitt [33] shows that the conductivity enhancement is favoured when the ionic radius of the dopant is close to that of host Na+ ion (l.16 A). This is to be expected because when the dopant and the host ions are comparable in size, the solid solutions are easily formed and excess of vacancies produced. The maximum en,

560

441

352

i

I

l

Temperoture in °C 282 227 181 i

t

hancement in conductivity obtained in Na2SO 4 La2(SO4) 3 system is due to complete stabilization of Na2SO4-I at room temperature. Fig. 9 shows that the Na2SO 4 containing 2 m/o of La 3+ and Sm 3+ have almost the same conductivity over the entire temperature range. The results for Na2SO4 containing 2 m/o In 3+ are also indentical at T > 240°C, the phase V -+ I transition temperature of Na2SO 4 . Apparently 2 m/o In 3+ is unable to stabilize a-Na 2 SO4 at lower ternperatures from those of La 3+ and Sm 3+. From these results we infer that hVm = 0.58 + 0.02 eV. From the log o versus 103/Tplot shown in fig. 10 for 4 m/o compositons of different dopants, it is observed that the Na2SO 4 + 4 m/o La2(SO4) 3 has the least activation energy and the highest conductivity. Besides La 3+, the Dy 3+ doping shows significant enhancement in o with an activation energy of 0.75 eV. Here the effect of "wrong size" also plays a role in enhancement of o as the ionic radii o f N a + and Dy 3+ are quite different. On the basis of our studies, we conclude that the Na 2 SO4-La2(SO4) 3 system appears to be the best choice for further development. The high temperature,

560

i

14/*

112

i

I

1

441 i

•

i

Temperature in *C 282 227 181 I

i

i

• o P, •

Lo

z~ Sm

o

144

i

112 i

Na2SO4 * 4m/oNzlso~) 3 (M=Lo, Dy,Sm,In)

No,SOL. + 2 m/, Mz(SO~)3 (M =La,Sm, Zn)

-~

352

In

Lo Sm Dy

In

-3 -3

'E -4 T

E -~

E

¥

b -5

b -5 -6

-6 NazSOt. -7 -7

-81.0

I 1.2

i 1./,

/ 1~_

I I 1.8 2.0 103/T (K -1 )

I 2.2

I 2.4

I 2.6

-81,0 2.8

Fig. 9. The log a versus temperature of Na2SO4 containing 2 m/o of La s÷, SinS+ and In s+.

1.12

1.~

1/6

1.8 2!0 I03/T (K -~ )

2!2

2!4

2!6

2.8

Fig. 10. The log a versus temperature plots for 4 m/o compositions of different dopants.

G. Prakash, K. ~hahi/Ionic transport in Na2SO4-Na2 NO4 and IVa2SO4 - M 2(S04) 3 systems

highly conducting a-Na2SO 4 (.phase I) has been successfully stabilized at room temperature. The highest conductivity achieved is 1.08 X 10 -3 [2 -1 cm - I at 290°C which is quite good and is only an order of magnitude less than the best Na + ion conductors available. Li2SO4 is similar to Na2SO 4 in the sense that both have high temperature modifications which are highly conducting; while in Na2SO 4 the (orthorhombic ~ hexagonal) transformation occurs at 241 ° C, the monoclinic -* fcc transformation in Li~ SO4 occurs at 575°C. It therefore appears reasonable to suggest that these trivalent dopants should be used to stabilize the high temperature, highly conducting fcc Li2SO 4 to lower temperatures.

References [1] P. Vashishta, J.N. Mundy and G.K. Shenoy, eds., in: Fast ion transport in solids (North-Holland, Amsterdam, 1979). [2] A.B. Lidiaxd, Handbuch der Physik, ed. S. Flugge, Vol. 20 (Springer, Berlin, 1957) p. 246. [3] Solid State Ionics 3/4 (1981), Prec. 3rd Intern. Meeting Solid Electrolytes-Solid State Ionics and Galvanic Cells (Tokyo, 1980). [4] P. Hagenmuller and W. van Gool, eds., in: Solid electrolytes, general principles, characterization, materials and application (Academic Press, New York, 1978). [5] K.T. Jacob and D.B. Rao, J. Electrochem. Soc. 126 (1979) 1842. [6] E.L. Kreidl and I. Simon, Nature 181 (1958) 1529. [7] Y. Saito, K. Kobayashi and T. Maruyama, Solid State Ionics 3/4 (1981) 393. [8] W.L Worcli and Q.G. IJu, (1983) unpublished data. [9] ,H.F. Fischmeister, Acta Cryst. 7 (1954) 776. [10] H.F. Fischmeister, Montash. Chem. 93 (1962) 420. [11] B.N. Mehrotra, Th. Hahn, H. Arnold and W. Eysel, Acta Cryst. A 31 (1975) S 79. [12] V. Amrithalingam, M.D. Kaxkhanavala and U.R.K. Rao, Acta Cryst. A 31 (1977) 522.

163

[13] L. Denielou, J. Petitet and C. Tequi, Thermochim. Acta 9 (19.54) 135. [14] M.A. Careem and B.E. Meliander, Solid State lonics 1.5 (1985) 327. [15] K.L Keester, W. Eysel and Th. Hahn, Acta Cryst. A 31 (1975) S 79. [16] M. Natarajan and E.A. Seeco, Can. J. Chem. 53 (1975) 1542. [17] H.H. Hofer, W, Eysel and U. yon Alpen, Mater. Ras. Bull. 13 (1978) 265. [18] H.H. Hofer, W. Eysel and U. yon Alpen, J. Solid State Chem. 36 (1981) 365. [19] R.M. Murray and E.A. Secco, Can. J. Chem. 56 (1978) 2616. [20] F.C. Kracek and C.J. Ksanda, J. Phys. Chem. 34 (1930) 1741. [21] P.H. Bottelberghs, in: Proc. NATO sponsored Advanced Study Institute on Fast Ion Transport in Solids, Solid State Batteries and Devices, ed. W. van Gool (NorthHolland, Amsterdam, 1973) p. 637. [22] R.J. Friauf, Physics of electrolytes, ed. J. Hladik, Vol. I (Academic Press, New York, 1972). [23] K. Singh and V.K. Deshpande, Solid State Ionics 12 (1984) 511. [24] K. Shahi and J.B. Wagner, Phys. Rev. B 23(12) (1981) 6417. [25] K, Shahi and J.B. Wagner Jr., J. Solid State Chem. 42 (1982) 107. [26] K. Shahi and LB. Wagner Jr., Solid State Ionics 12 (1984) 511. [27] K. Shahi and J.B. Wa~ner Jr., J. Phys. Chem. Solids 43 (1982) 713. [28] K. Shahi and J.B. Wagner Jr., J. Phys. Chem. Solids 44 (1983) 89. [29] K. Shahi and J.B. Wagner Jr., Appl. Phys. Letters 37 (1980) 757. [30] T. Jow and LB. Wagner Jr., J. Electrochem. Soc. 126 (1979) 1963. [31 ] S. Pack, in: Electrochemical SOc.Meeting (Los Angeles, Oct. 1979) Abstract 133. [32] J.C. Wang and N.J. Dudney, Solid State Ionics 18/19 (1986) 112. [33] R.D. Shannon and C.T. Prewitt, Acta Cryst. B 25 (1969) 925. [34] N. Imanaka, G.Y. Adachi and J. Shiokawa, Can. J. Chem. 61 (1983) 1557.