Ionic conductivity in the new series MSbTeO6 (M = K, Rb, Cs, Tl, Ag)

Solid State lonics 21 (1986) 195-201 North-Holland, Amsterdam

IONIC CONDUCTIVITY IN THE NEW SERIES MSbTeO 6 (M = K, Rb, Cs, TI, Ag) X. TURRILLAS *, G. DELABOUGLISE and J.C. JOUBERT Laboratoire de Matdriaux et de Gdnie Physique, E N S I E G - BP 46 - 3 8 4 0 2 St. Martin D 'Hdres, France

Received 7 May 1985 Revised manuscript received 20 January 1986

The series of pyrochlores MSbTeO6 (M = K, Rb, Cs, T1, Ag) were prepared and characterized by means of powder X-ray diffraction methods, ac measurements by the complex impedance method were performed at different temperatures. The conductivity of AgSbO3 was also measured to be compared with the conductivity of AgSbTeO6 .

1. Introduction Defect pyrochlore oxides of general formula AB206 were reported for the first time by Babel et al. [1 ]. In recent years, several publications have dealt with the ionic conductivity of these kinds of compounds. Grins et al. [2] studied the cation conductivity in the Tll+xMI+xW 1 _xO6 (M = Ta, Nb) system. In another paper, the same authors [3] gave conductivity data of the Kl+xTal+xW l_xO6, nH20 system. The series MNbWO6, MTaWO6 and MSbWO6 were studied by Michel et al. [ 4 - 7 ] , RbNbWO6 by Pannetier [8] and the series MNbTeO 6 and MTaTeO 6 by Darriet et al. [9]. CsSbTeO 6 was obtained by Babel et al. [1]. The pyrochlore structure belongs to the Fd3m space group and can be described in a general way after Bystrom [10], as a framework of oxygen nearoctahedra BO 6 (B being the smaller cations) sharing corners. In most AB206 pyrochlores the large cations occupy 8a sites (origin at 16c) and they can move along four [111] directions towards the empty neighbouring 16c sites. This paper reports the compounds of the series

* PIesent address: WolfsonUnit for Solid State Ionics, Department of Metallurgy and Materials Science, Imperial College, Prince Consort Rd., London SW7 2BP, UK.

MSbTeO 6 (M = K, Rb, Cs, T1, Ag) and their ionic conductivities.

2. Experimental 2.1. Sample preparation

The following reagent grade chemicals were used: KSb (OH) 6 , Te (OH) 6 , Sb203 , RbNO 3 , CsNO 3 , TINO3 and AgNO3 . In order to prepare KSbTeO6, the stoichiometric amounts of KSb(OH)6 and Te(OH)6 were mixed and ground in an agate mortar, then heated at 300°C in an alumina crucible for 2 h. After that, the mixture was ground and heated again at 700°C for 24 h. RbSbTeO 6, CsSbTeO 6, and T1SbTeO6 were prepared by mixing the stoichiometric amounts of the corresponding nitrate with Sb203 and Te(OH)6. After grinding in an agate mortar, they were heated for 24 h, at 450°C and for 24 h at 700°C in alumina crucibles. In order to prepare AgSbTeO6 .H20 , first KSbTeO 6 was exchanged in sulphuric acid following the procedure described in [11] and [12], to obtain H 3 OSbTeO 6 . AgSbTeO 6 .H20 was then obtained by an exchange reaction in an aqueous solution of AgNO3 weakly acidified with HNO 3 (pH = 2). 0.8957 g of H 3OSbTeO6, dried under vacuum at 130°C for 5 h was added to 50 ml of a 0.1065 N AgNO3 solu-

196

X. Turrillas et al./lonic conductivity in MSb TeO 6 pyrochlores

tion and left for 12 h. After that, the solution was filtered, a red-orange powder was separated and the solution titrated with a thiocyanate solution following Volhard's classical method [ 13]. The silver standard solutions were prepared b y dissolving the appropriate amounts o f AgNO 3 (analytical grade) previously dried at 140°C for 4 h in deionized water. The anhydrous form AgSbTeO 6 was obtained by heating AgSbTeO 6 X H 2 0 at 350°C for several hours. AgSbO 3 was prepared b y mixing and grinding the stoichiometric amounts o f Sb203 and AgNO 3 and then heating for 17 h at 750°C and for 10 h at 950°C.

2.3. X-ray diffraction

The powder X-ray diffraction patterns were made by means of an automatic Siemens Kristalloflex DS01 diffractometer. The samples were scanned in step by step mode with a step o f 0.01 0 with a time o f five seconds per step and with a Feowavelength (?~m = 1.93728 A). The position o f the peaks was corrected after goniometer calibration with a silicon standard pellet. The routine work of X-ray diffraction to follow the reactions and to control the purity of the samples was accomplished with Guinier and D e b y e Scherrer camera.

2.2. Electrical measurements

To perform the electrical measurements, cylindrical pellets were prepared. The apparent density o f all the pellets was about 80% o f the theoretical. The flat surfaces were covered with platinum paint. The conductivity measurements at different temperatures were made b y the complex impedance method with a Solartron 1174 analyzer at frequencies ranging between 1 MHz and 10 Hz at 10 points per decade. The time between two consecutive temperature steps was at least two hours. Both analyzer and furnace were controlled by means o f a Commodore 4032 microcomputer, and the data acquisition made automatically [14].

3.Results

X-ray diffraction patterns obtained with a Guinier camera, in all cases, showed the pyrochlore structure reflections (without impurities). The colours o f the compounds were as follows: KSbTeO 6 grey, RbSbTeO 6 dark grey, CsSbTeO 6 black, T1SbTeO 6 dark greenish, AgSbTeO 6 .H20 redorange and AgSbTeO 6 yellow. The observed and theoretical weight loss in the reactions of preparation are shown in table 1. In all cases the loss o f weight of the corresponding preparations were controlled except for AgSbTeO 6 .H20. For this compound the loss of weight registered corresponded to the reaction of decomposition, after heating for 48 h at 900°C. On

Table 1 Comparison between experimental and theoretical weight loss for the following reactions. See text for experimental conditions. Weight lo ss (%)

Exp.

Theor.

KSb(OH) 6 + Te(OH) 6 = KSbTeO 6 + 6H20

21.1

21.9

19.3

17.6

17.4

16.1

15.6

14.3

41.0

41.1

1

gO2 + RbNO 3 + ½Sb203 + Te(OH)6 = RbSbTeO 6 + NO2 + 3H20 ~-02 + CsNO3 + ~-Sb203 + Te(OH) 6 = CsSbTeO 6 + NO 2 + 3H2 O 1

102 + T1NO3 + ~ Sb203 + Te(OH)6 = TISbTeO 6 + NO2 + 3H20 1

AgSbTeO6"H20 = AgSbO3 + TeO2 + 502 + H20

x. Turrillas et al./lonic conductivity in MSb TeO 6 pyrochlores t h e o t h e r h a n d , t h e q u a n t i t a t i v e analysis r e f e r r e d t o t h e e x p e r i m e n t a l s e c t i o n was used t o follow t h e exc h a n g e r e a c t i o n rate. B e f o r e t h e e x c h a n g e , t h e 50 m l o f s o l u t i o n was 0 . 1 0 6 5 N in Ag, a n d a f t e r 12 h in cont a c t w i t h 0 . 8 9 5 7 g o f H 3 O S b T e O 6 , t h e s o l u t i o n was 0 . 0 5 9 7 5 N. T h i s implies a n e x c h a n g e rate o f a b o u t 95%. These c o m p o u n d s d e c o m p o s e at t e m p e r a t u r e s ranging b e t w e e n 9 0 0 ° C for K S b T e O 6 a n d a b o u t 1 0 0 0 ° C for t h e rest, e x c e p t A g S b T e O 6 . H 2 0 w h i c h loses first w a t e r yielding A g S b T e O 6 , a n d t h e n at a b o u t 6 0 0 ° C it d e c o m p o s e s yielding A g S b O 3 . By h e a t i n g A g S b T e O 6 - H 2 0 at 3 5 0 ° C for 2 0 h a yellow p o w d e r was o b t a i n e d . I m m e d i a t e l y a sample was int r o d u c e d i n t o a L y n d e m a n n glass capillary t u b e a n d

197

t h e n sealed. T h e cell p a r a m e t e r m e a s u r e d b y m e a n s o f a D e b y e - S c h e r r e r c a m e r a was 10.15 A. O n t h e o t h e r h a n d , t h e o b s e r v e d loss o f w e i g h t was 3.71%. T h a t m e a n s a loss o f 0.97 m o l e c u l e s o f w a t e r per form u l a . This r e a c t i o n is reversible. A f t e r b e i n g left to e q u i l i b r a t e o n t h e a m b i e n t c o n d i t i o n s for several days, t h e sample r e c o v e r e d its f o r m e r c o l o u r a n d X-ray d i f f r a c t i o n s h o w e d t h a t t h e cell p a r a m e t e r was again 10.23 A. T h e X-ray d i f f r a c t i o n d a t a for all t h e m e m b e r s o f this series c a n b e seen in table 2. A c o m p a r i s o n bet w e e n t h e i r X-ray d i f f r a c t i o n p a t t e r n s is s h o w n in fig. 1. T h e c o n d u c t i v i t y d a t a for K S b T e O 6 is n o t s h o w n b e c a u s e o f its low values ( a b o u t I 0 - 9 f2 - 1

Table 2 Experimental X-ray diffraction intensities and cell parameters for the series MSbTeO 6 (M = H30, K, Rb, Cs, T1) and AgSbTeO 6 .H 2 O. h

k

l

1 1 1 2 2 0 3 1 1 2 2 2 4 0 0 3 3 l 4 2 2 5 1 1 ,3 3 3 4 4 0 5 3 1 6 2 0 5 3 3 6 2 2 4 4 4 5 5 1 6 4 2 553, 731 8 0 0 7 3 3 6 6 0 555 ,751 6 6 2 8 4 0 7 5 3 ,9 l 1 6 6 4 9 3 1 8 4 4 755,7 7 1,933 Cell parameter

MSbTeO 6 observed intensities H30

K

Rb

Cs

T1

Ag

100.0

79.3 0.7 100.0 78.4 11.1 8.8

16.8 9.1 100.0 63.4 5.9 4.8

11.5 17.2 100.0 38.3 6.4 35.8 50.4 5.8 8.7 20.4 25.2 3.3 -

27.3 100.0 38.1 11.0 39.1 52.2 10.6 22.9 21.2 -

25.0 24.5 100.0 27.4 1.7 1.1 7.0 47.4 7.6 6.1 43.5 11.1 6.4

82.4 90.5 19.1 16.4 2.5 27.5 50.3 39.7 17.1 42.6 10.8 24.5

-

31.7 51.0 24.1 1.0 17.0 38.3 6.9 16.0 -

27.5 8.2 4.0 1.0 16.3 31.9 32.8 25.8

-

37.7 52.2 10.8 6.8 20.0 33.6 7.2 6.9 -

21.3 45.0 39.1

8.4 1.1 0.8 17.7 26.6 24.4 18.6 0.5 21.3 47.9 30.3

46.0 14.6 . . 22.2 31.3 19.5 13.1 39.8 63.7 16.6

10.1594

10-1243

10.1562

2 9 . 9

-

. .

41.8 12.7 . . 26.9 20.7 9.7 5.0 34.7 52.3 10.2113

9 . 5

-

38.9 12.7

7.5 7.4

25.3 20.7 10.3 34.2 50.0 -

4.9 27.0 27.9 6.7 6.3 34.6 9.5

10.1455

10.2302

. .

X. Turrillas et al./lonic conductivity in MSb TeO 6 pyrochlores

198

0

~¢,~

~-

HjO

0

0

~{ ~_.~.._~ L~ ~_~ ~_.

Rb

Cs

---~-..~t_

. J~

j

Fig. 1. Evolution of the intensities in the X-ray diffraction patterns for the series MSbTeO 6 (M = H30, K, Rb, Cs, T1).

-3

-4

4*

÷

b 0

-6

L

800

700

I

I

I

600

500

400

Fig. 2. RbSbTeO6 conductivity plot. Log a versus 1/T.

i

300 "C

199

X. Turrillas et al./lonic conductivity in MSb TeO 6 pyrochlores -3

÷

-4

t) Un 0

-5

÷

800

!

I

I

I

700

600

500

400

3~)0 "C

Fig. 3. CsSbTeO6 conductivity plot. Log o versus 1/T. cm -1 at 500°C) and the data corresponding to H3OSbTeO 6 are published in [11] and [12]. The conductivity values for AgSbO 3 are also given. This phase shows an X-ray diffraction pattern in agreement with refs. [15] and [16]. The graphs of log o versus 1 / T are shown in figs. 2 to 6.

4. Discussion The formulae proposed for these compounds agree with the weight loss observed in the preparation. For the pyrochlores o f Rb, Cs and T1 the agreement is less good probably because of a lack of monovalent

-3

-4 ÷

-5 b or, -6

0

÷

-7

-8

700

L

I

I

I

600

5OO

40O

300

Fig. 4. T1SbTeO6 conductivity plot. Log a versus 1/T.

L

200 "C

200

X. Turrillas e~ al./lonic conductivity in MSb TeO 6 pyrochlores -4

÷

-5

÷ ÷

÷

©

-6

I

5OO

I

I

I

300 200 Fig. 5. AgSbTeO 6 conductivity plot. Log o versus 1/T.

sO0

cations due to the use of slightly moisturized salts as starting material. Once MSbTeO 6 is formed, the excess of antimony and tellurium volatilize as Sb203 and TeO2, increasing the weight loss. The intensity values in the X-ray diffraction patterns are in agreement with the theoretical intensities obtained with monovalent cations occupying only the

•

100 C

8a sites except for AgSbTeO 6 .H20. In this last compound, Ag cations are only located on the 16c sites as A cations in normal pyrochlores A2B207 . In fig. 1 the progressive variation of the intensities as a function o f the number of electrons in the 8a site can be appreciated. On the other hand, the cell parameters vary according to the size o f the monovalent cation

-1

-2

c~ -3 O

-4

-5

I

I

600 500

I

I

I

LO0

300

200

I

100

Fig. 6. AgSbO 3 conductivity plot. Log a versus 1/T.

°C

X. Turrillas et al./lonic conductivity in MSb TeO 6 pyrochlores

in the expected way. Additional evidence o f the different occupation in Ag-pyrochlore is given by its cell parameter, the largest o f the series. The complex plane Z " versus Z ' diagrams are typically semicircles with a polarization effect at low frequencies, except for CsSbTeO 6 . The plots for this last compound are true semicircles, no polarization ~ffects being observed. The conductivity plots show a near linear behaviour. The activation energies are calculated by fitting experimental data to the equation log(aT) = log o 0 - e a / k T , by the least squares method. In all cases, the observed conductivity should be due to bulk and grain boundary contributions. Both contributions cannot be discriminated because o f poor apparent density o f the samples. The conductivity data shown were obtained on cooling. Hysteresis with differences no larger than 10% were observed between values measured on heating and on cooling. In the case o f AgSbTeO 6 .H20 , several peUets o f different thickness were used to perform measurements, and the conductivity patterns were always identical, showing a discontinuity at about 250°C certainly due to the releasing of a water molecule (see fig. 5). For RbSbTeO6, the activation energy between 600 and 760°C is 1.24 eV. For T1SbTeO 6 the activation energy is similar, 1.20 eV between 365°C and 650°C. In the case o f CsSbTeO6, the activation energy is 0.75 eV between 641°C and 435°C. The lack of polarization effects indicate the presence o f electronic conductivity, probably superimposed on the ionic conductivity. This could explain the black colour o f the sample. On the other hand, the conductivity of AgSbO 3 is relatively high as expected for a Ag÷-conductor. On the contrary, the conductivity o f AgSbTeO 6 is about 100 times lower. For AgSbTeO 6 the activation energy is 0.62 eV (above 250°C) and 0.65 eV for AgSbTeO 6 . H 2 0 (below 250°C). In the case of AgSbO 3 it is 0.57 eV between 247°C and 525°C. This different behaviour seems, at present, unexplicable, the main difference between them being the different occupancy o f the 16c sites (half occupied in AgSbTeO6). Other authors [ 17] give conductivity values of AgSbO 3 that agree with those observed in the present paper.

201

5. Conclusions The pyrochlores o f this series are poor ionic conductors. KSbTeO 6 is the worst conductor and CsSbTeO 6 is probably a mixed conductor. The conductivity in AgSbTeO 6 is about 100 times lower than in AgSbO 3 . The compounds MSbTeO 6 (M = K, Rb, Cs, T1) are defect pyrochlores with the 8a site occupied by M. The pyrochlores AgSbTeO 6 and AgSbTeO 6.H20 are different to the other members o f the series. Ag occupies one half o f the 16c sites o f the A 2 B 2 0 7 normal pyrochlore.

Acknowledgement One o f us (X.T.) is grateful to the Basque Government for the research grant which supported this work.

References [ 1 ] D. Babel, G. Pausewang and W. Viebahn, Z. Naturforsch. 22b (1967) 1219. [2] J. Grins, M. Nygren and T. WaUin, Rev. China. Min6r. 17 (1980) 299. [3 ] J. Grins, M. Nygren and T. Wallin, Mater. Res. Bull. 15 (1980) 53. [4] C. Michel, Th~se (Caen, 1975). [5] C. Michel, D. Groult, A. Deschanvres and B. Raveau, J. Inorg. Nucl. Chem. 37 (1975) 251. [6] C. Michel, D. Groult and B. Raveau, J. Inorg. Nucl. Chem. 37 (1975) 247. [7] D. Groult, C. Michel and B. Raveau, J. Inorg. Nucl. Chem. 36 (1974) 61. [8] J. Pannetier, Solid State Commun. 34 (1980) 405. [9] B. Darriet, M. Rat, J. Gall and P. Hagenmuller, Mater. Res. Bul. 6 (1971) 1305. [10] A. Bystrom, Arkiv Kemi Mineral. Geol. 18A (1945) 1. [ 11 ] X. TurriUas, Th~se (Grenoble, 1984). [ 12] X. TurriUas, T. Fournier, J. Muller, G. Delabouglise and J.C. Joubert, Solid State Ionics 17 (1985) 169. [ 13] N.H. Furman, ed., in: Standard methods of chemical analysis, 6th Ed., Vol. 1 (Van Nostrand, Princeton, 1962). [ 14] G. Delabouglise, Th~se (Grenoble, 1980). [ 15] N. Schrewelius, Z. Anorg. Allgem. Chem. 238 (1938) 241. [16] A.W. Sleight, Mater. Res. Bull. 4 (1969) 377. [ 17 ] H. Watelet, J.P. Besse, G. Baud and R. Chevalier, Mater. Res. Bull. 15 (1980) 875.