Electrical conductivity and ionic conduction mechanism in NaLiZrSi6O15 single crystals

Electrical conductivity and ionic conduction mechanism in NaLiZrSi6O15 single crystals

813 Solid State Ionics 9 & 10 (1983) 813-816 North-Holland Publishing Company ELECTRICAL CONDUCTIVITY AND IONIC CONDUCTION MECHANISM IN NaLiZrSi6...

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813

Solid State Ionics 9 & 10 (1983) 813-816 North-Holland Publishing Company

ELECTRICAL CONDUCTIVITY

AND IONIC CONDUCTION

MECHANISM

IN NaLiZrSi6015

SINGLE CRYSTALS

Alfred G. Duba 1 and Subrata Ghose 2 ILawrence Livermore Laboratory, 2Department of Geological Washington, U.S.A.

P.O. Box 808, Livermore, California, U.S.A.

Sciences, University of Washington,

Seattle,

The measurement of the ac conductivity in NaLiZrSi6015 single crystals under controlled oxygen fugacity conditions indicate conductivity of about 0.003 Siemens/ meter at 900°C. The conductivity is highest along [OOl], intermediate along [OlO], A changing conduction mechanism is indicated by a change and least along [loo]. in activation energy at 524°C. Below this temperature, the conduction presumably takes place through the migration of ionic and electronic defects. The ionic conduction above this temperature is facilitated by the partial migration of Na+ ions from the At high temperature, the Na(1) site to the Na(2) site, which is vacant at RT. Na(l)-Na(1) conduction path along [OOl] is favored over the Na(l)-Na(2) conduction path The long along [OlO] due to a difference in the bottlenecks along these paths. Na(l)-Na(1) nearest distance along [loo] hinders Na+ migration along this direction. 1.

Introduction

NaLiZrSi60I5 (zektzerite) is orthorhombic, soace arouo Cmca, with unit cell dimensions a= 14.330{2),‘b = 17.354(2), and c = 10.164(2) A. Its crystal-structure consisFs of corruoated six-tetrahedral-repeat corner-sharing double silicate chains and edae-sharino chains of alternating Li04 tetrahed;a and ZrO6 octahedra, both chains runnina parallel to the c axis. The sodium atoms occur in channels paraTle1 to the a axis formed by the corrugation of the siliIn addition, there is a cate double chains. vacant sodium site above and below the Zroctahedra along the a axis (Ghose and Wan, We elected that at hioh tem1978) (Fia.1). perature, some of the sodium ions will m-igrate from the occupied Na(1) site to the vacant Na(2) site, resulting in high electrical conTo test this idea and to determine ductivity. the detailed ionic conduction paths and the conduction mechanism, we have carried out ac electrical conductivity measurements along the three principal directions at temperatures up to 900 "C. In this paper, we briefly report our preliminary results and correlate them with the crvstal structure determinations of NaLiZrSi6015 at 400 and 600°C by Drs. Jean Vicat and Due Tran Oui of CNRS. Laboratoire de Cristallographie; Grenoble,.France (Vicat and Tran Qui, 1982; private communication). 2.

Experimental

Rectangular plates cut from slightly pinkish transparent natural NaLiZrSi6015 crystals in the three principal directions were covered with 0.025 mm thick gold foils to provide good electrical contact between the specimen and the platinum electrodes attached to thermocouples, which were also used for the temperature measurements. A gas mix of CO2:CO 0 167-2738/83/0000~0000/$

03.00 0 1983 North-Holland

(3O:l) was used in heating and cooling of all samples (Netherton and Duba, 1978). Temperature measurements were accurate to +3"C. Electrical conductivitv was calculated from samole Congeometry and conductance measurements. ductance less than 10w7S and areater than 10mgS were measured with a General-Radio 16028 capacitance measureing assembly. Conductance of 10-7s and higher were measured with a Hewlett Both bridges Packard 4274A Automatic Bridge. were operated at 1 kHz and measured ac conductance with an accuracy of 1%. 3.

Results

The electrical conductivity (u) measurements along [loo], [OlO] and [OOl] as a function of temperature are shown in Figs. 2 a,b,c. For [loo] the conductivity is linear with two distinct slopes. The high temperature portion of the conductivity has an activation energy of 1.90 eV as determined on heating. After the sample temperature was maintained at 900°C for 15 hours, the conductivity increased slightly, which persisted at all temperatures and increased slightly at lower temperatures. On cooling the activation energy was slightly lower (1.64 eV1. The conductivity along [OlO] as measured on heatinq showed a low temoerature mechanism with a slope of 0.25 eV, 'yielding to a high temperature mechanism with a slope of 1.59 eV: the transition zone between the‘two mechanisms spanned about 125-C (47D'C to 595'C). After 15 hours at 900: 5'C, the conductivity increased considerably as measured on cooling to about 540°C. After 15 hours at 543'C, the conductivity did not change on heating to 9OO'C. The activation energy was measured to be 1.46 eV.

814

A. G. Duba, S. Ghose 1 Electrical

corhctivity

atrd iorzic condurtior~

rnecl~anism

O-b Fig. 1: View of the NaLiZrSj60~5 structure down [lOO]; one half of the structure is shown; the other half is obtained by reflection across a mirror plane at x= l/2) passing through the oxygen atoms O(7), O(8) and O(9) (after mose and Wan, 1978).

b .O-

LO

b.0 -

i.0 -

1 8

10

-

-40

.

-50



-6.0

;.0

9

-30

8 ii

1

12 d/T(K-‘)

I

13

I

1

I-

i4

If

‘3

I 9

10

1$/T (K“)

Fig. 2 a, b, c. Electrical conductivity measurements as a function of temperature in NaLiZrSi60lS single crystals along the three principal directions. Up and down arrows indicate heating and cooling cycles respectively.

Li

12

IO’/T(K-‘)

,3

14

I5

A.G. Duba, S. Ghose 1 Electrical conductivity

After the sample was held at 9OO'C for 400 hours, the conductivity was measured on cooling; the activation energies for the high and low temperature portions were 1.50 and 0.67 eV, which are quite different from the initial slopes of 1.59 eV and 0.25 eV on heating respectively. The electrical conductivity along [OOl] on cooling increased by 0.4 of a log unit after 16 hours at 830 + 3'C compared to the heating The same results were obtained on cvcle. heating and cooling after 12 days at RT, followed bv 15 hours at 895 +3'C. The activation energies on heating and cooling are 1.56 eV and 1.43 eV respectively. The time dependence of conductivity along [OlO] was measured for the first 36 hours at 900°C for the CO2:CO (3O:l) gas mix; between 50 and 150 hours the gas mix was changed and the conductivity changed by few hundredths of an order of magnitude. In the next 10 days a small decrease, rather than an increase in conductivity was observed (Fig. 3).

Tamp

= 900

+ 3°C

and ionic conduction

4.

mechanism

815

Discussion

4.1 The ionic conduction mechanism Below 524°C the electrical conduction in NaLiZrSi6015 takes place presumably through the migration of intrinsic electronic and ionic From optical absorption spectra, Dr. defects. George R. Rossman of California Institute of Te hnology has detected very small amounts of Fez+ ions in the channels oarallel to the a-axis (Rossman, 1977; private communication). The presence of Fe2+ in the channels would require sodium ion vacancies in the Na(1) position. The occurrence of sodium in inter-~ The stitial sites is also a possibilitr. presence of very small amounts of Fe2+ and Fe3+ ions in the tetrahedral and octahedral sites of zektzerite [as in the mineral tuhualite, Nawould (Merlino, 1969)] Fe2+Fe3+Si6015 contribute to the electronic conductivity, particularly along the c axis. However, the very light pink color o'F the natural mineral rules out the presence of Fe2+ and Fe3+ ions except in minute traces. The chanqe in the low temperature activation energy ior electrical conduction alons TO101 from 0.25 eV on heatina to 0.67 eV on cooiing-indicates that the natur;? of these intrinsic defects is being altered by heating to 9OO'C. The high temperature mechanism which dominates above 524'C can be correlated with ionic conduction through the migration of Na+ ions, which is facilitated by the formation of considerable vacancies at the Na(1) site, through about 8% of the Na+ ions migrating to the Na(2) site at 600°C.

Fig. 3. Electrical conductivity measurements in NaLiZrSi6015 single crystals along [OlO] as a function of time. A break in the curve coincides with a change in oxygen fugacity.

At room temperature, the sodium atom at the Na(1) site shows a strong anisotropic thermal vibration; the longest axis of the thermal vibration ellipsoid lies in the ac plane, and points along 32" from the c ax= (Ghose and Wan, 1978). This axis increases in magnitude from 0.226 a at RT to 0.372 a at 600°C. Furthermore, the equivalent isotropic temperature factors of the sodi m atom at the Na(1) site increases from 2.1 12 at RT to 5.1 112 at 4OO'C, and finally to 10.3 82 at 6OO'C (Vicat and Tran Qui, 1982). The site occupancy refinements for Na at the Na(1) and Na(2) sites indicate that at 400°C the Na(2) site is virtually unoccupied, whereas at 6OO"C, the occupancy of the Na(2) site is 0.081. At high temperatures, the Si-0 and Zr-0 bond lengths remain virtually unchanged, whereas the Na-0 bond-lengths show considerable increase (Vicat and Tran Qui, 1982).

4.2

Ionic Conduction Paths

Our measurements indicate that the high temperature electrical conductivitv is hiahest along 5, intermediate along b and-least ilong the a axis. These facts are consistent with the Jetails of the crystal structure. The direction of the largest thermal vibration of the sodium atom at Na(1) (32' from c axis in the ac plane) indicates that c is the preferred direzion for sodium mobility along the Na(l)1) polyhedra share edges at RT] to form a chain parallel to the c axis; the nearest Na l)-Na(1) distance within-this chain is 5.21 . Along the b axis, the nearest Na(l)-Na(2) distance is much-shorter, 3.85 a. For the Na+ ion migration along the Na(l)-Na(2) path, however, the Na+ ions must pass through one of two rhombus-shaped bottlenecks [formed by 0(2)-0(3)-0(4)-0(4) and 0(4)-0(5)-0(6)-O(2)] (fig.l), within which th nearest oxygen-oxygen distances are 3.39 il [0(3)-O(4)] and 3.35 X [O(2)O(5)] at RT. These bottlenecks, which are narrower than that for the Na(l)-Na(1) path, prevent considerable ionic migration along the Na(l)-Na( ) path. The long Na(l)-Na(1) distance of 7.17 g along the a axis strongly hinders ionic migration along This direction.

816

4.3

A.G. Duba, S. Ghose /Electrical

conductivity

and ionic conduction

The Hysterisis Effect

The hysterisis effect in the conductivity measurements on heatinq and coolinq in NaLiZrSi6015 can be explained by two mechanisms: (a) time dependence of the deqree of sodium disorder between Na(1) and Na(2) sites and (b) alteration of some of the intrinsic defects including occurrence of sodium atoms in interstitial positions at high temperatures. We are carrying out further conductivity measurements as a function of time to clarify some of these mechanisms. Acknowledgements We are indebted to Mr. Bart Cannon, SeWashington, for the donation of attle, NaLiZrSi6015 single crystals and to Drs.Jean Vicat and Due Tran Qui, Grenoble, France and Dr. George R. Rossman, Pasadena, California for sharing their unpublished research results. This research has been supported by the Office of Basic Energy Sciences of the Department of Energy under contract W-S7405-ENG-48 with the University of California (A.G.D.) and the National Science Foundation, grant no. EAR8206526 (S.G.)

mechanism

References Ghose, S. and Wan, C., Zektzerite, NaLiZr Si6015: A silicate with six-tetrahedral repeat double chains, Amer. Mineral. 63 (1978) 304-310. Merlino, S. Tuhualite crystal structure,Science, 166 (1969) 1399-1401. Netherton, R. and Duba, A. G. An apparatus for simultaneously measuring electrical conductivity and oxygen fugacity,Lawrence Livermore Laboratory Report, UCRL-52394 (1978). Rossman, G. R.

Private Communication

Vicat, J. and Tran Qui, D. Private Communication (1982).

(1977).