Chapter 3 ' ' '

Studies on A-A -M-M-0 (A=Na or K; A =Ca or Sr; M= Ti or Zr; '

M = Nb or Ta) system

The oxide ion conductors in the system A-A'-M-M0

-0 (A=Na or K; A' =Ca or Sr; M = Ti

or Zr; M0

= Nb or Ta) were prepared by solid state reaction method. Phase identification

by X-ray powder diffraction shows they have perovskite type structure. The effects of

microstructure on the properties were studied using scanning electron microscopy (SEM).

Ionic conductivity of the oxides was measured using ac impedance spectroscopy as a

function of temperature ranging from 300-750°C.

3.1 Introduction

Inorganic oxides having high ionic conductivity are currently of great importance,

primarily because of their possible application as solid electrolytes in solid oxide fuel

cells (SOFC) [1]. They have a wide variety of other applications also like oxygen sensors

[2] and pumps [3] as well as membranes for oxygen separation [ 4]. Currently, yttria

stabilized zirconia (YSZ) is commonly used as an electrolyte in the conventional SOFCs

[5]. However, it is associated with some limitations such as high operating temperature

(1073-1273K), relatively low ionic conductivity below l000°C, problems about long term

stability of the cells, etc. Lowering the operating temperature can reduce some of these

problems.

Therefore, there is intense research going on worldwide to find new materials

h_aving high ionic conductivity at lower operating temperatures. Some useful materials

have already been developed in a few different systems such as samarium doped ceria

(SDC) [6], rare eath gallates [7] and lanthanum molybdate, La2Mo209 [8] and

NaBhV2010 [9].

Perovskite structured oxide ion conducting compositions with conductivities high

enough for consideration are developed recently. Perovskite type oxides have two sites

for aliovalent doping and thus creating vacancies in oxygen sublattice, which help the

migration of oxygen ions through the lattice [ 10-12]. Lanthanum gallate is one candidate

for application as electrolyte in SOFCs [13, 14]. Doubly doped (Sr and Mg) LaGa03

based perovskite system exhibits oxide ion conductivity higher than that of YSZ at 800°C

[15-17]. In the present study, high ionic conductivity in new perovskite type oxides: A­

A0

-M-M0

-0 (A=Na or K; A0

=Ca or Sr; M= Ti or Zr; M0

= Nb or Ta) is reported.

3.2 Experimental

High temperature solid state reaction technique is used for the preparation of the

oxides in the A-A0

-M-M0

-0 (A=Na or K; A0

=Ca or Sr; M= Ti or Zr; M0

= Nb or Ta)

systems. High purity chemicals Na2C03 (99%, NICE), K2C03(99+%, Acros

Organics),CaC03 (99+%, Acros Organics), SrC03 (98.5%, OTTO KEMI), Ta205 (99.9%,

47

Acros Organics), Ti02 (99%, Merck) Zr02 (99%, Acros Organics) and Nb205(Sigma­

Aldrich Chemicals) were wet mixed with acetone as the wetting medium in an agate

mortar and dried in an air oven at 100°C for I hr. The process of grinding and heating was

repeated thrice to get a homogeneous mixture.

The samples of NaA'TiM,06 (A' =Ca orSr; M' = Nb or Ta) were calcined at

1300°C in platinum crucible for l 2hrs. Calcined powders were ground well and

uniaxially pressed into compacts of 10mm diameter and 2.5mm thickness using hydraulic

press by applying a pressure of 2.5 tons. Then the pellets were sintered at 1450°C for

6hrs.

NaA'ZrM'06 (A' =Ca orSr; M' = Nb or Ta) samples were calcined at 1300°C for

12hr. The calcined powder was uniaxially pressed into pellets of 10mm diameter and

2.5mm thickness using hydraulic press by applying a pressure of 2.5 tons. Then the

pellets were sintered at 1450°C for 12hr.

KSrTi Nb06 sample was calcined at l000°C for 3hr and the calcined powder was

pelletised using hydraulic press by applying a pressure of about 2.5 tons and sintered at

1300°C for 3hr. The potassium compound shows some hygroscopic nature while

preparation.

The crystalline phase of the samples was examined by powder X-ray diffraction

method (XRD) with a Philips X-pert Pro X-ray diffractometer using Ni-filtered CuKa

radiation. The differential thermal analysis is carried out on the powdered samples using

Slimadzu 50H Thermal Analyzer.

The surfaces of the polished and thermally etched (near the sintering temperature

for 30 min) pellets were photographed using scanning electron microscope, JEOL JSM

5600LV to study grain morphology. For the electrical conductivity measurements the

pellets were electroded with long silver leadwires using silver paste. The measurements

were taken using Solatron 1260 Impedance Analyzer in the air and oxygen atmosphere.

48

3.3 Results and Discussion

3.3.1 AA1

TiM1

06 (A=Na or K; A'=Ca or Sr; M1

= Nb or Ta) system

3.3.1.1 XRD and Structure

Powder XRD patterns of the AA1

TiM1

06 (A=Na or K; A0

=Ca or Sr; M1

=

Nb or Ta) system is given in fig 3. l a-c. All the peaks in the patterns can be indexed on

the basis of perovskite type structure. The XRD patterns of these compounds show the

formation of a single phase pure perovskite type compound. The tolerance factor

calculated for this system is -1, the optimum tolerance factor to obtain the largest

electrical conductivity in perovskite type oxides [ 18].

NaCaTiNb06 and NaCaTiTa06 give XRD patterns which can be indexed based

on orthorhombic perovskite structure. The d spacing is in good agreement with the

JCPDS powder diffraction file (PDF) No.51-133 [19]. The prominent peaks are (101),

(200) and (220) reflections.

NaSrTiNb06, NaSrTiTa06 and KSrTiNb06 can be indexed on the basis of cubic

perovsl<ite structure. These patterns are similar to that of earlier reported XRD patterns of

perovskite type compounds such as KTa03 and are in good agreement with the PDF

No.38-1470 [20]. The strong diffraction peaks corresponds to (110), (100) and (200)

planes. The lattice parameters of these oxides are given in table 3.1.

49

0

10 20

0

0

N

30 40

0

N

N

29

�

0 N

N

<"> N

50

NaCaTiTa06

60 70 80

Fig. 3.la X-ray diffraction pattern of the NaCaTiM'o6 (M0

= Nb or Ta)

10 20 30 40

20

0

0

N

-

0

�

N

50

NaSrTiTa06

�

N

0

N

N

60 70

Fig. 3.lb X-ray diffraction pattern of the NaSrTiM0

06 (M' = Nb or Ta)

50

10 20 30 40

20

a

a

50 60

a

"'

"'

70

Fig. 3.lc X-ray diffraction pattern of KSrTi Nb06

3.3.1.2 Differential thermal analysis (DT A)

The DTA curves for the NaA'TiM'06 (A' =Ca orSr; M' = Nb or Ta) system are

shown in �g.3.2 a-d. DTA study of the new system is carried out to see whether there is

any phase change occurs during heating. There is no phase change is observed in the

DTAplots.

51

80

60

20

0

100

80

60

40

20

0

0 200 400

Temperature (°C)

600 800

Fig. 3.2a DTA curve for NaCaTiNb06

0 � � � � � � � � �

Temperature (°C)

Fig. 3.2b DTA curve for NaCaTiTa06

52

100 ,----------------�

80

60

20 /

0

0 200 400 600 800 1000

Temperrature (0

C)

Fig. 3.2c DT A curve for N aSrTiNb06

100

80

// 60

40 // 20

/0

0 200 400 600 800 1000

Temperature (0

C)

Fig. 3.2d DTA curve for NaSrTiTa06

53

3.3.1.3 SEM and microstructure

The SEM microstructure of the polished thermally etched surfaces of the sintered

pellets is shown in fig.3.3a-e. The samples show well sintered and well grown grains with

minimum porosity except in the case of KSrTiNb06• The density calculated for these

materials is in the range 4.6-4.9 glee. This can be confirmed from the microstructure.

The average grain size of this material is Sµm. The micrographs reveal single phase

morphology of the compounds without any secondary phase. The micrograph indicates

that KSrTiNb06 is porous and this affected its conductivity value.

54

a. NaCaTiNb06

c. NaSrTiNb06

b.NaCaTiTaO6

d. NaSrTiTa06

e. KSrTi NbO6

Fig. 3.3 SEM micrographs ofAA'TiM'06(A=Na or K; A' =Ca or Sr; M' =Nb or Ta) system

55

3.3.1.4 Electrical conductivity studies

Conductivity in solids can be either electronic or ionic. Electronic conductivity is

usually of the order of -10-3 Siem (semiconductors) to -10 10 Siem (metals) and such

materials will usually blackish. Ionic conductivity would be of the order of 10·5 - 10 1

Siem and such materials would be usually colorless.

The electrical conductivity of NaA'TiM,06 (A' =Ca orSr; M' = Nb or Ta) system

has been measured in the temperature range 300-750°

C in air as well as oxygen

atmosphere. The conductivity values at 700°

C are given in table 3.2. The electrical

conductivity at 700°

C is in the order of I 04 Siem for all compounds except KSrTiNb06•

The Arrhenius plots of the conductivity, cr as a function of 1/T (T=absolute

temperature) is shown in fig. 3.4a-e. The conductivity value obtained for the system in air

atmosphere is higher than the value obtained in oxygen atmosphere. In oxygen deficient

systems the conductivity takes place by the hopping of 02· ions in the vacancies. Since,

the number of vacancy decreases in oxygen atmosphere the rate of hopping of 02· ions

into vacancy sites also decreases [21]. Hence the ionic conductivity decreases in the

oxygen atmosphere.

As can be seen from the figures, the curves are approximately linear indicating

that cr nearly follows the Arrhenius type equation,

cr = cr0 exp (-Ea/kT). In this relation, cr0 represents a pre exponential factor, Ea is

the apparent activation energy of conduction process, k is the Boltzman's constant and T

is the absolute temperature.

The activation energy for conduction was calculated from the slope of the graph

in the temperature range 500-600°

C. Kilner and Brook [22] calculated that the intrinsic

activation energy for the perovskite structure should be 0.8 ± 0.2 eV. The higher

activation energy compared to fluorite structure is due to the smaller opening through

which the oxide ion must pass in the perovskite structure. It is seen that activation energy

of current carriers in strontium compound is more than the calcium substituted

compounds. The reason for this may be that the larger cation Sr2+ increases the

polarizability of ions to give the bonding character to be covalent [ 18]. Kreuer proposed

56

that the perovskite with Sr in A site absorb water in vacancies and stabilize oxide ion in

the lattice [23].

The activation energy in the oxygen atmosphere is higher than that in the air

atmosphere. This corresponds to the decrease in conductivity due to the decrease in rate

of hopping of 02• ions in the vacancy sites in the system [24]. The systems with Ta show

more activation energy. This may be due to the change in ionic radius of Nb and Ta.

The typical Cole-Cole plots of the NaA'TiM,06 (A'=ca or Sr; M' = Nb or Ta)

system at different temperatures (600-700°C) are shown in fig.3.5a-i. The intersection of

the semicircle on the low frequency side ( right of the fig.) with abscissa was taken as the

total resistance of the sample. There is only one semicircle is observed which

corresponds to the grain conduction [25]. Frequency decreases with the increasing real

component of impedance. For some of the compounds the plot doesn't pass through the

origin. This is because there are other arcs appear at higher frequencies and also Roo >O.

The presence of pores modifies the impedance plots by altering the diameter of the arcs

[26]. The distortion of the semicircle at low frequency region (right of figure) is due to

the material electrode interaction [27].

57

-3,-----------------�

-4 ·\· •"-

-5

-7

-8

'\:"-. I • NaCaTiNbO,(Alr) I "" "' -• · NaCaTiNb0,(02)

·"'·� ·""·�. ." �.,"

�:� �-"'

."'---�• ,�.

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K1)

Fig. 3.4a logcr vs l 000/T for NaCaTiNb06 in air and oxygen atmosphere

-3

-4

� -5

t> -6 �

-7

-8 -

0.9

�--�. •�. I • NaCaTiTaO,(Alr)

\ •�

•- NaCaTiTa0,(02)

1.0 1.1

� ·,. --�.

··��=�

�,·� •-............. ,, �· •

1.2 1.3 1.4 1.it 1.6

1000IT(¥_)

1.7 1.8

Fig. 3.4b logcr vs l 000/T for NaCaTiTa06 in air and oxygen atmosphere

58

-3.0

-3.5

-4.0 /?-

�-4.5 VJ "-:; -5.0

..g -5.5

-6.0

-6.5

-7.0

•:::::: ·�. ·�--. '•� �-

I m- NaSrTINbO,(Air) \ m-- NaSrTiNb0,(02)

-�.-�-�

'• � �--�-.

'•� "-._•

•

-7.5 +-�-r--r-,--�,--,--...�--.-���-,-���...--1 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K.1}

Fig. 3.4c logcr vs l 000/T for NaSrTiNb06 in air and oxygen atmosphere

-4

-5

-8

-9

I I

I •· NaSrTiTaO,(Air) \ •- NaSrTiTa0,(0

2)

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K')

Fig. 3.4d logcr vs l 000/T for NaSrTiTa06 in air and oxygen atmosphere

59

-4.0

-4.5

-5.0-

K � -5.5

t)

C)

.Q -6.0-

-6.5

-7.0

I I o I

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K1)

Fig. 3.4e log cr vs 1000/T for KSrTiNb06 in air atmosphere

-3.0e3 t--1----1--+--+---+--+---1---+--+--+----,1----1

,..... Ill

-2.5e3

8, -2.0e3

"' C:

·5

.§ -1.5e3

., " C:

� -1.0e3

o.oe,rfflm--------------_...!!!l._ _____ ..;z!II _ __J

0. 1.0e3 4.0e3 6.0e3

Impedance real (ohms)

Fig. 3.Sa Cole-Cole plot for NaCaTiNb06 in air atmosphere (a) 600°

C (b) 650°

C and (c)

700°

C

60

,.... -1.2e3

! c.'

·l -8.0e2 §.,"ffi al t -4.oe2

_ .......... _ (a) .. . ......

.,,. ....

Impedance real (ohms)

..........

2.4e3

Fig. 3.5b Cole-Cole plot for NaCaTiNb06 at 700°C in (a) oxygen and (b) air atmosphere

-6.0e4

,.;;-� -5.0e4 e. c.' l:'l -4.0e4 ·a, ..

E -; -3.0e4 " C: ..

"O

� -2.0e4 .s

6 .Oe4 8 .Oe4 1.0e5 1 .2e5

Impedance real (ohms)

Fig. 3.5c Cole-Cole plot for NaCaTiTa06 in air atmosphere (a) 600°C (b) 650

°C and (c)

700°C

61

-2.0e4t--+---+---+---+---+---+--+--+---+----

'[ -1.5e4

e.. C'

•! § -1.0e4

f -5.0e3

\

\ 4

\ O.OeH-1�------------------=--.l

O.OeO 1.0e4 2.0e4 3.0e4 4.0e4 5.0e4

Impedance real (ohms)

Fig. 3.5d Cole-Cole plot for NaCaTiTa06 at 700°C in (a) oxygen and (b) air atmosphere

-1.2e3 T"-l----+----+---+----+---+---+------1

'[ -9.0e2

e. � ·�§ -6.0e2 "' " C .. .,,,

f -3.0e2

(a)

1.0e3 2.0e3 3.0e3 4.0e3

Impedance real (ohms)

Fig. 3.5e Cole-Cole plot for NaSrTiNb06 in air atmosphere (a) 600°C (b) 650

°C and (c)

700°C

62

-8.0e2,--+---I----+---+----+--+----+---+-----i

i -6.0e2

e~

'"c:t ·4.0e2

~

~'"..§ -2.0e

(a)

O.OeO.1..---------------......--.....------l5.0e2 1.0e3 1.5e3 2.0e3 2.5e3

Impedance real (ohms)

Fig.3.5f Cole-Cole plot for NaSrTiNb06 at 700°C in (a) air and (b) oxygen atmosphere

-6.0e2 ,I--+--+--+--+--+--+--I--I--I--I--I--I--I--+--+--I--I--+--+--+--+--I

(a)

O.OeO.L-----------------~- .....-----'

-5.0e2'"'..E.t::e. ·4.0e2~~'0~ ·3.0e2

Impedance real (ohms)

Fig. 3.5 g Cole-Cole plot for NaSrTiTa06 in air atmosphere (a) 600°C (b) 650°C and (c)

700°C

63

-6.De2 r--1----1---1----1---+----1---i---1----1

j ·4.De2

e, I!' ..

. 5

l -2.De2

.. () C: ..

] § D.DeD

�·-·.._, (a)

�,,,,..� ..... ....

2.De2.L.--------------------__J 4.De2 8.De2 1.2e3 1.6e3 2.De3

Impedance real (ohms)

Fig. 3.5h Cole-Cole plot for NaSrTiTa06 at 700°C in (a) air and (b) oxygen atmosphere

-4.De3 +---+---l-+---+---t-+---1---1--i--1--+---+-+---1---+---<

j -3.De3 ,:;;;

0 ,..,, I!' .. C:

·5, .§ -2.0e3

"' ()

Iii .,, �..5 -1.De3

1.0e4 1.2e4 1.4e4 1 .6e4

Impedance real (ohms)

Fig. 3.5i Cole-Cole plot for KSrTiNb06 in air atmosphere (a) 600°C (b) 650

°C and (c)

700°C

64

Table 3.1

Crystal system and lattice parameters of AA0

TiM0

06 (A=Na or K; A0=Ca or Sr; M

0

= Nb or Ta) system

Composition Crystal Lattice parameter (A)

System a b C

NaCaTiTa06 Orthorhombic 5.432 7.738 5.479

NaSrTiTa06 Cubic 3.895

NaCaTiNb06 Orthorhombic 5.483 7.728 5.445

NaSrTiNb06 Cubic 3.917

KSrTiNb06 Cubic 3.925

Table 3.2

Comparison of conductivity and activation energy ofNaA0

TiM0

06 (A0

=Ca or Sr; M0

= Nb or Ta) compounds at 700°C

Composition Conductivity Conductivity Activation Activation

(air) (S cm-1) (02) (S · cm- 1) energy (air) energy (02)

(eV) (eV)

NaCaTiNb06 1.6lxl0-4 l.43x10-4 1.068 1.171

NaCaTiTa06 1.49x10-4 1.37xl0-4 1.241 1.269

NaSrTiNb06 3.77xl0-4 3.2lxl0-4 1.083 1.180

NaSrTiTa06 2.03xl0-4 1.99xl0-4 1.262 1.310

65

3.3.2 NaA0

ZrM0

06 (A' =Ca or Sr; M0

= Nb or Ta) system

3.3.2.1 XRD studies

XRD patterns of NaA0

ZrM0

06 (A0

=Ca or Sr; M0

= Nb or Ta) system is shown in

fig.3.3.6a-c. The patterns are similar to that obtained in the earlier system; the peaks are

not sharp. The patterns of NaCaZrNb06 and NaCaZrTa06 are indexed on the basis of

orthorhombic perovskite structure. The d spacing is in good agreement with the PDF

No.51-133 [19] and prominent planes are (101), (200) and (220).

NaSrZrNb06, and NaSrZrTa06 can be indexed on the basis of cubic perovslcite

structure and are in good agreement with the PDF No.38-1470 [20]. The strong

diffraction peaks corresponds to ( 110), ( 100) and (200) planes. The lattice parameters of

these oxides are given in table 3.3.

10 20

0

0

N

30 20

NaCaZrTa06

NaCaZrNb06

40 50 60 70

Fig. 3.6a X-ray diffraction pattern ofNaCaZrM0

06 (M0

= Nb or Ta)

66

~

0,...,...~

~

00,... NaSrZrTaO~

~

~ ,...0

~

,...0 0 N ~

N ,... ~ 0~

N N~ N

• ;;.........

NaSrZrNbOe

.11. ~

10 20 30 40

29

50 60 70

Fig. 3.6b X-ray diffraction pattern ofNaSrZrM'06 (M' = Nb or Ta)

3.3.2.2. Differential thermal analysis (DTA)

The DTA curves for the NaA'ZrM'06 (A' =Ca or Sr; M' = Nb or Ta) were shown

in fig.3.7 a-d. The system is studied for determination of any phase change during

heating. There is no phase change is observed for this system.

67

80

70

60

50

:[ 40

�

20

10

0

-10 0 200 400 600 800 1000

Temperature (0C)

Fig. 3.7a DTA curve for NaCaZrNb06

0 200 400 600 800 1000

Temperature (0C)

Fig. 3.7b DTA curve for NaCaZrTa06

68

80..--- --,

70

60

50

~4OCl:b 30

20

10

o

o 200 400 600

Temperature l°C)800 1000

Fig. 3.7c DTA curve for NaSrZrNb06

70

60

50

40

~30

~0

20

10

0

·100 200 400 600 800 1000

Temperature (oC)

Fig. 3.7d DTA curve for NaSrZrTa06

69

3.3.2.3. SEM and Microstructure

The SEM micrographs of the system NaA0

ZrM0

06 (A' =Ca or Sr; M' = Nb or Ta)

were given in fig.3.3.8 a-d. The density measured for NaCaZrM0

06 (M' = Nb or Ta)

system is in the range 4.9g/cc and for the other system, NaSrZrM0

06 (M0

= Nb or Ta), it is

in the range 4.2g/cc. The micrograph confirms these results. The SEM micrograph of

NaCaZrTa06 shows pores within the grains. The micrograph indicates that NaSrZrTa06

is porous and this affected its conductivity value. The average grain size is in the range 1-

3µm.

70

I

3.3.2.4 Electrical conductivity studies

The electrical conductivity of the system NaA'ZrM'06 (A' =Ca or Sr; M' = Nb or

Ta) has been measured by impedance analyzer over a frequency range 100mHz-IOMHz

in the air as well as oxygen atmosphere. The measurement temperature range was

between 300-750°C. Fig.3.9 a-d shows Arrhenius plots of conductivity for NaA'ZrM'06

(A' ==Ca or Sr; M' == Nb or Ta) system. The graph obtained is almost straight line

indicating that the conductivity obeys the Arrhenius equation. It can be seen from the

Arrhenius plots that the electrical conductivity increases linearly with rising temperature

and no abrupt conductivity change is observed. The conductivity values at 700°C are

given in table 3.4.

The conductivity values obtained for the system in oxygen atmosphere is less than

the values obtained in air atmosphere. This is due to the decrease in the rate of diffusion

of 0 2- ions into the vacancy due to the decrease in the number of vacancy in oxygen

atmosphere [24].

The activation energy for conduction was calculated from the slope of the graph

in the temperature range 500-600°C. The activation energy of the system in oxygen

atmosphere is higher than that in air. This is expected from the decrease in ionic

conductivity.

The conductivity is a function of the elements in the structure, the size of the

elements and the degree of order or disorder in the structure. Sammells et al. [28, 29]

observed that there is a strong correlation between the conductivity and the size of the

critical radius. The critical radius refers to the radius of the smallest area through which

the oxide ion 0 2- must pass.

The degree of distortion seems to be the most important parameter controlling the

oxide ion conductivity and the distortion seems in turn to be closely related to the degree

of deviation from matching ionic radii of the cation dopants that are used to introduce the

oxide vacancies [30]. The charge of the B ion (in this case Ti4+, Zr4+, Nb5+ and Ta5+) is

also seems to be important in the AB03 perovskites.

72

The typical Cole-Cole plots of the NaA'TiM'06 (A'=Ca or Sr; M' = Nb or Ta)

system at different temperatures (600-700°C) are shown in fig.3.10a-h. The intersection

of the semicircle on the low frequency side (right of the fig.) with abscissa was taken as

the total resistance of the sample. There is only one semicircle which corresponds to the

grain conduction. The Cole-Cole plot of NaCaZrTa06, does not pass through the origin,

since Roo>O. Presence of pores modifies the impedance plot by altering the diameter of

the arcs [26].

-2,------------------,

-3

-4

-5

E -6~~

I:> -7~

-8

-9

-10

\-.. NaCaZrNbO,(Air) \-e- NaCaZrNbO,(O,)

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

10001T (K')

Fig.3.9a log cr vs 1000/T for NaCaZrNb06 in air and oxygen atmosphere

73

-2

-3

-4

-5

-6

-7 ..Q

-8

-9

.,, ., "• '•"'--

"-:',, - a- NaCaZrTaO,(Air)

, •, a- NaCaZrTa0,(02)

'• ' �-� • •

"".�. �� ·�· .�.

""�.� �·

""'·

0,9 1-0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K1

)

Fig.3.9b log cr vs I 000/T for NaCaZrTa06 in air and oxygen atmosphere

..Q

-2

-3

-4

-5

-6

-7

-8

-9

-10

• NaSrZrNbO,(Air) - • - NaSrZrNb0,(0

2)

Q9 1� 1j 12 1� 1.4 1� 1� 1J 1B

1000/T (K1)

Fig.3.9c log cr vs 1000/T for NaSrZrNb06 in air and oxygen atmosphere

74

e

-2

-3

-4

-5

-6

-7

-8

-9

-10

•· NaSrZrTaO,(Air) .__. NaSrZrTaO,(O,)

...

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

1000/T (K"1)

Fig.3.9d log er vs l 000/T for NaSrZrTa06 in air and oxygen atmosphere

-6.0e3 +--+--+---t>----+--+---+--+--+-----+-----+-----1

,-..

..

-5.0e3

e. -4.0e3

"'

.s

i -3.0e3

� -2.0e3

-1.0e3

0.0eO 0. 4.0e3 6.0e3 8.0e3 1.0e4 1.2e4 1.4e4

Impedance real (ohms)

Fig. 3.10a Cole-Cole plot for NaCaZrNb06 in air atmosphere (a) 600°C (b) 650

°C and (c)

1oo·c

75

-6.0e2 t-----t---+---+----+---1-----11----+-----1

I 8. -4.0e2

� <:

"6, ..

..5 ... () <: {1l -2.0e2

!

I

I

5.0e2 1.0e3

Impedance real (ohms) 1.5e3

\ \

4-

4

2.0e3

Fig. 3.10b Cole-Cole plot for NaCaZrNb06 at 700°

C in (a) oxygen and (b) air

atmosphere

,-.. ..

E ..c::

-6.0e2-r--+---+----+---+----+------il---+---+---I

e. -4.0e2 (a) 2:-

-� "'..

..5 ... () <:

t ..5

(b)

8.0e2 1.2e3 2.0e3

Fig. 3.10c Cole-Cole plot for NaCaZrTa06 in air atmosphere (a) 600°

C (b) 650°

C and (c)

700°

C

76

·1.2e2,-+---+----+---+---+_--+---~f__-_...,

·1.0e2

6.0e25.0e24.0e2

Impedance real (ohms)

-2.0el

O.OeO"-----"''---1t-1-----------------.....---l

¥&.e. ·S.Oel~

.~co~ ·6.0el

g~ ·4.0et.,<>.

..5

Fig. 3.10d Cole-Cole plot for NaCaZrTa06 at 700°C in (a) oxygen and (b) air atmosphere

-1.6e5t---I--------+---+--~---+---+---j

·1.4e5

¥ ·t.2e5

e~ -1.0e5..c'6>~ -8.0e4

.,g -6.0e4...".,r -4.0e4

(c)

t .Oe5 2.0e5

(a)

4.0e5

Impedance real (ohms)

FIg. 3.10e Cole-Cole plot for NaSrZrNb06in air atmosphere (a) 600°C (b) 650°C and (c)

700°C

77

·8.0e4t-----t---I---+----+---+----t----+-----t

-' ' _A.- . - (a).... ' ..,....~,

~ -6.0e4

ec- .Illl! I'0~ -4.0e4 ~

..~ Ii ,..§ -2.0e4 ;.. •__,~..

(b)

1.0e5

Impedance real (ohms) .

\,\~..\

1.5e5 2.0e5

Fig. 3.10f Cole-Cole plot for NaSrZrNb06at 700°C in (a) oxygen and (b) air atmosphere

·6.0e5 +---I---+--I--_+_-+----+--I--+---I--+--_+_-1

-5.0e5

~.£:e -4.0e5c-..<:'0~ -3.0e5

OJ<><:-l!l -2.0e5OJ

.§

O.OeO.OeO 2.0e5 4.0e5 6.0e5 8.0e5

Impedance real (ohms)1.0e6 1.2e6

Fig. 3.10g Cole-Cole plot for NaSrZrTa06 in air atmosphere (a) 600°C (b) 650°C and (c)

700°C

78

-4.0e5 t---+---+--l-+---+--+--+-+---+--+--+---+-t---+--+-i

~ -3.0e5..ce.i'5~ -2.0e5

.."c:.....,..t -1.0e5

_ ' .....' _ , _ , ...... (a)

....-' --,.,.- .... ,.....~

'.,.~

\...O.Oe_----------_~-------l .......

o.OeO 1.0e5 2.0e5 3.0e5 4.0e5 5.0e5 6.0e5 7.0e5 S.Oe5- .. -. .

Impedance real (ohms)

Fig. 3.10h Cole-Cole plot for NaSrZrTa06 at 700°C in (a) air and (b) oxygen atmosphere

Table 3. 3

Comparison of conductivity and activation energy of NaA'ZrM'06 (A'=Ca or Sr;

M' = Nb or Ta) compounds at 700°C

Composition Crystal System Lattice parameter (A)

a b c

NaCaZrTa06 Orthorhombic 5.532 7.746 5.553

NaSrZrTa06 Cubic 3.932

NaCaZrNb06 Orthorhombic 5.584 7.734 5.543

NaSrZrNb06 Cubic 3.946

79

Table 3. 4

Comparison of conductivity and activation energy of NaA'zrM'o6 (A'=ca or Sr;

M' = Nb or Ta) compounds at 700°C

Composition Conductivity Conductivity Activation Activation

(air) ( S cm-1) (02) (S cm-1) energy (air) energy (02)

(eV) (eV)

NaCaZrNb06 3.62 xl0-J 2.73 xl0-j 1.858 2.124

NaCaZrTa06 1.78 X 10-J l.17xlo-j 2.157 2.201

NaSrZrNb06 2.56x 10-J 2.31xl0-j 1.893 2.198

NaSrZrTa06 1.22 X 104 l .09 x 10-4 2.321 2.524

3.4 Conclusion

The A-A'-M-M'-o (A=Na or K; A' =Ca or Sr; M= Ti or Zr; M'= Nb or Ta)

systems were prepared and electrical properties were studied. The X-ray diffraction study

shows the formation of perovskite structured compounds with single phase. The high

value of ionic conductivity is obtained for the NaA'ZrM'06 (A'=ca or Sr; M' = Nb or Ta)

system, except NaSrZrTa06 compound. The conducting species in this system is the 02-

ions. From the ionic conductivity studies, it can be concluded that this systems, especially

Zr substituted compounds, are potential candidates as solid electrolytes.

80

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