1

EXAFS study of La and Zr local

environment in amorphous precursors of

La2Zr2O7 ceramics prepared by a nitrate-

modified alkoxide synthesis route

E.-D. Ion1,2, B. Malic1, I. Arcon1, 3, J. Padeznik Gomilsek4 A. Kodre1,5, M. Kosec1

1Jožef Stefan Institute, Slovenija, 2 National Institute of Materials Physics, Romania, 3University of Nova Gorica, Slovenia, 4Faculty of Mechanical Engineering, University of Maribor,

5Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

2

Outline

• Introduction

• Aim of the work

• Experimental

• Results

• Conclusions

3

• Aim of the

work

• Experimental

• Results

• Conclusions

• La2Zr2O7 - facts:– Crystal system cubic,

– Space group F d -3 m,

– Unit cell dimensions a = 10.7860,

– Z=8,

– Crystal structure pyrochlore,

– Melting point: 2280oC.

• Why is it studied?– candidate material for thermal barrier coatings: high thermal stability, chemical

resistance and low thermal conductivity,

– SOFCs (undesired reaction product between electrode and electrolyte).

• How is it prepared?– Solid-state synthesis,

– Hydrothermal synthesis,

– Sol-gel synthesis.

• Introduction

4

Crystalline powder

Sol

La(NO3)3

Zr(OC4H9)4

CH3OC2H4OH

Mixing

Reflux

Drying

CIP, 1400oC/10h Heat

treatment

Nitrate-modified alkoxide synthesis for La2Zr2O7 ceramic

Precursor

powder

• Aim of the

work

• Experimental

• Results

• Conclusions

Pure pyrochlore phase

100oC lower than reported temperatures

? ?

CERAMIC

r=97.9%

• Introduction

5

• Investigation of the Zr and La local environment

upon transition from the sol to the amorphous

powder in order to learn about the distribution of the

constituent metal atoms at the sub-nanometer level.

• Introduction

• Experimental

• Results

• Conclusions

• Aim of the

work

6

• Sol: TG/DTA, EXAFS

• Powders: TG/DTA, XRD, SEM, EXAFS

Characterization

TG/DTA - Netzsch STA 409, flowing air, Pt crucible, with a heating rate

of 10oC/min

XRD - Bruker AXS D4 Endeavor diffractometer CuK radiation

SEM - Zeiss SUPRA 35VP

EXAFS - Data aquisition: XAFS (BL 11.1) beamline at ELETTRA and

beamlines E4 and C at HASYLAB, DESY in a standard

transmission mode at RT

Liquid samples (1 M ) - prepared in inert atmosphere and

sealed in thin vacuum-tight plastic

bags to prevent hydrolysis in air.

Powder samples - pellets (sample material + BN)

- Data analysis: IFEFFIT using FEFF6 code

• Introduction

• Aim of the

work

• Results

• Conclusions

• Experimental

7

X-ray Absorption Fine Structure (XAFS)

EXAFSdistances, coordination number type of the neighbours

EXAFS

XANESformal oxidation state coordination chemistry

XANES

k, k2 or k3 - space

FT

R - space

• Introduction

• Aim of the

work

• Results

• Conclusions

• Experimental

8

Thermal decomposition of the sol at 150oC

1 min 3 min 4 min 4.2 min

• Introduction

• Aim of the

work

• Experimental

• Conclusions

Evaporation of the solvent

Swelling and

foaming

Expansion of the volume for more than 100 times

Self-ignition and burning

TimeReference

time 0 min• Results

9

0 200 400 600 800 1000 120020

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 120020

30

40

50

60

70

80

90

100

-0.4

-0.2

0.0

0.2

0.4

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

TG

(%

)T

G (

%)

total mass loss=73.04%

Temperature (oC)

total mass loss=11.14%

Temperature (oC)

DT

A (V

/mg)

LZ - sol

DT

A (V

/mg)LZ - dried

Thermal behaviour of the sol and the dried powder• Introduction

• Aim of the

work

• Experimental

• Conclusions

• Exothermic reaction between

nitrates and organics - nitrates act

as an oxidizing agent for the

organic groups and enable their

removal.

• Gradual decomposition of the

dried powders. The residual

functional groups - unable to

initiate another exothermic

reaction, and they just decompose

during heating.

• Results

10

Crystallization and Morphology• Introduction

• Aim of the

work

• Experimental

• Conclusions

20 30 40 50 60 70

0

50

100

*

*

*

2 (deg.)

LZ_dried

(22

2)

0

50

100

LZ_900oC

LZ_800oC

LZ_700oC

LZ_500oC

0

50

100

*

0

300

600

0

400

800

1200

* - sample holder

*

*

Inte

nsi

ty (

arb

. u

nits)

(40

0)

(44

0)

(62

2)

(44

4)

1 m

1 m

• crystallite size (XRD peak broadening) = 67 nm,• dBET = 71nm

300nm

• Results

11

Qualitative Zr-K EXAFS analysis

6 8 10 12 14

0

10

20

30

40

k3

(k)

k (A-1)0 2 4 6

0

5

10

15

20

25

FT

ma

gn

itud

e (

arb

. u

nits

)R(A)

• Introduction

• Aim of the

work

• Experimental

• Conclusions

LZ - sol

LZ - dried

LZ – 500oC

La2Zr2O7

1100oC

The k3 weighted Zr-K spectra of LZ samples and their Fourier transforms

• Results

12

Quantitative Zr-K EXAFS analysis

Scattering atom

N R(Å) 2 R-factor

Zr – O 7.1(6) 2.09(1) 0.010

0.017Zr – C 1.9(8) 2.70(3) 0.005

Zr – Zr 5.6±1.1 3.36(2) 0.015

Zr – O 7.3(5) 2.11(1) 0.007

0.012Zr – C 2.7(9) 2.7 (2) 0.004

Zr – Zr 5.9±1.0 3.43(2) 0.013

Zr – O 6.8(3) 2.124(9) 0.007

0.009Zr – C 2.0(8) 2.72(2) 0.009

Zr – Zr 5.8(9) 3.43(3) 0.013

• Introduction

• Aim of the

work

• Experimental

• Conclusions

0 2 4 6

0

5

10

15F

T m

agni

tude

(ar

b. u

nits

)

R(A)

The k3 weighted Fourier transforms of Zr-K spectra of LZ samples. Solid line – experiment, dashed line ----- EXAFS model

Parameters of the nearest coordination shell around Zr atom: scattering atom, average number N, distance R, and Debye – Waller factor 2

LZ - sol

LZ - dried

LZ – 500oC• Results

13

Qualitative La-L3 EXAFS analysis• Introduction

• Aim of the

work

• Experimental

• Conclusions

The k3 weighted La-L3 spectra of LZ samples and their Fourier transforms

• Results

2 4 6 8 10

-5

0

5

10

15

20

25

k (A-1)

k3 (

k)

0 2 4 6

0

5

10

15

F

T m

ag

nitu

de

(a

rb.

un

its)

R(A)

LZ _500

LZ_sol

La(NO3)3

sol

LZ_dried

14

Scattering

atom

N R(Å) 2 R-factor

La – O 5.4(4) 2.48(1) 0.015 0.008

La – O 8.3(6) 2.56(1) 0.013 0.012

La – O 9.3(7) 2.575(8) 0.009

0.004La – N 1.9(2) 3.11(1) 0.005

La – N 1.1(3) 3.51(4)

La – O 7.3(8) 3.94(1) 0.007

La – O 6.0±1.3 4.40(1) 0.008

La – O 9.6(7) 2.577(2) 0.009

0.004La – N 1.9(2) 3.11(1) 0.002

La – N 1.1(3) 3.45(3)

La – O 5.8±1.0 3.95(1) 0.008

La – O 7.1±1.5 4.41(1) 0.008

• Introduction

• Aim of the

work

• Experimental

• Conclusions

Quantitative La-L3 EXAFS analysis

The k3 weighted Fourier transforms of Zr-K spectra of LZ samples. Solid line – experiment, dotted line ....... EXAFS model

Parameters of the nearest coordination shell around La atom: scattering atom, average number N, distance R, and Debye – Waller factor 2

LZ - sol

La(NO3)3 - sol

• ResultsLZ - dried

LZ_500

15

• Zr and La local environment in lanthanum zirconate precursors, prepared by a nitrate-

modified alkoxide sol-gel route, was studied.

• Zr – O – Zr links are formed already in the liquid sol - homocondensation of Zr species

in nano-clusters.

• In the liquid to amorphous solid transition the local environment of Zr does not

change.

• In the amorphous to crystalline solid transition at 500oC the homocondensation of Zr

is preserved, but the Zr-O-Zr distances are shortened.

• The La environment is not changed from the LN sol to LZ sol by the Zr butoxide

addition, but it is changed after drying and heating due to the nitrates groups

decomposition.

• There are no La – O – Zr links in the sol or in the amorphous powder.

• This synthesis route allows a good mixing of the species at the nanometer leavel.

• Introduction

• Aim of the

work

• Experimental

• Results

Conclusions

• Conclusions

16

• Ministry of Higher Education and Technology of Republic of Slovenia (P2 – 0105 and 1-

0112 ),

• EU project SICER (G1MA-CT-2002-04029),

• DESY and European Community  support under Contract RII3-CT-2004-506008 (IA-SFS),

• Access to synchrotron radiation facilities of HASYLAB (beamline E4 and C, project II-04-

065) and ELETTRA (beamline XAFS, project 2005156) is acknowledged,

• We would like to thank Edmund Welter of HASYLAB and Luca Olivi of ELETTRA for expert

advice on beamline operation,

• National Institute of Chemistry – FE-SEM,

• Dr. Tadej Rojac for crystallites size calculation,

• Tanja Urh and Mrs. Jena Cilensek for help in the experimental work.

Acknowledgement