1 EXAFS study of La and Zr local environment in amorphous precursors of La 2 Zr 2 O 7 ceramics prepared by a nitrate- modified alkoxide synthesis route E.-D. Ion 1,2, B. Malic 1, I. Arcon 1, 3, J. Padeznik Gomilsek 4 A. Kodre 1,5, M. Kosec 1 1 Jožef Stefan Institute, Slovenija, 2 National Institute of Materials Physics, Romania, 3 University of Nova Gorica, Slovenia, 4 Faculty of Mechanical Engineering, University of Maribor, 5 Faculty 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 La 2 Zr 2 O 7 - facts: –Crystal system cubic, –Space group F d -3 m, –Unit cell dimensions a = , –Z=8, –Crystal structure pyrochlore, –Melting point: 2280 o C. 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(NO 3 ) 3 Zr(OC 4 H 9 ) 4 CH 3 OC 2 H 4 OH Mixing Reflux Drying CIP, 1400 o C/10h Heat treatment Nitrate-modified alkoxide synthesis for La 2 Zr 2 O 7 ceramic Precursor powder Aim of the work Experimental Results Conclusions Pure pyrochlore phase 100 o C 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 10 o C/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) EXAFS distances, coordination number type of the neighbours EXAFS XANES formal oxidation state coordination chemistry XANES k, k 2 or k 3 - space FT R - space Introduction Aim of the work Results Conclusions Experimental

8 Thermal decomposition of the sol at 150 o C 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 Time Reference time 0 min Results

9 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 1  m crystallite size (XRD peak broadening) = 67 nm, d BET = 71nm 300nm Results

11 Qualitative Zr-K EXAFS analysis Introduction Aim of the work Experimental Conclusions LZ - sol LZ - dried LZ – 500 o C La 2 Zr 2 O o C The k 3 weighted Zr-K spectra of LZ samples and their Fourier transforms Results

12 Quantitative Zr-K EXAFS analysis Scattering atom NR(Å) 22 R-factor Zr – O7.1(6)2.09(1) Zr – C1.9(8)2.70(3)0.005 Zr – Zr5.6± (2)0.015 Zr – O7.3(5)2.11(1) Zr – C2.7(9)2.7 (2)0.004 Zr – Zr5.9± (2)0.013 Zr – O6.8(3)2.124(9) Zr – C2.0(8)2.72(2)0.009 Zr – Zr5.8(9)3.43(3)0.013 Introduction Aim of the work Experimental Conclusions The k 3 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 – 500 o C Results

13 Qualitative La-L 3 EXAFS analysis Introduction Aim of the work Experimental Conclusions The k 3 weighted La-L 3 spectra of LZ samples and their Fourier transforms Results LZ _500 LZ_sol La(NO 3 ) 3 sol LZ_dried

14 Scattering atom NR(Å) 22 R-factor La – O5.4(4)2.48(1) La – O8.3(6)2.56(1) La – O9.3(7)2.575(8) La – N1.9(2)3.11(1)0.005 La – N1.1(3)3.51(4) La – O7.3(8)3.94(1)0.007 La – O6.0± (1)0.008 La – O9.6(7)2.577(2) La – N1.9(2)3.11(1)0.002 La – N1.1(3)3.45(3) La – O5.8± (1)0.008 La – O7.1± (1)0.008 Introduction Aim of the work Experimental Conclusions Quantitative La-L 3 EXAFS analysis The k 3 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(NO 3 ) 3 - sol Results LZ - 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 500 o C 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

16 Ministry of Higher Education and Technology of Republic of Slovenia (P2 – 0105 and ), EU project SICER (G1MA-CT ), DESY and European Community support under Contract RII3-CT (IA-SFS), Access to synchrotron radiation facilities of HASYLAB (beamline E4 and C, project II ) and ELETTRA (beamline XAFS, project ) 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