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Dipartimento di Chimica Fisica “M. Rolla” – Università di Pavia

Dipartimento di Chimica Fisica “M. Rolla” – Università di Pavia. N ATURE OF THE M ONOCLINIC TO C UBIC P HASE T RANSITION IN THE F AST O XYGEN I ON C ONDUCTOR La 2 Mo 2 O 9 (LAMOX)

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Dipartimento di Chimica Fisica “M. Rolla” – Università di Pavia

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  1. Dipartimento di Chimica Fisica “M. Rolla” – Università di Pavia NATUREOFTHE MONOCLINICTO CUBIC PHASE TRANSITIONIN THE FAST OXYGEN ION CONDUCTOR La2Mo2O9 (LAMOX) Lorenzo Malavasi, Simon J.L. Billinge, Gaetano Chiodelli, Giorgio Flor, Hyunjeong J. Kim, Cristina Tealdi Dipartimento di Chimica Fisica “M.Rolla” e IENI-CNR, Università di Pavia, ITALY Department of Physics and Astronomy, Michigan State University, USA VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali – Perugia – 12-15 Giugno 2007

  2. General Considerations WHATISTHE ATOMIC PAIR DISTRIBUTION FUNCTION (PDF) Atomic Pair Density Function Atomic Pair Distribution Function g(r) GIVES THE PROBABILITY OF FINDING TWO ATOMS SEPARATED BY THE DISTANCE r ‘g(r) is like a distance map of the inside of the solid’ This is a method of “local” crystallography Which is the interest for the crystalline solids?

  3. General Considerations THROUGH THE PDF WE CAN STUDY THELOCAL DEVIATIONSOF THEAVERAGESTRUCTURE TRADITIONAL CRYSTALLOGRAPHIC METHODS Analysis of Bragg Peaks Diffuse Scattering CONTAINS INFORMATION ABOUT THE SHORT-RANGE AND INTERMEDIATE ORDER It has a weak dependence with Q and forms a continuous background Diffuse Scattering is usually discarded in crystallographic analysis TOTAL SCATTERING TECHNIQUE Bragg Peaks -Diffuse Scattering TOTAL SCATTERING STRUCTURE FUNCTION S(Q)

  4. G (Å-2) r(Å ) General Considerations TOTAL SCATTERING STRUCTURE FUNCTION S(Q) AND THE PDF Reduced Pair Distribution Function DIRECTLY MEASURED QUANTITY In principle it requires a measure up to Q infinite High Q-range of measure: HIGH RESOLUTION AND ACCURACY OF THE PDF

  5. General Considerations • Through the PDF analysis we can obtain information on: • Direct Information from the PDF • Atom-Pair Separation from Peak Positions • Coordination Number from Peak Integrated Intensity • Atom-Pair Probability Distribution from the Peak-Shape • Additional Information and Advanced Modelling • Joint Real- and Reciprocal-Space Refinements • Difference Modelling

  6. SOURCE LOCATION INSTRUMENT Intense pulsed neutron source (IPNS) Argonne National Lab. - USA SEPD, GLAD, GPPD ISIS Rutherford Appleton Lab - UK POLARIS, GEM KENS Tsukuba, JP HTT-II Spallation Neutron Source (SNS) Los Alamos National Lab. – USA HIPD, HIPPO, NPDF Experimental Details – Data Collection • ACCURACY IN THE DETERMINATION OFQ-VALUES • ACCURACY IN THE DETERMINATION OF INTENSITY Neutron Scattering Experiment SPALLATION NEUTRON SOURCES Large Flux of EPITHERMAL Neutrons  Short-wavelength High-Q (up to 100 Å-1)

  7. SOURCE E0 (keV) λ (Å) Qmax (Å-1) Cu 8.05 1.538 8.0 Mo 17.48 0.708 17.5 Ag 22.16 0.559 22.0 W 59.32 0.209 59.0 Experimental Details X-ray Scattering Experiment LABORATORY SOURCES Short Q-range -Long acquisition time - Relatively low-resolution SYNCHROTRON SOURCES Second Generation Sources Can Be Used (relatively low real-space resolution) THIRD GENERATION SOURCES (CHESS, ESRF; APS and Spring8)  Q-max 50 Å-1

  8. Experimental Details • REQUIREMENTS: • Time-stable incident flux • Low Background (collimation and shielding) • Stable Detectors and Detectors Electronic • Stable Beam Monitor • Stable Moderator • Solid State Detectors (preferred – X-rays) • Care in the instrument alignement

  9. Experimental Details – Data Analysis OBTAIN THE NORMALIZED TOTAL SCATTERING STRUCTURE FUNCTION S(Q) Measured Intensity  Scattering from the Sample and from the “Addenda” Addenda is Modified by Sample Absorption Multiple Scattering (in the sample and apparatus) Normalization to the Incident Flux Polarization Effects Sample Absorption ... ANY IMPERFECTION IN THE CORRECTION WILL AFFECT THE OUTCOME Good News: Most of the Corrections Can Be Reliably Estimated The structural Information in the PDF is “fairly robust” with respect to analysys errors (slowly varying with Q)

  10. Experimental Details – Data Analysis OBTAIN THE PDF FROM THE DIFFRACTION EXPERIMENT (PDFgetX1 – PDFgetN2) CALCULATE THE PDF FROM A STRUCTURAL MODEL REAL SPACE RIETVELD ANALYSIS PDFFIT3 REVERSE MONTE-CARLO DISCUSS4 [1] Peterson, P. F. et al., J. Applied Crystallography (2000), 33, 1192. [2] Jeong, I. K. et al., J. Applied Crystallography (2001), 34, 536. [3] Proffen, Th. Et al., J. Applied Crystallography (1999), 32, 572. [4] Proffen, Th. Et al., J. Applied Crystallography (1997), 30, 171.

  11. 542°C 563°C Introduction OXIDE ION CONDUCTORS • FUORITE TYPE (stabilized zirconia, ceria, δ-Bi2O3) • PEROVSKITES (doped LaGaO3) • INTERGROWTH PEROVSKITE/BI2O2 LAYERS (BIMEVOX) • PYROCHLORES (Gd2Zr2O7, Gd2Ti2O7) • LAMOX αβ Transition (monoclinic to cubic) at ~550°C

  12. Introduction - Structure β-LAMOX (cubic) Atomic coordinates Atom Wyck. Occ. x y z La1 4a 1.00 0.8501(5) 0.8501(5) 0.8501(5) Mo1 4a 1.00 0.1613(8) 0.1613(8) 0.1613(8) O1 4a 1.00 0.3155(8) 0.3155(8) 0.3155(8) O2 12b 0.66 0.9875(9) 0.1724(15) 0.3266(14) O3 12b 0.34 0.9152(28) 0.6213(27) 0.5670(18) Crystal data Crystal system cubic Space group P213 (no. 198) Unit cell dimensions a = 7.2421(2) Å Cell volume 379.83(1) Å3 Z = 4 F. Goutenoire et al., J. Mater. Chem. 2001,11; 119.

  13. Short O-O bond lenghts; • Partial Occupancy of O2 and O3 sites • Very high B-factors in O2 and O3 Introduction - Structure β-LAMOX (cubic) CONDUCTION MECHANISM INVOLVING O2 AND O3 SITES

  14. Introduction - Structure α-LAMOX (monoclinic) 2  3  4 Superstructure Relative to the Cubic High-temeprature Form and Small Monoclinic Distortion I.R. Evans et al., Chem. Mater. 2005,17; 4074.

  15. β-SnWO4β-La2Mo2O9 β-SnWO4β-La2Mo2O9 α-La2Mo2O9 Introduction - Structure La2Mo2O9 (LAMOX) αβ Transition (monoclinic to cubic) at ~560°C I.R. Evans et al., Chem. Mater. 2005,17; 4074.

  16. LAMOX – Some Results NPDF Measurements at 500 and 600°C (before and after the αβ transition) 600°C CUBIC MONOCLINIC 500°C d-space L. Malavasi et al., J. Am. Chem. Soc. 2007, 129, 6903 Neutron Diffraction Reveals Significant Contribution from Diffuse Scattering

  17. LAMOX – Some Results COMPARISONOF PDF AT THE TWO TEMPERATURES THE LOCAL STRUCTURES ARE SIMILAR, EVEN THOUGH THE AVERAGE ONES ARE MARKEDLY DIFFERENT EXPECTED CHANGE TO THE PDF FOR A CHANGE IN THE LOCAL STRUCTURE FROM MONOCLINIC TO CUBIC The use of “reasonable “ a.d.p. leads to sharp peaks in the calculated PDF

  18. LAMOX – Some Results Fit Parameters: a, b, c, γ, and  500°C Data Fitted with the Monoclinic Model Rwp 14.7% 600°C Data Fitted with the Monoclinic Model Rwp 15.5% 600°C Data Fitted with the Cubic Model Rwp 38.4%

  19. LAMOX – Some Results Fit Parameters: a, b, c, γ, and  and Atomic Positions and a.d.p. Rwp 20.0% Meaningless Occupancies… SOME DEFICIENSIS IN THE MODEL

  20. LAMOX – Conclusion The Local Structure of the Cubic Model is Basically the Same as in the Monoclinic Structure The Monoclinic  Cubic Phase Transition is a transition from long-range ordered to a dynamic short range-ordered distribution of the oxygen defects while preserving the monoclinic local structure Monoclinic Structure = Mo-O polyhedra with coordination 4, 5 and 6 Cubic Structure = 4.5 Local change with time of the Mo-O coordination Conduction Mechanims from a “donor” to an “acceptor” polyhedra Mo-O-Vo-Mo distance is roughly the same as Mo-O2-O3-Mo

  21. LAMOX – Conclusion First application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor A clear and reliable description of the local atom arrangement in LAMOX structure can be only achieved through the application of a local probe such as the PDF We directly determined that the transition from the monoclinic to the cubic phase of LAMOX is a transition from a static to a dynamic distribution of the oxygen defects while preserving the monoclinic local structure Useful tool to study the solid state ionics materials in order to obtain a more detailed description of their local structure which can lead to a better comprehension of the structure-property correlation

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