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Synergies between powder diffraction and electron microscopy

Synergies between powder diffraction and electron microscopy. L. D. Marks Northwestern University Acknowledgements Ken Poeppelmeier , Northwestern University Maryvonne Hervieu , Laboratoire CRISMAT. Mission Possible. Find an unknown crystal structure without growing a single crystal

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Synergies between powder diffraction and electron microscopy

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  1. Synergies between powder diffraction and electron microscopy L. D. Marks Northwestern University Acknowledgements Ken Poeppelmeier, Northwestern University Maryvonne Hervieu, Laboratoire CRISMAT

  2. Mission Possible • Find an unknown crystal structure without growing a single crystal • Check if the material is what one thinks it is • Try and ensure that the answer is correct

  3. But, the classic phase problem • We measure |F(k)|, the modulus • r(r)=òexp(2pik.r)|F(k)|exp(if(k))dk • Phase information, f(k) is lost • Does this matter?

  4. Phase: Apples & Oranges FT Aa exp(-i a) FT Ao exp(-i o) + { Oranle ? Appge? Ao exp(-i a)IFT Phase of Apple + Amplitude of Orange = ?

  5. FT-1 {Ao exp(-i a) } Apple Phase of Apple = Apple Phase is more important than amplitude

  6. Consequence • There may exist more than one arrangement of atoms/defects which fits diffraction data (or images) – what is called “a non-convex problem” • Good scientists check to ensure that they have the best (most correct) answer, not just a possible solution

  7. Classic Methodology • Offshoot of high-Tc superconductor research • Take power diffraction pattern (x-ray, neutron) • Analyze, based upon initial model (guess?) • Check with TED/HREM • Correct Spacegroup (CBED, precession) • Any superstructures (TED, HREM) • Twinning, defects (TED, HREM) • Chemistry (EDX, EELS)

  8. Powder X-ray Statistical average Quick, high precision Light elements can be difficult (use neutrons) Texture can cause problems Often requires initial guess TEM/TED Local probe Often takes time Measurement precision often low (~1% for spacings) Good for light elements, weak superstructures Thicker samples more complicated (can be better) Brief Comparison

  9. Pitfalls: I Systematic Absences • F(hkl) which are forbidden due to screw axes, glide planes allowed due to dynamical diffraction • F(hkl) which are accidentally forbidden due to positions of atoms are allowed due to dynamical diffraction • Checks: • CBED • Precession • Simulation (HREM/TED/CBED)

  10. Textbook case: Andalusite <110> Conventional Precessed Non-precessed Precessed • Forbidden spots extinct in precession pattern • Better intensity ordering Kinematically Forbidden, Dynamically Allowed Spots

  11. Pitfalls: 2 Symmetry • Symmetry in HREM images is in general lower than real symmetry • Incorrect orientation and lens aberrations reduce the symmetry • Astigmatism (2 & 3 fold) • Beam Tilt • Non isoplanaric illumination (FEG) • Crystal Tilt (thicker samples) • Checks • Simulation • CBED

  12. Pitfalls: 3 Incorrect model • Refinement always improves the fit between experimental data and a model • If the model is incorrect, it is rarely possible to know • No statistical method (e.g. Hamilton tests) can tell the difference between two different models • Checks: • Brainpower only…

  13. Pitfalls: 4 Incorrect material • Almost no material is phase pure when examined by TEM • Is: • The material not what expected (often true) • The TEM data from an anomalous region which is not statistically representative • Checks • Boring… look at many different regions

  14. Data Calculated Difference A Simple Example: Sr3CaRu2O9(Courtesy of Ken Poeppelmeier) Neutron Diffraction: suggests large cell – is this right? 1.5 1.0 Counts/sec (x 103) 0.5 0.0 High-resolution backscatter bank (144°) -0.5 1.0 2.0 3.0 4.0 d-spacing Collected on the SEPD at IPNS at Argonne National Laboratory

  15. Ca1 Ru1 Ru2 Ca2 Ru2 c Ru1 Ca1 a Yes -- Electron Diffraction [010] zone axis From ED: P21/c a = 17.1 Å b = 5.7 Å c = 9.8 Å ß = 125° V = 786 Å3 [001] zone axis Monoclinic unit cell is four times larger than the trigonal subcell Space group P21/c (#14) Larger cell allows all peaks in the PXD to be indexed EDS & TGA confirm Sr3CaRu2O9 ratios 010 300

  16. A complicated example A New Manganite With An Original Composite Tunnel Structure Ba6Mn24O48 Journal of Solid State Chem.132, 239 (1997). Ph. BOULLAY, M. HERVIEU and B. RAVEAU • Ba0.25MnO2 • Chemical composition • cationic ratio by EDS analyses • oxygen content by chemical analyses • X-ray powder diffraction •  tetragonal lattice - I centring •  a≈1.8 nm and c≈0.28nm

  17. But…. Wrong Cell 1) Basic cell: I-4/m or 4/mmm a≈1.8 nm, c≈0.28nm + 2) Extra reflections : an incommensurate structure + 3) Diffuse scattering a disordered structure

  18. + Diffuse Scattering (Disorder) A composite structure with a disordered subsystem = Basic cell seen by x-rays Modulated Structure s*=ha*+kb*+lc*+mq*, q=0.36c

  19. Verified by HREM Intergrowth of rutile and hollandite tunnels

  20. Summary • Combining x-ray (neutron) diffraction and TEM (Diffraction + Imaging) is a classic combination primarily in solid state chemistry • Normally nothing new is found, but without the check GIGO is possible

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