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Structural r espon s e to p ressure i nduced e lectronic t ransitions in TM-compounds

Structural r espon s e to p ressure i nduced e lectronic t ransitions in TM-compounds. Moshe Paz-Pasternak, Tel Aviv University, ISRAEL. Beware of false knowledge; it is more dangerous than ignorance George Bernard Shaw.

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Structural r espon s e to p ressure i nduced e lectronic t ransitions in TM-compounds

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  1. Structural response to pressure induced electronictransitionsin TM-compounds Moshe Paz-Pasternak, Tel Aviv University, ISRAEL Beware of false knowledge; it is more dangerous than ignorance George Bernard Shaw ERICE 2009

  2. What types of electronic transitions may lead to structural phase transition in TMC’s? • high to low spin transitions • Intra-band overlap; the Mott-Hubbard correlation breakdown. • Cationic inter-bandoverlap; valence exchange • and more…. ERICE 2009

  3. How to detect and measure electronic transitions at high pressures? • Appropriate electronic spectroscopy methods with radiation that can be transmitted through diamonds such as: • K-edge X-rays of the TM-ion to be used for XAS, XANES, XES, EXAFS, etc. • Mössbauer spectroscopy in iron-containing samples. • Optical spectroscopy - and • Wires for resistance and other electrical measurements ERICE 2009

  4. The d-shell (Hund’s rules) Fe3+ Fe3+(LS))5↑↓ ↑↓ ↓ 1/2 3 Fe3+(LS))5↑↓ ↑↓ ↓ 1/2 3 Fe2+ Fe2+(LS))5↑↓ ↑↓ ↑↓ 0 3 ERICE 2009

  5. The high spin state is unstable at high-pressure Fe3+ Fe2+ P “Spin crossover” P ERICE 2009

  6. Radius of TMHS > Radius of TMLS Fe3+(HS))5↑ ↑↑ ↑ ↑ Fe3+(LS))5↑↓↑↓ ↓ EuFeO3 Fe3+(HS) Fe3+(LS) ERICE 2009

  7. Mott Hubbard insulator B - electronic configuration of the TM ion The strong on-site Coulomb repulsion produces an energy gap, within the 3d band, known as the Mott-Hubbard gap (U). The insulating gap may also arise from a finite ligand-to-metal p-d charge-transfer energy Δ. In the case of Δ <U we have a Charge-Transfer insulator.  > U (L - ligand hole) U >  ERICE 2009

  8. electronic/magnetic consequences of Mott-Hubbard correlation-breakdown ERICE 2009

  9. Mössbauer spectroscopy currently the best experimental method at the atomic scale for studying magnetism at very high pressures Rudolf. Nobel 1961 The nuclear scattering cross-section of 57Fe(14.4 keV) gamma-rays is ~ 109 barns! That’s why we can use absorbers with diam. <0.1 mm ERICE 2009

  10. Nuclear resonant scatterer ±v detector Synchrotron monochromatic beam ERICE 2009

  11. Mössbauer spectroscopy for pedestrians • The hyperfine interaction in 57Fe • Effect of pressure upon HHyp • The Isomer Shift • Determining relative abundance of components ERICE 2009

  12. The Hyperfine Interaction in 57Fe ±3/2 QS ±1/2 Two quadrupole-split components 1/2 +3/2 ~µHhyf +1/2 -1/2 -3/2 Magnetic splitting -1/2 +1/2 2Γ 57Co e.c decay Γ, t1/2 t1/2 ~ 5x10-7 sec!!! Γ~ 0.5 μeV!!! I*=3/2 14. 4 keV ERICE 2009 I=1/2 57Fe

  13. The effect of Pressure upon the Hyperfine Field • With <Lz > = 0 the orbital term is quenched and HO = 0. • With pressure increase HO→0 “S”spin term, “O”  orbital term. *HOis P-dependent! The Fe magnetic-moment: ERICE 2009

  14. ΔR/R is a nuclear constant.ρs(0) is the s-electrons density at the nucleus Isomer shift; an unique atomic-scale densitometer Decrease in IS Increase in the densityat the vicinity of the Fe site ERICE 2009

  15. Determining the component-abundance ni ERICE 2009

  16. Structural Response to PI electronic transitions in Fe2+ compounds • FeO (wüstite) NaCl structure • FeX2 (X=Cl, I) • Fe(OH)2 CdI2 structure ERICE 2009

  17. Experimental proof of Hund’s rule Pmechanical > PCoulombic HS > LS starting at ~ 90 GPa No symmetry or appreciable volume change ever detected. LS HS NaCl structure ERICE 2009

  18. Mg0.9Fe0.1O ERICE 2009

  19. FeI2 ERICE 2009

  20. The curious case of iron hydroxide Fe(OH)2 Parise et al ERICE 2009

  21. pressure-induced oxidation of Fe(II) T >> TN Paramagnetic Fe2+ T << TN anti-ferromagnetic Fe2+ ERICE 2009

  22. oxidation is related to the orientationally deformed O-H dipoles H Lateral displacement (Parise et al) Fe3+ abundance ERICE 2009

  23. No change in structure! ERICE 2009

  24. Conclusion The irreversible oxidation process is attributed to: • the orientation-disorder of the O-H dipoles caused by the pressure-induced OH----HO coulomb repulsion, and, • to the exceptional small electron binding energy of Fe2+ Within the HP band-structure of Fe(OH)2 a new, localized band is formed populated by the “ousted” electrons ERICE 2009

  25. Structural response to PI electronic transitions in Fe3+ oxides • Fe2O3 (hematite) • R FeO3(R= rare-earth iron perovskites) • CuFeO3 (delafossite) ERICE 2009

  26. Fe2O3a correlation breakdown ΔV/V0 = 0.1 ERICE 2009 Rutile > Rh2O3 II

  27. Fe2O3a catastrophic correlation breakdown INSULATOR-METAL TRANSITION COLLAPSE OF MAGNETISM ERICE 2009

  28. Summary • Correlation breakdown triggersa 1st-order structural phase transition • Similar transitions are observed in GaFeO3and FeOOH, pointing to a structural instability of (Fe3+O6) species at • P > 50 GPa. ERICE 2009

  29. The exceptional case of RFeO3 perovskites ERICE 2009

  30. All R FeO3 (R3+ rare earth ) undergo HS>LS transition at ~ 40 GPa At P > 100 GPa they remain paramagnetic (<S>≠0) down to 4K. ERICE 2009

  31. IM takes place at ~ 120 GPa ERICE 2009

  32. A 1st (or 0th) order structural phase transition occurs at the HS>LS crossover with 3-5% volume reduction but with no symmetry change! No hysteresis The perovskite structure remains stable at least to 170 GPa ERICE 2009

  33. The Mott Hubbard phase diagram ERICE 2009

  34. The peculiar case of delaffosite (CuFeO3) At ambient pressure: spin-frustrated a.f. Hexagonal structure, very anisotropic Fe3+ (S=5/2), Cu1+ (S=0) Finally at ~19 GPa a 3D super-exchange is realized. TN ~ 50 K ERICE 2009

  35. valence exchange ... how did that happened? 27 GPa Cu2+ 4 GPa Cu1+ ERICE 2009

  36. The rigidity of the O2- – Cu1+- O2- dumbbell and its orientation along the c-axis are responsible for the large anisotropy in delafossite. With pressure increase the is doomed to collapse ERICE 2009

  37. A series of: 1 - PI structure transition 2 – Followed by PI electronic phase transition 3 - Which in turn leads to another structural phase transition LP HP1 HP2 LP HP1 HP2 ERICE 2009

  38. serendipity: the ability to make fortunate discoveries by accident • We thus conclude a serendipitous voyage into the extremities of matter. Discovery of fundamentals of physics DAC Discovery of SC in Hg Kamerlingh-Ohnes (1911) Discovery of America! ERICE 2009 Pinta and Santa Maria

  39. ERICE 2009

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