2. 30 May, 2009 ERICE 2009 1 Structural response to pressure induced electronic transitions in TM-compounds Moshe Paz-Pasternak, Tel Aviv University, ISRAEL Beware of false knowledge; it is more dangerous than ignorance George Bernard Shaw George Bernard Shaw

3. 30 May, 2009ERICE 2009 What types of electronic transitions may lead to structural phase transition in TMC’s? o high to low spin transitions o Intra-band overlap; the Mott-Hubbard correlation breakdown. o Cationic inter-band overlap; valence exchange o and more….

4. 30 May, 2009ERICE 2009 –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. –M ö ssbauer spectroscopy in iron-containing samples. –Optical spectroscopy - and –Wires for resistance and other electrical measurements

5. 30 May, 2009ERICE 2009 The d-shell (Hund’s rules) Fe 3+ Fe 3+ (LS)) 5 ↑↓ ↑↓ ↓ 1/2 3 Fe 2+ Fe 2+ (LS)) 5 ↑↓ ↑↓ ↑↓ 0 3 Fe 3+ (LS)) 5 ↑↓ ↑↓ ↓ 1/2 3

6. 30 May, 2009ERICE 2009 P Fe 3+ Fe 2+ The high spin state is unstable at high-pressure P “Spin crossover”

7. 30 May, 2009ERICE 2009 Fe 3+ (LS)) 5 ↑↓ ↑↓ ↓ Fe 3+ (HS)) 5 ↑ ↑ ↑ ↑ ↑ Radius of TM HS > Radius of TM Radius of TM HS > Radius of TM LS EuFeO 3 Fe 3+ (LS) Fe 3+ (HS)

8. 30 May, 2009ERICE 2009 Mott Hubbard insulator 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. (L - ligand hole) - electronic configuration of the TM ion U >   > U B

9. 30 May, 2009ERICE 2009 electronic/magnetic consequences of Mott-Hubbard correlation-breakdown correlated states Uncorr. states insulatormetallic Odd number of spins HSLS S ≠ 0 paramagnetic Even number of spins S ≠ 0 S = 0 paramagneticdiamagnetic

10. 30 May, 2009ERICE 2009 Mössbauer spectroscopy That’s why we can use absorbers with diam. <0.1 mm The nuclear scattering cross- section of 57 Fe(14.4 keV) gamma-rays is ~ 10 9 barns! currently the best experimental method at the atomic scale for studying magnetism at very high pressures Rudolf. Nobel 1961

11. 30 May, 2009ERICE 2009 ±v±v±v±v detector Nuclear resonant scatterer Synchrotron monochromatic beam

12. 30 May, 2009ERICE 2009 Mössbauer spectroscopy for pedestrians The hyperfine interaction in 57 Fe The hyperfine interaction in 57 Fe Effect of pressure upon H Hyp Effect of pressure upon H Hyp The Isomer Shift The Isomer Shift Determining relative abundance of components Determining relative abundance of components

13. 30 May, 2009ERICE 2009 t 1/2 ~ 5x10 -7 sec!!! Γ ~ 0.5 μeV!!! Two quadrupole- split components Magnetic splitting ±3/2 ±1/2 1/2 QS +3/2 +1/2 -1/2 - 3/2 -1/2 +1/2 ~µH hyf Γ, t 1/2 57 Co e.c decay 14. 4 keV 2Γ2Γ2Γ2Γ I=1/2 I * =3/2 The Hyperfine Interaction in 57 Fe 57 Fe

14. 30 May, 2009ERICE 2009 The effect of Pressure upon the Hyperfine Field With = 0 the orbital term is quenched and H O = 0. With = 0 the orbital term is quenched and H O = 0. With pressure increase H O → 0 With pressure increase H O → 0 *H O is P-dependent! “S”  spin term, “O”  orbital term. The Fe magnetic-moment:

15. 30 May, 2009ERICE 2009 ΔR/R is a nuclear constant. ρ s (0) is the s-electrons density at the nucleus Decrease in IS  Increase in the density at the vicinity of the Fe site Isomer shift; an unique atomic-scale densitometer

16. 30 May, 2009ERICE 2009 Determining the component-abundance n i

17. 30 May, 2009ERICE 2009 Structural Response to PI electronic transitions in Fe 2+ compounds FeO (wüstite) NaCl structure FeO (wüstite) NaCl structure FeX 2 (X=Cl, I) FeX 2 (X=Cl, I) Fe(OH) 2 Fe(OH) 2 CdI 2 structure

18. 30 May, 2009ERICE 2009 HS > LS starting at ~ 90 GPa No symmetry or appreciable volume change ever detected. LSLS HSHS NaCl structure Experimental proof of Hund’s rule P mechanical > P Coulombic

19. 30 May, 2009ERICE 2009 Mg 0.9 Fe 0.1 O

20. 30 May, 2009ERICE 2009 FeI 2

21. 30 May, 2009ERICE 2009 Fe(OH) 2 Parise et al

22. 30 May, 2009ERICE 2009 T >> T N Paramagnetic Fe 2+ T << T N anti- ferromagnetic Fe 2+

23. 30 May, 2009ERICE 2009 H Lateral displacement (Parise et al) Fe 3+ abundance

24. 30 May, 2009ERICE 2009 No change in structure!

25. 30 May, 2009ERICE 2009 Conclusion the orientation-disorder of the O-H dipoles caused by the pressure-induced OH----HO coulomb repulsion, and, 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 Fe 2+ to the exceptional small electron binding energy of Fe 2+ The irreversible oxidation process is attributed to: Within the HP band-structure of Fe(OH) 2 a new, localized band is formed populated by the “ousted” electrons

26. 30 May, 2009ERICE 2009 Structural response to PI electronic transitions in Fe 3+ oxides Fe 2 O 3 (hematite) Fe 2 O 3 (hematite) R FeO 3 (R= rare-earth iron perovskites) R FeO 3 (R= rare-earth iron perovskites) CuFeO 3 (delafossite) CuFeO 3 (delafossite)

27. 30 May, 2009ERICE 2009 Fe 2 O 3 a correlation breakdown Rutile > Rh 2 O 3 II ΔV/V 0 = 0.1

28. 30 May, 2009ERICE 2009 Fe 2 O 3 a catastrophic correlation breakdown INSULATOR-METAL TRANSITION COLLAPSE OF MAGNETISM

29. 30 May, 2009ERICE 2009 Correlation breakdown triggers a 1 st -order structural phase transition Similar transitions are observed in GaFeO 3 and FeOOH, pointing to a structural instability of (Fe 3+ O 6 ) species at P > 50 GPa. Summary

30. 30 May, 2009ERICE 2009

31. 30 May, 2009ERICE 2009 All R FeO 3 (R 3+ rare earth ) undergo HS>LS transition at ~ 40 GPa At P > 100 GPa they remain paramagnetic ( ≠0) down to 4K.

32. 30 May, 2009ERICE 2009 IM takes place at ~ 120 GPa

33. 30 May, 2009ERICE 2009 A 1 st (or 0 th ) 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

34. 30 May, 2009ERICE 2009

35. 30 May, 2009ERICE 2009 At ambient pressure: spin-frustrated a.f. Hexagonal structure, very anisotropic Fe 3+ (S=5/2), Cu 1+ (S=0) Finally at ~19 GPa a 3D super-exchange is realized. T N ~ 50 K

36. 30 May, 2009ERICE 2009 27 GPa Cu 2+ 4 GPa Cu 1+

37. 30 May, 2009ERICE 2009 The rigidity of the O 2- – Cu 1+ - O 2- dumbbell and its orientation along the c-axis are responsible for the large anisotropy in delafossite. With pressure increase the is doomed to collapse

38. 30 May, 2009ERICE 2009 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 HP1 HP2 LP

39. 30 May, 2009ERICE 2009 We thus conclude a serendipitous voyage into the extremities of matter. We thus conclude a serendipitous voyage into the extremities of matter. serendipity : the ability to make fortunate discoveries by accident Pinta and Santa Maria Discovery of America ! Discovery of fundamentals of physics DAC Discovery of SC in Hg Kamerlingh-Ohnes (1911)

40. 30 May, 2009ERICE 2009