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Structural response to pressure induced electronic transitions in TM-compounds Moshe Paz-Pasternak, Tel Aviv University, 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 1

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

– Appropriate electronic spectroscopy methods with radiation that can be transmitted through diamonds such – Appropriate electronic spectroscopy methods with radiation that can be transmitted through diamonds such as: – – – - 30 May, 2009 and – 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 Wires for resistance and other electrical measurements ERICE 2009

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

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

Radius of TMHS > Radius of TMLS Fe 3+(HS)) 5 ↑↑↑↑↑ Fe 3+(LS)) 5 Radius of TMHS > Radius of TMLS Fe 3+(HS)) 5 ↑↑↑↑↑ Fe 3+(LS)) 5 ↑↓ ↑↓ ↓ Eu. Fe. O 3 Fe 3+(HS) 30 May, 2009 Fe 3+(LS) ERICE 2009

Mott Hubbard insulator B - electronic configuration of the TM ion The strong on-site Mott Hubbard insulator B - electronic configuration of the TM ion The strong on-site Coulomb repulsion produces an energy gap, within the 3 d band, known as the Mott-Hubbard >U 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 30 May, 2009 ERICE 2009

electronic/magnetic consequences of Mott-Hubbard correlation-breakdown correlated states Uncorr. states insulator HS metallic Odd number electronic/magnetic consequences of Mott-Hubbard correlation-breakdown correlated states Uncorr. states insulator HS metallic Odd number of spins S ≠ 0 LS S≠ 0 paramagnetic Even S ≠ 0 S=0 number paramagnetic diamagnetic of spins 30 May, 2009 ERICE 2009

Mössbauer spectroscopy currently the best experimental method at the atomic scale for studying magnetism 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 57 Fe(14. 4 ke. V) section of 9 barns! gamma-rays is ~ 10 That’s why we can use absorbers with diam. <0. 1 mm 30 May, 2009 ERICE 2009

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

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

The Hyperfine Interaction in 57 Fe ± 3/2 QS ± 1/2 Two quadrupolesplit components The Hyperfine Interaction in 57 Fe ± 3/2 QS ± 1/2 Two quadrupolesplit components 1/2 +3/2 +1/2 -3/2 ~µHhyf -1/2 Magnetic splitting +1/2 2Γ 57 Co e. c decay Γ, t 1/2 I*=3/2 30 May, 2009 I=1/2 57 Fe 14. 4 ke. V t 1/2 ~ 5 x 10 -7 sec!!! Γ ~ 0. 5 μe. V!!! ERICE 2009

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

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

Determining the component-abundance ni 30 May, 2009 ERICE 2009 Determining the component-abundance ni 30 May, 2009 ERICE 2009

Structural Response to PI electronic transitions in Fe 2+ compounds n Fe. O (wüstite) Structural Response to PI electronic transitions in Fe 2+ compounds n Fe. O (wüstite) n Fe. X 2 (X=Cl, I) n Fe(OH)2 30 May, 2009 Na. Cl structure Cd. I 2 structure ERICE 2009

Experimental proof of Hund’s rule Pmechanical > PCoulombic HS > LS starting at ~ Experimental proof of Hund’s rule Pmechanical > PCoulombic HS > LS starting at ~ 90 GPa LS No symmetry or appreciable volume change ever detected. H S 30 May, 2009 ERICE 2009 Na. Cl structure

Mg 0. 9 Fe 0. 1 O 30 May, 2009 ERICE 2009 Mg 0. 9 Fe 0. 1 O 30 May, 2009 ERICE 2009

Fe. I 2 30 May, 2009 ERICE 2009 Fe. I 2 30 May, 2009 ERICE 2009

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

T >> TN Paramagnetic Fe 2+ T << TN antiferromagnetic Fe 2+ 30 May, T >> TN Paramagnetic Fe 2+ T << TN antiferromagnetic Fe 2+ 30 May, 2009 ERICE 2009

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

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

Conclusion The irreversible oxidation process is attributed to: n n the orientation-disorder of the Conclusion The irreversible oxidation process is attributed to: n n 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+ Within the HP band-structure of Fe(OH)2 a new, localized band is formed populated by the “ousted” electrons 30 May, 2009 ERICE 2009

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

Fe 2 O 3 a correlation breakdown ΔV/V 0 = 0. 1 30 May, Fe 2 O 3 a correlation breakdown ΔV/V 0 = 0. 1 30 May, 2009 Rutile > Rh 2 O 3 II ERICE 2009

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

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

30 May, 2009 ERICE 2009 30 May, 2009 ERICE 2009

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

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

A 1 st (or 0 th) order structural phase transition occurs at the HS>LS 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 30 May, 2009 ERICE 2009

30 May, 2009 ERICE 2009 30 May, 2009 ERICE 2009

At ambient pressure: spin-frustrated a. f. Hexagonal structure, very anisotropic Fe 3+ (S=5/2), Cu 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 3 D super-exchange is realized. TN ~ 50 K 30 May, 2009 ERICE 2009

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

The rigidity of the O 2 - – Cu 1+ - O 2 - 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 to collapse 30 May, 2009 ERICE 2009 is doomed

A series of: 1 - PI structure transition 2 – Followed by PI electronic 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 HP 1 HP 2 LP HP 1 30 May, 2009 ERICE 2009 HP 2

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

30 May, 2009 ERICE 2009 30 May, 2009 ERICE 2009