Electronic Structure and Transport Properties of Iron Compounds: Spin-Crossover Effects. Viktor Struzhkin

Collaborations M. Eremets, I. Eremets Max-Planck Institute, Mainz, Germany A. Gavriliuk, I. Lyubutin Institute of Crystallography, RAS, Moscow, RUSSIA Optics and Theory A. Goncharov, GL S. Ovchinnikov Institute of Physics, Siberian Branch of RAS, Krasnoyarsk, RUSSIA NFS, XES W. Sturhahn, J. Zhao, S. Kharlamova, P. Chow, M. Y. Hu APS, ANL, Argonne, USA J. F. Lin LLNL

Spins and Magnetism P < P c P > P c Scope: electronic structure of Fe 2+ and Fe 3+ in octahedral sites

In the Mott-Hubbard theory charge fluctuations d i n d j n  d i n-1 d j n+1 are completely suppressed due to strong exchange and Coulomb d - d interaction U J.Zaanen, G.Sawadzky, J.Allen [PRLett 1985] showed that an another type of charge transfer (  ) can be considered d i n  d i n+1 L, where L – is a hole in p – valence band of anion Depending on the ratio of parameters, which are related to hybridization Т, the system can be (from the point of view the nature of the gap Е gap ) : 1) Mott-Hubbard insulator d-d type U <  (Е gap  U), 2) insulator (or semiconductor) with charge transfer U >  (Е gap   ), 3) d – metal  < U and U < W/2 4) p – metal U <  and  < W Theoretical approach

Bandwidth- versus filling-controlled metal-insulator transition (Fujimori)

Mott-Hubbard transition under high pressure: bandwidth control

Tanabe-Sugano diagram for Fe +3 ion and Spin crossover egeg t 2g d Fe 3+ - LS (S = 1/2) 10Dq Fe 3+ - HS (S = 5/2) U sp

Magnetic collapse in transition metal oxides Cohen, Mazin, Isaak, Science 1997 R. E. Cohen et al., MRS Symp proc High-spin to low-spin transition I. Jackson and A. E. Ringwood (1981) ΔG = ΔE – PΔV + TΔS ΔE = Nn{π - Δ(r)}, Δ~ Δ 0 (r 0 /r) 5 For cubic (B1) FeO: P tr =50 GPa

X-ray emission spectroscopy as a local magnetic probe

High-spin to low-spin transition in FeS J.-P. Rueff, C.-C. Kao,V. V. Struzhkin, J. Badro, J. Shu, R. J. Hemley, and H. K. Mao, Phys. Rev. Lett. (1999)

J. Badro et. al., Science (2003) Mg 0.83 Fe Spin-crossover transition in ferropericlase

Nuclear inelastic scattering set-up (W. Sturhahn, E. Alp, M. Hu)

FeBO 3 Mössbauer spectroscopy and NFS (Lyubutin et al.)

10Dq

Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle A. Goncharov, V. Struzhkin, and S. Jacobsen The observed changes in absorption are in contrast to prediction and are attributed to d-d orbital charge transfer in the Fe 2+ ion. The results indicate low-spin (Mg,Fe)O will exhibit lower radiative thermal conductivity than high-spin (Mg,Fe)O, which needs to be considered in future geodynamic models of convection and plume stabilization in the lower mantle.

Theory

Comparison of Mössbauer and X-ray emission results for FeO M. P. Pasternak et al. Phys. Rev. Lett.(1997) J. Badro et al. Phys. Rev. Lett. (1999)

NFS FeO (wüstite)

Magnetic phase diagram of FeO

41 GPa113 GPa Insulator-metal transition in FeO

- Fe B O 3 - R Fe 3 (BO 3 ) 4 (R = Gd) - Y 3 Fe 5 O 12 Fe 3+ Samples: Singe crystals enriched with the Fe-57 isotope

Changes in the crystal color under pressure increase and decrease 19 GPa 41 GPa 50 GPa 13 GPa0 GPa32 GPa Electronic transition Y 3 Fe 5 O 12

at P = 46 GPa the insulator- semiconductor transition FeBO 3 Structural, magnetic, electronic and spin transitions at high pressures at 53 GPa  collapse of the unit-cell volume by ~ 9 % at P = 46 GPa magnetic collapse with the HS  LS transition

at P = 43 GPa the insulator- semiconductor transition GdFe 3 (BO 3 ) 4 Structural, electronic and spin transitions at high pressures at 26 GPa  collapse of the unit-cell volume by ~ 8 % at P = 43 GPa the HS  LS transition

at P = GPa the insulator- metal transition Structural, magnetic, electronic and spin transitions at high pressures at 48 GPa  srtuctural amorphyzation at P = 48 GPa magnetic collapse with the HS  LS transition Y 3 Fe 5 O 12

BiFeO 3 - belongs to ferro-magneto-electric materials (multiferroics) which have both a spontaneous electrical polarization and a spontaneous magnetization. Between known multiferroics, it has a record high the antiferromagnetic Neel temperature (T N = 643 K) and the ferroelectric Curie temperature (T C = 1083 K) Bi Fe O 3 : Multiferroic

BiFeO 3 Electronic transition from the insulating to highly conducting state. Mott ? 7.2 GPa 54.5 GPa

Pressure – temperature dependence of resistivity BiFeO 3 At 40 – 55 GPa the resistance drops by  10 7 (metallization)

at P = GPa the insulator- metal transition Structural, magnetic, electronic and spin transitions at high pressures near 45 GPa  srtuctural transition at P = 47 GPa magnetic collapse with the HS  LS transition Bi Fe O 3

Main parameter  is the effective Hubbard energy U eff U eff = E 0 (d 4 ) + E 0 (d 6 ) - 2 E 0 (d 5 ) LPP  P < P c  S = 2 S = 2 S = 5/2 HPP  P > P c  S = 1 S = 0 S = 1/2 S. G. Ovchinnikov [JETP Letters, 2003] Theoretical approach

ELECTORON STRUCTURE of FeBO 3 and GdFe 3 (BO 3 ) 4 [S.G. Ovchinnikov and S.A. Kharlamova, JETP Letters, 2003; 2004] SEC for Fe: а ) d - d - transitions b) charge transfer transitions p 6 d 5  p 5 d 6  electron creation Fe 3+  Fe 2+ :  с = E ( 5 Т 2, d 6 ) – E ( 6 A 1, d 5 ) hole creation Fe 3+  Fe 4+ :  = E 0 ( 6 A 1,d 5 ) – E 0 ( 6 A 1,d 4 ) By Raccah parameters :  с =  d + 5 A + 14 B – 0.4   =  d + 4 A – 14 B  Then the Hubbard effective parameter is: U eff =  c –  = А + 28 В –  = 4.2 eV

The effective Hubbard parameter : U eff =  c -  = E 0 (d 4 ) +E 0 (d 6 ) - 2E 0 (d 5 ) P  P c S=2 S=2 S=5/2 P  P c S=1 S=0 S=1/2 U eff =  c -  = А + 9В -7С  1.45 eV U eff РсРс 4.2 Р 1.45 Density of electronic states of GdFe 3 (BO 3 ) 4 in within multi-electronic p - d model at ambient and high pressure ELECTRON STRUCTURE of FeBO 3 and GdFe 3 (BO 3 ) 4 in MULTIELECTRON MODEL at AMBIENT and HIGH PRESSURE 7 S.G.Ovchinnikov. JETP Lett. (2003)

S. G. Ovchinnikov [JETP Letters, 2003] Theoretical approaches Fe 2+ Fe 3+ FeO?

Bandwidth- versus filling-controlled metal-insulator transition (Fujimori) HS-LS U-control