Magnetite the Enigma of the Verwey transition finally solved or Magnetite it never ceases to surpris

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Magnetite the Enigma of the Verwey transition
finally solved!orMagnetite it never ceases
to surprise
together with Weiming Xu and Gregory Rozenberg
(TAU)R. D. Taylor (LANL), and R. Jeanloz (UCB)
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Magnetite (Fe3O4), a cubic inverse spinel
Fe3
B-site (6-fold)
Fe3
Fe3

• Mixed-valence oxide.
• Fe3 is in a A-site (inverse spinel) and
  Fe2 Fe3 in a B - site.
• Fe3/Fe2 2/1
• Ferrimagnet (TN 850K)with Ms4µB
• A good conductor at RT.

Fe2
A-site (4-fold)
Normal Spinel Structure of Magnetite(?)
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The enigma started with this experiment..
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and then came Mott.
T lt TV

• Mott adopted Verweys concept and proposed that
  the electron assembly in magnetite above TV is a
  Wigner glass and the electron assembly in the
  coordinated sites is subject to long range order
  
• At T TV the Wigner glass transforms
  discontinuously into a Wigner crystal and the
  electrons are locked.
• N F. Mott, Festkörperproblem 19, 331 (1979)

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An unfortunate consequence of a Dogma

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The Verwey transition, known experimental facts

• At T TV a discontinuous increase in
  resistance, by 3 4 orders of magnitude.
• A structural phase transition of the first-order
  at T lt TV.
• TV 121 1 K for highly stoichiometric Fe3O4

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Experimental evidences for mixed Fe valence
within B-sites at T gt TV

• Mössbauer Spectroscopy
• No evidence for explicit Fe3 and Fe2 in
  B-sites. Proposed scheme Fe3 (Fe3 e-) or
  Fe2.5 Fe2.5 with possible electron hopping
  with time scale lt 10-11 s.
• Resonant Fe x-ray scattering and 57Fe NMR
• B-site iron atoms are electronically equivalent
  in a time scale lt 10-16s.

?
B-site
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Experimental search for "d" localization!
70s today XRD Transport 57Fe NMR 57Fe
Mössbauer Spectroscopy Raman Neutrons Absorption
Fe K-edge spectroscopy Etc., etc.
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Contradictory results (1)
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Contradictory results (2)
2001
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Contradictory results (3)
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Contradictory results (4 and 5)
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EXPERIMENTAL

• Sample
• Fe2O3 Fe ? Fe3O4 solid-solid reaction of
  stoichiometric amounts of Fe and Fe2O3 enriched
  with 25 57Fe in an evacuated quartz tube at
  900C.
• Diamond-Anvils Cell
• Mössbauer Spectroscopy, XRD (ESRF)

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57Fe Mössbauer Effect
The Mössbauer signature of the Verwey
transition at ambient pressure
T gt TV 2-components

• The best probe for detecting charge order (TltTV)
  or disorder (TgtTV) in magnetite

2-components spectrum
3-components spectrum
T lt TV 2-components?
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a serendipitous discovery...
Magnetite at 10 GPa, 200 K is metallic. The MS
spectrum should show the T gt TV spectral
features! Whats going on?

• Spinel structure of magnetite extends to 30
  GPa. It undergoes a 1st order phase transition to
  a CaTi2O4-type structure (Bbmm) accompanied by a
  large volume shrinkage in which Fe2 becomes
  8-fold coordinated (according to Fei et al.)
• We were interested in studying the spin-state of
  Fe2 and transport properties of the new HP phase
  (P gt 30 GPa)
• But Weiming insisted in investigating the HP
  properties of the spinel magnetite (P lt 30 GPa).

Localization signature?
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Pressure-induced coordination crossover,
transition from inverse to normal spinel
(Fe3)A(Fe2.5,Fe2.5)B ? (Fe2)A(Fe3,Fe3)B
P
Spectrum of a normal spinel !!!
Fe3A
Fe2A
Typical spectrum for T lt TV
Fe2A Fe3Fe3B. Fe2A / Fe3Fe3B 1/2
Intermediate spinel
Typical spectrum for T gt TV at ambient pressure.
Fe3A Fe2.5Fe2.5B.
Fe3A / Fe2.5Fe2.5B 1/2
Fe3A
Fe2.5A
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inverse?normal spinel transformation reversible
with temperature and pressure
Normal
Inverse
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(No Transcript)
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The Progressive transformation of inverse ?
normal spinel
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Are there any other (experimental) evidences for
the inverse?normal conversion?YES!
INVERSE
NORMAL
INVERSE
NORMAL
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Are there any other (experimental) evidences for
the inverse?normal conversion?YES!
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Magnetic moment of the ferrimagnetic Inverse
Spinel (5/22)µB - (5/2)µB 2µB
B-site
A-site
Magnetic moment of the ferrimagnetic Normal
Spinel (5/25/2)µB - (2)µB 3µB
B-site
A-site
HPNP-2 (Cologne 2004)
a 50 enhancement
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P-T phase diagram of magnetite
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and finally
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The origin of the Verwey transition
charge-ordering at the B-sites?
it's due to the Orthorhombic Distortion (OD)

• TV(P) scales with OD(P) Probable Cause of MI
  transition
• Opening of a p-p gap within the orthorhombic
  phase

º cubic
º orthorhombic
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EPILOGUE

• Beware of false knowledge it is more dangerous
  than ignorance things
• George Bernard Shaw