Electronic Materials and Extreme Conditions J. Paul Attfield - - PowerPoint PPT Presentation

Electronic Materials and Extreme Conditions

• J. Paul Attfield

School of Chemistry and Centre for Science at Extreme Conditions (CSEC), University of Edinburgh

Compound Tc (K) Nb3Ge 23 PbMo 6S8 16 LiTi2O4 13 Ba0.6K0.4BiO3 30 HgBa2Ca2Cu3O8+δ 136 (ET)2Cu(NCS)2* 13 Cs3C60 34 Li0.2HfNCl 25 MgB2 39 S S S S S S * ET =

High temperature superconductors (1986-)

Superconductivity – correlated motion

• f electron pairs below a critical

temperature (Tc), characterised by zero electrical resistance and perfect diamagnetism; Low-Tc era 1911-1986

• metals and alloys

High-Tc era 1986-

• copper oxides (etc)

La0.7Ca0.3MnO3 - ferromagnetic and conducting  Colossal Magnetoresistances (CMR) for sensors, spintronic devices etc. La0.5Ca0.5MnO3 - nonmagnetic (antiferromagnetic) and insulating  localisation and long range order of;

• charges (Mn3+/Mn4+ states),
• d-orbitals (Mn3+Jahn-Teller distortion)
• spins (Mn3+/Mn4+ magnetic moments)

CMR Manganese oxides (1995-)

High Pressure Perovskites

SrCrO3

Orbitally driven phase separation Ortega San Martin et al, PRL 2007

PbRuO3

Symmetry-reversing orbital transition Kimber et al, PRL 2009

BiNiO3 (Kyoto)

Colossal NTE Azuma et al, Nature Comm. 2011

MnVO3

Helimagnetic A site spin order Markkula et al, PRB 2011

Bi0.95La0.05NiO3

SrCrO3

‘Hard-soft’ synthesis  SrCrO2.80  SrCrO2.75 Arevalo et al ACIE 2012

The Verwey Structure of Magnetite (Fe3O4)

Mark Senn, Jon Wright & JPA, Nature (2012)

Magnetite and magnetism

biomagnetism lodestones spintronics compass ferrites geomagnetism

• Evidenced by a first order transition in

resistivity, heat capacity and magnetisation at 125 K

• Complex superstructure

Fe3+[Fe2.5+]2O4 →Fe3+[Fe2+Fe3+]O4

Verwey, E. J. W. (1939). "Electronic conduction of magnetite (Fe3O4) and its transition point at low temperatures." Nature 144: 327-328. Fe2+ Fe3+ Fe3+

Low temperature properties – the Verwey transition

Theoretical approaches:

• Verwey (1939) Fe2+/Fe3+ charge order (Verwey model, 1946)
• Order-disorder of 2 electron-B4 tetrahedra (Anderson, 1956)
• CO from U/W band instability (Cullen & Callen, 1970)
• Polaron (bi-, molecular-) CO (Mott, Chakraverty, Yamada 1970-1980)
• Bond-dimerisation (no CO) - Fe2

5+ dimers (Seo, Khomskii 2002-)

Use microcrystals from previous powder (Fe3-3dO4, d < 0.0001 - Prof. J. Honig):

• Twinning, multiple scattering, extinction problems reduced by using microcrystallites.
• Microcrystal beamline ID11@ESRF - 100 μm focused monochromatic beam.
• Hard X-rays (74 keV, λ = 0.16653(1) Å) reduces absorption, accesses high Q.
• Magnetic alignment (~1 T field from permanent magnet while cooling through TV)
• Refinement software for twinned crystals (SHELXL)
• Try many microcrystals – be lucky

Full structure solution (Senn, Wright, JPA) 2006-2012

h = 50 (hkl) sections

Best microcrystal: 

• approx. spherical, ~40 μm
• two twins at 90 K, 89:11
• Cc structure determined using

91,433 unique Bragg intensities

• model uniqueness checked

against 2,000 randomised starting models Second best microcrystal: 

• irregular, ~100 μm
• four twins
• refined structure same as above

Fe2+/Fe3+ charge

• rder to first

approximation

Electronic order in the Verwey state of magnetite

….and orbital order

• f Fe2+ states….

….but Fe2+ ions also weakly bonded to two neighbours – trimeron units. Trimeron order. Significance?

• Ground state

unexpected, not simple charge order.

• Prevalence of orbital

molecules (trimerons)?

• Dynamics above 125K?

Thanks

Wei-Tin Chen Lucy Clark Shigeto Hirai Andrea Marcinkova Mikael Markkula George Penny Marek Senn Alex Sinclair Congling Yin Minghui Yang Angel Arevalo-Lopez Anna Kusmartseva Martin Misak Jenny Rodgers