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

PbMo6S8 16

LiTi2O4 13

Ba0.6K0.4BiO3 30

HgBa2Ca2Cu3O8+δ 136

(ET)2Cu(NCS)2* 13

Cs3C60 34

Li0.2HfNCl 25MgB2 39

S

S

S

S

S S* ET =

High temperature superconductors (1986-)

Superconductivity – correlated motion of 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 order to first approximation

Electronic order in the Verwey state of magnetite

….and orbital order of 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