Showing papers by "J. Paul Attfield published in 2012"

Charge order and three-site distortions in the Verwey structure of magnetite

Abstract: X-ray diffraction is used to show that the structural distortion of magnetite below 125 kelvin is to a first approximation caused by charge ordering of its constituent iron ions, but that the localized electrons are distributed over three iron sites to form ‘trimeron’ quasiparticles. In a letter to Nature in 1939, Evert Johannes Willem Verwey described the first example of a low-temperature charge-ordering transition in a solid — the Verwey transition — in the mineral magnetite, Fe3O4 (see go.nature.com/3h2pp1 ). This phenomenon has since been observed in other transition metal oxides, yet despite decades of study, the precise structure of the charge-ordered state in magnetite has remained elusive. The complex structural distortions that characterize the Verwey state have now been determined, revealing an anomalous shortening of some inter-atomic distances. These are suggestive of an unusual charge configuration in which the localized electrons are each distributed over three neighbouring Fe sites. The mineral magnetite (Fe3O4) undergoes a complex structural distortion and becomes electrically insulating at temperatures less than 125 kelvin. Verwey proposed in 1939 that this transition is driven by a charge ordering of Fe2+ and Fe3+ ions1, but the ground state of the low-temperature phase has remained contentious2,3 because twinning of crystal domains hampers diffraction studies of the structure4. Recent powder diffraction refinements5,6,7 and resonant X-ray studies8,9,10,11,12 have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models13,14,15,16,17,18,19. Here we report the full low-temperature superstructure of magnetite, determined by high-energy X-ray diffraction from an almost single-domain, 40-micrometre grain, and identify the emergent order. The acentric structure is described by a superposition of 168 atomic displacement waves (frozen phonon modes), all with amplitudes of less than 0.24 angstroms. Distortions of the FeO6 octahedra show that Verwey’s hypothesis is correct to a first approximation and that the charge and Fe2+ orbital order are consistent with a recent prediction17. However, anomalous shortening of some Fe–Fe distances suggests that the localized electrons are distributed over linear three-Fe-site units, which we call ‘trimerons’. The charge order and three-site distortions induce substantial off-centre atomic displacements and couple the resulting large electrical polarization to the magnetization. Trimerons may be important quasiparticles in magnetite above the Verwey transition and in other transition metal oxides.

Cation-Size-Mismatch Tuning of Photoluminescence in Oxynitride Phosphors

Abstract: Red or yellow phosphors excited by a blue light-emitting diode are an efficient source of white light for everyday applications. Many solid oxides and nitrides, particularly silicon nitride-based m...

Electronic orders in the Verwey structure of magnetite

Abstract: Electronic structure calculations of the Verwey ground state of magnetite, Fe${}_{3}$O${}_{4}$, using density functional theory with treatment of on-site Coulomb interactions (DFT+U scheme), are reported. These calculations use the recently published experimental crystal structure coordinates for magnetite in the monoclinic space group $Cc$. The computed density distribution for minority spin electron states close to the Fermi level demonstrates that charge order and Fe${}^{2+}$-orbital order are present at the $B$-type lattice sites to a first approximation. However, Fe${}^{2+}$/Fe${}^{3+}$ charge differences are diminished through weak bonding interactions of the Fe${}^{2+}$ states to specific pairs of neighboring iron sites that create linear, three-$B$-atom trimeron units that may be regarded as orbital molecules. Trimerons are ordered evenly along most Fe atom chains in the Verwey structure, but more complex interactions are observed within one chain.

Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9.

Abstract: We show that the valence electrons of Ba3NaRu2O9, which has a quasimolecular structure, completely crystallize below 210 K. Using an extended Hubbard model, we show that the charge ordering instability results from long-range Coulomb interactions. However, orbital ordering, metal-metal bonding, and formation of a partial spin gap enforce the magnitude of the charge separation. The striped charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition with a quasidegenerate charge-melted phase under photoexcitation at low temperature. Our results establish a broad class of simple metal oxides as models for emergent phenomena at the border between the molecular and solid states.

Subextensive entropies and open order in perovskite oxynitrides.

Abstract: Unusual subextensive configurational entropies that vary with particle size and tend to zero per atom in macroscopic samples are predicted for AMO3–zNz oxynitrides with perovskite type crystal structures. These materials are crystallographically disordered on the atomic scale, but local anion order produces chains of M–N–M bonds that undergo a 90° turn at each M cation, giving rise to subextensive entropies in materials such as SrTaO2N, LaNbON2, and EuWO1.5N1.5. A general Pauling ice-rules formula is used to calculate the extensive molar entropies for other cases such as SrMoO2.5N0.5 and BaTaO2N. The subextensive oxynitrides are usefully classified as showing an “open order”, related to the correlated order of displacements in ferroelectric perovskites such as BaTiO3. This raises the possibility that further open-ordered oxynitride or molecular structures may be accessible, and other states such as spins and charges may also show novel open orders.

Cation-Size-Mismatch Tuning of Photoluminescence in Oxynitride Phosphors.

Abstract: The oxonitridoaluminosilicates M1.95Eu0.05Si5-xAlx N8-xOx (M: Ca, Sr, Ba; 0 ≤ x ≤ 1) are synthesized from stoichiometric mixtures of Ba3N2, Sr3N2, Ca3N2, α-Si3N4, EuN, and Al2O3 (BN crucible, 0.5 MPa N2, 1600 °C, 2 h).

Structural, Magnetic, and Electrical Properties of Bi1-xLaxMnO3 (x=0.0, 0.1, and 0.2) Solid Solutions

Abstract: Possible ferromagnetic and ferroelectric orders in ceramic Bi1–xLaxMnO3 (x = 0.0, 0.1, and 0.2) samples prepared under 3–6 GPa pressure have been investigated. Rietveld fits to powder neutron diffraction data show that BiMnO3 and Bi0.9La0.1MnO3 adopt a monoclinic C2/c perovskite superstructure whereas Bi0.8La0.2MnO3 has orthorhombic Pnma symmetry. Both structural analysis and Curie–Weiss fits to magnetic susceptibility data show that high spin d4 Mn3+ is present with no significant Bi deficiency or Mn4+ content apparent. La substitution suppresses the magnetic Curie temperature of the monoclinic phase from 105 K for x = 0 to 94 K at x = 0.1, but the x = 0.2 material shows antiferromagnetic order similar to that of LaMnO3. Impedance spectroscopy and dielectric measurements on the x = 0.1 and 0.2 materials show modest bulk permittivity values (45–80) down to 50 K, and there is no strong evidence for ferroelectric behavior. The two samples have thermally activated conductivities with activation energies of 0...