The nature of the Verwey transition occurring at T V ≈ 125 K in magnetite (Fe 3 O 4 ) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above T V are widely reported, but the origin of the underlying correlations and hence of the Verwey transition remains unclear. Here we show that local structural fluctuations in magnetite emerge below the Curie transition at T C ≈ 850 K, through X-ray pair distribution function analysis. Around 80% of the low temperature correlations emerge in proportion to magnetization below T C . This confirms that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and hence the origin of the Verwey transition. Such hidden instabilities may be important to other spin-polarised conductors and orbitally degenerate materials.

A remarkably complex electronic order of Fe(2+)/Fe(3+) charges, Fe(2+) orbital states, and weakly metal-metal bonded Fe3 units known as trimerons, was recently discovered in stoichiometric magnetite (Fe3O4) below the 125 K Verwey transition. Here, the low temperature crystal structure of a natural magnetite from a mineral sample has been determined using the same microcrystal synchrotron X-ray diffraction method. Structure refinement demonstrates that the natural sample has the same complex electronic order as pure synthetic magnetite, with only minor reductions of orbital and trimeron distortions. Chemical analysis shows that the natural sample contains dopants such as Al, Si, Mg and Mn at comparable concentrations to extraterrestrial magnetites, for example, as reported in the Tagish Lake meteorite. Much extraterrestrial magnetite exists at temperatures below the Verwey transition and hence our study demonstrates that the low temperature phase of magnetite represents the most complex long-range electronic order known to occur naturally.

Complex oxygen ordering evidenced for the oxygen membrane cathode material Pr₂NiO_4.25 at room temperature with translational periodicities attaining almost 100 Å by single-crystal synchrotron diffraction studies.

Frustrated magnetic materials can show unconventional correlations such as quantum spin liquid states and monopole excitations in spin ices. These phenomena are observed on uniformly frustrated lattices such as triangular, kagome or pyrochlore types, where all nearest neighbour interactions are equivalent. Here we report incommensurate long-range spin amplitude waves in the spinels Fe 2 GeO 4 and γ-Fe 2 SiO 4 at low temperatures, which indicate that the degree of frustration may itself be a fluctuating quantity that can spontaneously order without a lattice distortion as a 'frustration wave'. Fe 2 GeO 4 with propagation vector ( 2 / 3 + δ 2 / 3 + δ 0) has ordered Fe 2+ moments that vary between fully saturated 4 μB and 0 values, consistent with a frustration wave order. γ-Fe 2 SiO 4 has a more complex (¾ + δ ¾ + δ 0) order that coexists with an ordered spin ice phase. Dynamic orbital fluctuations are proposed to give rise to locally correlated patterns of ferromagnetic and antiferromagnetic interactions consistent with the observed orders.

Charge ordering creates a spontaneous array of differently charged ions and is associated with electronic phenomena such as superconductivity, colossal magnetoresistances (CMR), and multiferroicity. Charge orders are usually suppressed by chemical doping and site selective doping of a charge ordered array has not previously been demonstrated. Here we show that selective oxidation of one out of eight distinct Fe 2+ sites occurs within the complex Fe 2+ /Fe 3+ ordered structure of 2%-doped magnetite (Fe 3 O 4), while the rest of the charge and orbitally ordered network remains intact. This 'charge order within a charge order' is attributed to the relative instability of the trimeron distortion surrounding the selected site. Our discovery suggests that similar complex charge ordered arrays could be used to provide surface sites for selective redox reactions, or for storing information by doping specific sites.

Oxygen deficient Sr2ScGaO5 single-crystals with cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing 1/6 of all oxygen positions vacant. While neutron single crystal diffraction, followed by Maximum Entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO6 octahedra and GaO4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short range order. Temperature dependent X-ray diffraction powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr2ScGaO5 to be thermodynamically stable only above 1400°C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr2ScGaO5 remains surprisingly kinematically stable upon heating from ambient to 1300°C, indicating a huge inertia for the retransformation towards the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10 -4 S/cm at 600°C, while oxygen 18 O/ 16 O isotope exchange indicates a free oxygen mobility to set in at around 500°C.

No abstract

The spinel structure is widely occurring in natural and synthetic materials. The oxides of this family exhibit the general formula AB2O4, where the oxygen forms a ccp arrangement, the B cation coordinates in edge-sharing octahedra and the A cation occupies tetrahedra. The technological and scientific significance of these materials has risen in the last few decades, but their geological relevance had focused their characterisation under high-temperature and high-pressure conditions. The low temperature behaviour of most materials in this class is remarkably under-explored.GeFe2O4 is a normal spinel, where the octahedral site is occupied by Fe(II) cations and the tetrahedral site is occupied by Ge(IV). The 3D arrangement of the Fe(II) outlines a pyrochlore lattice; as such, the structure has the potential for frustration upon magnetic ordering of the iron moment, which is reported as antiferromagnetic (T ~ 10 K) by early physical measurements. [1] We report a full magnetic structure solution, derived from neutron powder diffraction data collected at D2B and D20 at the ILL reactor, France. Contrary to reports on other germanate spinels in the transition metal series (GeCo2O4, GeNi2O4 [2][3]), the GeFe2O4 structure shows no signs of distortion in the crystal structure and remains cubic Fd-3m below the Néel temperature. The appearance of magnetic Bragg peaks reveals the ordering temperature to be T = 8.6 K. Keeping the cubic symmetry, the magnetic structure was solved with incommensurate propagation vector k = [ ⅔+δ, ⅔+δ, 0 ], with δ = 0.025.Due to the inherent frustration of the pyrochlore lattice, a complex spin structure is required to achieve antiferromagnetic ordering without distorting the cubic symmetry. This unusual magnetic ordering and its implications will be presented.