This Review highlights the application of high-temperature solutions for exploratory crystal growth and materials discovery of novel complex oxides. It provides an overview of the method of flux crystal growth of complex oxides and can function as a "how to" guide for those interested in oxide crystal growth. The most commonly used fluxes are discussed in terms of their applicability for dissolving specific elements and the typical reaction conditions are compiled. A large variety of recent quaternary and higher oxides that have been grown as crystals from fluxes are used to illustrate the power of the flux method to grow oxide crystals containing specific elements.

A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba 1 À x Na x Fe 2 As 2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr(2)Fe(1.5)Mo(0.5)O(6-δ) (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group Pnma. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr(2)Fe(1.5)Mo(0.5)O(5.90(2)) (δ = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for its impressive electronic conductivity.

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of a hole-doped BaFe2As2. This observation was interpreted as indirect evidence of a new double-Q magnetic structure, but alternative models of orbital order could not be ruled out. Here, we present Mössbauer data that show unambiguously that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. We show that this state is naturally explained as the interference between two spin-density waves, demonstrating the itinerant character of the magnetism of these materials and the primary role played by magnetic over orbital degrees of freedom.

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In this Perspective we discuss a variety of methods that are broadly applicable to the syntheses of solid-state compounds. To illustrate the application of these methods we use solid-state actinide compounds, an area of chemistry that has been the focus of this laboratory for the last decade. We have synthesized single crystals, primarily of actinide chalcogenide compounds, by a variety of techniques. The benefits and drawbacks of each will be discussed in detail. Because of their propensity towards high coordination numbers and their ability to take on a variety of formal oxidation states, actinide compounds adopt structures illustrated here that are often different from those adopted by transition-metal or even lanthanide compounds. Often, there are corresponding differences in their physical properties, and these are touched upon here.

We find evidence for long-range and short-range (ζ = 70Å at 4 K) incommensurate magnetic order on the quasi-face-centered-cubic (FCC) lattices of the monoclinic double perovskites La2NaRuO6 and La2NaOsO6 respectively. Incommensurate magnetic order on the FCC lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbour and next nearest neighbour exchange interactions. In the Ru system with long-range order, inelastic neutron scattering also reveals a spin gap ∆ ∼ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally-coordinated 3d 3 systems, so the large gap observed for La2NaRuO6 may result from the significantly enhanced value of spin-orbit coupling in this 4d 3 material. In the context of the interplay between geometric frustration and SOC, there has been less interest in 4d and 5d DPs with the electronic configuration d 3 . One downside is that d 3 systems are generally assumed to possess spin-only S = 3/2 ground states with quenched orbital angular momentum according to the usual L − S coupling scheme, since the magnetic B ′ ions are in a local octahedral environment, and this configuration should minimize the effects of SOC. Another issue is d3 DP systems are expected to behave more classically due to the large spins, and for almost all known cases long-range magnetic order is found [12]. Although magnetic order cannot be stabilized on the FCC lattice solely by NN AFM exchange interactions J 1 > 0, finite next nearest neighour (NNN) exchange J 2 or magnetic anisotropy can alleviate the classical ground state degeneracy [13] and allow the systems to order. The phase diagram of the J 1 -J 2 model has been determined theoretically for the FCC lattice using mean field theory (MFT) [14,15]. Four different collinear magnetic phases are found depending on the sign and magnitude of J 1 and J 2 , including ferromagnetism and Type I, Type II, and Type III antiferromagnetism. All four phases have been realized in d Recently, we investigated the magnetism of the monoclinic d 3 DPs La 2 NaRuO 6 and La 2 NaOsO 6 by magnetic susceptibility, heat capacity and neutron powder diffraction (NPD) [25]. The magnetic susceptibility shows a deviation from the Curie-Weiss law (θ CW = -57 K) at a temperature of 15 K for the Ru system, accompanied by a λ anomaly in the specific heat at the same temperature. While the magnetic susceptibility of the Os system shows a similar deviation from Curie-Weiss law behaviour (θ CW = -74 K) around 12 K, only a broad feature is observed in the specific heat. Furthermore, in contrast to the expected collinear magnetic ground states for d 3 systems, we found incommensurate long-range order in La 2 NaRuO 6 with a moment size of 1.87 µ B and no magnetic Bragg peaks for La 2 NaOsO 6 down to 4 K [25]. This behaviour is difficult to understand in the general context of d 3 DPs.In this letter, we have investigated these d 3 systems with muon spin relaxation (µSR) and time-of-flight neutron scattering measuremen...

The recently discovered C4 tetragonal magnetic phase in hole-doped members of the iron-based superconductors provides new insights into the origin of unconventional superconductivity. Previously observed in Ba1-xAxFe2As2 (with A = K, Na), the C4 magnetic phase exists within the well studied C2 spin-density wave (SDW) dome, arising just before the complete suppression of antiferromagnetic (AFM) order but after the onset of superconductivity. Here, we present detailed x-ray and neutron diffraction studies of Sr1−xNaxFe2As2 (0.10 ≤ x ≤ 0.60) to determine their structural evolution and the extent of the C4 phase. Spanning ∆x ∼ 0.14 in composition, the C4 phase is found to extend over a larger range of compositions, and to exhibit a significantly higher transition temperature, Tr ∼ 65K, than in either of the other systems in which it has been observed. The onset of this phase is seen near a composition (x ∼ 0.30) where the bonding angles of the Fe2As2 layers approach the perfect 109.46• tetrahedral angle. We discuss the possible role of this return to a higher symmetry environment for the magnetic iron site in triggering the magnetic reorientation and the coupled re-entrance to the tetragonal structure. Finally, we present a new phase diagram, complete with the C4 phase, and use its observation in a third hole-doped 122 system to suggest the universality of this phase.