We report new solid state and hydrothermal synthetic routes to (Li,Na)2FePO4F that incorporate carbon-containing additives and result in good electrochemical properties of this Li (or Na) ion electrode material. Single crystal X-ray diffraction analysis of Na2FePO4F prepared by flux growth confirms the unusual structural features of this compound that include pairs of face-sharing metal octahedra and [6 + 1] coordination of the sodium ions. Facile Na−Li ion-exchange occurs upon reflux with lithium salts, upon electrochemical cycling in a cell (vs. Li), and also in a cell simply equilibrated at OCV. The material does not exhibit typical two-phase behavior on electrochemical cycling. A combination of a redox process which occurs with little structural strain, and ion scrambling give rise to a solid solution-like sloping voltage profile on charge−discharge, although localization of the Fe2+/3+ in the mixed valence single phase intermediate, Na1.5FePO4F drives a very small structural distortion. Temperature-dependent Mössbauer spectroscopy measurements confirm this localization, at least on short time scales (10−8 s), which persists to 370 °C. Finally, polycrystalline powders of other members of this family of compounds (Na2CoPO4F, Na2NiPO4F) were synthesized for the first time. Na2CoPO4F has a Co2+/Co3+ potential near 4.8 V. Mixed-metal phosphates of the form Na2(Fe1−x M x )PO4F, where M = Co, Mg, were also synthesized and found to be promising positive electrode materials for Li-ion or Na-ion energy storage devices.

We have found a remarkably large response of the transition temperature of CaFe 2 As 2 single crystals grown from excess FeAs to annealing and quenching temperature. Whereas crystals that are annealed at 400 • C exhibit a first-order phase transition from a high-temperature tetragonal to a low-temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960 • C exhibit a transition from a high-temperature tetragonal phase to a low-temperature, nonmagnetic, collapsed tetragonal phase below 100 K. By use of temperature-dependent electrical resistivity, magnetic susceptibility, x-ray diffraction, Mössbauer spectroscopy, and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing or quenching temperature from 400 • to 850 • C with the low-temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal, nonmagnetic for transition temperatures below 90 K. This suppression of the orthorhombic-antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal, nonmagnetic phase is similar to what has been observed for CaFe 2 As 2 under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature-dependent width of formation of CaFe 2 As 2 with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960 • C there is a fine (of order 10 nm) semiuniform distribution of precipitate that can be associated with an average strain field, whereas for samples annealed at 400 • C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe 2 As 2 lattice.

The magnetic ordering of superconducting single crystals of K0.80Fe1.76Se2.00 has been studied between 10 K and 550 K using 57 Fe Mössbauer spectroscopy. Despite being superconducting below Tsc ∼ 30 K, the iron sublattice in K0.80Fe1.76Se2.00 clearly exhibits magnetic order from well below Tsc to its Néel temperature of TN = 532 ± 2 K. The iron moments are ordered almost parallel to the crystal c-axis. The order collapses rapidly above 500 K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.PACS numbers:

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We have observed the effects of phonon mode softening at the ferroelectric transition in Eu 0.5 Ba 0.5 TiO 3 by 151 Eu Mössbauer spectroscopy. Both Eu 2+ and Eu 3+ spectral components are observed in the relative area ratio of 90% : 10% and both show a decrease in subspectral area at the transition, centred at 175 K, due to phonon mode softening. Surprisingly, the temperature dependence of the f-factor shows a much stronger response in the Eu 3+ component than in the Eu 2+ one. Preliminary analysis of neutron powder diffraction data rules out the possibility that some of the europium might be located on titanium sites.