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Conventional high-temperature reactions limit the control of coordination polyhedra in transition-metal oxides to those obtainable within the bounds of known coordination geometries for a given transition metal. For example, iron atoms are almost exclusively coordinated by three-dimensional polyhedra such as tetrahedra and octahedra. However, recent works have shown that binary metal hydrides act as reducing agents at low temperatures, allowing access to unprecedented structures. Here we show the reaction of a perovskite SrFeO3 with CaH2 to yield SrFeO2, a new compound bearing a square-planar oxygen coordination around Fe2+. SrFeO2 is isostructural with 'infinite layer' cupric oxides, and exhibits a magnetic order far above room temperature in spite of the two-dimensional structure, indicating strong in-layer magnetic interactions due to strong Fe d to O p hybridization. Surprisingly, SrFeO2 remains free from the structural instability that might well be expected at low temperatures owing to twofold orbital degeneracy in the Fe2+ ground state with D(4h) point symmetry. The reduction and the oxidation between SrFeO2 and SrFeO3 proceed via the brownmillerite-type intermediate SrFeO2.5, and start at the relatively low temperature of approximately 400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.

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Using neutron diffraction, 170 Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb 2 Ti 2 O 7 , where the Yb 31 S 0 1͞2 ground state has planar anisotropy. Below ϳ0.24 K, the temperature of the known specific-heat l transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb 31 spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime. DOI: 10.1103/PhysRevLett.88.077204 PACS numbers: 75.40. -s, 75.25. +z, 76.75. +i, 76.80. +y Geometrically derived magnetic frustration arises when the spatial arrangement of the spins is such that it prevents the simultaneous minimization of all interaction energies [1 -4]. In the crystallographically ordered pyrochlore structure compounds R 2 Ti 2 O 7 , the rare earth ions ͑R͒ form a sublattice of corner sharing tetrahedra and a number of these compounds have been observed to exhibit frustration related behavior [5][6][7][8][9][10][11][12]. The low temperature magnetic behavior associated with a particular rare earth depends on the properties of the crystal field ground state and on the origin (exchange/dipole), size, and sign of the interionic interactions. For example, the recently identified spin-ice configuration [6,10,11] has been linked with an Ising-like anisotropy and a net ferromagnetic interaction. Most of the published work on the pyrochlores have concerned rare earth ions with Ising-like characteristics [6 -12] and there has also been some interest in the properties of weakly anisotropic Gd 31 [5]. Less attention has been paid to the case, considered here, where the ion has planar anisotropy.To date, in systems where geometrical frustration may be present, two different low temperature magnetic ground states have been considered. First, under the influence of the frustration the system does not experience a magnetic phase transition and remains in a collective paramagnetic state with the spin fluctuations persisting as T ! 0 [3,4,[6][7][8][9][10][11]13]. Second, a long range ordered state is reached through a phase transition which may be first order [5,14,15]. Our results for Yb 2 Ti 2 O 7 , obtained using neutron diffraction, 170 Yb Mössbauer spectroscopy, and muon spin relaxation (mSR), evidence a novel scenario: there is a first-order transition which does not correspond to a transition from a paramagnetic state to a (long or short range) magnetically ordered state. The transition chiefly concerns the time domain, and involves an abrupt slowing down of the dynamics of short range correlated spins; below the transition temperature, these spins continue to fluctuate at a temperature independent rate.We have established the background magnetic characteristics for Yb 2 Ti 2 O 7 in a separate study [16,17]. The Yb 31 ion crystal field ground state is a very well isolated Kramers doubl...

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties and the discovery of new materials is a challenging research focus. Here we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1) and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell which will combine the advantages of both oxide ion and proton conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.