Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

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We investigated the crystal structure of Rb 2 Cu 3 SnF 12 and its magnetic properties using single crystals. This compound is composed of Kagomé layers of corner-sharing CuF 6 octahedra with a 2a  2a enlarged cell as compared with the proper Kagomé layer. Rb 2 Cu 3 SnF 12 is magnetically described as an S ¼ 1=2 modified Kagomé antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagomé cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated. Antiferromagnets on highly frustrated lattices produce a rich variety of physics.1,2) In particular, a two-dimensional Heisenberg Kagomé antiferromagnet (2D HKAF) is of great interest from the viewpoint of the interplay of the frustration and quantum effects. There are many theoretical studies on the 2D HKAF. The spin wave theory for a large spin value predicted an ordered ground state with the so-called ffiffi ffi 3 p  ffiffi ffi 3 p structure, which is selected by quantum fluctuation from infinite classical ground states, 3,4) whereas for a small spin value, a disordered ground state was observed by various approaches.5-9) Recent careful analyses and numerical calculations for an S ¼ 1=2 case demonstrated that the ground state is a spin liquid state composed of singlet dimers only, and that the ground state is gapped for triplet excitations, but gapless for singlet excitations.10-12) Consequently, magnetic susceptibility has a rounded maximum at T $ ð1=6ÞJ=k B and decreases exponentially toward zero with decreasing temperature, while specific heat exhibits a power law behavior at low temperatures. 8,13) Specific heat also shows an additional structure, peak or shoulder at low temperatures after exhibiting a broad maximum at T $ ð2=3ÞJ=k B .The experimental studies of the S ¼ 1=2 HKAF have been limited, and the above-mentioned intriguing predictions have not been verified experimentally. The cupric com- 27) Unfortunately, these systems undergo structural phase transitions at T t ¼ 220 and 170 K, respectively, and also magnetic phase transitions at T N ' 24 K. 27) However, the magnetic susceptibilities observed at T > T t can be perfectly described using theoretical results for an S ¼ 1=2 HKAF with large exchange interactions J=k B $ 250 K. 28)In the present work, we synthesized the new hexagonal compound Rb 2 Cu 3 SnF 12 with a similar crystal structure as Cs 2 Cu 3 ZrF 12 and performed magnetic susceptibility and high-field magnetization measurements using single crystals. As shown below, we found that the ground state is a disordered singlet with a finite gap for magnetic excitations.Rb 2 Cu 3 SnF 12 crystals were synthesized via the chemical reaction 2RbF þ 3CuF 2 þ SnF 4 ! Rb 2 Cu 3 SnF 12 . RbF, CuF 2 , and SnF 4 were dehydrated by heating in vacuum at 60 -100 C for three days....

The oxide-ion conductivity of NdBaInO 4 has been increased by Sr doping. Nd 0.9 Sr 0.1 BaInO 3.95 showed the highest electrical conductivity among Nd 1Àx Sr x BaInO 4Àx/2 (x ¼ 0.0, 0.1, 0.2, and 0.3). The oxide-ion conductivity s ion of Nd 0.9 Sr 0.1 BaInO 3.95 (s ion ¼ 7.7 Â 10 À4 S cm À1 ) is about 20 times higher than that of NdBaInO 4 (s ion ¼ 3.6 Â 10 À5 S cm À1 ) at 858 C, and the activation energy of oxide-ion conduction is a little lower for Nd 0.9 Sr 0.1 BaInO 3.95 (0.795(10) eV) than that for NdBaInO 4 (0.91(4) eV). The structure analysis based on neutron powder diffraction data revealed that the Sr exists at the Nd site and oxygen vacancies are observed in Nd 0.9 Sr 0.1 BaInO 3.95 . This result indicates that the increase of the oxide-ion conductivity is mainly due to the increase of the carrier concentration. The bond valence-based energy landscape indicated two-dimensional oxide-ion diffusion in the (Nd,Sr) 2 O 3 unit on the bc-plane and a decrease of the energy barrier by the substitution of Nd with Sr cations. † Electronic supplementary information (ESI) available: A document containing the crystallographic data of Nd 0.9 Sr 0.1 BaInO 3.95 , additional experimental information, and a crystallographic information le (CIF) of Nd 0.9 Sr 0.1 BaInO 3.95 . See

While cation order-disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order-disorder transition from the anion-ordered fluorite structure ( P4/ nmm) for larger Ln ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln (Sm-Er). Structural analysis reveals that with the increase of Ln radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn and larger HLn tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.

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Multiferroic CuFe1−xAlxO2 (x = 0.02) exhibits a ferroelectric ordering accompanied by a proper helical magnetic ordering below T = 7K under zero magnetic field. By polarized neutron diffraction and pyroelectric measurements, we have revealed a one-to-one correspondence between the spin helicity and the direction of the spontaneous electric polarization. This result indicates that the spin helicity of the proper helical magnetic ordering is essential for the ferroelectricity in CuFe1−xAlxO2. The induction of the electric polarization by the proper helical magnetic ordering is, however, cannot be explained by the Katsura-Nagaosa-Balatsky model, which successfully explains the ferroelectricity in the recently explored ferroelectric helimagnets, such as TbMnO3. We thus conclude that CuFe1−xAlxO2 is a new class of magnetic ferroelectrics.PACS numbers: 75.80.+q, 75.25.+z, 77.80.-e Novel types of couplings between dielectric property and magnetism, which produce colossal magnetoelectric (ME) effects, have been extensively investigated since a gigantic ME effect was discovered in RMnO 3 (R is a rare earth material) [1]. Among several types of couplings between spins and electric polarizations, a ferroelectricity induced by noncollinear spin arrangements has been most widely investigated experimentally and theoretically [2,3,4,5,6,7,8]. Katsura, Nagaosa and Baratsky (KNB) proposed that the local electric dipole moment p, which arises between neighboring two spins S i and S i+1 , can be described in the form of p ∝ e i,i+1 × (S i × S i+1 ), where e i,i+1 is a unit vector connecting two spins [2]. This formula successfully explains the ferroelectric property in cycloidal or conical magnetic orderings of some transition metal oxides, such as RMnO 3 (R=Tb, Tb 1−x Dy x ), Ni 3 V 2 O 8 , MnWO 4 and CoCr 2 O 4 [4,5,6,7,8]. Moreover, a recent polarized neutron diffraction study on TbMnO 3 demonstrated that the spin helicity, clockwise or counterclockwise, correlates with the direction of the electric polarization, as predicted in the formula [9]. It is, however, recently reported that ferroelectricity in a helical magnetic ordering of a delafossite multiferroic CuFe 1−x Al x O 2 cannot be explained by the above formula [10]. Therefore, CuFe 1−x Al x O 2 provides an opportunity to explore an another type of spin-polarization coupling.CuFeO 2 , which is one of model materials of a triangular lattice antiferromagnet, has been extensively investigated as a geometrically frustrated spin system for last fifteen years [11,12,13]. The ground state of CuFeO 2 is a collinear commensurate 4-sublattice (↑↑↓↓) state with the magnetic moments along the c axis, which is normal to the triangular lattice layers, in spite of the Heisenberg spin character expected from the electronic configuration of Fe. When a magnetic field is applied along the c axis at low temperature, CuFeO 2 exhibits a multi-step magnetization process consisting of several magnetization plateaus and slopes, which is accompanied by stepwise changes of lattice constants [14...

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