The crystal and magnetic structures of polycrystalline BiCoO 3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO 3 (space group P4mm; Z ) 1; a ) 3.72937(7) Å and c ) 4.72382(15) Å at room temperature; tetragonality c/a ) 1.267) is isotypic with BaTiO 3 and PbTiO 3 in the whole temperature range. BiCoO 3 is an insulator with a Ne ´el temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k ) ( 1 / 2 , 1 / 2 , 0). In this model, magnetic moments of Co 3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)µ B and 2.93(2)µ B , respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO 3 . BiCoO 3 decomposed in air above 720 K to give Co 3 O 4 and sillenite-like Bi 25 -CoO 39 .

Structural properties of polycrystalline single-phased BiMnO3 samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO3 crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) A, b = 5.61263(8) A, c = 9.8632(2) A, beta = 110.6584(12) degrees at 300 K and a = 9.5866(3) A, b = 5.59903(15) A, c = 9.7427(3) A, beta = 108.601(2) degrees at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO3 at 300 K melts above TOO = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, mueff = 4.69 microB and theta = 138.0 K at 300-450 K and mueff = 4.79 microB and theta = 132.6 K at 480-600 K.

A new-type structural transition has been found in Li 2 RuO 3 with honeycomb lattice of edge-sharing RuO 6 -octahedra. With decreasing temperature T, the electrical resistivity exhibits an anomalous increase at T=T c~5 40 K, suggesting the (metal to insulator)-like transition and the magnetic susceptibility also shows a sharp decrease. Detailed structure analyses have revealed that the high temperature space group C2/m changes to P2 1 /m at T c . The most striking fact is that a significant reduction of the bond lengths is found between two of six Ru-Ru pairs of the hexagon in the low temperature phase, indicating a new type phase transition by the mechanism of the formation of molecular orbits of these Ru-Ru pairs.KEYWORDS: Li 2 RuO 3 , honeycomb structure, structural transition *Corresponding author: e43247a@nucc.cc.nagoya-u.ac.jpCompounds with the honeycomb lattice often present interesting behavior originating from their characteristic structures. For example, in the course of the studies on the physical properties of localized spin systems of A 3 T 2 SbO 6 (A=Na, Li; T=Cu, Ni, Co) and Na 2 T 2 TeO 6 on the (distorted) honeycomb lattice, spin gap behaviors have been found for T=Cu, [1][2][3] while the magnetic transitions to the spin-ordered state have been observed for T=Co and Ni. 4)As one of possible examples of conductive electrons on honeycomb lattice, we have investigated physical properties of Li 2 RuO 3 . It has the layers of the honeycomb lattice of edge-sharing RuO 6 octahedra with a LiO 6 octahedron at the center of each hexagon of RuO 6 (Fig. 1). The Ru valence is +4 and the four electrons exist in the 4d t 2g orbits. For this system, we have found a phase transition at temperature T=T c~5 40 K, where the crystal symmetry changes from a monoclinic (space group C2/m) to another monoclinic (space group P2 1 /m) one with decreasing T. As described in detail later, the transition is associated with the molecular orbit formation of Ru 4+ -Ru 4+ ions of the edge-sharing RuO 6 pair, presenting a new mechanism of structural transitions.Polycrystalline samples of Li 2 RuO 3 were prepared by sintering pellets of mixtures of RuO 2 and Li 2 CO 3 with proper molar ratio at 1000 ˚C for 24 h in air. 5,6) The powder neutron diffraction patterns of these samples indicate that a small amount of RuO 2 (molar fraction of ∼1.20 %) exists. There also exists an impurity peak of the unidentified phase, which has the integrated intensity of ~4.5 % of the maximum integrated intensity of the main phase as shown later. The magnetic susceptibilities χ were measured using a Quantum Design SQUID magnetometer under a magnetic field H=1 T in the temperature range of 2-700 K. The electrical resistivities ρ were measured by the standard four-terminal method using an ac-resistance bridge from 4.6 K to 695 K. The specific heats C were measured by a thermal relaxation method in the temperature range of 5-60 K using a Physical Property Measurement System (PPMS, Quantum Design). Powder X-ray diffraction measurements were carrie...

Utilizing the high stability of calcium and rare earth hydrides, CaFeAsF 1−x H x (x = 0.0-1.0) and SmFeAsO 1−x H x (x = 0.0-0.47) have been first synthesized using high pressure to form hydrogen-substituted 1111 type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H − ions occupying F − sites in the blocking layer of CaFeAsF. The resulting CaFeAsF 1−x H x is non-superconducting, whereas SmFeAsO 1−x H x is a superconductor, with an optimal T c = 55 K at x~ 0.2. It was found that up to 40% of the O 2− ions can be replaced by H − ions, with electrons being supplied into the FeAs-layer to maintain neutrality (O 2− = H − + e − ). When x exceeded 0.2, T c was reduced corresponding to an electron over-doped region.

With neutron powder diffraction, electron diffraction, and second-harmonic generation, we have shown that BiScO3 has a structure closely related to that of multiferroic BiMnO3, but BiScO3 crystallizes in the centrosymmetric space group of C2/c. These results bring up a question about the origin of ferroelectricity in BiMnO3. BiScO3 may serve as a model system to understand the role of Mn3+ ions in the ferroelectricity of BiMnO3.

No abstract

The crystal and magnetic structures of polycrystalline BiCrO3 were determined by the Rietveld method from neutron diffraction data measured at temperatures from 7 to 490 K. BiCrO3 crystallizes in the orthorhombic system above 420 K (space group Pnma; Z = 4; a = 5.54568(12) Å, b = 7.7577(2) Å, and c = 5.42862(12) Å at 490 K) in the GdFeO3-type structure. Below 420 K down to 7 K, a monoclinic structure is stable with C2/c symmetry (a = 9.4641(4) Å, b = 5.4790(2) Å, c = 9.5850(4) Å, and β = 108.568(3)° at 7 K). A possible model for antiferromagnetic order below T N = 109 K is proposed with a propagation vector of k = (0, 0, 0). In this model, magnetic moments of Cr3+ ions are coupled antiferromagnetically in all directions, forming a G-type antiferromagnetic structure. Refined magnetic moments at 7, 50, and 80 K are 2.55(2)μB, 2.43(2)μB, and 2.09(2)μB, respectively. The structure refinements revealed no deviation from stoichiometry in BiCrO3.

The crystal structure of metastable tetragonal zirconia prepared via the alkoxide method has been investigated at temperatures up to 1473 K, to clarify the similarity between this metastable phase and the tetragonal phase at high temperature. The lattice constants, tetragonality, oxygen shift parameter, and equivalent isotropic thermal parameter of the metastable tetragonal phase are proportional to the temperature. These parameters, when extrapolated to the high-temperature range, are very similar to those of the high-temperature tetragonal phase. Present results indicate that the structure of the metastable tetragonal phase is the same as that of the high-temperature tetragonal phase.