A Rhombic Dodecahedral Honeycomb Structure with Cation Vacancy Ordering in a γ‑Ga₂O₃ Crystal

The crystal structure of a γ-Ga2O3 layer grown epitaxially on an MgO substrate by a vapor phase transport method was investigated by transmission electron microscopy, electron diffraction, and scanning transmission electron microscopy with aberration correctors. Some forbidden reflections were excited in electron diffraction patterns by double reflection from the vicinity of the substrate interface. Phase boundaries are observed in atomic column images using high-angle annular dark field images. A structure model is proposed to explain the experimental results. Cation vacancy ordering is introduced in the structure model to distort the γ-Ga2O3 crystal lattice along one axis and reduce the lattice mismatch with the substrate. Some grains are formed and alter the directions to reduce the distortion for the other axis. The grains are stacked with {110} phase boundaries and form a rhombic dodecahedral honeycomb. The rhombic dodecahedral honeycomb structure model with cation vacancy ordering is stabilized by the lattice mismatch between the γ-Ga2O3 crystal and the MgO substrate, and it disappears at a depth of 170 nm from the interface

Helical Carbon and Graphitic Films Prepared from Iodine-Doped Helical Polyacetylene Film Using Morphology-Retaining Carbonization

Helical Carbon and Graphitic Films Prepared from Iodine-Doped Helical Polyacetylene Film Using Morphology-Retaining Carbonizatio

Chemical States of Overcharged LiCoO₂ Particle Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy

Deterioration mechanisms of LiCoO₂ electrode materials for lithium ion batteries remain unclear. Using electron energy-loss spectroscopy and transmission electron microscopy, this study investigated chemical states of LiCoO₂ particles on first overcharging. We present a scheme for quantification of the Li/Co atomic ratio. Using quantitative Li mapping and comprehensive probing of Li–K, Co–M_2,3, Co–L₃, and O–K edges, we observed that overcharging causes the progression of Co³⁺/Co²⁺ reduction with oxygen extraction from the particle surface to the interior. A gradual change in the chemical composition at and around the particle surfaces after charging of 60% revealed the presence of Co₃O₄-like and CoO-like phases at surface regions. We also observed nanocracks with deficient Li ions. These results are key factors affecting degradation on overcharging

Single Crystallization of Olivine Lithium Phosphate Nanowires using Oriented Attachments

Electrospinning enables fabrication of nanowires (NWs) of various materials from a polymer solution. Nevertheless, few reports have described single crystallization of oxide and polyanion NWs. Its mechanism remains unknown. This report presents transmission electron microscopy observations of conversion from electrospun amorphous NWs to single-crystalline olivine lithium phosphate NWs. After nucleation and grain growth, single crystallization is achieved by the attachment of adjacent crystal grains with common crystallographic orientations in an amorphous phase confined to self-forming carbon shells. The present NW axes have no specific orientation. These results imply that self-forming shells play a key role in achieving single-crystalline NWs in electrospinning

High-Pressure Synthesis, Crystal Structures, and Properties of Perovskite-like BiAlO₃ and Pyroxene-like BiGaO₃

New oxides, BiAlO3 and BiGaO3, were prepared using a high-pressure high-temperature technique at 6 GPa and 1273−1473 K. BiAlO3 is isotypic with multiferroic perovskite-like BiFeO3 and has octahedrally coordinated Al3+ ions. Structure parameters of BiAlO3 were refined from laboratory X-ray powder diffraction data (space group R3c; Z = 6; a = 5.37546(5) Å and c = 13.3933(1) Å). BiGaO3 has the structure closely related to pyroxene-like KVO3. Structure parameters of BiGaO3 were refined from time-of-flight neutron powder diffraction data (space group Pcca; Z = 4; a = 5.4162(2) Å, b = 5.1335(3) Å, and c = 9.9369(5) Å). The GaO4 tetrahedra in BiGaO3 are joined by corners forming infinite (GaO3)3- chains along the a axis. Bi3+ ions in BiGaO3 have 6-fold coordination. Both BiAlO3 and BiGaO3 decompose at ambient pressure on heating above 820 K to give Bi2M4O9 and Bi25MO39 (M = Al and Ga). Vibrational properties of BiAlO3 and BiGaO3 were studied by Raman spectroscopy. In solid solutions of BiAl1-xGaxO3, a C-centered monoclinic phase structurally related to PbTiO3 with lattice parameters of a = 5.1917(4) Å, b = 5.1783(4) Å, c = 4.4937(3) Å, and β = 91.853(3)° was found

High-Pressure Synthesis, Crystal Structure Determination, and a Ca Substitution Study of the Metallic Rhodium Oxide NaRh₂O₄

The sodium rhodate NaRh2O4 was synthesized for the first time and characterized by neutron and X-ray diffraction studies and measurements of magnetic susceptibility, specific heat, electrical resistivity, and the Seebeck coefficient. NaRh2O4 crystallizes in the CaFe2O4-type structure, which is comprised of a characteristic RhO6 octahedral network. The compound is metallic in nature, probably reflecting the 1:1 mixed valence character of Rh(III) and Rh(IV) in the network. For further studies of the compound, the Rh valence was varied significantly by means of an aliovalent substitution:  the full-range solid solution between NaRh2O4 and CaRh2O4 was achieved and characterized as well. The metallic state was dramatically altered, and a peculiar magnetism developed in the low Na concentration range

High-Pressure Synthesis, Crystal Structure Determination, and a Ca Substitution Study of the Metallic Rhodium Oxide NaRh₂O₄

The sodium rhodate NaRh2O4 was synthesized for the first time and characterized by neutron and X-ray diffraction studies and measurements of magnetic susceptibility, specific heat, electrical resistivity, and the Seebeck coefficient. NaRh2O4 crystallizes in the CaFe2O4-type structure, which is comprised of a characteristic RhO6 octahedral network. The compound is metallic in nature, probably reflecting the 1:1 mixed valence character of Rh(III) and Rh(IV) in the network. For further studies of the compound, the Rh valence was varied significantly by means of an aliovalent substitution:  the full-range solid solution between NaRh2O4 and CaRh2O4 was achieved and characterized as well. The metallic state was dramatically altered, and a peculiar magnetism developed in the low Na concentration range

Spinel-to-CaFe₂O₄-Type Structural Transformation in LiMn₂O₄ under High Pressure

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) Å, b = 2.83387(18) Å, and c = 10.6535(7) Å (Pnma). Electron diffraction patterns indicated an order of superstructure 3a × b × c, which might be initiated by Li vacancies. The exact composition is estimated at Li0.92Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to ∼0.27 eV from ∼0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at ∼10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle

Spinel-to-CaFe₂O₄-Type Structural Transformation in LiMn₂O₄ under High Pressure

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) Å, b = 2.83387(18) Å, and c = 10.6535(7) Å (Pnma). Electron diffraction patterns indicated an order of superstructure 3a × b × c, which might be initiated by Li vacancies. The exact composition is estimated at Li0.92Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to ∼0.27 eV from ∼0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at ∼10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle

High-Pressure Synthesis, Crystal Structure Determination, and a Ca Substitution Study of the Metallic Rhodium Oxide NaRh₂O₄

The sodium rhodate NaRh2O4 was synthesized for the first time and characterized by neutron and X-ray diffraction studies and measurements of magnetic susceptibility, specific heat, electrical resistivity, and the Seebeck coefficient. NaRh2O4 crystallizes in the CaFe2O4-type structure, which is comprised of a characteristic RhO6 octahedral network. The compound is metallic in nature, probably reflecting the 1:1 mixed valence character of Rh(III) and Rh(IV) in the network. For further studies of the compound, the Rh valence was varied significantly by means of an aliovalent substitution:  the full-range solid solution between NaRh2O4 and CaRh2O4 was achieved and characterized as well. The metallic state was dramatically altered, and a peculiar magnetism developed in the low Na concentration range