High-Pressure Synthesis and Properties of Solid Solutions between BiMnO₃ and BiScO₃

Solid solutions BiMn1-xScxO3 (0 ≤ x ≤ 1) were prepared at 6 GPa and 1383−1443 K. Selected area and convergent beam electron diffraction showed that BiMn0.9Sc0.1O3 crystallizes in the centrosymmetric space group C2/c at room temperature. The structure parameters of BiMn0.9Sc0.1O3 were refined by the Rietveld method from laboratory X-ray diffraction data (Z = 8; a = 9.6029(3) Å, b = 5.60988(14) Å, c = 9.7690(3) Å, β = 108.775(2°) at 293 K). The Mn−O bond lengths suggest that the orbital order present in BiMnO3 at 300 K disappears in BiMn0.9Sc0.1O3. Therefore, the monoclinic-to-monoclinic phase transition observed in BiMnO3 at 474 K and associated with the orbital melting was not detected in BiMn1-xScxO3 for x ≥ 0.05 down to 133 K. BiMn1-xScxO3 were characterized by dc and ac magnetization, specific heat, and differential scanning calorimetry measurements. The long-range ferromagnetic order seems to survive for x = 0−0.2. For x ≥ 0.4, the samples showed spin-glass-like features. The Weiss temperature deduced from the fitting of magnetic susceptibilities was positive for all the compositions with x x. The temperature of the magnetic transitions decreased and the temperature of the structural monoclinic-to-orthorhombic phase transition increased (from 768 K for x = 0 to 840 K for x = 0.3) with increasing x

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

Structural and Physical Properties of Heavily Doped Yttrium Vanadate: Y_0.6Cd_0.4VO₃

Structural properties of Y0.6Cd0.4VO3 were investigated by electron diffraction and laboratory and synchrotron X-ray powder diffraction methods. Y0.6Cd0.4VO3 crystallizes in space group Pnma (GdFeO3-type perovskite structure) between 12 and 300 K (a = 5.45887(3) Å, b = 7.57250(4) Å, and c = 5.27643(2) Å at 300 K). The lattice parameters showed anomalous behavior on temperature. The c parameter linearly decreased from 12 to 120 K, and then it lineally increased from 160 to 300 K. The b parameter was constant between 12 and 120 K, demonstrated a drop from 120 to 200 K, and then lineally increased from 200 to 300 K. The c/a ratio had a rather sharp maximum at 150 K. In Y0.6Cd0.4VO3 the V−O distances in the ac plane began to split to shorter and longer ones below 150 K, indicating that orbital fluctuations are involved. The phase transition near 150 K in Y0.6Cd0.4VO3 is accompanied by a broad anomaly on the specific heat and change of the slope of the inverse magnetic susceptibility. Other members of the Y1-xCdxVO3 solid solution with x = 0.3, 1/3, and 0.5 did not show this kind of phase transition. This kind of a phase transition has never been detected in other doped vanadates, R1-xMxVO3 (R = Y and rare earths and M = Ca and Sr)

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

Origin of the Monoclinic-to-Monoclinic Phase Transition and Evidence for the Centrosymmetric Crystal Structure of BiMnO₃

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) Å, b = 5.61263(8) Å, c = 9.8632(2) Å, β = 110.6584(12)° at 300 K and a = 9.5866(3) Å, b = 5.59903(15) Å, c = 9.7427(3) Å, β = 108.601(2)° 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, μeff = 4.69μB and θ = 138.0 K at 300−450 K and μeff = 4.79μB and θ = 132.6 K at 480−600 K