Synthesis, Structure, and Physical Properties of <i>A</i>-site Ordered Perovskites <i>A</i>Cu₃Co₄O₁₂ (<i>A</i> = Ca and Y)

A-site ordered perovskites CaCu3Co4O12 and YCu3Co4O12, and their solid solutions Ca1−xYxCu3Co4O12 (x = 0.25, 0.50, and 0.75), were synthesized under high pressure (9 GPa) and high temperature (1273 K). They all have a CaCu3Ti4O12-type structure (cubic, space group: Im3̅) and the lattice constant a slightly decreased from 7.1226(5) Å of CaCu3Co4O12 to 7.1195(3) Å of YCu3Co4O12. Rietveld refinement based on synchrotron powder X-ray diffraction suggests that the valence state of CaCu3Co4O12 is close to CaCu3+3Co3.25+4O12, which differs from those of other CaCu2+3B4+4O12 perovskites. Substitution of Ca with Y at the A-site changes the metallic CaCu3Co4O12 to an insulating YCu3Co4O12, and a metal−insulator transition occurs at x = 0.50−0.75. The electrical resistivity, thermoelectricity, and specific heat results reveal that electrons are doped into the Co 3d band and that the valence state changes from CaCu3+3Co3.25+4O12 to YCu3+3Co3+4O12

Weak Ferroelectricity in <i>n</i> = 2 Pseudo Ruddlesden–Popper-Type Niobate Li₂SrNb₂O₇

Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden–Popper-type compounds, which are generally formed by intergrowth of two-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates the coexistence of spontaneous polarization and an anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles calculations and single-crystal X-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane antiferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden–Popper-type compounds

Fabrication and Thermoelectric Properties of Freestanding Ba_1/3CoO₂ Single-Crystalline Films

Thermoelectric energy conversion has attracted attention as an energy-harvesting technology for converting waste heat into electricity via the Seebeck effect. Conducting oxide-based thermoelectric materials that exhibit a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Among many conducting oxides, Ba1/3CoO2 epitaxial films exhibit the highest figures of merit. For the practical use of Ba1/3CoO2, bulk ceramics or single-crystalline Ba1/3CoO2 is necessary. Here, we show that freestanding Ba1/3CoO2 single-crystalline films can be fabricated by peeling Ba1/3CoO2 epitaxial films from the substrate. We fabricated Ba1/3CoO2 epitaxial films and immersed them in 40 °C hot water for several tens of minutes. Subsequently, the Ba1/3CoO2 epitaxial film spontaneously peeled off and floated on the surface of the water like seaweed. We measured and analyzed the crystal structure, chemical composition, and thermoelectric properties before and after peeling and realized that there was no significant difference. The present results provide a useful method for fabricating freestanding single-crystalline oxide films for thermoelectrics

Current-Induced Metallic State in an Organic (EDT-TSF)₂GaCl₄ Conductor

A newly prepared organic conductor, (EDT-TSF)2GaCl4, shows considerable nonlinear conductance in the insulating state below 20 K, and a metallic state is restored by the application of moderate currents. This conductor has a stacking structure with a quasi-one-dimensional open Fermi surface, similar to the sulfur analogues. Since the interchain interaction is enhanced by the selenium substitution, the static magnetic susceptibility as well as ESR does not show any anomaly around 20 K, and low-temperature X-ray investigation does not show any extra spots. Isostructural (EDT-TSF)2FeCl4 shows similar conducting properties, although the magnetic interaction of the anion is weak. Like this, nonlinear conductance is a versatile tool to restore a metallic state when the metal−insulator transition is almost suppressed

Current-Induced Metallic State in an Organic (EDT-TSF)₂GaCl₄ Conductor

A newly prepared organic conductor, (EDT-TSF)2GaCl4, shows considerable nonlinear conductance in the insulating state below 20 K, and a metallic state is restored by the application of moderate currents. This conductor has a stacking structure with a quasi-one-dimensional open Fermi surface, similar to the sulfur analogues. Since the interchain interaction is enhanced by the selenium substitution, the static magnetic susceptibility as well as ESR does not show any anomaly around 20 K, and low-temperature X-ray investigation does not show any extra spots. Isostructural (EDT-TSF)2FeCl4 shows similar conducting properties, although the magnetic interaction of the anion is weak. Like this, nonlinear conductance is a versatile tool to restore a metallic state when the metal−insulator transition is almost suppressed