Observation of Anion Order in Pb₂Ti₄O₉F₂

The observation of anion order is indispensable for the investigation of oxyfluorides. However, the negligible contrast between O^2– and F^– in both X-ray and neutron diffraction obscures the distinct anion sites for Rietveld refinement. Therefore, the difference in the chemical bonding of M–O^2– and M–F^– is the key to determining anion order. In this study, bond-valence-sum calculations and determination of the electron density distribution by the maximum entropy method illustrated anion order in the newly synthesized oxyfluoride Pb₂Ti₄O₉F₂. These results demonstrate a promising method to determine anion order in mixed anion systems

Temperature-Independent, Large Dielectric Constant Induced by Vacancy and Partial Anion Order in the Oxyfluoride Pyrochlore Pb₂Ti₂O_6−δF_2δ

In mixed-anion systems, partial anion order can be the key to realizing enhanced dielectric properties. A novel A2B2X6X′0.5-type oxyfluoride pyrochlore, Pb2Ti2O5.4F1.2, was prepared through the conventional solid-state reaction. Characterization of the oxyfluoride pyrochlore by synchrotron X-ray diffraction and electron diffraction revealed the existence of partial O2–/F– order at the X site, in which F– is involved in fac-type Ti­(O/F)6 coordination, associated with the O2–/vacancy order at the X′ site. Although Pb2Ti2O5.4F1.2 adopts a centrosymmetric structure, the refined occupancy factors suggested the existence of local polarization. Partial O2–/F– anion order stabilized the large displacement of Ti4+, leading to a high dielectric constant (as high as 800) with a low-temperature coefficient. These results suggest that mixing heteroanions in pyrochlores provides a basis for exploring novel dielectric materials

Reductive Formation of a Vanadium(IV/V) Oxide Cluster Complex [V₈O₁₉(4,4′‑^<i>t</i>Bubpy)₃] Having a <i>C</i>₃‑Symmetric Propeller-Shaped Nonionic V₈O₁₉ Core

A novel <i>C</i>₃-symmetric propeller-shaped vanadium­(IV/V) oxide cluster complex, [V₈O₁₉(4,4′-^<i>t</i>Bubpy)₃] (<b>V</b>_<b>8</b><b>′</b>), has been synthesized from the reaction of the windmill-shaped vanadium­(V) oxide cluster complex [V₈O₂₀(4,4′-^<i>t</i>Bubpy)₄] (<b>V</b>_<b>8</b>) with PPh₃ under N₂, whereas refluxing <b>V</b>_<b>8</b> in methanol or ethanol under N₂ provides tetranuclear oxido­(alkoxido)­vanadium­(IV/V) complexes [V₄O₆(OR)₆(4,4′-^<i>t</i>Bubpy)₂] [R = Me (<b>V</b>_<b>4</b><b>′-Me</b>) and Et (<b>V</b>_<b>4</b><b>′-Et</b>)]. The mixed-valent vanadium­(IV/V) clusters <b>V</b>_<b>8</b><b>′</b> and <b>V</b>_<b>4</b><b>′</b> are converted back to <b>V</b>_<b>8</b> under O₂. Interconversions of <b>V</b>_<b>4</b><b>′</b> and the oxido­(alkoxido)­vanadium­(V) complexes [V₄O₈(OMe)₄(4,4′-^<i>t</i>Bubpy)₂] (<b>V</b>_<b>4</b>) and [V₇O₁₇(OEt)­(4,4′-^<i>t</i>Bubpy)₃] (<b>V</b>_<b>7</b><b>-Et</b>) are also presented

Suppressed Activation Energy for Interfacial Charge Transfer of a Prussian Blue Analog Thin Film Electrode with Hydrated Ions (Li⁺, Na⁺, and Mg²⁺)

Interfacial charge transfer is one of the most important fundamental steps in the charge and discharge processes of intercalation compounds for rechargeable batteries. In this study, temperature-dependent electrochemical impedance spectroscopy was carried out to clarify the origin of the high power output of aqueous batteries with Prussian blue analog electrodes. The activation energy for the interfacial charge transfer, <i>E</i>_a, was estimated from the temperature dependence of the interfacial charge transfer resistance. The <i>E</i>_a values with Li⁺ and Na⁺ aqueous electrolytes were considerably smaller than those with organic electrolytes. The small <i>E</i>_a values with aqueous electrolytes could result from the fact that the Coulombic repulsion at the interface is largely suppressed by the screening effect of hydration

Electrospinning Synthesis of Wire-Structured LiCoO₂ for Electrode Materials of High-Power Li-Ion Batteries

An application of the Li-ion batteries to advanced transportation systems essentially requires the enhancement of the rate capability; thus, the fabrication of nanostructured cathode materials with the large surface area and short Li-ion diffusion length is particularly important. In this study, an electrospinning method was adopted for the synthesis of wire-structured LiCoO₂. The diameter of the as-spun fiber obtained from the precursor solution with multiwalled carbon nanotubes (vapor-grown carbon fiber, VGCF) was thinner than that of as-spun fiber obtained from the solution without VGCF. After the heat treatment, wire-structured LiCoO₂ was successfully obtained regardless of the existence of dispersed VGCF in the precursor solution, although the particle size of LiCoO₂ fabricated with VGCF was smaller than that of LiCoO₂ fabricated without VGCF. The charge/discharge and rate-capability experiments revealed that both resulting materials show the reversible Li-ion insertion/extraction reaction. However, due to the existence of a small irreversible capacity at the initial cycles, the interfacial resistance increases, resulting in the poor cyclability and lower charge/discharge rate capability, especially for nanowire LiCoO₂ fabricated with VGCF

Structural Chemistry and Magnetic Properties of Ln₁₈Li₈Rh_5−<i>x</i>Fe_<i>x</i>O₃₉ (Ln = La, Nd)

Polycrystalline samples of Ln18Li8Rh5−xFexO39 (Ln = La, Nd; 0.5 ≤ x ≤ 5) have been synthesized by a solid-state method and studied by a combination of dc and ac magnetometry, neutron diffraction, and Mössbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3̅n, a0 ∼ 12 Å) based on intersecting ⟨111⟩ chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Ln−O framework. At low values of x, iron preferentially occupies the smaller (2a) of the two distinct octahedral sites as low-spin Fe(IV). The Rh(III) on the larger (8e) octahedral site is replaced by high-spin Fe(III). Nd-containing compositions having x > 1 show spin-glass-like behavior below ∼5 K. La-containing compositions having x > 1 show evidence of a magnetic transition at ∼8 K, but the nature of the transition is unclear. This contrasting behavior demonstrates that, although the structural chemistry of the two systems is essentially the same, the magnetic character of the Ln cations plays an important role in determining the properties of these compounds

Precise Electrochemical Control of Ferromagnetism in a Cyanide-Bridged Bimetallic Coordination Polymer

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K_0.2Ni­[Fe­(CN)₆]_0.7·4.7H₂O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe³⁺/Fe²⁺ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature <i>T</i>_C gradually decreases due to reduction of paramagnetic Fe³⁺ to diamagnetic Fe²⁺, and the ferromagnetic transition is completely suppressed for Li_0.6(NiFe-PBA). On the other hand, <i>T</i>_C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe²⁺ to paramagnetic Fe³⁺, and the ferromagnetic transition is nearly recovered for Li₀(NiFe-PBA). Furthermore, the plots of <i>T</i>_C as a function of the amount of inserted/extracted Li ion <i>x</i> are well consistent with the theoretical values calculated by the molecular-field approximation