日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

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Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

Journal of Physics Condensed Matter: Preface

This is the first targeted review of the synthesis – microstructure – electrochemical performance relations of MoS2 – based anodes and cathodes for secondary lithium ion batteries (LIBs). Molybdenum disulfide is a highly promising material for LIBs that compensates for its intermediate insertion voltage (∼2 V vs. Li/Li+) with a high reversible capacity (up to 1290 mA h g−1) and an excellent rate capability (e.g. 554 mA h g−1 after 20 cycles at 50 C). Several themes emerge when surveying the scientific literature on the subject: first, we argue that there is excellent data to show that truly nanoscale structures, which often contain a nanodispersed carbon phase, consistently possess superior charge storage capacity and cycling performance. We provide several hypotheses regarding why the measured capacities in such architectures are well above the theoretical predictions of the known MoS2 intercalation and conversion reactions. Second, we highlight the growing microstructural and electrochemical evidence that the layered MoS2 structure does not survive past the initial lithiation cycle, and that subsequently the electrochemically active material is actually elemental sulfur. Third, we show that certain synthesis techniques are consistently demonstrated to be the most promising for battery applications, and describe these in detail. Fourth, we present our selection of synthesis methods that we believe to have a high potential for creating improved MoS2 LIB electrodes, but are yet to be tried.

Lithium ion battery applications of molybdenum disulfide (MoS2) nanocomposites

Transport and magnetic properties of polycrystalline ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6\ensuremath{-}\ensuremath{\delta}},$ $0l~\ensuremath{\delta}l~0.05,$ have revealed the existence of two distinguishable monoclinic ferromagnetic phases separated by a two-phase domain $0.02l~\ensuremath{\delta}l~0.05$ and a pseudotetragonal $(c/al\sqrt{2})$ phase with $\ensuremath{\delta}g~0.05$ that was prepared at 600 \ifmmode^\circ\else\textdegree\fi{}C. A nearly oxygen-stoichiometric sample having a magnetization $M(5\mathrm{K},50\mathrm{kOe})=5.78{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ with a Curie temperature ${T}_{c}\ensuremath{\approx}226\mathrm{K}$ is identified as atomically ordered ${\mathrm{La}}_{2}{\mathrm{Co}}^{2+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ containing about 1.8% antiferromagnetic spins at antisites. This ferromagnetic phase is an n-type polaronic conductor that progressively traps mobile electrons at the oxygen vacancies that introduced them on lowering the temperature. Although the x-ray-diffraction pattern can be indexed in orthorhombic (Pbnm) or monoclinic ${(P2}_{1}/n)$ symmetry with $\ensuremath{\beta}\ensuremath{\approx}90\ifmmode^\circ\else\textdegree\fi{},$ atomic order identifies the space group as ${P2}_{1}/n.$ A second monoclinic, ferromagnetic phase with ${T}_{c}l150\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05$ has a large, positive thermoelectric power that increases progressively with decreasing temperature. Quenching a $\ensuremath{\delta}\ensuremath{\approx}0.02$ sample from 1350 \ifmmode^\circ\else\textdegree\fi{}C into liquid ${\mathrm{N}}_{2}$ gave a single phase with ${T}_{c}=134\mathrm{K}$ and $\ensuremath{\delta}\ensuremath{\approx}0.05.$ A sample with $\ensuremath{\delta}g~0.05$ that was synthesized at 600 \ifmmode^\circ\else\textdegree\fi{}C was pseudotetragonal $(c/al\sqrt{2})$ and had a paramagnetic Weiss constant $\ensuremath{\theta}l{T}_{c}\ensuremath{\approx}225\mathrm{K}$ as well as a significantly smaller magnetization, but its magnetization curve $M(T)$ showed no evidence of spin-glass behavior; its large, positive thermoelectric power was characteristic of polaronic conduction without trapping of mobile charge carriers at lower temperatures. Interpretation of the two phases with $\ensuremath{\delta}\ensuremath{\approx}0.05$ is based on the hypothesis that introduction of high-spin ${\mathrm{Mn}}^{3+}$ by the oxygen vacancies creates around it additional ${\mathrm{Mn}}^{3+}$ and intermediate-spin ${\mathrm{Co}}^{3+}$ at neighboring sites; the resulting gain in elastic energy from cooperative, dynamic Jahn-Teller deformations at these ions must be sufficient to overcome the cost of about 0.2 eV for the electron transfer from a ${\mathrm{Co}}^{2+}\mathrm{ion}$ to a ${\mathrm{Mn}}^{4+}\mathrm{ion}.$

Multiple magnetic phases of La 2 CoMnO 6 − δ ( 0 δ 0 . 0 5 )

Nominal La 2 NiMnO 6 prepared by solid-state reaction in air in accordance with earlier reports is shown to contain excess oxygen as well as the coexistence of two ferromagnetic phases of comparable Curie temperatures, onemonoclinic and the other rhombohedral. As originally predicted, ordering of Ni 2 + and Mn 4 + ions gives ferromagnetic Ni 2 + -O-Mn 4 + interactions and transforms the orthorhombic Pbnm space group to monoclinic P2 1 In with β90° and the rhombohedral R3c space group to R3m or R3. Synthesis by the Pechini method in Ar, air, and O 2 atmospheres under different thermal treatments also consistently gave O 6 + δ ; the lowest δ=0.05(1) was attained for a single P2 1 /n phase reacted under Ar at 1350°C. Lowering the mean A-site atomic radius in La 2 - x □ x NiMnO 6 and Nd 2 NiMnO 6 also stabilizes the monoclinic phase, and near oxygen stoichiometry was attained in La 2 - x □ x NiMnO 6 for x0.09. Excess oxygen is accommodated in the perovskite structure by the creation of cation vacancies, and it is shown that lanthanum vacancies create deep three-hole acceptor traps. Comparison with the double perovskite La 2 CoMnO 6 and La 2 NiRuO 6 + δ versus La 2 CoRuO 6 signals that stabilization of lanthanum vacancies is associated with a Ni 3 + /Ni 2 + redox couple that is stabilized by a counter octahedral-site cation M having a strong covalent component to its M-O bonding. It is therefore proposed that in the presence of Ni 2 + , but not Co 2 + , a lanthanum vacancy is stabilized by the formation of two holes trapped deeply in molecular orbitals of an O 1 2 cluster of the oxygen atoms that neighbor a lanthanum vacancy. Transport data also indicate a lowering of the separation of the Mn 4 + /Mn 3 + and Ni 3 + /Ni 2 + redox couples from E g ≥0.8 eV to E g 0.3 eV in the ordered Ni 2 + , Mn 4 + array. This lowering and a motional enthalpy ΔH m n 0.1 eV of electrons is attributed to locally cooperative Jahn-Teller deformations of low-spin Ni 3 + and high-spin Mn 3 + octahedral sites. The magnetization M(5 K, 50 kOe) is lowered by both local atomic disorder and the formation of antiphase boundaries. It is shown that a prolonged anneal at 800 °C reduces the local atomic disorder, but it does not remove the antiphase boundaries. Synthetic strategies to increase the magnetization must be designed to reduce the concentration of antiphase boundaries and cation vacancies as well as the atomic disorder.

Oxygen stoichiometry, ferromagnetism, and transport properties of La 2-x NiMnO 6+δ

Transport measurements in a high-temperature superconductor provide evidence that the so-called pseudogap phase ends at a quantum critical point located inside the superconducting dome in the phase diagram of cuprates.

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Linear temperature dependence of resistivity and change in the Fermi surface at the pseudogap critical point of|[nbsp]|a|[nbsp]|high-Tc superconductor

The temperature dependence of thermal conductivity, $\ensuremath{\kappa}(T),$ and magnetic susceptibility, $\ensuremath{\chi}(T),$ have been measured on single crystals of $L{\mathrm{CoO}}_{3}$ $(L=\mathrm{La},$ Pr, Nd) grown by the floating-zone method. The susceptibility measurement shows a progressive stabilization of the low-spin (LS) state of Co(III) with decreasing size of the ${\mathrm{L}}^{3+}$ ion, and the population of excited intermediate-spin (IS) or high-spin (HS) state Co(III) ions begins to increase at 200 K and 300 K for ${\mathrm{PrCoO}}_{3}$ and ${\mathrm{NdCoO}}_{3}$ compared with 35 K in ${\mathrm{LaCoO}}_{3}.$ The low-temperature Curie-Weiss paramagnetic susceptibility of $L{\mathrm{CoO}}_{3}$ is an intrinsic property arising from surface cobalt and, possibly, a LS ground state bearing some IS character caused by the virtual excitation to the IS state. The transition from a LS to a IS/HS state introduces bond-length fluctuations that suppress the phonon contribution to $\ensuremath{\kappa}(T)$ below 300 K. The suppressed $\ensuremath{\kappa}(T)$ could be further reduced by dynamic Jahn-Teller distortions associated with the IS/HS species. A smooth transition in $\ensuremath{\rho}(T)$ and $\ensuremath{\alpha}(T)$ and a nearly temperature independent $\ensuremath{\alpha}(T)\ensuremath{\approx}20\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ above 600 K do not support a thermally induced, homogeneous Mott-Hubbard transition model for the high-temperature transition of ${\mathrm{LaCoO}}_{3}$ from an insulating to a conductive state. A two-phase process is proposed for the interval $300\mathrm{K}lTl700\mathrm{K}$ with a conductive phase growing in a matrix of IS states with localized $e$ electrons that are stabilized by Jahn-Teller distortions that may be dynamic.

Bond-length fluctuations and the spin-state transition in L CoO 3 ( L = La , Pr, and Nd)

The floating-zone method and high-pressure synthesis have been used to obtain the hexagonal and the perovskite $R\mathrm{Mn}{\mathrm{O}}_{3}$ $(R=\mathrm{Y},\phantom{\rule{0.3em}{0ex}}\mathrm{Ho},\phantom{\rule{0.3em}{0ex}}\mathrm{Er},\phantom{\rule{0.3em}{0ex}}\mathrm{Tm},\mathrm{Yb},\phantom{\rule{0.3em}{0ex}}\mathrm{Lu})$ compounds. We have refined the crystal structure and characterized the compounds with measurements of magnetic susceptibility $\ensuremath{\chi}(T)$ and thermal conductivity $\ensuremath{\kappa}(T)$. The systematic change of $\ensuremath{\kappa}(T)$ below ${T}_{N}$ found in all members of the hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ family shows that some spin-independent bond-length fluctuation plays an important role in the suppression of $\ensuremath{\kappa}(T)$ below ${T}_{N}$ as well as in the paramagnetic phase. The responsible soft vibrational mode is identified. In contrast, the perovskite $R\mathrm{Mn}{\mathrm{O}}_{3}$ shows a phonon-like $\ensuremath{\kappa}(T)$ below room temperature, but with an anomalously large critical scattering at ${T}_{N}$. A phase diagram of transition temperatures versus the ${R}^{3+}$-ion radius for both hexagonal and perovskite phases is also given.

J. B. Goodenough

Papers

Multiple magnetic phases of La 2 CoMnO 6 − δ ( 0 δ 0 . 0 5 )

Oxygen stoichiometry, ferromagnetism, and transport properties of La 2-x NiMnO 6+δ

Linear temperature dependence of resistivity and change in the Fermi surface at the pseudogap critical point of|[nbsp]|a|[nbsp]|high-Tc superconductor

Bond-length fluctuations and the spin-state transition in L CoO 3 ( L = La , Pr, and Nd)

Hexagonal versus perovskite phase of manganite R Mn O 3 ( R = Y , Ho , Er , Tm , Yb , Lu )