http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.

http://science.sciencemag.org/content/sci/288/5465/462.full.pdf

Orbital Physics in Transition-Metal Oxides

BACKGROUND Heat and electricity are two forms of energy that are at opposite ends of a spectrum Heat is ubiquitous, but with low quality, whereas electricity is versatile, but its production is demanding Thermoelectrics offers a simple and environmentally friendly solution for direct heat-to-electricity conversion A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity Electricity also can drive a TE device to work as a solid-state heat pump for distributed spot-size refrigeration TE devices are free of moving parts and feasible for miniaturization, run quietly, and do not emit greenhouse gasses The full potential of TE devices may be unleashed by working in tandem with other energy-conversion technologies Thermoelectrics found niche applications in the 20th century, especially where efficiency was of a lower priority than energy availability and reliability Broader (beyond niche) application of thermoelectrics in the 21st century requires developing higher-performance materials The figure of merit, ZT, is the primary measure of material performance Enhancing the ZT requires optimizing the adversely interdependent electrical resistivity, Seebeck coefficient, and thermal conductivity, as a group On the microscopic level, high material performance stems from a delicate concert among trade-offs between phase stability and instability, structural order and disorder, bond covalency and ionicity, band convergence and splitting, itinerant and localized electronic states, and carrier mobility and effective mass ADVANCES Innovative transport mechanisms are the fountain of youth of TE materials research In the past two decades, many potentially paradigm-changing mechanisms were identified, eg, resonant levels, modulation doping, band convergence, classical and quantum size effects, anharmonicity, the Rashba effect, the spin Seebeck effect, and topological states These mechanisms embody the current states of understanding and manipulating the interplay among the charge, lattice, orbital, and spin degrees of freedom in TE materials Many strategies were successfully implemented in a wide range of materials, eg, V2VI3 compounds, VVI compounds, filled skutterudites and clathrates, half-Heusler alloys, diamond-like structured compounds, Zintl phases, oxides and mixed-anion oxides, silicides, transition metal chalcogenides, and organic materials In addition, advanced material synthesis and processing techniques, for example, melt spinning, self-sustaining heating synthesis, and field-assisted sintering, helped reach a much broader phase space where traditional metallurgy and melt-growth recipes fell short Given the ubiquity of heat and the modular aspects of TE devices, these advances ensure that thermoelectrics plays an important role as part of a solutions package to address our global energy needs OUTLOOK The emerging roles of spin and orbital states, new breakthroughs in multiscale defect engineering, and controlled anharmonicity may hold the key to developing next generation TE materials To accelerate exploring the broad phase space of higher multinary compounds, we need a synergy of theory, machine learning, three-dimensional printing, and fast experimental characterizations We expect this synergy to help refine current materials selection and make TE materials research more data driven We also expect increasing efforts to develop high-performance materials out of nontoxic and earth-abundant elements The desire to move away from Freon and other refrigerant-based cooling should shift TE materials research from power generation to solid-state refrigeration International round-robin measurements to cross-check the high ZT values of emerging materials will help identify those that hold the most promise We hope the renewable energy landscape will be reshaped if the recent trend of progress continues into the foreseeable future

http://science.sciencemag.org/content/sci/357/6358/eaak9997.full.pdf

Advances in thermoelectric materials research: Looking back and moving forward

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

Combining electron diffraction, x-ray diffraction, and high-resolution electron microscopy techniques, a structural model for the cobaltite $``{\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}''$ has been found. This compound is a misfit-layered oxide consisting in two monoclinic subsystems with identical a, c, and \ensuremath{\beta} parameters, but different b parameters: $a=4.8376(7)\AA{},$ $c=10.833(1)\AA{},$ $\ensuremath{\beta}=98.06(1)\ifmmode^\circ\else\textdegree\fi{},$ ${b}_{1}=4.5565(6)\AA{},$ and ${b}_{2}=2.8189(4)\AA{}.$ The structure is built up from the stacking along c of triple rock salt-type layers ${\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}$ (first subsystem) with single ${\mathrm{CdI}}_{2}$-type ${\mathrm{CoO}}_{2}$ layers (second subsystem). Two different sets of Co-O distances are involved which are interpreted as the existence of cobalt with three different oxidation states 2+, 3+, and 4+, in agreement with x-ray appearance near-edge structure spectra at the Co K edge. At about 420 K, both resistivity and susceptibility show an anomaly which results from a spin-state transition of cobalt at this temperature. Below 300 K, the resistivity measured along the ${\mathrm{CoO}}_{2}$ layers shows a metal-insulator transition as T decreases, whereas the much larger out-of-plane resistivity values show the anisotropic behavior of this phase. The application of a magnetic field induces a negative magnetoresistance which reaches -35% for 7 T. Moreover, thermoelectric power measurements yield a high positive value of \ensuremath{\approx}125 \ensuremath{\mu}V ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ at 300 K with a weak temperature dependence in between 100 and 300 K. This result contrasts with the metallic in-plane resistivity.

Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca 3 Co 4 O 9

Structural and Magnetic Studies of Ordered Oxygen-Deficient PerovskitesLnBaCo2O5+δ, Closely Related to the “112” Structure

The magnetic phase diagrams of four ${L}_{1\ensuremath{-}x}{A}_{x}{\mathrm{MnO}}_{3}$ series $(L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m};$ $A=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r})$ have been established combining neutron diffraction, electron microscopy, and magnetotransport measurements in order to understand and optimize the colossal magnetoresistance (CMR) properties of these compounds. A complementary study of the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ phase diagram $(xg~0.8)$ is also performed. The comparison of these diagrams demonstrates that low $〈{r}_{A}〉$ values are required to obtain CMR properties on the ${\mathrm{Mn}}^{4+}$ rich side, i.e., when a competition between charge order and cluster glass phases occurs ${(L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3};$ $L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m}).$ As a consequence of the low $〈{r}_{A}〉$ values on the rich ${\mathrm{Mn}}^{3+}$ side $(xl0.5$ in ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}),$ the ferromagnetic metallic (FMM) state is never reached, whereas for the same x values, systems with larger $〈{r}_{A}〉$ ${(L}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3};$ $L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m})$ exhibit CMR properties related to the FMM state. A particular attention is paid to the $x\ensuremath{\sim}0.5$ compositions of these ${L}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ series for which the Curie and N\'eel temperatures are ${T}_{N}l{T}_{C}$ and ${T}_{C}l{T}_{N}$ for Pr and Sm, respectively. The importance of the chemical factors on the charge ordering (studied vs temperature by electron diffraction) and on the magnetic structures (from neutron diffraction) is also emphasized.

Magnetic phase diagrams of L 1 − x A x MnO 3 manganites ( L = P r , S m ; A = C a , S r )

New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7 T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.

Magnetoresistance in the oxygen deficient LnBaCo2O5.4 (Ln=Eu, Gd) phases

Single crystals of the one-dimensional phase Ca3Co2O6 of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co2O6] chains ( K) antiferromagnetically coupled ( K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (100 to 103 Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m=1/4, 1/2, 1, 2, 3 where m=3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m=1 magnetization. A magnetic phase diagram is finally proposed.

A. Maignan

Papers

Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca 3 Co 4 O 9

Structural and Magnetic Studies of Ordered Oxygen-Deficient PerovskitesLnBaCo2O5+δ, Closely Related to the “112” Structure

Magnetic phase diagrams of L 1 − x A x MnO 3 manganites ( L = P r , S m ; A = C a , S r )

Magnetoresistance in the oxygen deficient LnBaCo2O5.4 (Ln=Eu, Gd) phases

Single crystal study of the one dimensional Ca Co O compound: five stable configurations for the Ising triangular lattice