Narrow Results

The tungsten oxides and bronzes have been extensively studied since their discovery in the last century, because of their brilliant colors and high electrical conductivity. More recently the driving interest resulted from their potential use in electrochromic displays and other electrochemical systems. Their crystalline structures are generally based on the corner sharing of WO₆ octahedra giving tunnels of variable size and shape leading to exciting intercalation chemistry. These structures readily undergo redox reactions, and in the last quarter century these reactions have often involved soft chemistry. Most recently hydrothermal techniques have been used to prepare new sodium tungstates with the hexagonal tungsten bronze and the pyrochlore structures. The phase formed is a function of the pH of the reaction medium. The pyrochlore phase readily undergoes ion-exchange with a wide range of monovalent cations giving the compounds, M_xW₂O_6+x/2 · yH₂O; the value of y is strongly dependent on the identity of the cation, M. WO₃ with the pyrochlore structure could be formed from the hydronium and ammonium complexes. Lithium can be readily intercalated either chemically and electrochemically into both these phases, just as in the previously-known bronze phases. Surprisingly more lithium is incorporated in most cases in the hexagonal than in the pyrochlore phase. The ions in the pyrochlore structure show rapid ionic mobility, with the hydronium ion showing the greatest mobility.

We have investigated a different manganese oxide material in which the colossal magnetoresistance (CMR) is observed. As many recent evidences suggest that a superexchange interaction is dominated in the pyrochlore Tl₂Mn₂O₇, it is found that the superexchange interaction is a chief mechanism driving the ferromagnetism. We specially formulate the superexchange interaction to calculate the resistivity in analogy with the t-scattering matrix of the Kondo formalism. We compare our calculation for resistivity with results of experiments and obtain the Curie temperature, T_c.

(Nd_0.7Yb_0.3)₂Zr₂O₇ and ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ (0 < x ≤ 0.25) ceramics have been synthesized by pressureless sintering by tailoring the chemical compositions. Microstructure and electrical conductivity of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ were investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and AC impedance spectroscopy. (Nd_0.7Yb_0.3)₂Zr₂O₇ ceramic exhibits a mixed crystal structure of defect fluorite and pyrochlore. After doping with Sm³⁺ cations, the structure changes from a mixed type of (Nd_0.7Yb_0.3)₂Zr₂O₇ to a single pyrochlore type of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇, as the addition of Sm³⁺ reduces the difference in ionic radius between Nd³⁺ and Yb³⁺. However, ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ ceramics contain the localized short-range disorder despite the structural order overall in the pyrochlore. The measured total conductivities of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ obey the Arrhenius relation. Doping of Sm³⁺ enhances the electrical conductivity of (Nd_0.7Yb_0.3)₂Zr₂O₇ ceramic significantly, which is closely related to the variations in the concentration of oxygen vacancies at 48f sites, relatively low activation energy and high pre-exponential factor caused by the long-range order and short-range disorder.

Transparent ceramics have potential applications in various areas, including aerospace, relativistic optics industries, medical cares and defense. Specifically, they can be used as laser gain media, armor windows, IR domes, solid-state phosphors, scintillators and electro-optical components. From crystal structure point of view, transparent ceramic materials should have high crystallographic symmetries (cubic, tetragonal and hexagonal), which have minimal birefringent effect. Currently, transparent ceramics are dominantly based on oxide materials, although there are also nonoxides, such as fluorides and nitrides (oxynitrides). Transparent ceramics with pyrochlore and fluorite structures have attracted much attention in recent years, whereas fluorides are not well described in the open literature. Therefore, this paper is aimed to deliver an overview on the progress of the two categories of transparent ceramics, from material processing and characterization point of view.

The hidden ordered state of the frustrated pyrochlore oxide Tb_2+xTi_2-xO_7+y is possibly one of the two electric multipolar, or quadrupolar, states of the effective pseudospin-1/2 Hamiltonian derived from crystal-field ground state doublets of non-Kramers Tb³⁺ ions. These long-range orders are antiparallel or parallel alignments of transverse pseudospin components representing electric quadrupole moments, which cannot be observed as magnetic Bragg reflections by neutron scattering. However, pseudospin waves of these states are composite waves of the magnetic-dipole and electric-quadrupole moments, and can be partly observed by inelastic magnetic neutron scattering. We calculate these spin-quadrupole waves using linear spin-wave theory and discuss previously observed low-energy magnetic excitation spectra of a polycrystalline sample with x = 0.005(T_c = 0.5 K).

We have studied the magnetic response of itinerant spin ice, by applying the cluster dynamical mean field theory (CDMFT) to the Ising Kondo lattice model on a pyrochlore lattice. As a result, we found a characteristic interplay between magnetization curve and spin ice correlation developed at low temperatures. The magnetization develops a kink-like structure at the 2/3 of its saturation value, reminiscent of kagome ice plateau. Accompanied with the magnetization process, the monopole density shows nonmonotonic magnetic field dependence with a clear minimum, reflecting a subtle energetics of spin configurations. The spin ice correlation also affects the transport properties of the system, and brings about negative magnetoresistivity with its slope strongly dependent on the magnitude of spin ice correlation. We discuss these behaviors in comparison with the magnetic response observed in Pr₂Ir₂O₇.