Oxide pyrochlores — A review

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

Ferroelectric Metal–Organic Frameworks

Summary The theory of the dielectric and crystallographic properties of barium titanate is considered. By expanding the free energy as a function of polarization and strain and making reasonable assumptions about the coefficients, it is found possible to account for the various crystal transitions. Calculations are made of the dielectric constants, crystal stains, internal energy, and self polarization as functions of temperature. Finally relations are obtained between the coefficients in the free energy and the ionic force constants. These are used to estimate some of the coefficients which are not completely determined by experimental data.

XCVI. Theory of barium titanate

Transition metal carbides (TMCs) are a large family of materials with many intriguing properties and applications, and high-quality 2D TMCs are essential for investigating new physics and properties in the 2D limit. However, the 2D TMCs obtained so far are chemically functionalized, defective nanosheets having maximum lateral dimensions of ∼10 μm. Here we report the fabrication of large-area high-quality 2D ultrathin α-Mo2C crystals by chemical vapour deposition (CVD). The crystals are a few nanometres thick, over 100 μm in size, and very stable under ambient conditions. They show 2D characteristics of superconducting transitions that are consistent with Berezinskii–Kosterlitz–Thouless behaviour and show strong anisotropy with magnetic field orientation; moreover, the superconductivity is also strongly dependent on the crystal thickness. Our versatile CVD process allows the fabrication of other high-quality 2D TMC crystals, such as ultrathin WC and TaC crystals, which further expand the large family of 2D materials. Chemical vapour deposition is used to grow stable, ultrathin crystals of α-Mo2C and other transition metal carbides with lateral size up to 100 μm. α-Mo2C shows a superconducting behaviour with 2D character, strongly dependent on the crystal thickness.

Large-area high-quality 2D ultrathin Mo2C superconducting crystals

About eighty transition metal compounds comprising borides, carbides, nitrides, oxides, silicides, and germanides of metals of Groups 4A, 5A, and 6A were tested for superconductivity down to 1.20\ifmmode^\circ\else\textdegree\fi{}K, using a magnetic method. Among the specimens were most of the known compounds of the above type not examined magnetically for superconducting behavior by previous workers, and in all cases the structures were checked by x-ray diffraction analysis. The following eleven new superconductors were discovered, with the transition temperatures (\ifmmode^\circ\else\textdegree\fi{}K) shown in parentheses: ${\mathrm{W}}_{2}$B (3.10\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{Nb}}_{2}$C (9.18\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{Ta}}_{2}$C (3.26\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{Nb}}_{4}$${\mathrm{N}}_{3}$ (7.2\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{V}}_{3}$Si (17.1\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{V}}_{3}$Ge (6.01\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{Mo}}_{3}$Si (1.30\ifmmode^\circ\else\textdegree\fi{}), ${\mathrm{Mo}}_{3}$Ge (1.43\ifmmode^\circ\else\textdegree\fi{}), $\ensuremath{\alpha}$-Th${\mathrm{Si}}_{2}$ (3.16\ifmmode^\circ\else\textdegree\fi{}), $\ensuremath{\beta}$-Th${\mathrm{Si}}_{2}$ (2.41\ifmmode^\circ\else\textdegree\fi{}), and ${\mathrm{W}}_{3}$${\mathrm{Si}}_{2}$ (2.84\ifmmode^\circ\else\textdegree\fi{}). These compounds include the first superconducting germanides, ${\mathrm{V}}_{3}$Ge and ${\mathrm{Mo}}_{3}$Ge, which, together with ${\mathrm{V}}_{3}$Si and ${\mathrm{Mo}}_{3}$Si, crystallize in the cubic $\ensuremath{\beta}$-tungsten structure. The transition temperature of ${\mathrm{V}}_{3}$Si is apparently the highest known for any binary superconducting compound.

The Superconductivity of Some Transition Metal Compounds

The solid solution alloys formed by the incomplete dshell metals in groups 4, 5, 6, and 7 were tested for superconductivity down to 1 deg K. For alloys formed between neigh boring elements in a given row of the periodic table, two transition temperature maxima are observed with valence numbers approximately equal to 4.7 and 6.4, respectively, the only exception being the first long period, in which the upper maximum is absent. Similar maxima occur when the constituent elements are selected from different rows of the periodic table, thus confirming the dominant role of the d-shell electrons. It is known that the normal density-ofstates function, N(O), passes through a series of maxima as the d-shell is filled up, two of these peaks lying at about the same composition as the two transition temperature peaks observed in the present work. The relationship of T/sub c/ to N(O) for the transition metal alloys is discussed. Transition temperature data are also presented for alloys composed of neighboring elements in a given column of the periodic tabular. In this case, the form of the relationship between T/sub c/ and electronic or lattice properties is still obscure. (auth)

Superconducting Solid Solution Alloys of the Transition Elements

A Search for New Superconducting Compounds

The surface diffusion and adsorption of oxygen on tungsten have been studied by a field emission technique, by utilizing the high sticking coefficient, and negligible vapor pressure of O2 at 42–20°K Three types of migration were noted At ∼27°K diffusion occurs with a sharp boundary and an activation energy Ed≈09 kcal This is believed to be migration of physically adsorbed O2 over the chemisorbed layer, with precipitation on the bare surface The distance traversed by the boundary was found to be limited by desorption above ∼40°K A study of this effect yielded an estimate of 23 kcal for the heat of adsorption, Ha, in the physisorbed layer At 400–500°K and high coverages, diffusion occurs within the monolayer with activation energies of 23–25 kcal Boundaries moving radially outward from the close‐packed faces are observed, and believed to result from precipitation of O atoms in trap sites on the rough regions of the surface At low coverages boundary free diffusion with Ed≃30 kcal is observed and i

Adsorption and Diffusion of Oxygen on Tungsten

Changes in susceptibility of GeTe with temperature and sample composition are shown in transition curves for four pressed and sintered samples. Measurements show that GeTe becomes superconductivity in the predicted tempernture range. A plot of the transition temperatures as a function of the carrier concentration is also shown. (L.B.S.)

John K. Hulm

Papers

The Superconductivity of Some Transition Metal Compounds

Superconducting Solid Solution Alloys of the Transition Elements

A Search for New Superconducting Compounds

Adsorption and Diffusion of Oxygen on Tungsten

Superconductivity in Germanium Telluride