The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

Oxide pyrochlores — A review

First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

https://pubs.rsc.org/en/content/articlepdf/2014/cy/c4cy00669k

Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction

It is well known that the breakthrough of Bednorz and Muller1 in discovering superconductivity in (La, Ba)2CuO4 was inspired in part by their knowledge of the superconducting properties of Ba(Pb, Bi)O3 (ref. 2). With a transition temperature, Tc, of ∼12 K, that compound was not generally considered anomalous despite the fact that its Tcis 3–5 times higher than that of traditional superconductors with comparable density of states3–5. The increases in Tc for copper-oxide-based materials continue to generate worldwide excitement, but from both a chemical and theoretical point of view it would also be exciting if high-Tcsuperconductivity were observed in another class of materials. Here we report the results of experiments leading us to the single-phase perovskite Ba0.6K0.4BiO3, which has a magnetically determined onset temperature of 29.8 K—a Tc considerably higher than that of conventional superconductors and surpassed only by copper-containing compounds. Superconductivity in this compound occurs within the framework of a three dimensionally connected bismuth-oxygen array. These results suggest that further research toward exploring the limiting Tcs for bismuth-oxide-based, high-temperature superconductors might be fruitful.

J.L. Gillson

Papers

Crystal chemistry of metal dioxides with rutile-related structures

Chemistry of noble metal oxides. III. Electrical transport properties and crystal chemistry of ABO2 compounds with the delafossite structure

High-temperature superconductivity in the BaPb1−xBixO3 system

Single crystal synthesis and electrical properties of CdSnO3, Cd2SnO4, In2TeO6 and Cdln2O4

New transition metal dichalcogenides formed at high pressure