The composition-controlled metal-insulator transition in the perovskite systems LaNi1−x M x O3 (M = Cr, Mn, Fe, and Co) has been investigated by transport measurements over the temperature range 12–300 K. These systems, which have critical electron densities (n c) in the range (1–2) × 1020 electrons cm−3, exhibit sharp metal-insulator transitions at the base temperature. The corresponding minimum metallic conductivity (σ min), separating the localized and itinerant electronic regimes, is of the order of 102 ohm−1 cm−1. Particular attention is paid to the idea of σ min scaling with n c, and our present results are compared with earlier studies of the metal-insulator transition in low (e.g., Ge:Sb) and high (e.g., metal-ammonia, supercritical Hg) electron-density systems. A link is established between the transport and magnetic properties of the title systems at the metal-insulator transition.
Chemical doping of graphene is necessary to generate a band gap that is valuable for a range of applications. Chemical doping of graphene with elements like nitrogen and boron gives rise to useful properties. In this context, recent studies of borocarbonitrides, BxCyNz, comprising carbon, and the two elements on either side of it, are of significance. While uniformly homogeneous compositions of borocarbonitrides may be hard to generate, there have been efforts to synthesize them by solid state as well as gas phase routes. The products obtained show evidence for the presence of B-C and C-N bonds besides B-N and C-C bonds (but no N-N bonds), and possible occurrence of random BCN networks in addition to graphene and BN domains. Properties of borocarbonitrides depend on the composition, and the method of synthesis, enabling one to traverse from the insulating BN to the conducting graphene. In this account, we present important features of borocarbonitrides including synthesis, characterization, properties and potential applications. Surface oxygen functionalities and amine-groups of borocarbonitrides have been quantitatively determined by the fluorescence labeling of surface species (FLOSS) technique. Typical applications are in gas adsorption and energy devices such as supercapacitors and fuel cells as well as electrochemical sensors. Performance of borocarbonitrides as a HER catalysts is impressive, showing electrochemical activity close to that of Pt. It is possible to covalently link a BCN layer to other 2D nanosheets and the materials obtained by such cross-linking with layers of C3N4, MoS2 and MoSe2 show outstanding HER performance and other useful characteristics. Interestingly, heterostructures of BCN with nanosheets of MoS2 and other 2D materials can be formed reversibly by supramolecular means, which show good visible-light driven photochemical hydrogen evolution activity.
Solid solutions of the formula La2−xLnxCuO4 (Ln = Pr, Nd) possess the orthorhombic structure of La2CuO4 for small values of x and transform to the tetragonal Nd2CuO4 structure at a critical value of x. At the critical composition, there is an abrupt change in specific volume as well as the c a ratio. The material exhibits temperature-independent electrical resistivity below the critical value x and semiconducting behaviour above it. The specific volume and c a ratio smoothly decrease with increase in x in the La2Cu1−xNixO4 system, although the solid solution possess the tetragonal K2NiF4 structure when x>0.1. Compositions with x>0.1 exhibit a gradual semiconductor metal transition similar to that of La2NiO4, the transition temperature decreasing with increasing x.
Several complexes of iron and cobalt transform gradually or abruptly from a low-spin state to a high-spin state with increase in temperature and such transitions have been investigated widely in the past few years. Spin-state transitions in complexes are often accompanied by changes in enthalpy and crystal structure and many of them exhibit characteristics of first-order phase transitions. Transitions in certain complexes are affected by pressure, dopant metal ions and grinding and not infrequently show a plateau in the magnetic susceptibility-temperature plots. Seemingly unknown to coordination chemists, there is a fair body of information on the spin-state transitions in transition metal oxides and other extended solids. The transitions in rare earth cobaltates exhibit a plateau or a maximum in the inverse susceptibility-temperature plots, changes in enthalpy and crystal structure and other characteristics, not unlike those of the complexes. An attempt is made in this article, to bring together the essential features of spin-state transitions in metal complexes and oxides, particularly with respect to the nature of the phase transitions associated with spin cross-over. Models for spin-state transitions are examined and scope for further research indicated.
Layered transition metal dichalcogenides (TMDCs) are extensively investigated as catalyst materials for a wide range of electrochemical applications due to their high surface area and versatile electronic and chemical properties.
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An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
