Systematics in the electrical and magnetic properties of transition metal perovskites LnBO₃ (Ln=rare-earth ion, B=3d transition metal) with the variation of Ln and B ions are reviewed. The electrical resistivity and activation energy of LnBO₃ compounds increase with the decreasing size of the Ln³⁺ ion for a given B ion. The low-spin to high-spin transition temperature of Co³⁺ ion in LnCoO₃ similarly increases with the decrease in size of Ln³⁺ while the magnetic ordering temperatures in LnVO₃, LnFeO₃, LnCrO₃ and LnSrCo₂O₆ decreases with decreasing size of the rare-earth ion. These results may be understood in terms of the increasing acidity of the rare earth ion with decreasing size and the competition between the Ln³⁺ and the B³⁺ ions for covalency with the oxygen ions. The effect of this competition on the metal oxygen covalency and crystal field parameter is discussed in relation to the results obtained and Goodenough’s phase diagram. The possibility ofd-f exchange interaction in La_1−xLn_x NiO₃ is also discussed in the light of ESR results.

A few types of phase transitions in solids of interest to the author are discussed after a brief presentation of the general features of phase transitions. The different systems discussed include low-dimensional solids, polytypes, organic solids, plastic crystals, glasses and silver selenide. Spin-state transitions and dipole glasses are also briefly examined.

An increasing number of inorganic solids forming long-period structures due to recurrent intergrowth of two chemically distinct but structurally related units are getting to be known in recent years. These novel structures have given rise to new chemistry at solid-solid interfaces. Besides intergrowth structures with long-range order, many solids with random intergrowth (similar to stacking faults in polytypes) are known. Ordered integrowth gives rise to homologous series of structures in many systems. Barium ferrites, the Aurivillius family of oxides and other perovskite-related oxides, siliconiobates, and tungsten oxide bronzes are some of the systems exhibiting ordered intergrowth structures. Both ordered and disordered intergrowths are fruitfully investigated by high resolution electron microscopy. The main emphasis in this article is on intergrowth structures where the component units are compositionally different. These systems are obviously most fascinating since compositional change occurs across each interface (intergrowth plane), unlike in polytypic materials where the composition remains constant. Even in ordered intergrowth structures, there is always some disorder. If order in an intergrowth structure does not prevail over large distances, but occurs only over shorter stretches (say, a few repeats of the sequence), it becomes difficult to describe the solid except in terms of the gross composition and where possible, the unit cell dimensions. Structures with occasional intergrowths are found in a variety of materials such asβ-alumina, Magnéli phases, silicates, ferrites and several other oxide systems. In addition to examining the structural features of various intergrowths, the origin of the intergrowth phenomenon is discussed.

Solid state chemistry was in its infancy when the author got interested in the subject. In this article, the author outlines the manner in which the subject has grown over the last four decades, citing representative examples from his own contributions to the different facets of the subject. The various aspects covered include synthesis, structure, defects, phase transitions, transition metal oxides, catalysts, superconductors, metal clusters and fullerenes. In an effort to demonstrate the breadth and vitality of the subject, the author shares his own experiences and aspirations and gives expression to the agony and ecstacy in carrying out experimental research in such a frontier area in India.

Scanning tunneling microscopy of C₇₀ films deposited on HOPG and gold substrates has been carried out to investigate the 2D packing, defects and disorder. Besides providing direct evidence for orientational disorder, high resolution images showing the carbon skeleton as well as the molecular arrangement in a solid solution of C₇₀ and C₆₀ are presented. Tunneling conductance measurements indicate a small gap in the C₇₀ film deposited on HOPG substrate.

Supramolecular synthesis is being increasingly employed in materials design. In this article, the design of porous solids is discussed as a case study. Examples from recent work in the areas of open-framework inorganic materials, mesoporous solids and organic porous solids are presented to illustrate this important aspect of materials chemistry.

Discovery of the amazing properties of graphene has aroused great interest in other 2D materials. 2D inorganic analogues of graphene such as the transition metal dichalcogenides have been investigated widely and these materials,especially MoS$_2$, exhibit many properties of interest. In particular, they possess properties of direct use in energy devices. In this article we review the synthesis and properties of the 2D materials of relevant transistors, sensors, photodetectors, supercapacitors and batteries as well as in oxygen reduction and hydrogen evolution reactions.

Two-dimensional layered PbFCl and BaFCl have been prepared by solid-state reactions and the crystal structures were subjected to Rietveld refinement to obtain structural parameters. The compounds characterized by severalmethods including X-ray photoelectron spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, etc. have been subjected to ultrasonic exfoliation in different solvents such as water,dimethylformamide and $N$-methyl-2-pyrrolidone. Water is found to yield single layers of both these halides. The supercapacitor performances of PbFCl, BaFCl and the exfoliated materials have been studied. Exfoliated 1–2 layered PbFCl exhibits a high-specific capacitance of 158 F g$^{-1}$ at a scan rate of 10 mV s$^{-1}$.

MoS$_x$Se$_{(2-x)}$ and MoSe$_x$Te$_{(2-x)}$ solid solutions with various S:Se and Se:Te ratios have been prepared by high temperature solid-state reactions, and thinned down to few-layers by Li-intercalation followed by exfoliation. Photocatalytic as well as electrocatalytic hydrogen evolution reaction (HER) activity of exfoliated MoS$_x$Se$_{(2-x)}$/MoSe$_x$Te$_{(2-x)}$2D nanosheets have been studied. It is found that Se-rich compositions exhibit good HER activity. The MoS$_{0.5}$Se$_{1.5}$ nanosheets show high photocatalytic HER activity yielding 29.6 mmol h$^{-1}$ g$^{-1}$ of H$_2$, while MoS$_{1.0}$Se$_{1.0}$ displays good electrocatalytic activity with an onset potential of $-$0.220 V. Amongst MoSe$_x$Te$_{(2-x)}$ solid solutions, MoSe$_{1.8}$Te$_{0.2}$ shows relatively high photocatalytic HER activity (5.0 mmol h$^{-1}$ g$^{-1}$), while MoSe$_{1.0}$Te$_{1.0}$ exhibits a low onset potential ($-$0.190 V vs. RHE).