Mechanical milling and subsequent annealing in air at temperatures between 210 and $1200°\mathrm{C}$ have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from $76\pm 1\text{to}22\pm 0.5\mathrm{nm}$ ) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above $425°\mathrm{C}$ annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature $(>700°\mathrm{C})$ annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.

Three new tetranuclear copper(II) complexes [Cu(HL¹)]₄·4EtOH (1·4EtOH), [Cu(HL²)]₄ (2) and [Cu(H₂L³)]₄(NO₃)₄·2H₂O (3·2H₂O) have been synthesized using three different hydrazone Schiff base ligands derived from the condensation of the aromatic acid hydrazides 2-hydroxybenzo-, 2-aminobenzo- or benzo-hydrazide, with 2,3-dihydroxybenzaldehyde. Complexes 1 and 3 have been characterized by single crystal X-ray diffraction analysis. The coordinating behaviour of the ligand depends on the nature of the ortho substituent present in the hydrazide moiety. The ligands bearing a strong electron donating group (by resonance) in the ortho position undergo complexation via enolization and deprotonation, whereas the absence of such an effect leads to complexation via the keto form, and two different types of tetranuclear Cu(II) clusters, viz. open-cubane and cubane, are obtained. Variable temperature magnetic susceptibility measurements of complexes 1 and 3 have been carried out to examine the nature of magnetic interaction between the Cu(II) centres. All the three complexes (1–3) act as good catalyst precursors towards mild hydrocarboxylation of linear and cyclic alkanes into carboxylic acids in water–acetonitrile medium.