Twenty years on and 164 reviews later since the appearance of volume 1 of this prestigious series, the rare earths continue to perplex, if not mystify us. There is no doubt that in the next breakthrough to a higher level of human understanding of nature, the rare earth elements and their compounds will play an important part in its elaboration. The reviews contained in this 25th volume continue this quest.

The first chapter reveals the effects of the addition of the rare earths on steel. These elements were introduced to the steel-making industry in the 1920's to affect deoxidization and desulfurization. Today it is possible to extend their usefulness to provide steels with desired characteristics. The description of why and how additions of the rare earths affect micro- and nano-structures in steels which provide these beneficial modifications is explored.

Chapter two reviews ternary and higher-order nitride materials. The synthesis of ternary compounds is examined and the products are categorized by their composition and the nature of the chemical bonding they exhibit. The structures of the compounds are discussed where possible.

The spectral intensities of f-f transitions in lanthanide compounds are discussed at length in the following chapter. The authors present an exhaustive consideration of the unique appearance of intraconfigurational electronic transitions in f-type materials, particularly the lanthanides.

The final chapter reviews the prodigious literature on the organometallic &pgr; complexes of the f-elements. The authors consider the main classes of complexes of both the lanthanide and actinide organometallics, and synthesis and structure are dealt with in particular.