The topic of this book is the physics of transition metal compounds. In all their properties strong electron correlations play a crucial role. However TM compounds are not the only materials in which electron correlations are extremely important. Other such systems are substances containing rare earth elements with partially filled 4 f shells or actinide compounds with 5 f -electrons. These systems show a lot of very interesting special properties such as mixed valence and heavy fermion behavior. And though these phenomena were discovered and are mostly studied in 4 f and 5 f systems, similar effects, maybe less pronounced, are also observed in some TM compounds. The main concepts, and also the main problems in the physics of rare earth (and actinide) compounds are very similar to those in TM systems. Therefore we also include in this book, formally devoted to TM materials, this short chapter in which we summarize the main phenomena discovered in 4 f and 5 f systems, and compare them and their description with that of TM systems. Some of the sephenomena were even discovered first in materials with TM ions, but later proved to be essential in treating rare earth systems; while other notions were introduced for rare earth compounds and later transferred to the study of transition metal systems.

There exists a significant body of literature devoted specifically to some of the topics discussed briefly below. One can find detailed descriptions for example in Hewson (1993) or Coleman (2007).