Journal of Magnetism and Magnetic Materials 100 (1991) 126-138 North-Holland Valence fluctuation and heavy fermion behaviour in rare earth and actinide based compounds D.T. Adroja Indian Institute of Technology, Powai, Bombay 400 076, India and S.K. Malik Tata Institute of Fundamental Research, Bombay 400 005, India A brief historical review of some of the experimental work reported on the mixed valent (MV) or valence fluctuating (VF) and heavy fermion (HF) systems is given. The characteristic physical properties of MV and HF systems are discussed. The salient features of the theoretical models are outlined. Results on some systems are presented. I. Introduction The elements starting from lanthanum (atomic number Z = 57) and ending at lutetium (Z = 71) are known as the lanthanides or the rare earths and are characterized by the progressive filling of their 4f shell. Scandium (Z = 21) and yttrium (Z = 39) are also included in the category of rare earths [1] because of their similar physical prop- erties. Rare earths form a variety of intermetallic compounds and alloys with transition metals and metalloids [2-10]. Experimental investigations on these intermetallic compounds and alloys in the recent past have revealed that some of the com- pounds containing Ce, Eu and Yb (and also at times Pr, Sm and Tm) exhibit interesting physical properties/phenomena associated with the 4f electrons, such as magnetic ordering with anoma- lously high ordering temperature [11-13], Kondo effect [14-15], superconductivity [16-17], heavy fermion behavior [18-24] and mixed valence or valence fluctuation behaviour [25-37] (We shall use mixed valence, valence fluctuation or inter- configuration fluctuations synonymously to mean the same). Among these, the phenomena of heavy fermion (HF) and valence fluctuation (VF) have attracted considerable attention in recent years both experimentally (see refs. [18-37] and other references cited therein) and theoretically [38- 48]. In spite of the availabilit~ of a large number of experimental data and a variety of theoretical models [38-48] on the VF and HF systems, there is still no single theoretical model which explains all the physical properties of these systems in a satisfactory way. Therefore, this has sustained interest in identifying new VF and HF systems and carrying out systematic studies using differ- ent experimental techniques to completely bring out various aspects of these systems. In this paper we review briefly some of the earlier work re- ported on MV and HF behaviour of rare earth intermetallic compounds. The characteristic phys- ical properties of these systems are discussed. 0304-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved