Susceptibility of rare earth compounds in the intermediate valence state

The thermodynamic potential of rare earth compounds in a mixed valent state is investigated by perturbation theoretical methods. Two types of contributions must be distinguished, the impurity terms and terms corresponding to interactions between different rare earth sites. For a large class of substances the latter are shown to be of minor importance. The susceptibility of the resulting theory is compared directly with the renormalization group calculations of Krishna-murthy, Wilson and Wilkins and with measured susceptibilities of-Ce, SmB₆, YbAl₃.