Relativistic f⇆f transitions in crystal fields

A relativistic model of f <-> f transitions based on the transformation of all intershell tensor operators to effective relativistic double tensor operators is developed. The transition amplitude is expressed in terms of effective operators acting within the f(N) shell due to a partial closure performed upon the relativistic intershell operators. The final expression is discussed in the light of its reduction to the non-relativistic case that describes f <-> f transitions in the language of the standard Judd-Ofelt theory.