[en] The main goal of this research work is to rationalize the rich vibronic structure of lanthanide type crystals, such as Cs₂NaDyCl₆ and Cs₂NaHoCl₆, in the space group Fm3m(O_h⁵). These systems are known to be highly relativistic and as a consequence, major corrections to previous model calculations should be taken into account so as to explain from a semi-quantitative viewpoint, the observed spectral intensities. We have decided to tackle this study taking special care, of both the physics and the chemistry involved with special emphasis on the theoretical model to be employed as well as, in the strategy to be followed to rationalize the available experimental data. This paper aims to advance our understanding of the intensity mechanisms, associated with observed radiative transitions, say for complex highly relativistic systems, in the solid state physics. The spectral intensities associated with superpositions (juxtapositions) of peaks and/or bands in the absortion and emission spectra are considered in detail and a preliminary working methodology is put forward with reference to the Cs₂NaDyCl₆ and Cs₂NaHoCl₆ crystals