SPECTROSCOPIC STUDIES, THEORETICAL-MODELS AND STRUCTURAL CHARACTERIZATION - I - THE ELPASOLITES CS(2)NALNCL(6), WHERE LN(3+) = ER3+, YB3+

The stoichiometric elpasolites of the type Cs(2)NaLnCl(6) are known to belong to the space group Fm3Sm(O-h(5)). The structural data of these elpasolites is both scare and rather limited and therefore a througho ut experimental study is needed. Over the last few years, our research group has been engaged in a project to undertake: the synthesis, the structural and spectroscopic characterization for these materials. We aim to gain understanding about the most likely intensity mechanisms a ssociated with the one and the two photon spectroscopies. These theore tical studies are indeed, very complex, since they involve transitions of the kind f --> d and/or f --> g at the central metal Bs well as li gand subsystem excitations, among terminal states of different parity. The above described mechanism is currently being used, though its app arent conceptual simplicity, nevertheless we recognize the intrinsic l imitations of the model calculation. We could, in principle relax many of these approximation by including, for instance; higher order vibro nic interactions, the coupling among the internal and the external vib rations of the same symmetry species and also the static and dynamic m agnetic dipole contributions to the total transition dipole moments. T he latter are known as vibronically induced magnetic-dipole transition s, appearing at false origins defined by single quanta of the gerade v ibrational modes of the cluster LnCl(6)(3+). In this paper, we shall r eport new and updated structural data, spectroscopic studies (FTIR, Ra man and luminescence) and also a physical model to account for the obs erved spectral intensities for these two elpasolites.