Electric and Magnetic Dipole Strengths of f-f Transitions in Cubic Cs₂NaYCl₆:Ln³⁺ Systems

Abstract

Optical spectra of trivalent lanthanide ions (Ln³⁺) doped into the cubic elpasolite host, Cs₂NaYCL₆, have proved to be of special value in theoretical studies of Ln³⁺-crystal field interactions (1). In the Cs₂NaYCl₆:Ln³⁺ systems, the Ln³⁺ ions are each surrounded by six nearest-neighbor chloride (Cl⁻) ions and each resides at a site of exact octahedral (O_h) symmetry. The O_h site symmetry allows the lanthanide crystal field to be described entirely in terms of just two crystal field coefficients (excluding the spherically symmetric components of the crystal field), and it precludes any electric dipole contributions to the intensities of no-phonon (origin) crystal field transitions associated with lanthanide f-f excitations and de-excitations. In general, the optical absorption and emission spectra of the Cs₂NaYCl₆ systems may be analyzed entirely in terms of pure magnetic dipole lines associated with f-f (crystal field) origins, and sets of one-phonon vibronic lines which acquire electric dipole intensity via a vibronic coupling mechanism. Invariably, the most intense vibronic lines are associated with the three ungerade vibrational modes localized within the LnCl ³⁻₆ chromophoric clusters. Vibronic lines associated with non-cluster vibrational modes (e.g., lattice modes) are frequently observed, but with lesser intensities.