Abstract
The Ce3+ ion in Cs2NaCe(NO2)6 (I), which comprises the unusual Th site symmetry of the Ce(NO2)63− ion, demonstrates the largest Ce-O Stokes shift of 8715 cm−1 and the low emission quenching temperature of 53 K. The activation energy for quenching changes with temperature, attributed to relative shifts of the two potential energy curves involved. The splitting of the Ce3+ 5d1 state into two levels separated by 4925 cm−1 is accounted for by a first principles calculation using the crystal structure data of I. The NO2− energy levels and spectra were investigated also in Cs2NaLa(NO2)6 and modelled by hybrid DFT. The vibrational and electronic spectral properties have been thoroughly investigated and rationalized at temperatures down to 10 K. A comparison of Stokes shifts with other Ce-O systems emphasizes the dependence upon the coordination number of Ce3+.
Highlights
Hexanitrito complexes of transition metals, such as [TM(NO2)6]4− TM = Cu2+, Co2+, exhibit Jahn-Teller distortion of the TM-N6 octahedron[1]
In view of the high coordination number of the lanthanide ion in the hexanitritolanthanate anion, we envisaged that the properties of the cerium complex would be of interest
The features due to NO2− and Ce3+ ions in the electronic spectra have been rationalized by theory
Summary
Hexanitrito complexes of transition metals, such as [TM(NO2)6]4− TM = Cu2+, Co2+, exhibit Jahn-Teller distortion of the TM-N6 octahedron[1] It was unexpected[2] that the analogous complexes of lanthanide ions (Ln3+) exhibit a different coordination geometry, with the ligand oxygen rather than nitrogen being coordinated to Ln3+. Since the Ln3+ ion is situated at a centrosymmetric site, pure and forced electric dipole allowed transitions are forbidden in the 4fN – 4fN optical spectra of hexanitritolanthanates. The electronic emission and absorption spectra comprise zero phonon lines enabled by the magnetic dipole mechanism together with sidebands of ungerade vibrations. The lowest energy zero phonon line of NO2− (at ~500 nm; ~20000 cm−1) is due to the spin-forbidden 1A1 → 3B1 (C2v) (S0 → T1) transition[9]. It was demonstrated that the Stokes shift is the largest ever reported for Ce3+ emission in oxygen coordination
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