Luminescence spectroscopy of anomalous Eu3+ in ternary aluminates: Experimental evidence for covalent bonding of 4f-orbitals of lanthanides

Luminescence spectroscopy of Eu3+ doped into strontium and calcium aluminate nanoparticles (NPs) sheds light on two longstanding arguments: 1) covalent bonding of the 4f-orbitals, and 2) the so-called anomalous luminescence of Eu3+ ion. Similar to a few reports on crystalline compounds in the literature, we observe an intense 5D0→7F0 emission accompanied with a low-energy oxygen to Eu3+ charge transfer (CT) band, however, in the amorphous phase. In the strontium aluminate NPs, the 5D0→7F0 transition appears at a record-high energy of 17675 cm−1 (565.75 nm), whereas its corresponding CT band can shift to a record low energy of 29200 cm−1 (342 nm). Systematic investigation of these peculiarities leads to unprecedented information about the local environment of the anomalous Eu3+. Spectroscopic probing of the Eu3+ local environment suggests that the electropositivity of one specific neighboring metal-ion determines the energy of the 5D0→7F0 transition through electron delocalization, which directly involves the 4f-orbitals of the Eu3+ ion.