Mixed occupation, double valence and tunable emission of Eu in BaIn2(P2O7)2

The luminescence behavior of rare earth Eu as the famous fluorescent probe not only depends on its chemical valence, but also on the concentration and occupation in the lattice, except for the interaction with the host. Herein, previously reported monoclinic BaIn2(P2O7)2 (BIP) phosphate acts as the host, and part of In atoms have been experimentally designed to be replaced by Eu. The lattice occupation and luminescence behavior of Eu in BIP were discussed. In the PL spectra, the intrinsic emission of BIP and the emission of Eu2+ were observed in addition to the characteristic emission of Eu3+, all of which were also emerged in the cathodoluminescence spectrum. EPR analysis confirmed the existence of Eu2+, that is, Eu3+ is spontaneously reduced in the BIP lattice. The red/orange ratio of Eu3+ characteristic emission changes with the change of concentration, indicating the evolution of lattice occupation. The charge density before and after Eu doped BIP was analyzed by DFT calcu-lation. From the perspective of charge exchange, it was further confirmed that partially Eu3+ was reduced to Eu2+ at different In or Ba crystallographic sites. The present work demonstrates a unique case of mixed occu-pation, double valence and tunable emission of Eu in phosphate.