Synthesis and luminescent properties of Eu3+- and Eu2+-activated sodium pyrophosphate Na4P2O7

The luminescent properties of Eu3+ and Eu2+ ions in sodium pyrophosphate, Na4P2O7, have been studied. The excitation spectrum of the Eu3+ emission in Na4P2O7 consists of several sets of bands in the range 280–535 nm due to 4f–4f transitions of Eu3+ ions and a broad band with a maximum at about 240 nm interpreted to be due to a charge transfer (CT) transition from oxygen 2p states to empty states of the Eu3+ 4f6-configuration. Although the CT band energy is large enough, the quantum efficiency (η) of the Eu3+ emission in Na4P2O7 under CT excitation was estimated to be very low (η ≤ 0.01). In terms of a configurational coordinate model, this fact is interpreted as a result of the high efficiency of a radiationless relaxation from the CT state to the 7F0 ground state of Eu3+ ions occupying sodium sites in Na4P2O7. A strong reducing agent is required in order to stabilize Eu2+ ions in Na4P2O7 during the synthesis. Several nonequivalent Eu2+ luminescence centers in Na4P2O7 were found.