Surprising luminescent properties of the polyphosphates Ln(PO3)3 : Eu (Ln = Y, Gd, Lu)

Abstract

The optical emission properties of the lanthanoid catena-polyphosphates Ln(PO₃)₃ (Ln = Y, Gd, Lu) doped with europium were investigated. Incommensurately modulated β-Y(PO₃)₃ : Eu (super space groupCc (0|0.364|0)0) and Gd(PO₃)₃ : Eu (space groupI2/a) show the usual emission characteristics of Eu³⁺, while in Lu(PO₃)₃ : Eu (space groupCc) the europium is unprecedentedly partially reduced to the divalent state, as proven by both a broad emission band at 406 nm excited at 279 nm and an EPR spectroscopic investigation. ¹⁵¹Eu-Mössbauer spectroscopy showed that only a very small part of the europium is reduced in Lu(PO₃)₃ : Eu. An explanation for this unusual behaviour is given.