Metal-to-metal charge transfer emission, its mechanism and quenching in Y2Sn2O7:Ce3+

Abstract

Polycrystalline yttrium pyro-stannate phosphors with Ce3+ were studied with respect to their potential use as luminescent materials in solid state lighting (SSL) based on light emitting diodes (LEDs). Peculiar and previously not seen luminescence in this pyrochlore activated with trivalent cerium ions was observed at 525 nm and investigated. This unexpected emission was absent in single crystals of this material described earlier. The uncharacteristic emission was analyzed on the basis of temperature-dependent luminescence spectra and decay curves. The emission lifetime as recorded at 77 K and under a direct 4f → 5d excitation of Ce3+ is only 5 ns. It is independent of temperature up to 250–300 K, after which it shows strong temperature quenching and non-single exponential decay behavior. It was found that the luminescence observed is not that of the well-known and understood trivalent cerium interconfigurational 5d → 4f nature, but is rather assigned to the metal-to-metal charge transfer (MMCT) origin. The observation comes as a surprise as MMCT states in inorganic compounds were only known until now as efficient quenching states for luminescence and no reports on such emission have been published to date. The luminescence mechanism is demonstrated using the configurational coordinate diagram.