Interionic energy transfer by electric multipole interaction in rare-earth pentaphosphates

Abstract

We present a thorough investigation of nonradiative energy-transfer processes in various rare-earth ($R$) pentaphosphates ($R{\mathrm{P}}_{5}{\mathrm{O}}_{14}$). Using time-resolved fluorescence spectroscopy, different crystals with high and low concentration of the interacting ${R}^{3+}$ ions were investigated. It turns out that energy transfer in $R{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ causes both spatial energy migration of excited states and fluorescence quenching. At high rare-earth concentration the concentration dependence of fluorescence quenching is shown to be governed by fast energy migration. From low-concentration measurements the dominant interionic coupling mechanism could be determined employing a microscopic picture for the energy-transfer process. A particular statistical model is introduced to combine the results obtained in the low- and high-concentration limit. The investigations yield that energy transfer in $R{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ is due to electric multipole interactions within the entire range of rare-earth concentrations, even at interionic spacings of 5 \AA{}.