Energy transfer between different Eu2+ ions in the white phosphor Ba7F12Cl2:Eu2+

We have studied in detail the emission spectra of the white phosphor Ba7F12Cl2:Eu2+ as a function of Europium content, excitation wavelength and temperature. The change of the emission spectrum with excitation wavelength shows a systematic shift in the CIE chromaticity diagram from warm white upon excitation in the near UV (370 nm) to cold white upon excitation at shorter wavelengths. The observed intensity changes with europium concentration confirm that energy transfer takes place, which is both concentration and temperature dependent. Temperature and sample dependent lifetime studies show that the observed lifetimes do not change within experimental error between dilute and concentrated Eu-doped samples, and they remain constant between 5 K and room temperature. The second observation confirms the previous results that the thermal quenching of the white emission occurs at high temperature (200 degrees C). The combination of all observations suggests that the energy transfer takes place first between the normal 4f(6)5 d(1) states of the Eu ions located on the 3 different crystallographic sites of Ba, and is followed by subsequent relaxation to anomalous emission states whose emission lifetimes remain constant with sample concentration and temperature from 5 K to 300 K.

Automated summary

The study shows that the emission spectrum of Ba7F12Cl2:Eu2+ white phosphor shifts systematically from warm white to cold white with changes in excitation wavelength, and that energy transfer is concentration and temperature dependent. The observation of constant emission lifetimes at different temperatures and concentrations suggests that energy transfer first occurs between the normal states of the Eu ions, followed by relaxation to anomalous emission states.