Are activators in micron-sized particles insensitive to their environment? Electronic spectra of LaPO₄:Eu³⁺ particles in liquids, glasses and phosphor in glasses

Abstract

The behavior of nanoparticles in media of different refractive index has received considerable theoretical and experimental investigation. However, the behavior of larger particles, of micron size, has not been established. We have chosen Eu3+ as a marker of environmental changes. Dilution of this ion into LaPO4 gives a simple, fingerprint emission spectrum which has been studied via its luminescent lifetime, the R2 ((5D0 → 7F2)/(5D0 → 7F1) intensity ratio) and the electric dipole:magnetic dipole intensity ratio, (ED:MD), of spectral features. Measurements of micron-sized particles of LaPO4:Eu3+ synthesized by several methods have been made for dispersions in alcohols, in compressed glass discs, in melted glass discs, in phosphor in glass (PiG) and in melted PiG. The experimental results are discussed and rationalized. The Eu3+ 5D0 emission lifetime is reasonably constant in the bulk powders, powders dispersed in alcohols or compressed discs but is smaller in PiGs and melted glass discs. In the PiGs the structural integrity of LaPO4:Eu3+ is maintained until they are melted at higher temperature. The change in lifetime, when varying the composition of the PiG, follows the Virtual Cavity Model. For dispersal of the particles in solvents there is a cut-off particle size below 1 μm where the electronic spectra of the moiety become insensitive to the environment and the properties are determined by refractive index. The remarkable change in R2 ratio in the different systems considered – by a factor of nearly 600 % - and the corresponding decrease in 5D0 lifetime – by a factor of nearly 60 % - illustrates the versatility of the lanthanide ion.