Dynamic local structural symmetries and luminescence properties of the yellow emitting phosphor Ce3+-doped Y3Al5O12

Licentiate thesis, 2016

This thesis centers on investigations of the local structure and vibrational properties of the oxide garnet Y3Al5O12 (YAG), which when substituted with a few percent of the activator ion Ce3+ to replace Y3+ (Y3−xCexAl5O12, YAG:Ce3+) is one of the most important phosphors for solid state lighting. The study builds on a comprehensive analysis of the nature of the phonons and localized vibrational modes in YAG:Ce3+ and how these depend on the Ce3+ concentration and temperature and how they affect key optical properties, such as the intensity and frequency of the emitted light. The investigations have been carried out by using a combination of Raman, infrared, luminescence, and neutron spectroscopies, together with mode-selective vibrational excitation, and are further supported by computer modeling. The results show that the static and dynamic structure of YAG:Ce3+ are dependent on both Ce3+ concentration and temperature. The substitution of the heavier Ce3+ ions compared to Y3+ is found to lower the vibrational frequencies of most of the phonons and localized vibrational modes around the Ce3+ ions, implying that they become more populated at a given temperature. As the temperature increases, vibrational modes of higher and higher frequency are activated, some of which induce significant dynamical tetragonal distortions around the Ce3+ ions. These distortions are shown to lead to a red-shift of the frequency of the emitted light. In addition, some of the high-frequency phonons are shown to be notably related to non-radiative relaxation of the excited-state electrons of the Ce3+ ions via electron-phonon coupling, which decreases the emission intensity when these phonons are activated. The reduction in emission intensity at elevated temperature is however a complicated process as it is found to relate also to thermal ionization of the excited-state electrons into the conduction band of the host crystal, which may be followed by charge trapping by defects.