Research on materials for energy applications represents one of the most fast-growing and important topics in materials science. This thesis focuses on investigations of, so called, phosphor materials for use in white-light Light Emitting Diodes (LEDs). These devices aim to replace the old and inefficient white lighting technologies that have been phased out (incandescent lamps) or are problematic from an environmental point of view (compact fluorescent lamps). However, a poor correlated color temperature and a poor color rendering index, which are measures of how warm and natural the light is perceived by the human eye, and a too low thermal stability of present-day white-light LED devices, hamper their wider usage and market breakthrough. The development of new devices depends on a better understanding of the fundamental properties of the light-matter interactions, but such understanding is, at present, lacking.
In order to take on this challenge, I have investigated in detail key fundamental properties, such as the crystal structure and dynamics of atoms and their correlation to light-matter interactions in a family of promising phosphors. In particular, I have used a combination of advanced experimental techniques and theoretical methodologies, available at Chalmers as well as at large-scale research facilities in the Netherlands, United Kingdom, and the United States, that allowed to obtain unique insights into the effects of atomic structure and dynamics on both the color and intensity of the emitted light generated in white-light LED devices. Generally, the results show that the color and intensity of the emitted light of the studied phosphors are very sensitive to even minute changes of the composition of the material as well as on temperature. More specifically, it is shown that an increase in the "local structural and dynamics disorder", as reflected by, e.g., a softening of the chemical bonds of the crystal lattice and a decrease of the vibrational frequency of certain vibrational modes of the material, is correlated with a general decrease of the emission intensity. Therefore, the new understanding of the relationships between the light-matter interactions and optical properties established in this thesis provides effective design principles for the future developments of new, more efficient, LED materials.