High-pressure investigations of materials have been shown to be a valuable tool concerning the synthesis of new compounds with fascinating structures and outstanding properties. Besides contributions to the well-studied field of rare-earth oxoborates, the main goal of this thesis was to establish the high-pressure/high-temperature route for rare-earth fluoride and fluorido borates. Here, only two different rare-earth fluoride borates, synthesized under ambient pressure, were known at the beginning of my investigations. Systematic research on this field led to numerous new compounds with...