In Gießen, Hoppe continued his extensive research in the field of solid state chemistry with a focus on the synthesis and characterization of oxo- and fluorometalates of the alkali metals. During his research he published over 650 articles in international and national peer-review journals. In addition, he had been the scientific editor for the German Journal of Inorganic and General Chemistry (Zeitschrift für Anorganische und Allgemeine Chemie).