This PhD thesis presents experimental results from the synthesis, crystal structure and microwave dielectric characterization of BaO-rich phases within the BaO-Nb2O5 and BaONb2O5- WO3 system. I was primarily interested in two compounds with unknown crystal structure, i.e., the binary Ba4Nb2O9 and the ternary Ba6Nb2WO14, which lie on a common virtual …