Using X-ray diffraction analysis, scanning electron microscopy, thermogravimetry, and measurements of the dielectric properties up to the MW frequency range, the characterization of ..., ..., and ... compounds, which all include ..., was performed. As the processes of ... reduction and the evaporation of ...-containing species contribute to the presence of secondary phases, the preparation of single-phase ceramics is rather difficult. To minimize the amount of secondary phases during the firing process, the pellets were muffled in a corresponding compound and then fired in an autoclave furnace under 10 bars of oxygen pressure. By sintering the ..., ..., and ... between 840... and 1010...C, ceramics with ... ranging from 36 to 350, Q x f values from 220 to 12 500 GHz, and ... from +41 to +2600 ppm/K were obtained. (ProQuest: ... denotes formulae/symbols omitted.)