Phase analysis and dielectric properties of oxides obtained in the MgO-(1-x)Nb₂O₅-(x)Ta₂O₅ system

Abstract

MgNb_2-xTa_xO₆ (0 ≤x ≤2) phases can be obtained as the major phase (75 to 90%) by solid state reactions starting from oxides. These oxides crystallize in the orthorhombic columbite structure till x=1·75 and the tetragonal trirutile structure for MgTa₂O₆ (x = 2·0). For all the compositions there exist secondary phases like Nb₂O₅ or Ta₂O₅ in addition to the major AB₂O₆ phase. Sintered disks (1200°C) show dielectric constants varying between 14·8 and 16·0 for the entire range of composition at a frequency of 500 kHz. The dielectric loss is nearly constant around 0·025 to 0·03 between 0 ≤x ≤ 1 but increases to 0·17 for the MgTa₂O₆ phase (x= 2·0). Scanning electron micrographs reveal a gradual decrease in grain size with increase in Ta concentration with a size of 3 micron forthe x=0 composition (sintered at 1200°C) while the x=2 phase shows a grain size of approximately 0·5 microns. The microwave dielectric constant at ~14 GHz is found to be 20·9 for the x=0 composition and 17·7 for the x=2 composition.