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 tillx = 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 0025 to 003 between 0 ≶x ≶ 1 but increases to 017 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 thex = 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 thex = 0 composition and 17.7 for thex = 2 composition.