Abstract
We report on the phase relation in the BaO–MgO–Ta2O5 system equilibrated at 1450 °C. We found three ternary phases, namely 3:1:1 perovskite Ba3MgTa2O9, 9:1:7 TTB (tetragonal tungsten bronze)-type Ba9MgTa14O45 and 4:1:5 Ba4MgTa10O30 closely related to the tungsten bronze structure. Another ternary phase, Ba10Mg0.25Ta7.9O30, with a 10 layer hexagonal perovskite structure can be only obtained after equilibration at 1600 °C. MgO solubility in the TTB-like Ba4Ta10O29, BaTa4O11 and Ba6.63Ta34.95O95 does not exceed 2.0 mol%. Only the hexagonal polymorph of the BaTa2O6 phase was found along the Mg4Ta2O9–BaTa2O6 join. Measurements of dielectric properties in the 20 Hz–2 MHz range revealed that Ba9MgTa14O45 has a dielectric constant of 99 with no frequency dispersion and low dielectric loss. We have also clarified several contradictory reports related to the ternary and binary phases in the BaO–MgO–Ta2O5 composition field.
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