Phase equilibria in the BaO–MgO–Ta2O5 system

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, Ba₁₀Mg_0.25Ta_7.9O₃₀, 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 Ba_6.63Ta_34.95O₉₅ 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.