Phase Relationships in the BaO–Ga2O3–Ta2O5 System and the Structure of Ba6Ga21TaO40

Phase relationships in the BaO–Ga2O3–Ta2O5 ternary system at 1200 °C were determined. The A6B10O30 tetragonal tungsten bronze (TTB) related solution in the BaO–Ta2O5 subsystem dissolved up to ∼11 mol % Ga2O3, forming a ternary trapezoid-shaped TTB-related solid solution region defined by the BaTa2O6, Ba1.1Ta5O13.6, Ba1.58Ga0.92Ta4.08O13.16, and Ba6GaTa9O30 compositions in the BaO–Ga2O3–Ta2O5 system. Two ternary phases Ba6Ga21TaO40 and eight-layer twinned hexagonal perovskite solid solution Ba8Ga4–xTa4+0.6xO24 were confirmed in the BaO–Ga2O3–Ta2O5 system. Ba6Ga21TaO40 crystallized in a monoclinic cell of a = 15.9130(2) Å, b = 11.7309(1) Å, c = 5.13593(6) Å, β = 107.7893(9)°, and Z = 1 in space group C2/m. The structure of Ba6Ga21TaO40 was solved by the charge flipping method, and it represents a three-dimensional (3D) mixed GaO4 tetrahedral and GaO6/TaO6 octahedral framework, forming mixed 1D 5/6-fold tunnels that accommodate the Ba cations along the c axis. The electrical property of Ba6Ga21TaO40 was characterized by using ac impedance spectroscopy.