Crystal structures and dielectric properties of ordered double perovskites containing Mg and Ta

The ordered double perovskites ALaMgTaO₆ (A=Ba, Sr, Ca) and La₂Mg(Mg_1/3Ta_2/3)O₆ have been prepared and characterized. Synchrotron X-ray powder diffraction analyses show that all four compounds exhibit a rock-salt type ordering of the B-site cations (Mg²⁺/Ta⁵⁺) and a random distribution of A-site cations (A²⁺/La³⁺). The space group symmetries are determined to be for BaLaMgTaO₆, and P2₁/n for SrLaMgTaO₆, CaLaMgTaO₆, and La₂Mg(Mg_1/3Ta_2/3)O₆. Diffuse-reflectance spectroscopy shows these ordered perovskites have optical band gaps in the range of 4.6−4.8 eV. These values are roughly 1 eV wider than the ternary perovskite oxides of Ta⁵⁺ such as KTaO₃, due to narrowing of the conduction bandwidth which results from Mg²⁺/Ta⁵⁺ ordering. These compounds are insulators with dielectric permittivities of κ=18-23, dielectric losses of tan δ=0.004-0.007, and small temperature coefficients of capacitance <100 ppm/K over the temperature range 20-150 °C. BaLaMgTaO₆ is of particular interest because it possesses a near-zero temperature dependence of capacitance.