Crystal structures and dielectric properties of ordered double perovskites containing Mg2+ and Ta5+

The ordered double perovskites ALaMgTaO6 (A=Ba, Sr, Ca) and La2Mg(Mg1/3Ta2/3)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 image for BaLaMgTaO6, and P21/n for SrLaMgTaO6, CaLaMgTaO6, and La2Mg(Mg1/3Ta2/3)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.