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

The ordered double perovskites ALaMgTaO(6) (A = Ba, Sr, Ca) and La2Mg(Mg1/3Ta2/3)O-6 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 (Mg-2/Ta5+) and a random distribution of A-site cations (A(2)/La-)(3+). The space group symmetries are determined to be Fm $(3) over bar $m for BaLaMgTaO6, and P2(1)/n for SrLaMgTaO6, CaLaMgTaO6, and La2Mg(Mg1/3Ta2/3)O-6. 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 kappa = 18-23, dielectric losses of tan (delta = 0.004-0.007, and small temperature coefficients of capacitance < 100 ppm/K over the temperature range 20-150 degrees C. BaLaMgTaO6 is of particular interest because it possesses a near-zero temperature dependence of capacitance. (C) 2007 Elsevier Inc. All rights reserved.