The structures of TaO6 and NbO6 oxygen octahedra in xBa(Mg1/3Ta2/3)O3+(1−x)Ba(Mg1/3Nb2/3)O3 perovskite ceramics with x = 0, 0.25, 0.50, 0.75, and 1.0 were investigated by the extended x-ray absorption fine structure method. The decline in the microwave dielectric constant as x increases is caused mainly by the decrease of the mean volume of the oxygen octahedra, regardless of the 1:2 ordered structure and the distortion of the oxygen octahedron. The low Qf values of the TaO6 and NbO6 mixed samples are caused by not only the degrading of the 1:2 ordered structure but also the distortion of oxygen octahedral cages.
