Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy1 - xCax)(2)Ti2O7 (-) (delta) (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 degrees C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5-500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500-600 degrees C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 degrees C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy2Ti2O7 leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively. (C) 2010 Elsevier B.V. All rights reserved.