Apparent giant dielectric constants, dielectric relaxation, and ac-conductivity of hexagonal perovskites La1.2Sr2.7 BO7.33 (B=Ru, Ir)

We present a thorough dielectric investigation of the hexagonal perovskites La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature range, supplemented by additional infrared measurements. The occurrence of giant dielectric constants up to 105 is revealed to be due to electrode polarization. Aside of dc and ac conductivity contributions, we detect two intrinsic relaxation processes that can be ascribed to ionic hopping between different off-center positions. In both materials we find evidence for charge transport via hopping of localized charge carriers. In the infrared region, three phonon bands are detected, followed by several electronic excitations. In addition, these materials provide further examples for the occurrence of a superlinear power law in the broadband ac conductivity, which recently was proposed to be a universal feature of all disordered matter. - Graphical abstract: Frequency-dependent dielectric constant and conductivity of La1.2Sr2.7RuO7.33 at various temperatures. A frequency range of more than 16 decades is covered, revealing giant values of the dielectric constant, hopping conduction, a superlinear power law of the conductivity, and phononic and electronic excitations in the infrared region