The dielectic response of K _x Al _x Ti_8−x O₁₆ and K _x Mg _x/2 Ti_8−x/2 O₁₆

Abstract

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8−x O₁₆ and K_x Mg_x/2 Ti_8−x/2 O₁₆ have been synthesized in the composition range 1.6⩽x⩽2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10⁻³ to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the “universal” dielectric relation in which the loss follows the law x″(ω) ∢ ωⁿ⁻¹, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.